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First proton--proton collisions at the LHC as observed with the ALICE detector: measurement of the charged particle pseudorapidity density at sqrt(s) = 900 GeV

by The Alice Collaboration, K Aamodt, N Abel, U Abeysekara, A Abrahantes Quintana, … A Acero, D Adamova, M M Aggarwal, G Aglieri Rinella, A G Agocs, S Aguilar Salazar, Z Ahammed, A Ahmad, N Ahmad, S U Ahn, R Akimoto, A Akindinov, D Aleksandrov, B Alessandro, R Alfaro Molina, A Alici, E Almaraz Avina, J Alme, T Alt, V Altini, S Altinpinar, C Andrei, A Andronic, G Anelli, V Angelov, C Anson, T Anticic, F Antinori, S Antinori, K Antipin, D Antonczyk, P Antonioli, A Anzo, L Aphecetche, H Appelshauser, S Arcelli, R Arceo, A Arend, N Armesto, R Arnaldi, T Aronsson, I C Arsene, A Asryan, A Augustinus, R Averbeck, T C Awes, J Aysto, M D Azmi, S Bablok, M Bach, A Badala, Y W Baek, S Bagnasco, R Bailhache, R Bala, A Baldisseri, A Baldit, J Ban, R Barbera, G G Barnafoldi, L Barnby, V Barret, J Bartke, F Barile, M Basile, V Basmanov, N Bastid, B Bathen, G Batigne, B Batyunya, C Baumann, I G Bearden, B Becker, I Belikov, R Bellwied, E Belmont-Moreno, A Belogianni, L Benhabib, S Beole, I Berceanu, A Bercuci, E Berdermann, Y Berdnikov, L Betev, A Bhasin, A K Bhati, L Bianchi, N Bianchi, C Bianchin, J Bielcik, J Bielcikova, A Bilandzic, L Bimbot, E Biolcati, A Blanc, F Blanco, F Blanco, D Blau, C Blume, M Boccioli, N Bock, A Bogdanov, H Boggild, M Bogolyubsky, J Bohm, L Boldizsar, M Bombara, C Bombonati, M Bondila, H Borel, V Borshchov, C Bortolin, S Bose, L Bosisio, F Bossu, M Botje, S Bottger, G Bourdaud, B Boyer, M Braun, P Braun-Munzinger, L Bravina, M Bregant, T Breitner, G Bruckner, R Brun, E Bruna, G E Bruno, D Budnikov, H Buesching, K Bugaev, P Buncic, O Busch, Z Buthelezi, D Caffarri, X Cai, H Caines, E Camacho, P Camerini, M Campbell, V Canoa Roman, G P Capitani, G Cara Romeo, F Carena, W Carena, F Carminati, A Casanova Diaz, M Caselle, J Castillo Castellanos, J F Castillo Hernandez, V Catanescu, E Cattaruzza, C Cavicchioli, P Cerello, V Chambert, B Chang, S Chapeland, A Charpy, J L Charvet, S Chattopadhyay, S Chattopadhyay, M Cherney, C Cheshkov, B Cheynis, E Chiavassa, V Chibante Barroso, D D Chinellato, P Chochula, K Choi, M Chojnacki, P Christakoglou, C H Christensen, P Christiansen, T Chujo, F Chuman, C Cicalo, L Cifarelli, F Cindolo, J Cleymans, O Cobanoglu, J P Coffin, S Coli, A Colla, G Conesa Balbastre, Z Conesa Del Valle, E S Conner, P Constantin, G Contin, J G Contreras, Y Corrales Morales, T M Cormier, P Cortese, I Cortes Maldonado, M R Cosentino, F Costa, M E Cotallo, E Crescio, P Crochet, E Cuautle, L Cunqueiro, J Cussonneau, A Dainese, H H Dalsgaard, A Danu, I Das, S Das, A Dash, S Dash, G O V De Barros, A De Caro, G De Cataldo, J De Cuveland, A De Falco, M De Gaspari, J De Groot, D De Gruttola, A P De Haas, N De Marco, R De Rooij, S De Pasquale, G De Vaux, H Delagrange, G Dellacasa, A Deloff, V Demanov, E Denes, A Deppman, G D RErasmo, D Derkach, A Devaux, D Di Bari, C Di Giglio, S Di Liberto, A Di Mauro, P Di Nezza, M Dialinas, L Diaz, R Diaz, T Dietel, H Ding, R Divia, O Djuvsland, G Do Amaral Valdiviesso, V Dobretsov, A Dobrin, T Dobrowolski, B Donigus, I Dominguez, D M M Don, O Dordic, A K Dubey, J Dubuisson, L Ducroux, P Dupieux, A K Dutta Majumdar, M R Dutta Majumdar, D Elia, D Emschermann, A Enokizono, B Espagnon, M Estienne, D Evans, S Evrard, G Eyyubova, C W Fabjan, D Fabris, J Faivre, D Falchieri, A Fantoni, M Fasel, R Fearick, A Fedunov, D Fehlker, V Fekete, D Felea, B Fenton-Olsen, G Feofilov, A Fernandez Tellez, E G Ferreiro, A Ferretti, R Ferretti, M A S Figueredo, S Filchagin, R Fini, F M Fionda, E M Fiore, M Floris, Z Fodor, S Foertsch, P Foka, S Fokin, F Formenti, E Fragiacomo, M Fragkiadakis, U Frankenfeld, A Frolov, U Fuchs, F Furano, C Furget, M Fusco Girard, J J Gaardhoje, S Gadrat, M Gagliardi, A Gago, M Gallio, P Ganoti, M S Ganti, C Garabatos, C Garc, J Gebelein, R Gemme, M Germain, A Gheata, M Gheata, B Ghidini, P Ghosh, G Giraudo, P Giubellino, E Gladysz-Dziadus, R Glasow, P Glassel, A Glenn, R Gomez, H Gonzalez Santos, L H Gonzalez-Trueba, P Gonzalez-Zamora, S Gorbunov, Y Gorbunov, S Gotovac, H Gottschlag, V Grabski, R Grajcarek, A Grelli, A Grigoras, C Grigoras, V Grigoriev, A Grigoryan, B Grinyov, N Grion, P Gros, J F Grosse-Oetringhaus, J Y Grossiord, R Grosso, C Guarnaccia, F Guber, R Guernane, B Guerzoni, K Gulbrandsen, H Gulkanyan, T Gunji, A Gupta, R Gupta, H A Gustafsson, H Gutbrod, O Haaland, C Hadjidakis, M Haiduc, H Hamagaki, G Hamar, J Hamblen, B H Han, J W Harris, M Hartig, A Harutyunyan, D Hasch, D Hasegan, D Hatzifotiadou, A Hayrapetyan, M Heide, M Heinz, H Helstrup, A Herghelegiu, C Hernandez, G Herrera Corral, N Herrmann, K F Hetland, B Hicks, A Hiei, P T Hille, B Hippolyte, T Horaguchi, Y Hori, P Hristov, I Hrivnacova, S Hu, S Huber, T J Humanic, D Hutter, D S Hwang, R Ichou, R Ilkaev, I Ilkiv, P G Innocenti, M Ippolitov, M Irfan, C Ivan, A Ivanov, M Ivanov, V Ivanov, T Iwasaki, A Jacholkowski, P M Jacobs, L Jancurova, S Jangal, R Janik, K Jayananda, C Jena, S Jena, L Jirden, G T Jones, P G Jones, P Jovanovi, H Jung, W Jung, A Jusko, A B Kaidalov, S Kalcher, P Kali, T Kalliokoski, A Kalweit, A Kamal, R Kamermans, K Kanaki, E Kang, J H Kang, J Kapitan, V Kaplin, S Kapusta, T Karavicheva, E Karpechev, A Kazantsev, U Kebschull, R Keidel, M M Khan, S A Khan, A Khanzadeev, Y Kharlov, D Kikola, B Kileng, D J Kim, D S Kim, D W Kim, H N Kim, J Kim, J H Kim, J S Kim, M Kim, M Kim, S H Kim, S Kim, Y Kim, S Kirsch, I Kisel, S Kiselev, A Kisiel, J L Klay, J Klein, C Klein, M Kliemant, A Klovning, A Kluge, S Kniege, K Koch, R Kolevatov, A Kolojvari, V Kondratiev, N Kondratyeva, A Konevskih, E Kornas, R Kour, M Kowalski, S Kox, K Kozlov, J Kral, I Kralik, F Kramer, I Kraus, A Kravcakova, T Krawutschke, M Krivda, D Krumbhorn, M Krus, E Kryshen, M Krzewicki, Y Kucheriaev, C Kuhn, P G Kuijer, L Kumar, N Kumar, R Kupczak, P Kurashvili, A Kurepin, A N Kurepin, A Kuryakin, S Kushpil, V Kushpil, M Kutouski, H Kvaerno, M J Kweon, Y Kwon, P La Rocca, F Lackner, P Ladron De Guevara, V Lafage, C Lal, C Lara, D T Larsen, G Laurenti, C Lazzeroni, Y Le Bornec, N Le Bris, H Lee, K S Lee, S C Lee, F Lefevre, M Lenhardt, L Leistam, J Lehnert, V Lenti, H Leon, I Leon Monzon, H Leon Vargas, P Levai, Y Li, R Lietava, S Lindal, V Lindenstruth, C Lippmann, M A Lisa, O Listratenko, L Liu, V Loginov, S Lohn, X Lopez, M Lopez Noriega, R Lopez-Ramirez, E Lopez Torres, G Lovhoiden, A Lozea Feijo Soares, S Lu, M Lunardon, G Luparello, L Luquin, J R Lutz, M Luvisetto, K Ma, R Ma, D M Madagodahettige-Don, A Maevskaya, M Mager, A