TauSpinner Algorithms for Including Spin and New Physics Effects in \(\gamma \gamma \rightarrow \tau \tau \) Process

The possible anomalous New Physics contributions to the electric and magnetic dipole moments of the \(\tau \) lepton have brought renewed interest in development of new charge-parity violating signatures in the \(\tau \)-pair production at Belle II energies, and also at higher energies of the LHC and the FCC. In this paper, we discuss the effects of anomalous contributions to the cross section and spin correlations in the \(\gamma \gamma \to \tau ^-\tau ^+\) production processes, with \(\tau \) decays included. Such processes have been observed in the \(pp\) and PbPb collisions at CERN LHC experiments. Due to the complex nature of the resulting distributions, Monte Carlo techniques are useful, in particular for event reweighting with studied New Physics phenomena. For the \(\gamma \gamma \) processes, extensions of the Standard Model amplitudes are implemented in the <span class="sf">TauSpinner</span> program. This is mainly with the electric and magnetic dipole moments in mind, however the algorithm can easily be extended to other New Physics interactions, provided they can be encapsulated into the similar form-factors in the Standard Model structure of matrix elements. Basic formulas and algorithm principles are presented, and numerical examples are provided for illustration. Information on how to use the program is given in Appendix of the paper. Abstract Published by the Jagiellonian University 2025 authors