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.
Highlights
With the data collected so far by LHC experiments, there was not much interest to explore physics of τ-lepton decays, with the exception of exploiting τ leptons in searches for rare or Standard-Model-forbidden decay channels, see e.g. [1]
Τ-lepton signatures can provide a powerful tools in many areas, like studies of hard processes characteristics, measurements of properties of Higgs boson(s) [2,3], or in searches for New Physics [4,5,6]
In spite of a limited scope of the present version, like a lack of the loop-induced gluon coupling to the Higgs boson, or subprocesses with b-quarks, TauSpinner can already be used as a tool to obtain numerical results of interest for phenomenology
Summary
With the data collected so far by LHC experiments, there was not much interest to explore physics of τ-lepton decays, with the exception of exploiting τ leptons in searches for rare or Standard-Model-forbidden decay channels, see e.g. [1]. We explain details of the modification which we have introduced to the initialization of MadGraph generated amplitudes and emphasize the necessity of using the effective sin θeWf f for the calculation of coupling constants to correctly model the measured spin asymmetries in the Drell– Yan process This is even more important for the correct generation of angular distributions of leptons in the decay frame of intermediate Z bosons. After taking into account permutation of incoming and outgoing partons and CP symmetric states this gives in total 4 × 4 × 2 = 32 non-zero contributions to the sum of Eq (2) This is the case both for Drell–Yan–type background and Higgs-boson production processes.
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