Mahajan, D P Mahapatra, A Maire, I Makhlyueva, D Mal RKevich, M Malaev, I Maldonado Cervantes, M Malek, T Malkiewicz, P Malzacher, A Mamonov, L Manceau, L Mangotra, V Manko, F Manso, V Manzari, Y Mao, J Mares, G V Margagliotti, A Margotti, A Marin, I Martashvili, P Martinengo, M I Martinez, A Martinez Davalos, G Martinez Garcia, Y Maruyama, A Marzari Chiesa, S Masciocchi, M Masera, M Masetti, A Masoni, L Massacrier, M Mastromarco, A Mastroserio, Z L Matthews, B Mattos Tavares, A Matyja, D Mayani, G Mazza, M A Mazzoni, F Meddi, A Menchaca-Rocha, P Mendez Lorenzo, M Meoni, J Mercado Perez, P Mereu, Y Miake, A Michalon, N Miftakhov, J Milosevic, F Minafra, A Mischke, D Miskowiec, C Mitu, K Mizoguchi, J Mlynarz, B Mohanty, L Molnar, M M Mondal, L Montano Zetina, M Monteno, E Montes, M Morando, S Moretto, A Morsch, T Moukhanova, V Muccifora, E Mudnic, S Muhuri, H Muller, M G Munhoz, J Munoz, L Musa, A Musso, B K Nandi, R Nania, E Nappi, F Navach, S Navin, T K Nayak, S Nazarenko, G Nazarov, A Nedosekin, F Nendaz, J Newby, A Nianine, M Nicassio, B S Nielsen, S Nikolaev, V Nikolic, S Nikulin, V Nikulin, B S Nilsen, M S Nilsson, F Noferini, P Nomokonov, G Nooren, N Novitzky, A Nyatha, C Nygaard, A Nyiri, J Nystrand, A Ochirov, G Odyniec, H Oeschler, M Oinonen, K Okada, Y Okada, M Oldenburg, J Oleniacz, C Oppedisano, F Orsini, A Ortiz Velazquez, G Ortona, C Oskamp, A Oskarsson, F Osmic, L Osterman, P Ostrowski, I Otterlund, J Otwinowski, G Ovrebekk, K Oyama, K Ozawa, Y Pachmayer, M Pachr, F Padilla, P Pagano, G Paic, F Painke, C Pajares, S Pal, S K Pal, A Palaha, A Palmeri, R Panse, G S Pappalardo, W J Park, B Pastircak, C Pastore, V Paticchio, A Pavlinov, T Pawlak, T Peitzmann, A Pepato, H Pereira, D Peressounko, C Perez, D Perini, D Perrino, W Peryt, J Peschek, A Pesci, V Peskov, Y Pestov, A J Peters, V Petracek, A Petridis, M Petris, P Petrov, M Petrovici, C Petta, J Peyre, S Piano, A Piccotti, M Pikna, P Pillot, L Pinsky, N Pitz, F Piuz, R Platt, M Ploskon, J Pluta, T Pocheptsov, S Pochybova, P L M Podesta Lerma, F Poggio, M G Poghosyan, K Polak, B Polichtchouk, P Polozov, V Polyakov, B Pommeresch, A Pop, F Posa, V Pospisil, B Potukuchi, J Pouthas, S K Prasad, R Preghenella, F Prino, C A Pruneau, I Pshenichnov, G Puddu, P Pujahari, A Pulvirenti, A Punin, V Punin, M Putis, J Putschke, E Quercigh, A Rachevski, A Rademakers, S Radomski, T S Raiha, J Rak, A Rakotozafindrabe, L Ramello, A Ramirez Reyes, M Rammler, R Raniwala, S Raniwala, S Rasanen, I Rashevskaya, S Rath, K F Read, J Real, K Redlich, R Renfordt, A R Reolon, A Reshetin, F Rettig, J P Revol, K Reygers, H Ricaud, L Riccati, R A Ricci, M Richter, P Riedler, W Riegler, F Riggi, A Rivetti, M Rodriguez Cahuantzi, K Roed, D Rohrich, S Roman Lopez, R Romita, F Ronchetti, P Rosinsky, P Rosnet, S Rossegger, A Rossi, F Roukoutakis, S Rousseau, C Roy, P Roy, A J Rubio-Montero, R Rui, I Rusanov, G Russo, E Ryabinkin, A Rybicki, S Sadovsky, K Safarik, R Sahoo, J Saini, P Saiz, D Sakata, C A Salgado, R Salgueiro Dominques Da Silva, S Salur, T Samanta, S Sambyal, V Samsonov, L Sandor, A Sandoval, M Sano, S Sano, R Santo, R Santoro, J Sarkamo, P Saturnini, E Scapparone, F Scarlassara, R P Scharenberg, C Schiaua, R Schicker, H Schindler, C Schmidt, H R Schmidt, S Schreiner, S Schuchmann, J Schukraft, Y Schutz, K Schwarz, K Schweda, G Scioli, E Scomparin, G Segato, D Semenov, S Senyukov, J Seo, S Serci, L Serkin, E Serradilla, A Sevcenco, I Sgura, G Shabratova, R Shahoyan, G Sharkov, N Sharma, S Sharma, K Shigaki, M Shimomura, K Shtejer, Y Sibiriak, M Siciliano, E Sicking, E Siddi, T Siemiarczuk, A Silenzi, D Silvermyr, E Simili, G Simonetti, R Singaraju, R Singh, V Singhal, B C Sinha, T Sinha, B Sitar, M Sitta, T B Skaali, K Skjerdal, R Smakal, N Smirnov, R Snellings, H Snow, C Sogaard, O Sokolov, A Soloviev, H K Soltveit, R Soltz, W Sommer, C W Son, H S Son, M Song, C Soos, F Soramel, D Soyk, M Spyropoulou-Stassinaki, B K Srivastava, J Stachel, F Staley, I Stan, G Stefanek, G Stefanini, T Steinbeck, E Stenlund, G Steyn, D Stocco, R Stock, P Stolpovsky, P Strmen, A A P Suaide, M A Subieta Vasquez, T Sugitate, C Suire, M Sumbera, T Susa, D Swoboda, T J M Symons, A Szanto De Toledo, I Szarka, A Szostak, M Szuba, M Tadel, C Tagridis, A Takahara, J Takahashi, R Tanabe, J D Tapia Takaki, H Taureg, A Tauro, M Tavlet, G Tejeda Munoz, A Telesca, C Terrevoli, J Thader, R Tieulent, D Tlusty, A Toia, T Tolyhy, C Torcato De Matos, H Torii, G Torralba, L Toscano, F Tosello, A Tournaire, T Traczyk, P Tribedy, G Troger, D Truesdale, W H Trzaska, G Tsiledakis, E Tsilis, T Tsuji, A Tumkin, R Turrisi, A Turvey, T S Tveter, H Tydesjo, K Tywoniuk, J Ulery, K Ullaland, A Uras, J Urban, G M Urciuoli, G L Usai, A Vacchi, M Vala, L Valencia Palomo, S Vallero, A Van Den Brink, N Van Der Kolk, P Vande Vyvre, M Van Leeuwen, L Vannucci, A Vargas, R Varma, A Vasiliev, I Vassiliev, M Vassiliou, V Vechernin, M Venaruzzo, E Vercellin, S Vergara, R Vernet, M Verweij, I Vetlitskiy, L Vickovic, G Viesti, O Vikhlyantsev, Z Vilakazi, O Villalobos Baillie, A Vinogradov, L Vinogradov, Y Vinogradov, T Virgili, Y P Viyogi, A Vodopianov, K Voloshin, S Voloshin, G Volpe, B Von Haller, D Vranic, J Vrlakova, B Vulpescu, B Wagner, V Wagner, L Wallet, R Wan, D Wang, Y Wang, Y Wang, K Watanabe, Q Wen, J Wessels, J Wiechula, J Wikne, A Wilk, G Wilk, M C S Williams, N Willis, B Windelband, C Xu, C Yang, H Yang, A Yasnopolsky, F Yermia, J Yi, Z Yin, H Yokoyama, I K Yoo, X Yuan, I Yushmanov, E Zabrodin, B Zagreev, A Zalite, C Zampolli, Yu Zanevsky, S Zaporozhets, A Zarochentsev, P Zavada, H Zbroszczyk, P Zelnicek, A Zenin, A Zepeda, I Zgura, M Zhalov, X Zhang, D Zhou, S Zhou, S Zhou, J Zhu, A Zichichi, A Zinchenko, G Zinovjev, M Zinovjev, Y Zoccarato, V Zychacek show all authors
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The European Physical Journal C (2009)
Volume: 65, Issue: 1-2, Pages: 111-125

Abstract

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range eta < 0.5, we obtain dNch/deta = 3.10 0.13 (stat.) 0.22 (syst.) for all inelastic interactions, and dNch/deta = 3.51 0.15 (stat.) 0.25 (syst.) for non-single diffractive interactions. These results are consistent with previous measurements in proton-antiproton interactions at the same centre-of-mass energy at the CERN SppS collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase.

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First proton--proton collisions at the LHC as observed with the ALICE detector: measurement of the charged particle pseudorapidity density at sqrt(s) = 900 GeV

ALICE Collaboration: First proton{proton collisions at the LHC as observed with the ALICE...
