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TauSpinner algorithms for including spin and New Physics effects in $\bar q q \rightarrow Z/γ^* \to ττ$ process

The possible anomalous New Physics contributions to dipole and weak dipole moments of the $τ$ lepton bring renewed interest in development and revisiting charge-parity violating signatures in $τ$-pair production in $Z$-boson decay at energies of the LHC. In this paper, we discuss effects of anomalous contributions to polarisation and spin correlations in the $\bar q q \to τ^+ τ^-$ production processes, with $τ$ decays included. Because of the complex nature of the resulting distributions, Monte Carlo techniques are useful, in particular of event reweighing with studied New Physics phenomena. Extensions of the Standard Model spin amplitudes, within Improved Born Approximation used for matrix element, are implemented in the TauSpinner program. This is mainly with $τ$ dipole and weak dipole moments in mind, but is applicable to arbitrary New Physics interactions, provided they can be encapsulated into the Standard Model $2 \to 2$ structure of matrix element extensions. Implementation allows one also to introduce arbitrary phase-shift between vector and axial-vector couplings of $Z$ boson to $τ$ leptons, which would have impact on observed transverse spin correlations. Basic formulas and algorithm principles are presented, together with distributions for spin correlation matrix. Numerical examples of impact on experimental signatures are shown in case of $τ^\pm \to ρ^\pm ν_τ\to π^\pm π^0 ν_τ$ decays. Information on how to use and configure the TauSpinner program is given in Appendix.