Search for charged-lepton-flavour violating $e\mu tq$ interactions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for charged-lepton-flavour violation (cLFV) in top-quark production and decay is presented. The data analysed correspond to 140 fb$^{-1}$ of proton-proton collisions recorded at a centre-of-mass energy of $\sqrt{s}= 13$ TeV by the ATLAS detector at the Large Hadron Collider. The possible cLFV contribution is studied via two mechanisms. The first is via the production of a leptonically-decaying single-top quark in association with a pair of oppositely-charged different-flavour leptons. The second is the production of top-quark pairs, where one top quark decays into a pair of oppositely-charged different-flavour leptons and a $u$- or $c$-quark, while the other decays into a leptonically-decaying $W$ boson and a $b$-quark, according to the Standard Model. The signal processes contain three charged leptons, taken to be electrons or muons. No significant deviation from the Standard Model prediction is observed, and these results are interpreted within an Effective Field Theory framework. Upper limits are placed at 95% confidence level on six Wilson coefficients, ranging from $|C^{\mathrm{tensor}}_{e\mu tu}|/\Lambda^2<0.018$ TeV$^{-2}$ to $|C^{\mathrm{scalar}}_{e\mu tc}|/\Lambda^2<0.32$ TeV$^{-2}$ for the tensor $u$-quark and scalar $c$-quark couplings, respectively. These Wilson coefficient limits are translated into 95% confidence level upper limits on cLFV top-quark decay branching ratios, ranging from $B(t \to e\mu u)<6 \cdot 10^{-9}$ to $B(t \to e\mu c)<2.7 \cdot 10^{-7}$ for the scalar $u$-quark and tensor $c$-quark couplings, respectively.