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

Many Beyond-the-Standard-Model theories predict the existence of one or more additional neutral gauge bosons, or $Z'$ bosons, with masses at the TeV scale or higher. A search for resonances of $Z'$ bosons decaying to $\tau^+\tau^-$ pairs in $\sqrt{s}=8$ TeV $pp$ 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 $Z'$ signals. No statistically significant excess above the Standard Model expectation is observed in any channel. Bayesian $95\%$ credibility upper limits are placed on the $Z'$ production cross section times $Z'\to\tau\tau$ branching ratio as functions of the $Z'$ resonance mass for each channel and for a combination of the channels. Sequential Standard Model $Z'$ bosons with masses below 2.02 TeV are excluded at 95$\%$ credibility. The impacts on the cross section limits from varying the $Z'_{SSM}$ decay width and couplings to fermions are evaluated. Limits are also placed on the cross section times branching ratio of Non-Universal $G(221)$ $Z'$ bosons with enhanced couplings to third-generation fermions.