arXiv: 0911.5430 [hep-ex]
First proton{proton collisions at the LHC as observed with
the ALICE detector: measurement of the charged-particle
pseudorapidity density at
p
s = 900 GeV
ALICE collaboration
K. Aamodt78, N. Abel43, U. Abeysekara30, A. Abrahantes Quintana42, A. Acero63, D. Adamova86, M.M. Aggarwal25,
G. Aglieri Rinella40, A.G. Agocs18, S. Aguilar Salazar66, Z. Ahammed55, A. Ahmad2, N. Ahmad2, S.U. Ahn50i,
R. Akimoto100, A. Akindinov68, D. Aleksandrov70, B. Alessandro102, R. Alfaro Molina66, A. Alici13,
E. Almaraz Avi~na66, J. Alme8, T. Alt43ii, V. Altini6, S. Altinpinar32, C. Andrei17, A. Andronic32, G. Anelli40,
V. Angelov43ii, C. Anson27, T. Anticic113, F. Antinori40iii, S. Antinori13, K. Antipin37, D. Antonczyk37, P. Antonioli14,
A. Anzo66, L. Aphecetche73, H. Appelshauser37, S. Arcelli13, R. Arceo66, A. Arend37, N. Armesto92, R. Arnaldi102,
T. Aronsson74, I.C. Arsene78iv, A. Asryan98, A. Augustinus40, R. Averbeck32, T.C. Awes76, J. Aysto49, M.D. Azmi2,
S. Bablok8, M. Bach36, A. Badala24, Y.W. Baek50i, S. Bagnasco102, R. Bailhache32v, R. Bala101, A. Baldisseri89,
A. Baldit26, J. Ban58, R. Barbera23, G.G. Barnafoldi18, L. Barnby12, V. Barret26, J. Bartke29, F. Barile5, M. Basile13,
V. Basmanov94, N. Bastid26, B. Bathen72, G. Batigne73, B. Batyunya35, C. Baumann72v, I.G. Bearden28, B. Becker20vi,
I. Belikov99, R. Bellwied34, E. Belmont-Moreno66, A. Belogianni4, L. Benhabib73, S. Beole101, I. Berceanu17,
A. Bercuci32vii, E. Berdermann32, Y. Berdnikov39, L. Betev40, A. Bhasin48, A.K. Bhati25, L. Bianchi101, N. Bianchi38,
C. Bianchin79, J. Bielck81, J. Bielckova86, A. Bilandzic3, L. Bimbot77, E. Biolcati101, A. Blanc26, F. Blanco23viii,
F. Blanco63, D. Blau70, C. Blume37, M. Boccioli40, N. Bock27, A. Bogdanov69, H. Bggild28, M. Bogolyubsky83,
J. Bohm96, L. Boldizsar18, M. Bombara12ix, C. Bombonati79x, M. Bondila49, H. Borel89, V. Borshchov51,
C. Bortolin79, S. Bose54, L. Bosisio103, F. Bossu101, M. Botje3, S. Bottger43, G. Bourdaud73, B. Boyer77,
M. Braun98, P. Braun-Munzinger32;33ii, L. Bravina78, M. Bregant103xi, T. Breitner43, G. Bruckner40, R. Brun40,
E. Bruna74, G.E. Bruno5, D. Budnikov94, H. Buesching37, K. Bugaev52, P. Buncic40, O. Busch44, Z. Buthelezi22,
D. Ca arri79, X. Cai111, H. Caines74, E. Camacho64, P. Camerini103, M. Campbell40, V. Canoa Roman40,
G.P. Capitani38, G. Cara Romeo14, F. Carena40, W. Carena40, F. Carminati40, A. Casanova Daz38, M. Caselle40,
J. Castillo Castellanos89, J.F. Castillo Hernandez32, V. Catanescu17, E. Cattaruzza103, C. Cavicchioli40, P. Cerello102,
V. Chambert77, B. Chang96, S. Chapeland40, A. Charpy77, J.L. Charvet89, S. Chattopadhyay54, S. Chattopadhyay55,
M. Cherney30, C. Cheshkov40, B. Cheynis62, E. Chiavassa101, V. Chibante Barroso40, D.D. Chinellato21,
P. Chochula40, K. Choi85, M. Chojnacki106, P. Christakoglou106, C.H. Christensen28, P. Christiansen61,
T. Chujo105, F. Chuman45, C. Cicalo20, L. Cifarelli13, F. Cindolo14, J. Cleymans22, O. Cobanoglu101, J.-P. Con99,
S. Coli102, A. Colla40, G. Conesa Balbastre38, Z. Conesa del Valle73xii, E.S. Conner110, P. Constantin44,
G. Contin103x, J.G. Contreras64, Y. Corrales Morales101, T.M. Cormier34, P. Cortese1, I. Cortes Maldonado84,
M.R. Cosentino21, F. Costa40, M.E. Cotallo63, E. Crescio64, P. Crochet26, E. Cuautle65, L. Cunqueiro38,
J. Cussonneau73, A. Dainese59iii, H.H. Dalsgaard28, A. Danu16, I. Das54, S. Das54, A. Dash11, S. Dash11,
G.O.V. de Barros93, A. De Caro90, G. de Cataldo40xiii, J. de Cuveland43ii, A. De Falco19, M. de Gaspari44,
J. de Groot40, D. De Gruttola90, A.P. de Haas106 N. De Marco102, R. de Rooij106, S. De Pasquale90, G. de Vaux22,
H. Delagrange73, G. Dellacasa1, A. Delo 107, V. Demanov94, E. Denes18, A. Deppman93, G. D'Erasmo5, D. Derkach98,
A. Devaux26, D. Di Bari5, C. Di Giglio5x, S. Di Liberto88, A. Di Mauro40, P. Di Nezza38, M. Dialinas73, L. Daz65,
R. Daz49, T. Dietel72, H. Ding111, R. Divia40, . Djuvsland8, G. do Amaral Valdiviesso21, V. Dobretsov70,
A. Dobrin61, T. Dobrowolski107, B. Donigus32, I. Domnguez65, D.M.M. Don46 O. Dordic78, A.K. Dubey55,
J. Dubuisson40, L. Ducroux62, P. Dupieux26, A.K. Dutta Majumdar54, M.R. Dutta Majumdar55, D. Elia6,
D. Emschermann44xiv, A. Enokizono76, B. Espagnon77, M. Estienne73, D. Evans12, S. Evrard40, G. Eyyubova78,
C.W. Fabjan40xv, D. Fabris79, J. Faivre41, D. Falchieri13, A. Fantoni38, M. Fasel32, R. Fearick22, A. Fedunov35,
D. Fehlker8, V. Fekete15, D. Felea16, B. Fenton-Olsen28xvi, G. Feo lov98, A. Fernandez Tellez84, E.G. Ferreiro92,
A. Ferretti101, R. Ferretti1xvii, M.A.S. Figueredo93, S. Filchagin94, R. Fini6, F.M. Fionda5, E.M. Fiore5,
M. Floris19x, Z. Fodor18, S. Foertsch22, P. Foka32, S. Fokin70, F. Formenti40, E. Fragiacomo104, M. Fragkiadakis4,
U. Frankenfeld32, A. Frolov75, U. Fuchs40, F. Furano40, C. Furget41, M. Fusco Girard90, J.J. Gaardhje28, S. Gadrat41,
M. Gagliardi101, A. Gago64xviii, M. Gallio101, P. Ganoti4, M.S. Ganti55, C. Garabatos32, C. Garca Trapaga101,
J. Gebelein43, R. Gemme1, M. Germain73, A. Gheata40, M. Gheata40, B. Ghidini5, P. Ghosh55, G. Giraudo102,
P. Giubellino102, E. Gladysz-Dziadus29, R. Glasow72xix, P. Glassel44, A. Glenn60, R. Gomez31, H. Gonzalez Santos84,
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2L.H. Gonzalez-Trueba66, P. Gonzalez-Zamora63, S. Gorbunov43ii, Y. Gorbunov30, S. Gotovac97, H. Gottschlag72,
V. Grabski66, R. Grajcarek44, A. Grelli106, A. Grigoras40, C. Grigoras40, V. Grigoriev69, A. Grigoryan112,
B. Grinyov52, N. Grion104, P. Gros61, J.F. Grosse-Oetringhaus40, J.-Y. Grossiord62, R. Grosso80, C. Guarnaccia90,
F. Guber67, R. Guernane41, B. Guerzoni13, K. Gulbrandsen28, H. Gulkanyan112, T. Gunji100, A. Gupta48,
R. Gupta48, H.-A. Gustafsson61, H. Gutbrod32, . Haaland8, C. Hadjidakis77, M. Haiduc16, H. Hamagaki100,
G. Hamar18, J. Hamblen53, B.H. Han95, J.W. Harris74, M. Hartig37, A. Harutyunyan112, D. Hasch38, D. Hasegan16,
D. Hatzifotiadou14, A. Hayrapetyan112, M. Heide72, M. Heinz74, H. Helstrup9, A. Herghelegiu17, C. Hernandez32,
G. Herrera Corral64, N. Herrmann44, K.F. Hetland9, B. Hicks74, A. Hiei45, P.T. Hille78xx, B. Hippolyte99,
T. Horaguchi45xxi, Y. Hori100, P. Hristov40, I. Hrivnacova77, S. Hu7, S. Huber32, T.J. Humanic27, D. Hutter36,
D.S. Hwang95, R. Ichou73, R. Ilkaev94, I. Ilkiv107, P.G. Innocenti40, M. Ippolitov70, M. Irfan2, C. Ivan106,
A. Ivanov98, M. Ivanov32, V. Ivanov39, T. Iwasaki45, A. Jacho lkowski40, P. Jacobs10, L. Jancurova35, S. Jangal99,
R. Janik15, K. Jayananda30, C. Jena11, S. Jena71, L. Jirden40, G.T. Jones12, P.G. Jones12, P. Jovanovic12,
H. Jung50, W. Jung50, A. Jusko12, A.B. Kaidalov68, S. Kalcher43ii, P. Kalinak58, T. Kalliokoski49, A. Kalweit33,
A. Kamal2, R. Kamermans106, K. Kanaki8, E. Kang50, J.H. Kang96, J. Kapitan86, V. Kaplin69, S. Kapusta40,
T. Karavicheva67, E. Karpechev67, A. Kazantsev70, U. Kebschull43, R. Keidel110, M.M. Khan2, S.A. Khan55,
A. Khanzadeev39, Y. Kharlov83, D. Kikola108, B. Kileng9, D.J Kim49, D.S. Kim50, D.W. Kim50, H.N. Kim50,
J. Kim83, J.H. Kim95, J.S. Kim50, M. Kim50, M. Kim96, S.H. Kim50, S. Kim95, Y. Kim96, S. Kirsch40, I. Kisel43iv,
S. Kiselev68, A. Kisiel27x, J.L. Klay91, J. Klein44, C. Klein-Bosing40xiv, M. Kliemant37, A. Klovning8, A. Kluge40,
S. Kniege37, K. Koch44, R. Kolevatov78, A. Kolojvari98, V. Kondratiev98, N. Kondratyeva69, A. Konevskih67,
E. Kornas29, R. Kour12, M. Kowalski29, S. Kox41, K. Kozlov70, J. Kral81xi, I. Kralik58, F. Kramer37, I. Kraus33iv,
A. Kravcakova57, T. Krawutschke56, M. Krivda12, D. Krumbhorn44, M. Krus81, E. Kryshen39, M. Krzewicki3,
Y. Kucheriaev70, C. Kuhn99, P.G. Kuijer3, L. Kumar25, N. Kumar25, R. Kupczak108, P. Kurashvili107, A. Kurepin67,
A.N. Kurepin67, A. Kuryakin94, S. Kushpil86, V. Kushpil86, M. Kutouski35, H. Kvaerno78, M.J. Kweon44,
Y. Kwon96, P. La Rocca23xxii, F. Lackner40, P. Ladron de Guevara63, V. Lafage77, C. Lal48, C. Lara43, D.T. Larsen8,
G. Laurenti14, C. Lazzeroni12, Y. Le Bornec77, N. Le Bris73, H. Lee85, K.S. Lee50, S.C. Lee50, F. Lefevre73,
M. Lenhardt73, L. Leistam40, J. Lehnert37, V. Lenti6, H. Leon66, I. Leon Monzon31, H. Leon Vargas37, P. Levai18,
Y. Li7, R. Lietava12, S. Lindal78, V. Lindenstruth43ii, C. Lippmann40, M.A. Lisa27, O. Listratenko51, L. Liu8,
V. Loginov69, S. Lohn40, X. Lopez26, M. Lopez Noriega77, R. Lopez-Ramrez84, E. Lopez Torres42, G. Lvhiden78,
A. Lozea Feijo Soares93, S. Lu7, M. Lunardon79, G. Luparello101, L. Luquin73, J.-R. Lutz99, M. Luvisetto14,
K. Ma111, R. Ma74, D.M. Madagodahettige-Don46, A. Maevskaya67, M. Mager33x, A. Mahajan48, D.P. Mahapatra11,
A. Maire99, I. Makhlyueva40, D. Mal'Kevich68, M. Malaev39, I. Maldonado Cervantes65, M. Malek77, T. Malkiewicz49,
