Abstract
Lepton flavour violation in the charged lepton sector is an unambiguous signal of physics beyond the Standard Model. Searches for lepton flavour violation in decays of the ZZ boson with the ATLAS detector is reported, focusing on decays into an electron or muon and a hadronically decaying \tauτ-lepton, using pppp collisions data with a centre-of-mass energy of 13 TeV. Upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: \mathcal{B}(Z\to e\tau) < 5.8\times10^{-5}ℬ(Z→eτ)<5.8×10−5 and \mathcal{B}(Z\to \mu\tau) < 2.4\times10^{-5}ℬ(Z→μτ)<2.4×10−5. When combined with a previous ATLAS result based on pppp collisions data with a centre-of-mass energy of 8 TeV, an upper limit of \mathcal{B}(Z\to \mu\tau) < 1.3\times10^{-5}ℬ(Z→μτ)<1.3×10−5 is obtained.
Highlights
In the Standard Model (SM), flavour is not a conserved global symmetry in general
The outputs of the different neural network (NN) classifiers are combined into a final discriminant that is effective in discriminating the signal events from all the major backgrounds
The fake factors are combined into a weighted average F, where the individual fake factors are weighted by the fraction of expected contribution from the corresponding process to the total fakes in the signal region (SR) as predicted by Monte Carlo (MC) simulations
Summary
In the Standard Model (SM), flavour is not a conserved global symmetry in general. It is well known that flavour-changing processes exist in the quark and neutrino sectors. In the SM, lepton-flavour-violating (LFV) processes such as Z → μτ are possible beyond tree level with neutrino oscillations considered (Fig. 1), but with vanishingly small branching ratios (∼ 10−54) [1]. Searches for such processes are free from irreducible SM backgrounds, and any observation would be an unambiguous signal of Beyond-the-Standard-Model (BSM) phenomena. This makes LFV Z boson decays an interesting signal to probe BSM theories with LFV predictions, including models with heavy neutrinos [2], extended gauge [3] or supersymmetry [4]. This article presents the searches for the LFV decays of the Z boson into an electron or a muon, hereafter referred to as a light lepton or , and a hadronically decaying τ-lepton [9] with the ATLAS detector, using pp collisions data with a centre-of-mass energy of 13 TeV
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