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arXiv · 1310.1730

Predicting τ-lepton polarization at the LHC

Adomas Jelinskas
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Abstract

This is a master's thesis and it is written in Lithuanian language. The goal of this thesis is to predict the polarization of a τ-particle produced at the LHC and coming from a real W boson. This is achieved by calculating the projection of the expectation value of the polarization vector of the τ-particle. Calculations are done in the frame of the Standard Model. In this model only left-chiral currents couple to W bosons. The mass of the τ-particle is left non-zero, therefore one can see the influence of helicity mixing on polarization. The initial colliding particles at the LHC are protons and the most important channels for W production are quark fusion: $q\bar{q} -> W^+ -> τ^+ ν_τ$. Calculation for τ-lepton production from Z bosons and photons, $q\bar{q} -> Z^0, A^γ-> τ^+ τ^-$, are also included for investigation of possible background events. Because quarks are confined in a proton, this thesis presents a thorough treatment on discribing proton's inner structure with Parton distribution functions. The results section shows plots of differential cross sections for τ-lepton production from different quark flavours and different energy intervals of the τ-particle. A projection of the expectation value of the polarization vector depends on the chosen energy intervals. The vector on which it is projected is chosed to be the direction of the τ-particle's motion. The influence of the mass is best seen for low energy particles, because then the helicity mixing is higher. PACS numbers: 13.88.+e, 14.60.Fg

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Adomas Jelinskas. 2013-10-07. Predicting τ-lepton polarization at the LHC. https://arxiv.org/abs/1310.1730

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