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

TL;DR

This paper extends the TauSpinner algorithm to include 2 to 4 process matrix elements for tau lepton pair production with jets at the LHC, enabling reweighting of event dynamics without re-simulation.

Contribution

The paper introduces an extension of TauSpinner to incorporate 2 to 4 process matrix elements, allowing detailed reweighting of tau pair production with jets, including Higgs and electroweak effects.

Findings

Tau lepton polarization close to Z peak is unaffected by 2 to 4 matrix elements.

Including 2 to 4 matrix elements can cause up to 10% difference in tau polarization predictions.

Electroweak scheme choice significantly impacts reweighting results.

Abstract

The TauSpinner algorithm allows to modify the physics of the Monte Carlo generated samples due to the changed assumptions of event production dynamics, without re-generating events. To each event it attributes weights: the spin effects of tau-lepton production or decay, or the production mechanism are modified. There is no need to repeat the detector response simulation. We document the extension to 2 to 4 processes in which the matrix elements for the parton-parton scattering amplitudes into a tau-lepton pair and two outgoing partons are used. Tree-level matrix elements for the Standard Model processes, including the Higgs boson production are used. Automatically generated codes by MadGraph5 have been adapted. Tests of the matrix elements, reweighting algorithm and numerical results are presented. For averaged tau lepton polarisation, we perform comparison of 2 to 2 and 2 to 4 matrix elements used to calculate the spin weight in pp to tau tau j j events. We show, that for events with tau-lepton pair close to the Z-boson peak, the tau-lepton polarisation calculated using 2 to 4 matrix elements is very close to the one calculated using 2 to 2 Born process only. For the m_(tautau) masses above the Z-boson peak, the effect from including 2 to 4 matrix elements is also marginal, however when restricting into subprocesses qq,q bar q to tau tau j j only, it can lead to a 10% difference on the predicted tau-lepton polarisation. Choice of electroweak scheme can have significant impact. The modification of the electroweak or strong interaction can be performed with the re-weighting technique. TauSpinner v.2.0.0, allows to introduce non-standard couplings for the Higgs boson and study their effects in the vector-boson-fusion. The discussion is relegated to forthcoming publications.