A Search for Z' Gauge Bosons Decaying to Tau-Antitau Pairs in Proton-Proton Collisions with the ATLAS Detector

• Main Author: Leister, Andrew Gerard
• Language: EN
• Published: Yale University 2015
• Subjects: Physics|High energy physics
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3663660

<p>Many Beyond-the-Standard-Model theories predict the existence of one or more additional neutral gauge bosons, or <i>Z'</i> bosons, with masses at the TeV scale or higher. A search for resonances of <i>Z'</i> bosons decaying to &tau;⁺ &tau;^- pairs in [special characters omitted]<i>s</i> = 8 TeV <i>pp</i> collisions from the LHC is presented. The data was collected by the ATLAS detector and corresponds to an integrated luminosity of 19.5-20.3 fb^-1. The search is performed in ditau decay channels in which at least one tau decays hadronically. In each channel, the numbers of ditau events in high-mass regions of data are counted and compared to the expected numbers from Standard Model backgrounds and <i>Z'</i> signals. No statistically significant excess above the Standard Model expectation is observed in any channel. Bayesian 95% credibility upper limits are placed on the <i>Z'</i> production cross section times <i>Z'</i> &rarr; &tau;&tau; branching ratio as functions of the <i>Z'</i> resonance mass for each channel and for a combination of the channels. Sequential Standard Model <i>Z'</i> bosons with masses below 2.02 TeV are excluded at 95% credibility. The impacts on the cross section limits from varying the <i>Z_ssM</i> decay width and couplings to fermions are evaluated. Limits are also placed on the cross section times branching ratio of Non-Universal <i>G</i>(221) <i> Z'</i> bosons with enhanced couplings to third-generation fermions. These are evaluated as functions of the <i>Z'_NU</i> mass and another free parameter. <i>Z'_Nu</i> bosons with masses below 1.3-2.1 TeV are excluded at 95% credibility.</p>