P. Malzacher32, A. Mamonov94, L. Manceau26, L. Mangotra48, V. Manko70, F. Manso26, V. Manzari40xxiii,
Y. Mao111xxiv, J. Mares82, G.V. Margagliotti103, A. Margotti14, A. Marn32, I. Martashvili53, P. Martinengo40,
M.I. Martnez84, A. Martnez Davalos66, G. Martnez Garca73, Y. Maruyama45, A. Marzari Chiesa101, S. Masciocchi32,
M. Masera101, M. Masetti13, A. Masoni20, L. Massacrier62, M. Mastromarco5, A. Mastroserio5x, Z.L. Matthews12,
B. Mattos Tavares21, A. Matyja29, D. Mayani65, G. Mazza102, M.A. Mazzoni88, F. Meddi87, A. Menchaca-Rocha66,
P. Mendez Lorenzo40, M. Meoni40, J. Mercado Perez44, P. Mereu102, Y. Miake105, A. Michalon99, N. Miftakhov39,
J. Milosevic78, F. Minafra5, A. Mischke106, D. Miskowiec32, C. Mitu16, K. Mizoguchi45, J. Mlynarz34, B. Mohanty55,
L. Molnar18x, M.M. Mondal55, L. Monta~no Zetina64xxv, M. Monteno102, E. Montes63, M. Morando79, S. Moretto79,
A. Morsch40, T. Moukhanova70, V. Muccifora38, E. Mudnic97, S. Muhuri55, H. Muller40, M.G. Munhoz93, J. Munoz84,
L. Musa40, A. Musso102, B.K. Nandi71, R. Nania14, E. Nappi6, F. Navach5, S. Navin12, T.K. Nayak55, S. Nazarenko94,
G. Nazarov94, A. Nedosekin68, F. Nendaz62, J. Newby60, A. Nianine70, M. Nicassio6x, B.S. Nielsen28, S. Nikolaev70,
V. Nikolic113, S. Nikulin70, V. Nikulin39, B.S. Nilsen27xxvi, M.S. Nilsson78, F. Noferini14, P. Nomokonov35,
G. Nooren106, N. Novitzky49, A. Nyatha71, C. Nygaard28, A. Nyiri78, J. Nystrand8, A. Ochirov98, G. Odyniec10,
H. Oeschler33, M. Oinonen49, K. Okada100, Y. Okada45, M. Oldenburg40, J. Oleniacz108, C. Oppedisano102,
F. Orsini89, A. Ortz Velazquez65, G. Ortona101, C. Oskamp106, A. Oskarsson61, F. Osmic40, L. Osterman61,
P. Ostrowski108, I. Otterlund61, J. Otwinowski32, G. vrebekk8, K. Oyama44, K. Ozawa100, Y. Pachmayer44,
M. Pachr81, F. Padilla101, P. Pagano90, G. Paic65, F. Painke43, C. Pajares92, S. Pal54xxvii, S.K. Pal55, A. Palaha12,
A. Palmeri24, R. Panse43, G.S. Pappalardo24, W.J. Park32, B. Pastircak58, C. Pastore6, V. Paticchio6, A. Pavlinov34,
T. Pawlak108, T. Peitzmann106, A. Pepato80, H. Pereira89, D. Peressounko70, C. Perez64xviii, D. Perini40,
D. Perrino5x, W. Peryt108, J. Peschek43ii, A. Pesci14, V. Peskov65x, Y. Pestov75, A.J. Peters40, V. Petracek81,
A. Petridis4xix, M. Petris17, P. Petrov12, M. Petrovici17, C. Petta23, J. Peyre77, S. Piano104, A. Piccotti102,
M. Pikna15, P. Pillot73, L. Pinsky46, N. Pitz37, F. Piuz40, R. Platt12, M. P loskon10, J. Pluta108, T. Pocheptsov35xxviii,
S. Pochybova18, P.L.M. Podesta Lerma31, F. Poggio101, M.G. Poghosyan101, T. Poghosyan112, K. Polak82,
B. Polichtchouk83, P. Polozov68, V. Polyakov39, B. Pommeresch8, A. Pop17, F. Posa5, V. Pospsil81, B. Potukuchi48,
J. Pouthas77, S.K. Prasad55, R. Preghenella13xxii, F. Prino102, C.A. Pruneau34, I. Pshenichnov67, G. Puddu19,
P. Pujahari71, A. Pulvirenti23, A. Punin94, V. Punin94, M. Putis57, J. Putschke74, E. Quercigh40, A. Rachevski104,
A. Rademakers40, S. Radomski44, T.S. Raiha49, J. Rak49, A. Rakotoza ndrabe89, L. Ramello1, A. Ramrez Reyes64,
M. Rammler72, R. Raniwala47, S. Raniwala47, S. Rasanen49, I. Rashevskaya104, S. Rath11, K. F. Read53, J. Real41,
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3K. Redlich107, R. Renfordt37, A.R. Reolon38, A. Reshetin67, F. Rettig43ii, J.-P. Revol40, K. Reygers72xxix,
H. Ricaud99xxx, L. Riccati102, R.A. Ricci59, M. Richter8, P. Riedler40, W. Riegler40, F. Riggi23, A. Rivetti102,
M. Rodriguez Cahuantzi84, K. Red9, D. Rohrich40xxxi, S. Roman Lopez84, R. Romita5iv, F. Ronchetti38,
P. Rosinsky40, P. Rosnet26, S. Rossegger40, A. Rossi103, F. Roukoutakis40xxxii, S. Rousseau77, C. Roy73xii,
P. Roy54, A.J. Rubio-Montero63, R. Rui103, I. Rusanov44, G. Russo90, E. Ryabinkin70, A. Rybicki29, S. Sadovsky83,
K. Safark40, R. Sahoo79, J. Saini55, P. Saiz40, D. Sakata105, C.A. Salgado92, R. Salgueiro Dominques da Silva40,
S. Salur10, T. Samanta55, S. Sambyal48, V. Samsonov39, L. Sandor58, A. Sandoval66, M. Sano105, S. Sano100,
R. Santo72, R. Santoro5, J. Sarkamo49, P. Saturnini26, E. Scapparone14, F. Scarlassara79, R.P. Scharenberg109,
C. Schiaua17, R. Schicker44, H. Schindler40, C. Schmidt32, H.R. Schmidt32, S. Schreiner40, S. Schuchmann37,
J. Schukraft40, Y. Schutz73, K. Schwarz32, K. Schweda44, G. Scioli13, E. Scomparin102, G. Segato79, D. Semenov98,
S. Senyukov1, J. Seo50, S. Serci19, L. Serkin65, E. Serradilla63, A. Sevcenco16, I. Sgura5, G. Shabratova35,
R. Shahoyan40, G. Sharkov68, N. Sharma25, S. Sharma48, K. Shigaki45, M. Shimomura105, K. Shtejer42, Y. Sibiriak70,
M. Siciliano101, E. Sicking40xxxiii, E. Siddi20, T. Siemiarczuk107, A. Silenzi13, D. Silvermyr76, E. Simili106,
G. Simonetti5x, R. Singaraju55, R. Singh48, V. Singhal55, B.C. Sinha55, T. Sinha54, B. Sitar15, M. Sitta1,
T.B. Skaali78, K. Skjerdal8, R. Smakal81, N. Smirnov74, R. Snellings3, H. Snow12, C. Sgaard28, O. Sokolov65,
A. Soloviev83, H.K. Soltveit44, R. Soltz60, W. Sommer37, C.W. Son85, H.S. Son95, M. Song96, C. Soos40, F. Soramel79,
D. Soyk32, M. Spyropoulou-Stassinaki4, B.K. Srivastava109, J. Stachel44, F. Staley89, I. Stan16, G. Stefanek107,
G. Stefanini40, T. Steinbeck43ii, E. Stenlund61, G. Steyn22, D. Stocco101xxxiv, R. Stock37, P. Stolpovsky83, P. Strmen15,
A.A.P. Suaide93, M.A. Subieta Vasquez101, T. Sugitate45, C. Suire77, M. Sumbera86, T. Susa113, D. Swoboda40,
J. Symons10, A. Szanto de Toledo93, I. Szarka15, A. Szostak20, M. Szuba108, M. Tadel40, C. Tagridis4, A. Takahara100,
J. Takahashi21, R. Tanabe105, J.D. Tapia Takaki77, H. Taureg40, A. Tauro40, M. Tavlet40, G. Tejeda Mu~noz84,
A. Telesca40, C. Terrevoli5, J. Thader43ii, R. Tieulent62, D. Tlusty81, A. Toia40, T. Tolyhy18, C. Torcato de Matos40,
H. Torii45, G. Torralba43, L. Toscano102, F. Tosello102, A. Tournaire73xxxv, T. Traczyk108, P. Tribedy55, G. Troger43,
D. Truesdale27, W.H. Trzaska49, G. Tsiledakis44, E. Tsilis4, T. Tsuji100, A. Tumkin94, R. Turrisi80, A. Turvey30,
T.S. Tveter78, H. Tydesjo40, K. Tywoniuk78, J. Ulery37, K. Ullaland8, A. Uras19, J. Urban57, G.M. Urciuoli88,
G.L. Usai19, A. Vacchi104, M. Vala35ix, L. Valencia Palomo66, S. Vallero44, A. van den Brink106, N. van der Kolk3,
P. Vande Vyvre40, M. van Leeuwen106, L. Vannucci59, A. Vargas84, R. Varma71, A. Vasiliev70, I. Vassiliev43xxxii,
M. Vassiliou4, V. Vechernin98, M. Venaruzzo103, E. Vercellin101, S. Vergara84, R. Vernet23xxxvi, M. Verweij106,
I. Vetlitskiy68, L. Vickovic97, G. Viesti79, O. Vikhlyantsev94, Z. Vilakazi22, O. Villalobos Baillie12, A. Vinogradov70,
L. Vinogradov98, Y. Vinogradov94, T. Virgili90, Y.P. Viyogi11xxxvii, A. Vodopianov35, K. Voloshin68, S. Voloshin34,
G. Volpe5, B. von Haller40, D. Vranic32, J. Vrlakova57, B. Vulpescu26, B. Wagner8, V. Wagner81, L. Wallet40,
R. Wan111xxiv, D. Wang111, Y. Wang44, Y. Wang111, K. Watanabe105, Q. Wen7, J. Wessels72, R. Wheadon102,
J. Wiechula44, J. Wikne78, A. Wilk72, G. Wilk107, M.C.S. Williams14, N. Willis77, B. Windelband44, C. Xu111,
C. Yang111, H. Yang44, A. Yasnopolsky70, F. Yermia73, J. Yi85, Z. Yin111, H. Yokoyama105, I-K. Yoo85,
X. Yuan111xxxviii, I. Yushmanov70, E. Zabrodin78, B. Zagreev68, A. Zalite39, C. Zampolli40xxxix, Yu. Zanevsky35,
Y. Zaporozhets35, A. Zarochentsev98, P. Zavada82, H. Zbroszczyk108, P. Zelnicek43, A. Zenin83, A. Zepeda64,
I. Zgura16, M. Zhalov39, X. Zhang111i, D. Zhou111, S. Zhou7, S. Zhou7, J. Zhu111, A. Zichichi13xxii, A. Zinchenko35,
G. Zinovjev52, M. Zinovjev52, Y. Zoccarato62, and V. Zychacek81
Aliation notes
iAlso at26
iiAlso at36
iiiNow at80
ivNow at32
vNow at37
viNow at22
viiNow at17
viiiAlso at46
ixNow at57
xNow at40
xiNow at49
xiiNow at99
xiiiNow at6
xivNow at72
xvNow at: University of Technology and Austrian Academy of Sciences, Vienna, Austria
xviAlso at60
xviiAlso at40
xviiiNow at: Seccion Fsica, Departamentode de Ciencias, Ponti cia Universidad Catolica del Peru, Lima, Peru
xixDeceased
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4xxNow at74
xxiNow at105
xxiiAlso at: Centro Fermi { Centro Studi e Ricerche e Museo Storico della Fisica \Enrico Fermi", Rome, Italy
xxiiiNow at5
xxivAlso at41
xxvNow at101
xxviNow at30
xxviiNow at89
xxviiiAlso at78
xxixNow at44
xxxNow at33
xxxiNow at8
xxxiiNow at4
xxxiiiAlso at72
xxxivNow at73
xxxvNow at62
xxxviNow at: Centre de Calcul IN2P3, Lyon, France
xxxviiNow at55
xxxviiiAlso at79
xxxixAlso at14
Collaboration institutes
1 Dipartimento di Scienze e Tecnologie Avanzate dell'Universita del Piemonte Orientale and Gruppo Collegato INFN, Alessan-
dria, Italy
2 Department of Physics Aligarh Muslim University, Aligarh, India
3 National Institute for Nuclear and High Energy Physics (NIKHEF), Amsterdam, Netherlands
4 Physics Department, University of Athens, Athens, Greece
