Documentation of TauSpinner algorithms: program for simulating spin effects in $$\uptau $$τ-lepton production at LHC

Abstract$$\uptau $$τ-leptons produced in pp collisions allow to measure Standard Model parameters and offer probes for New Physics. The TauSpinner program can be used to modify spin (or production matrix elements) effects in any $$\uptau $$τ sample. It relies on the kinematics of outgoing particles: $$\uptau $$τ lepton(s) (also $$\upnu _\uptau $$ντ in case of W-mediated processes, optionally also four-moments of accompanying hard jets) and $$\uptau $$τ decay products. No other information is required from the event record. With calculated spin (or production/decay matrix element) weights, attributed on the event-by-event basis, modifications to the spin/decay/production features, is possible without the need for regenerating events. With TauSpinner algorithms, the experimental techniques developed over years since LEP 1 times are already used and extended for LHC applications. The purpose of the present publication is to systematically document physics basis of the program, and to overview its application domain and systematic errors.