Production of τ lepton pairs with high pT jets at the LHC and the TauSpinner reweighting algorithm

Abstract

The purpose of the TauSpinner algorithm is to provide a tool that allows to modify the physics model of the Monte Carlo generated samples due to the changed assumptions of event production dynamics, but without the need of re-generating events. To each event TauSpinner attributes the weights. In this way, for example, the spin effects of τ -lepton production or decay are modified, or the effect of the changes in the production mechanism are introduced according to a new physics model. Such an approach is useful, because there is no need to repeat the detector response simulation with each variant of the physics model considered. In addition, since only the event weights differ for the models, samples are correlated and statistical error of the modification is proportional to the reweighting only. We document the extension of the TauSpinner algorithm to (2 → 4) processes in which the matrix elements for the parton-parton scattering amplitudes into a τ -lepton pair and two outgoing partons are used. The method is based on tree-level matrix elements with complete helicity information for the Standard Model processes, including the Higgs boson production. For this purpose automatically generated codes by MadGraph5 have been adapted. Consistency tests of the implemented matrix elements, reweighting algorithm and numerical results are presented. For the sensitive observable, namely the averaged τ lepton polarisation, we perform a systematic comparison between (2 → 2) and (2 → 4) matrix elements used to calculate the spin weight in pp→ τ τ jj events. We show, that for events with τ -lepton pair close to the Z-boson peak, the τ -lepton polarisation calculated using (2 → 4) matrix elements is very close to the one calculated using (2 → 2) Born process only. For the mτ τ masses above the Z-boson peak, the effect from including (2 → 4) matrix elements is also marginal, however when taking into account only subprocesses qq, qq¯ → τ τ jj, it can lead to a 10 % difference on the predicted τ -lepton polarisation. On the other hand, we have found that the appropriate choice of electroweak scheme can have significant impact. We show that the modification of the electroweak or strong interaction initialization (including change of the electroweak schemes or analytic form of scale dependence for αS) can be performed with the re-weighting technique as well. The new version of TauSpinner ver.2.0.0 presented here, allows also to introduce non-standard couplings for the Higgs boson and study their effects in the vector-boson-fusion processes by exploiting the spin correlations of τ -lepton pair decay products. The discussion of physics effects is however relegated to forthcoming publications.