5 Dipartimento Interateneo di Fisica `M. Merlin' and Sezione INFN, Bari, Italy
6 Sezione INFN, Bari, Italy
7 China Institute of Atomic Energy, Beijing, China
8 Department of Physics and Technology, University of Bergen, Bergen, Norway
9 Faculty of Engineering, Bergen University College, Bergen, Norway
10 Lawrence Berkeley National Laboratory, Berkeley, California, United States
11 Institute of Physics, Bhubaneswar, India
12 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
13 Dipartimento di Fisica dell'Universita and Sezione INFN, Bologna, Italy
14 Sezione INFN, Bologna, Italy
15 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
16 Institute of Space Sciences (ISS), Bucharest, Romania
17 National Institute for Physics and Nuclear Engineering, Bucharest, Romania
18 KFKI Research Institute for Particle and Nuclear Physics, Hungarian Academy of Sciences, Budapest, Hungary
19 Dipartimento di Fisica dell'Universita and Sezione INFN, Cagliari, Italy
20 Sezione INFN, Cagliari, Italy
21 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil
22 Physics Department, University of Cape Town, iThemba Laboratories, Cape Town, South Africa
23 Dipartimento di Fisica e Astronomia dell'Universita and Sezione INFN, Catania, Italy
24 Sezione INFN, Catania, Italy
25 Physics Department, Panjab University, Chandigarh, India
26 Laboratoire de Physique Corpusculaire (LPC), Clermont Universite, Universite Blaise Pascal, CNRS{IN2P3, Clermont-
Ferrand, France
27 Department of Physics, Ohio State University, Columbus, Ohio, United States
28 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
29 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland
30 Physics Department, Creighton University, Omaha, Nebraska, United States
31 Universidad Autonoma de Sinaloa, Culiacan, Mexico
32 ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum fur Schwerionenforschung, Darmstadt, Germany
33 Institut fur Kernphysik, Technische Universitat Darmstadt, Darmstadt, Germany
34 Wayne State University, Detroit, Michigan, United States
35 Joint Institute for Nuclear Research (JINR), Dubna, Russia
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536 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universitat Frankfurt, Frankfurt, Germany
37 Institut fur Kernphysik, Johann Wolfgang Goethe-Universitat Frankfurt, Frankfurt, Germany
38 Laboratori Nazionali di Frascati, INFN, Frascati, Italy
39 Petersburg Nuclear Physics Institute, Gatchina, Russia
40 European Organization for Nuclear Research (CERN), Geneva, Switzerland
41 Laboratoire de Physique Subatomique et de Cosmologie (LPSC), Universite Joseph Fourier, CNRS-IN2P3, Institut Poly-
technique de Grenoble, Grenoble, France
42 Centro de Aplicaciones Tecnologicas y Desarrollo Nuclear (CEADEN), Havana, Cuba
43 Kirchho -Institut fur Physik, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany
44 Physikalisches Institut, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany
45 Hiroshima University, Hiroshima, Japan
46 University of Houston, Houston, Texas, United States
47 Physics Department, University of Rajasthan, Jaipur, India
48 Physics Department, University of Jammu, Jammu, India
49 Helsinki Institute of Physics (HIP) and University of Jyvaskyla, Jyvaskyla, Finland
50 Kangnung National University, Kangnung, South Korea
51 Scienti c Research Technological Institute of Instrument Engineering, Kharkov, Ukraine
52 Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine
53 University of Tennessee, Knoxville, Tennessee, United States
54 Saha Institute of Nuclear Physics, Kolkata, India
55 Variable Energy Cyclotron Centre, Kolkata, India
56 Fachhochschule Koln, Koln, Germany
57 Faculty of Science, P.J. Safarik University, Kosice, Slovakia
58 Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, Slovakia
59 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy
60 Lawrence Livermore National Laboratory, Livermore, California, United States
61 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden
62 Universite de Lyon 1, CNRS/IN2P3, Institut de Physique Nucleaire de Lyon, Lyon, France
63 Centro de Investigaciones Energeticas Medioambientales y Tecnologicas (CIEMAT), Madrid, Spain
64 Centro de Investigacion y de Estudios Avanzados (CINVESTAV), Mexico City and Merida, Mexico
65 Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico
66 Instituto de Fsica, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico
67 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia
68 Institute for Theoretical and Experimental Physics, Moscow, Russia
69 Moscow Engineering Physics Institute, Moscow, Russia
70 Russian Research Centre Kurchatov Institute, Moscow, Russia
71 Indian Institute of Technology, Mumbai, India
72 Institut fur Kernphysik, Westfalische Wilhelms-Universitat Munster, Munster, Germany
73 SUBATECH, Ecole des Mines de Nantes, Universite de Nantes, CNRS-IN2P3, Nantes, France
74 Yale University, New Haven, Connecticut, United States
75 Budker Institute for Nuclear Physics, Novosibirsk, Russia
76 Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
77 Institut de Physique Nucleaire d'Orsay (IPNO), Universite Paris-Sud, CNRS-IN2P3, Orsay, France
78 Department of Physics, University of Oslo, Oslo, Norway
79 Dipartimento di Fisica dell'Universita and Sezione INFN, Padova, Italy
80 Sezione INFN, Padova, Italy
81 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic
82 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
83 Institute for High Energy Physics, Protvino, Russia
84 Benemerita Universidad Autonoma de Puebla, Puebla, Mexico
85 Pusan National University, Pusan, South Korea
86 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez u Prahy, Czech Republic
87 Dipartimento di Fisica dell'Universita `La Sapienza' and Sezione INFN, Rome, Italy
88 Sezione INFN, Rome, Italy
89 Commissariat a l'Energie Atomique, IRFU, Saclay, France
90 Dipartimento di Fisica `E.R. Caianiello' dell'Universita and Sezione INFN, Salerno, Italy
91 California Polytechnic State University, San Luis Obispo, California, United States
92 Departamento de Fsica de Partculas and IGFAE, Universidad de Santiago de Compostela, Santiago de Compostela, Spain
93 Universidade de S~ao Paulo (USP), S~ao Paulo, Brazil
94 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia
95 Department of Physics, Sejong University, Seoul, South Korea
96 Yonsei University, Seoul, South Korea
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697 Technical University of Split FESB, Split, Croatia
98 V. Fock Institute for Physics, St.Petersburg State University, St.Petersburg, Russia
99 Institut Pluridisciplinaire Hubert Curien (IPHC), Universite de Strasbourg, CNRS-IN2P3, Strasbourg, France
100 University of Tokyo, Tokyo, Japan
101 Dipartimento di Fisica Sperimentale dell'Universita and Sezione INFN, Turin, Italy
102 Sezione INFN, Turin, Italy
103 Dipartimento di Fisica dell'Universita and Sezione INFN, Trieste, Italy
104 Sezione INFN, Trieste, Italy
105 University of Tsukuba, Tsukuba, Japan
106 Institute for Subatomic Physics, Utrecht University, Utrecht, Netherlands
107 Soltan Institute for Nuclear Studies, Warsaw, Poland
108 Warsaw University of Technology, Warsaw, Poland
109 Purdue University, West Lafayette, Indiana, United States
110 Zentrum fur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany
111 Hua-Zhong Normal University, Wuhan, China
112 Yerevan Physics Institute, Yerevan, Armenia
113 Rudjer Boskovic Institute, Zagreb, Croatia
Abstract. On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider
(LHC), two counter-rotating proton bunches were circulated for the rst time concurrently in the machine,
at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with
only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all
LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region
was centred very well in both the longitudinal and transverse directions and 284 events were recorded in
coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed
both online and oine. We have used these events to measure the pseudorapidity density of charged primary
particles in the central region. In the range jj < 0:5, we obtain dNch=d = 3:10 0:13(stat:) 0:22(syst:)
for all inelastic interactions, and dNch=d = 3:51  0:15(stat:)  0:25(syst:) for non-single di ractive
interactions. These results are consistent with previous measurements in proton{antiproton interactions at
the same centre-of-mass energy at the CERN SppS collider. They also illustrate the excellent functioning
and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment,
in this early start-up phase.
