TauSpinner : a tool for simulating CP effects in $H\rightarrow \tau \tau$ decays at LHC

In this paper, we discuss application of the TauSpinner package as a simulation tool for measuring the CP state of the newly discovered Higgs boson using the transverse spin correlations in the $H\rightarrow \tau \tau$ decay channel. We discuss application for its main background $Z/\gamma ^{*} \rightarrow \tau \tau$ as well. The TauSpinner package allows to add, with the help of weights, transverse spin correlations corresponding to any mixture of scalar/pseudoscalar state, on already existing events using information from the kinematics of outgoing $\tau$ leptons and their decay products only. This procedure can be used when polarimetric vectors of the $\tau$ s decays and density matrix for \tau -pair production are not stored with the event sample. We concentrate on the well-defined effects for the Higgs (or Higgs-like scalar) decays, which are physically separated from the production processes. TauSpinner also allows to reintroduce (or remove) spin correlations to events from Drell–Yan $Z/\gamma ^{*} \rightarrow \tau \tau$ process, the main background for the Higgs parity observables, again with the help of weights only. From the literature, we recall well-established observables, developed for measuring the CP of the Higgs, and use them as benchmarks for illustrating applications of the TauSpinner package. We also include a description of the code and prepared testing examples