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Observation of π-K+ and π+K- Atoms

Physical Review Letters (2016) 117(11)
DOI: 10.1103/PhysRevLett.117.112001
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Abstract

The observation of hydrogenlike πK atoms, consisting of π-K+ or π+K- mesons, is presented. The atoms are produced by 24 GeV/c protons from the CERN PS accelerator, interacting with platinum or nickel foil targets. The breakup (ionization) of πK atoms in the same targets yields characteristic πK pairs, called "atomic pairs," with small relative momenta Q in the pair center-of-mass system. The upgraded DIRAC experiment observed 349±62 such atomic πK pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic πK atom.

Figures

  • Figure 1. Dependence of the breakup probability Pbr on AπK lifetime for 108µm (solid blue line) and 98µm (dashed red line) Ni targets, and an example how lifetime could be obtained from experimentally measured breakup probability
  • Figure 2. General view of upgraded DIRAC setup (1 – target station; 2 – first shielding; 3 – micro drift chambers (MDC); 4 – scintillating fiber detector (SFD); 5 – ionization hodoscope (IH); 6 – second shielding; 7 – vacuum tube; 8 – spectrometer magnet; 9 – vacuum chamber; 10 – drift chambers (DC); 11 – vertical hodoscope (VH); 12 – horizontal hodoscope (HH); 13 – aerogel Cherenkov detector (ChA); 14 – heavy gas Cherenkov detector (ChF); 15 – nitrogen Cherenkov detector (ChN); 16 – preshower detector (PSh); 17 – muon detector (Mu).
  • Figure 3. a) difference of particle generation times for events with positively charged particle momenta (4.4÷ 4.5) GeV/c. Experimental data (histogram) are fitted by the event sum (black, solid): π−K+ (red, dashed), π+π− (blue, dotted), pπ− (magenta, dotted-dashed) and accidentals (green, constant). b) similar distributions for events with positively charged particle momenta (5.4 ÷ 5.5) GeV/c.
  • Figure 4. QL distribution of π−K+ pairs after applying different criteria: absence of signals in CHF and CHA detectors, proper TOFs between target and VH.
  • Figure 5. QL distribution of π+K− pairs after applying different criteria: absence of signals in CHF detector, proper TOFs between target and VH.
  • Figure 6. a) Distribution of π+K− and π−K+ pairs over Q, shown by points with error bars, is fitted by a sum of simulated distributions of “atomic” (red dotted-dashed), “Coulomb” (blue dashed) and “non-Coulomb” (magenta dotted) distributions. A sum of background distributions (“Coulomb” and “non-Coulomb”) is shown by a solid black line. b) Difference of experimental and background distributions is shown together with simulated distributions of “atomic pairs”.
  • Table 1. “Atomic pair” numbers nA obtained by analyzing the 1-dimensional Q and |QL| distributions and the 2-dimensional (|QL|,QT ) distribution. Only statistical errors are given.
  • Table 2. Estimations of systematic errors, which are induced by different sources, for analysis of data distribution over relative momentum Q, its longitudinal projection |QL| and two dimensional distribution over (|QL|,QT ).

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CITATION STYLE

APA

Adeva, B., Afanasyev, L., Allkofer, Y., Amsler, C., Anania, A., Aogaki, S., … Zrelov, P. (2016). Observation of π-K+ and π+K- Atoms. Physical Review Letters, 117(11). https://doi.org/10.1103/PhysRevLett.117.112001

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