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61 Introduction
The very rst proton{proton collisions at Point 2 of the
CERN Large Hadron Collider (LHC) [1] occurred in the
afternoon of 23rd November 2009, at a centre-of-mass en-
ergy
p
s = 900 GeV, during the commissioning of the ac-
celerator. This publication, based on 284 events recorded
in the ALICE detector [2] on that day, describes a deter-
mination of the pseudorapidity density of charged primary
particles1 dNch=d (  ln tan =2, where  is the polar
angle with respect to the beam line) in the central pseudo-
rapidity region. The purpose of this study is to compare
with previous measurements for proton{antiproton (pp)
collisions at the same energy [3], and to establish a ref-
erence for comparison with forthcoming measurements at
higher LHC energies.
The event sample collected with our trigger contains
three di erent classes of inelastic interactions, i.e. colli-
sions where new particles are produced: non-di ractive,
single-di ractive, and double-di ractive2. Experimentally
1 Here, primary particles are de ned as prompt particles pro-
duced in the collision and all decay products, except products
from weak decays of strange particles such as K0s and .
2 Inelastic pp collisions are usually divided into these classes
depending on the fate of the interacting protons. If one (both)
we cannot distinguish between these classes, which, how-
ever, are selected by our trigger with di erent eciencies3.
In order to compare our data with those of other exper-
iments, we provide the result with two di erent normal-
izations: the rst one (INEL) corresponds to the sum of all
inelastic interactions and corrects the trigger bias individ-
ually for all event classes, by weighting them, each with its
own estimated trigger eciency and abundance. The sec-
ond normalization (non-single-di ractive or NSD) applies
this correction for non-di ractive and double-di ractive
processes only, while removing, on average, the single-
di ractive contribution.
Multiparticle production is rather successfully describ-
ed by phenomenological models with Pomeron exchange,
which dominates at high energies [4, 5]. These models re-
incoming beam particle(s) are excited into a high-mass state,
the process is called single (double) di raction; otherwise the
events are classi ed as non-di ractive. Particles emitted in
di ractive reactions are usually found at rapidities close to
that of the parent proton.
3 We estimate the trigger eciency for each class using the
process-type information provided by Monte Carlo generators;
the values vary by up to a factor of two between classes and
are listed in Section 3. The relative abundance of each class is
taken from published data (see text).
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7late the energy dependence of the total cross section to
that of the multiplicity production using a small num-
ber of parameters, and are the basis for several Monte
Carlo event generators describing soft hadron collisions
(see for example [6{8]). According to these models, it is
expected that the charged-particle density increases by a
factor 1:7 and 1:9 when raising the LHC centre-of-mass
energy from 900 GeV to 7 and 14 TeV respectively (i.e.
intermediate and nominal LHC energies). The di erence
in charged-particle densities between pp and pp interac-
tions is predicted to decrease as 1=
p
s at high energies [9].
This di erence was last measured at the CERN ISR to be
in the range 1:5{3 % [10] at
p
s = 53 GeV. Extrapolating
these values to
p
s = 900 GeV, one obtains a very small
di erence of about 0:1{0:2 %. Therefore, we will compare
our measurement to existing pp data and also to di erent
Monte Carlo models.
This article is organized as follows: Section 2 describes
the experimental conditions during data taking; the main
features of the ALICE detector subsystems used for this
analysis are decribed in Section 3; Section 4 is dedicated
to the event selection and data analysis; the results are
discussed in Section 5 and Section 6 contains the conclu-
sion.
2 LHC and the run conditions
The LHC, built at CERN in the circular tunnel of 27 km
circumference previously used by the Large Electron{Posi-
tron collider (LEP), will provide the highest energy ever
explored with particle accelerators. It is designed to col-
lide two counter-rotating beams of protons or heavy ions.
The nominal centre-of-mass energy for proton{proton col-
lisions is 14 TeV. However, collisions can be obtained down
to
p
s = 900 GeV, which corresponds to the beam injec-
tion energy.
The results from the rst proton{proton collisions pre-
sented here were obtained during the early commissioning
phase of the LHC, when two proton bunches were circu-
lating for the rst time concurrently in the machine. The
bunches used were the so-called \pilot bunches": low in-
tensity bunches used during machine commissioning, with
a few 109 protons per bunch. The two beams were brought
into nominal position for collisions without a speci c at-
tempt to maximize the interaction rate. The nominal r.m.s.
size of LHC beams at injection energy is about 300m in
the transverse direction and 10:5 cm in the longitudinal
(z-axis) direction. However, at this early stage, the beam
parameters can deviate from these nominal values; they
were not measured for the ll used in this analysis. For
the previous ll, for which the longitudinal size was mea-
sured, it was found to be shorter, with an r.m.s. of about
8 cm. Assuming Gaussian beam pro les, the luminous re-
gion should be smaller than the beam size by a factor ofp
2 in all directions.
Shortly after circulating beams were established, the
ALICE data aquisition system [11] started collecting ev-
ents with a trigger based on the Silicon Pixel Detector
(SPD), requiring two or more hits in the SPD in coinci-
dence with the passage of the two colliding bunches as in-
ferred from beam pickup detectors. As a precaution, only
a small subset of the detector subsystems, including the
silicon tracking detectors and the scintillator trigger coun-
ters, was turned on, in order to assess the beam conditions
provided by the LHC.
The trigger rate was measured just before collisions
with the same trigger conditions. Without beams we mea-
sured a rate of 3104 Hz (in coincidence with one bunch
crossing interval per orbit). In coincidence with the pas-
sage of the bunch of one circulating beam the rate was
0:006 Hz. As soon as the second beam was injected in
the accelerator, the event rate increased signi cantly, to
0:11 Hz. The rst event that was analyzed and displayed
in the counting room by the oine reconstruction software
AliRoot [12] running in online mode is shown in Fig. 1.
This marked symbolically the keenly anticipated start of
the physics exploitation of the ALICE experiment4. The
online reconstruction software implemented in the High-
Level Trigger (HLT) computer farm [13] also analyzed the
events in real time and calculated the vertex position of
the collected events, shown in Fig. 2. The distributions
are very narrow in the transverse plane (sub-millimetre,
including contributions from detector resolution and resid-
ual misalignment), of about the expected size in the lon-
gitudinal direction and well positioned with respect to the
nominal centre of the ALICE detector. This provided im-
mediate evidence that a substantial fraction of the events
corresponded to collisions between the protons of the two
counter-rotating beams.
After 43 minutes, the two beams were dumped in order
to proceed with the LHC commissioning programme. In
total, 284 events were triggered and recorded during this
short, but important, rst run of the ALICE experiment
with colliding beams.
3 The ALICE experiment
ALICE, designed as the dedicated heavy-ion experiment
at the LHC, also has excellent performance for proton{
proton interactions [14]. The experiment consists of a large
number of detector subsystems [2] inside a solenoidal mag-
net (B = 0:5 T). The magnet was o during this run.
During the several months of running with cosmic rays
in 2008 and 2009, all of the ALICE detector subsystems
were extensively commissioned, calibrated and used for
data taking [15]. Data were collected for an initial align-
ment of the parts of the detector that had sucient ex-
posure to the mostly vertical cosmic ray
ux. Data were
also taken during various LHC injection tests to perform
timing measurements and other calibrations.
Collisions take place at the centre of the ALICE de-
tector, inside a beryllium vacuum beam pipe (3 cm in ra-
dius and 800m thick). The tracking system in the AL-
ICE central barrel covers the full azimuthal range in the
4 The event display started shortly after data taking and
therefore missed the rst few events.
