A self-calibrating, double-ratio method to test tau lepton universality in W boson decays at the LHC
S. Dysch, T. R. Wyatt
Measurements in W+W− events at LEP2 and ... provide intriguing hints of a violation of lepton universality in the charged current coupling of tau leptons relative to those for electrons and muons. We propose a novel, self-calibrating method to test tau lepton universality in W boson decays at the LHC. We compare directly the ratio of the numbers of selected ℓ−τ and e−μ final states in di-leptonic tt events with that in Z/γ∗→τ+τ− events. Here ℓ=e or μ and τ is a candidate semi-hadronic tau decay. This "double-ratio" cancels to first order sensitivity to systematic uncertainties on the reconstruction of e, μ, and τ leptons, thus improving very significantly the precision to which tau lepton universality can be tested in W boson decay branching ratios at the LHC. Using particle-level Monte Carlo events, and a parameterised simulation of detector performance, we demonstrate the effectiveness of the method ... Our studies indicate that a single experiment precision on the tau lepton universality test of around 1.4% is achievable with a data set of 140 fb−1 at √s = 13 TeV. ...