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 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' → tautau 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 ZssM 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. These are evaluated as functions of the Z'NU mass and another free parameter. Z'Nu bosons with masses below 1.3-2.1 TeV are excluded at 95% credibility.