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8Fig. 1. The rst pp collision candidate shown by the event display in the ALICE counting room (3D view, r- and r-z
projections), the dimensions are shown in cm. The dots correspond to hits in the silicon vertex detectors (SPD, SDD and SSD),
the lines correspond to tracks reconstructed using loose quality cuts. The ellipse drawn in the middle of the detector surrounds
the reconstructed event vertex.
Fig. 2. Online display of the vertex positions reconstructed by the High-Level Trigger (HLT). The gure shows, counter-
clockwise from top left, the position in the transverse plane for all events with a reconstructed vertex, the projections along the
transverse coordinates x and y, and the distribution along the beam line (z-axis).
Page 10
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9Fig. 3. Arrival time of particles in the VZERO detectors relative to the beam crossing time (time zero). A number of beam-
halo or beam{gas events are visible as secondary peaks in VZERO-A (left panel) and VZERO-C (right panel). This is because
particles produced in background interactions arrive at earlier times in one or the other of the two counters. The majority of
the signals have the correct arrival time expected for collisions around the nominal vertex.
pseudorapidity window jj < 0:9. It has been designed to
cope with the highest charged-particle densities expected
in central Pb{Pb collisions. The following four detector
subsystems were active during data taking and were used
in this analysis.
{ The Silicon Pixel Detector (SPD) consists of two cylin-
drical layers with radii of 3:9 and 7:6 cm and has about
9:8 million pixels of size 50  425m2. It covers the
pseudorapidity ranges jj < 2 and jj < 1:4 for the
inner and outer layers respectively, for particles orig-
inating at the centre of the detector. The e ective -
acceptance is larger due to the longitudinal spread of
the position of the interaction vertex. The detector is
read out by custom-designed ASICs bump-bonded di-
rectly on silicon ladders. Each chip contains 8192 chan-
nels and also provides a fast trigger signal if at least
one of its pixels is hit. The trigger signals from all
1200 chips are then combined in a programmable logic
unit which provides a level-0 trigger signal to the cen-
tral trigger processor. The total thickness of the SPD
amounts to about 2.3 % of a radiation length. About
83 % of the channels were operational for particle de-
tection and 77 % of the chips were used in the trigger
logic. The SPD was aligned using cosmic-ray tracks
collected during 2008 [16], and the residual misalign-
ment was estimated to be below 10 m for the modules
well covered by mostly vertical tracks. The modules
on the sides are likely to be a ected by larger residual
misalignment.
{ The Silicon Drift Detector (SDD) consists of two cylin-
drical layers at radii of 15:0 and 23:9 cm and covers
the region jj < 0:9. It is composed of 260 sensors
with an internal voltage divider providing a drift eld
of 500 V/cm and MOS charge injectors that allow
measurement of the drift speed via dedicated calibra-
tion triggers. The charge signal of each of the 133 000
collection anodes, arranged with a pitch of 294m,
is sampled every 50 ns by an ADC in the front-end
electronics. The total thickness of the SDD layers (in-
cluding mechanical supports and front-end electronics)
amounts to 2.4 % of a radiation length. About 92 % of
the anodes were fully operational.
{ The two layers of the double-sided Silicon Strip Detec-
tor (SSD) are located at radii of 38 and 43 cm respec-
tively, covering jj < 0:97. The SSD consists of 1698
sensors with a strip pitch of 95m and a stereo angle
of 35 mrad. The detector provides a measurement of
the charge deposited in each of its 2:5106 strips. The
position resolution is better than 20m in the r-' di-
rection and about 0.8 mm in the direction along the
beam line. The thickness of the SSD, including sup-
ports and services, corresponds to 2.2 % of a radiation
length. About 90 % of the SSD area was active during
data taking.
{ The VZERO detector consists of two arrays of 32 scin-
tillators each, which are placed around the beam pipe
on either side of the interaction region: VZERO-A at
z = 3:3 m, covering the pseudorapidity range 2:8 <
 < 5:1, and VZERO-C at z = 0:9 m, covering the
pseudorapidity range 3:7 <  < 1:7. The time reso-
lution of this detector is better than 1 ns. Its response
is recorded in a time window of 25 nsec around the
nominal beam crossing time. For events collected in
this run, the arrival times of particles at the detector
relative to this \time zero" is shown in Fig. 3. Note
that in general several particles are registered for each
event. Particles hitting one of the detectors before the
beam crossing have negative arrival times and are typ-
ically due to interactions taking place outside the cen-
tral region of ALICE.
More details about the ALICE experiment and its detector
subsystems can be found in [2].
The trigger used to record the events for the present
analysis is de ned by requiring at least two hit chips in
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10
the SPD, in coincidence with the signals from the two
beam pick-up counters indicating the presence of two pass-
ing proton bunches. The eciency of this trigger as well
as all other corrections have been studied using two dif-
ferent Monte Carlo generators, PYTHIA 6.4.14 [17] tune
D6T [18] and PHOJET [8], for INEL and NSD interac-
tions. The trigger eciencies for non-di ractive, single-
di ractive, and double-di ractive events were evaluated
separately, and found to be 98{99 %, 48{58 %, and 53{
76 % respectively. The ranges are determined by the two
event generators. These event classes were combined for
the corrections using the fractions measured by UA5 [19]:
non-di ractive 0:767  0:059; single-di ractive 0:153
0:031; double-di ractive 0:08  0:05. The resulting ef-
ciencies were found to be 87{91 % for the INEL normal-
ization and 94{97 % for the NSD normalization, again de-
pending on the event generator used.
The results presented in the following sections are those
obtained with PYTHIA. The di erence between results
corrected with PYTHIA and PHOJET is used in the es-
timate of the systematic uncertainty.
4 Data analysis
The data sample used in the present analysis consists of
284 events recorded without magnetic eld. The results
presented here are based on the analysis of the SPD data.
However, information from the SDD, SSD and VZERO
was used to crosscheck the identi cation and removal of
background events.
In the SPD analysis, the position of the interaction
vertex is reconstructed [20] by correlating hits in the two
silicon-pixel layers to obtain tracklets. The achieved reso-
lution depends on the track multiplicity and for this spe-
ci c vertex reconstruction is approximately 0:1{0:3 mm in
the longitudinal direction and 0:2{0:5 mm in the trans-
verse direction. For events with only one charged track,
the vertex position is determined by intersecting the SPD
tracklet with the mean beam axis determined from the
vertex positions of other events in the sample. A vertex
was reconstructed in 94 % of the selected events. The dis-
tribution of the vertex position in the longitudinal direc-
tion (z-axis) is shown in Fig. 4. For events originating
from the centre of the detector, the vertex-reconstruction
eciency was estimated, using Monte Carlo simulations,
to be 84 % for INEL interactions and 92 % for NSD col-
lisions. These eciencies decrease for larger jzj-values of
the vertex in low-multiplicity events; therefore, only events
with vertices within jzj < 10 cm were used. This allows for
an accurate charged-particle density measurement in the
pseudorapidity range jj < 1:6 using both SPD layers.
Using the reconstructed vertex as the origin, we calcu-
late the di erences in azimuthal (', bending plane) and
polar (, non-bending direction) angles of pairs of hits
with one hit in each SPD layer. These tracklets [21] are
selected by a cut on the sum of the squares of ' and ,
each normalized to its estimated resolution (80 mrad and
25 mrad, respectively). When more than one hit in a layer
matches a hit in the other layer, only the hit combination
Fig. 4. Longitudinal vertex distribution from hit correlations
in the two pixel layers of the ALICE inner tracking system.
Vertical dashed lines indicate the region jzj < 10 cm, where
the events for the present analysis are selected. A Gaussian t
with an estimated r.m.s. of about 4 cm to the central part is
also shown.
with the smallest angular di erence is used. This occurs
in only 2 % of the matched hits.
The number of primary charged particles is estimated
by counting the number of tracklets. This number was
corrected for:
{ trigger ineciency;
{ detector and reconstruction ineciencies;
{ contamination by decay products of long-lived parti-
cles (K0s , , etc.), gamma conversions and secondary
interactions.
The corrections are determined as a function of the z-
position of the primary vertex, and on the pseudorapidity
of the tracklet. For the analyzed sample the average cor-
rection factor for tracklets is about 1.5.
The beam{gas and beam-halo background events were
removed by a cut on the ratio between the number of
tracklets and the total number of hits in the tracking
system (SPD, SDD, and SSD); this ratio is smaller for
background events (as measured in the previous lls trig-
gering on the bunch passage from one side) than for col-
lisions [22]. In addition, the timing information from the
VZERO detector was used for background rejection by re-
moving events with negative arrival time (see Fig. 3). The
event quality and event classi cation was crosschecked by
a visual scan of the whole event sample. In total 29 events
(i.e. about 10 %) were rejected as beam induced back-
ground, which is consistent with the rate expected from
previous lls. The remaining background was estimated
from the vertex distribution and found to be negligible.
The contamination from coincidence with a cosmic event
was estimated to be one event in the full sample. Indeed,
two cosmic events were identi ed by scanning, both with-
out reconstructed vertex.
Particular attention has been paid to events having
zero or one charged tracklets in the SPD acceptance. The
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11
Fig. 5. Multiplicity dependence of the combined eciency
to select an event as minimum bias and to reconstruct its
vertex in SPD, for non-di ractive (crosses), single-di ractive
(squares), and double-di ractive (circles) events, based on
PYTHIA events.
vertex- nding eciency for events with one charged par-
ticle in the acceptance is about 80 %. The number of zero-
track events has been estimated by Monte Carlo calcula-
tions. The total number of collisions used for the normal-
ization was calculated from the number of events selected
for the analysis, corrected for the vertex-reconstruction in-
eciency. In order to obtain the normalization for INEL
and NSD events, we further corrected the number of se-
lected events for the trigger eciency for these two event
classes. In addition, for NSD events, we subtract the single-
di ractive contribution. These corrections, as well as those
for the vertex nding eciency, depend on the event char-
ged-particle multiplicity, see Fig. 5. The dependence of
the event- nding eciency (combining event selection and
vertex nding) on multiplicity was calculated for di er-
ent interaction types using our detector simulation, and is
above 98 % for events with at least two charged particles.
