Predicting \tau-lepton polarization at the LHC

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 \tau-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 \tau-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 \tau-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: $q\bar{q} -> W^+ -> \tau^+ \nu_{\tau}$. Calculation for \tau-lepton production from Z bosons and photons, $q\bar{q} -> Z^0, A^\gamma -> \tau^+ \tau^-$, 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 \tau-lepton production from different quark flavours and different energy intervals of the \tau-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 \tau-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