The averaged combined corrections for the vertex recon-
struction eciency and the selection eciency is 20 % for
INEL interactions and much smaller for NSD interactions,
due to the cancelation of some contributions.
The various corrections mentioned above were calcu-
lated using the full GEANT 3 [23] simulation of the AL-
ICE detector as included in the oine framework Ali-
Root. In order to estimate the systematic uncertainties,
the above analysis was repeated by:
{ applying di erent cuts for the tracklet de nition (vary-
ing the angle cut-o by 50 %);
{ varying by 10 % the density of the material in the
tracking system, thus changing the material budget;
{ using the non-aligned geometry;
{ varying by30 % the composition of the produced par-
ticle types with respect to the yields suggested by the
event generators;
{ varying the particle yield below 100 MeV=c by 30 %;
Table 1. Contributions to systematic uncertainties on the
measurement of the charged-particle pseudorapidity density.
Uncertainty
Tracklet selection cuts negl.
Material budget negl.
Misalignment 0.5 %
Particle composition negl.
Transverse-momentum spectrum 0.5 %
Contribution of di raction (INEL) 4 %
Contribution of di raction (NSD) 4.5 %
Event-generator dependence (INEL) 4 %
Event-generator dependence (NSD) 3 %
Detector eciency 4 %
SPD triggering eciency 2 %
Background events negl.
Total (INEL) 7.2 %
Total (NSD) 7.1 %
{ evaluating the uncertainty in the normalization to
INEL and NSD samples by varying the ratios of the
non-di ractive, single-di ractive and double-di ractive
cross sections according to their measured values and
errors [19] and using two di erent models for di rac-
tion kinematics (PYTHIA and PHOJET).
An additional source of systematic error comes from
the limited statistics used so far to determine the ecien-
cies of the SPD detector modules. In test beams, the SPD
eciency in active areas was measured to be higher than
99:8 %. This was crosschecked in-situ with cosmic data,
but only over a limited area and with limited statistics.
At this stage, we have assigned a conservative value of 4 %
to this uncertainty. The triggering eciency of the SPD
was estimated from the data itself, using the trigger infor-
mation recorded in the data stream for events with more
than one tracklet, and found to be very close to 100 %,
with an error of about 2 % (due to the limited statistics).
These contributions to the systematic uncertainty on
the charged particle pseudorapidity density are summa-
rized in Table 1. Our conclusion is that the total system-
atic uncertainty on the pseudorapidity density is less than
7:2 % for INEL collisions and 7:1 % for NSD collisions.
The largest contribution comes from uncertainties in cross
sections of di ractive processes and their kinematic simu-
lation.
More details about this analysis, corrections, and the
evaluation of the systematic uncertainties can be found
in [24].
5 Results
Figure 6 shows the charged primary particle pseudorapid-
ity density distributions obtained for INEL and NSD inter-
actions in the range jj < 1:6. The pseudorapidity density
obtained in the central region jj < 0:5 for INEL interac-
tions is 3:100:13(stat.)0:22(syst.) and for NSD interac-
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12
Table 2. Comparison of charged primary particle pseudorapidity densities at central pseudorapidity (jj < 0:5) for inelastic
(INEL) and non-single di ractive (NSD) collisions measured by the ALICE detector in pp interactions and by UA5 in pp
interactions [3] at a centre-of-mass energy of 900 GeV. For ALICE, the rst error is statistical and the second is systematic; no
systematic error is quoted by UA5. The experimental data are also compared to the predictions for pp collisions from di erent
models. For PYTHIA the tune versions are given in parentheses. The correspondence is as follows: D6T is tune (109); ATLAS
CSC is tune (306); Perugia-0 is tune (320).
Experiment ALICE pp UA5 pp [3] QGSM [25] PYTHIA [17] PHOJET [8]
Model (109) [18] (306) [26] (320) [27]
INEL 3:10 0:13 0:22 3:09 0:05 2.98 2.33 2.99 2.46 3.14
NSD 3:51 0:15 0:25 3:43 0:05 3.47 2.83 3.68 3.02 3.61
Fig. 6. Pseudorapidity dependence of dNch=d for INEL and
NSD collisions. The ALICE measurements (squares) are com-
pared to UA5 data (triangles) [3]. The errors shown are statis-
tical only.
tions is 3:510:15(stat.)0:25(syst.). Also shown in Fig. 6
are the previous measurements of proton{antiproton inter-
actions from the UA5 experiment [3]. Our results obtained
for proton{proton interactions are consistent with those
for proton{antiproton interactions, as expected from the
fact that the predicted di erence (0.1{0.2 %) is well below
measurement uncertainties. The measurements at central
pseudorapidity (jj < 0:5) are summarized in Table 2
together with model predictions obtained with QGSM,
PHOJET and three di erent PYTHIA tunes. PYTHIA
6.4.14, tune D6T, and PHOJET yield respectively the low-
est and highest charged particle densities. Therefore, these
two have been used for the evaluation of our systematic
errors. PYTHIA 6.4.20, tunes ATLAS CSC and Perugia-0,
are candidates for use by the LHC experiments at higher
LHC energies and are shown for comparison.
Figure 7 shows the centre-of-mass energy dependence
of the pseudorapidity density in the central region (jj <
0:5). The data points are obtained in the jj < 0:5 range
from this experiment and from references [3,10,28{31], and
are corrected for di erences in pseudorapidity range where
necessary, tting the pseudorapidity distribution around
 = 0. As noted above, there is good agreement between
pp and pp data at the same energy. The dashed and solid
Fig. 7. Charged-particle pseudorapidity density in the central
rapidity region in proton{proton and proton{antiproton inter-
actions as a function of the centre-of-mass energy. The dashed
and solid lines (for INEL and NSD interactions respectively)
indicate the t using a power-law dependence on energy.
lines (for INEL and NSD interactions respectively) are
obtained by tting the density of charged particles in the
central pseudorapidity rapidity region with a power-law
dependence on energy.
Using this parametrization, the extrapolation to the
nominal LHC energy of
p
s = 14 TeV yields dNch=d =
5:5 and dNch=d = 5:9 for INEL and NSD interactions
respectively.
6 Conclusion
Proton{proton collisions observed with the ALICE detec-
tor in the early phase of the LHC commissioning have been
used to measure the pseudorapidity density of charged pri-
mary particles at
p
s = 900 GeV. In the central pseudo-
rapidity region (jj < 0:5), we obtain dNch=d = 3:10 
0:13(stat:)  0:22(syst:) for all inelastic and dNch=d =
3:51  0:15(stat:)  0:25(syst:) for non-single di ractive
proton{proton interactions. The results are consistent with
earlier measurements of primary charged-particle produc-
tion in proton{antiproton interactions at the same energy.
They are also compared with model calculations.
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13
These results have been obtained with a small sam-
ple of events during the early commissioning of the LHC.
They demonstrate that the LHC and its experiments have
nally entered the phase of physics exploitation, within
days of starting up the accelerator complex in November
2009.
Acknowledgements
The ALICE collaboration would like to thank all its engineers
and technicians for their invaluable contributions to the con-
struction of the experiment. We would like to thank and con-
gratulate the CERN accelerator teams for the outstanding per-
formance of the LHC complex at start up, and for providing us
with the collisions used for this paper on such a short notice!
The ALICE collaboration acknowledges the following fund-
ing agencies for their support in building and running the AL-
ICE detector:
{ Calouste Gulbenkian Foundation from Lisbon and Swiss
Fonds Kidagan, Armenia;
{ Conselho Nacional de Desenvolvimento Cient co e Tecnol-
gico (CNPq), Financiadora de Estudos e Projeto (FINEP),
Fundac~ao de Amparo a Pesquisa do Estado de S~ao Paulo
(FAPESP);
{ National Natural Science Foundation of China (NSFC), the
Chinese Ministry of Education (CMOE) and the Ministry
of Science and Technology of China (MSTC);
{ Ministry of Education and Youth of the Czech Rebublic;
{ Danish National Science Research Council and the Carls-
berg Foundation;
{ The European Research Council under the European Com-
munity's Seventh Framework Programme;
{ Helsinki Institute of Physics and the Academy of Finland;
{ French CNRS-IN2P3, the `Region Pays de Loire', `Region
Alsace', `Region Auvergne' and CEA, France;
{ German BMBF and the Helmholtz Association;
{ Hungarian OTKA and National Oce for Research and
Technology (NKTH);
{ Department of Atomic Energy and Department of Science
and Technology of the Government of India;
{ Istituto Nazionale di Fisica Nucleare (INFN) of Italy;
{ MEXT Grant-in-Aid for Specially Promoted Research, Ja-
pan;
{ Joint Institute for Nuclear Research, Dubna;
{ Korea Foundation for International Cooperation of Science
and Technology (KICOS);
{ CONACYT, DGAPA, Mexico, ALFA-EC and the HELEN
Program (High-Energy physics Latin-American{European
Network);
{ Stichting voor Fundamenteel Onderzoek der Materie
(FOM) and the Nederlandse Organistie voor Wetenschap-
pelijk Onderzoek (NWO), Netherlands;
{ Research Council of Norway (NFR);
{ Polish Ministry of Science and Higher Education;
{ National Authority for Scienti c Research - NASR (Auton-
tatea Nationala pentru Cercetare Stiinti ca - ANCS);
{ Federal Agency of Science of the Ministry of Education and
Science of Russian Federation, International Science and
Technology Center, Russian Federal Agency of Atomic En-
ergy, Russian Federal Agency for Science and Innovations
and CERN-INTAS;
{ Ministry of Education of Slovakia;
{ CIEMAT, EELA, Ministerio de Educacion y Ciencia of
Spain, Xunta de Galicia (Consellera de Educacion), CEA-
DEN, Cubaenerga, Cuba, and IAEA (International Atomic
Energy Agency);
{ Swedish Reseach Council (VR) and Knut & Alice Wallen-
berg Foundation (KAW);
{ Ukraine Ministry of Education and Science;
{ United Kingdom Science and Technology Facilities Council
(STFC);
{ The United States Department of Energy, the United States
National Science Foundation, the State of Texas, and the
State of Ohio.
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2330

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