Searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ in √s = 13 TeV pp collisions with the ATLAS detector
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jats:titleAjats:scbstract</jats:sc> </jats:title>jats:pThis paper presents direct searches for lepton flavour violation in Higgs boson decays, jats:italicH</jats:italic> → jats:italiceτ</jats:italic> and jats:italicH</jats:italic> → jats:italicμτ</jats:italic>, performed using data collected with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy jats:inline-formulajats:alternativesjats:tex-math$$ \sqrt{s} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msqrt mml:mis</mml:mi> </mml:msqrt> </mml:math></jats:alternatives></jats:inline-formula> = 13 TeV, corresponding to an integrated luminosity of 138 fbjats:supjats:italic−</jats:italic>1</jats:sup>. Leptonic (jats:italicτ</jats:italic> → jats:italicℓν</jats:italic>jats:subjats:italicℓ</jats:italic></jats:sub>jats:italicν</jats:italic>jats:subjats:italicτ</jats:italic></jats:sub>) and hadronic (jats:italicτ</jats:italic> → hadrons jats:italicν</jats:italic>jats:subjats:italicτ</jats:italic></jats:sub>) decays of the jats:italicτ</jats:italic>-lepton are considered. Two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. No significant excess of events is observed and the results are interpreted as upper limits on lepton-flavour-violating branching ratios of the Higgs boson. The observed (expected) upper limits set on the branching ratios at 95% confidence level, jats:inline-formulajats:alternativesjats:tex-math$$ \mathcal{B} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miB</mml:mi> </mml:math></jats:alternatives></jats:inline-formula>(jats:italicH</jats:italic> → jats:italiceτ</jats:italic>) jats:italic<</jats:italic> 0jats:italic.</jats:italic>20% (0jats:italic.</jats:italic>12%) and jats:inline-formulajats:alternativesjats:tex-math$$ \mathcal{B} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miB</mml:mi> </mml:math></jats:alternatives></jats:inline-formula>(jats:italicH</jats:italic> → jats:italicμτ</jats:italic> ) jats:italic<</jats:italic> 0jats:italic.</jats:italic>18% (0jats:italic.</jats:italic>09%), are obtained with the MC-template method from a simultaneous measurement of potential jats:italicH</jats:italic> → jats:italiceτ</jats:italic> and jats:italicH</jats:italic> → jats:italicμτ</jats:italic> signals. The best-fit branching ratio difference, jats:inline-formulajats:alternativesjats:tex-math$$ \mathcal{B} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miB</mml:mi> </mml:math></jats:alternatives></jats:inline-formula>(jats:italicH</jats:italic> → jats:italicμτ</jats:italic>) → jats:inline-formulajats:alternativesjats:tex-math$$ \mathcal{B} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miB</mml:mi> </mml:math></jats:alternatives></jats:inline-formula>(jats:italicH</jats:italic> → jats:italiceτ</jats:italic>), measured with the Symmetry method in the channel where the jats:italicτ</jats:italic>-lepton decays to leptons, is (0jats:italic.</jats:italic>25 ± 0jats:italic.</jats:italic>10)%, compatible with a value of zero within 2jats:italic.</jats:italic>5jats:italicσ</jats:italic>.</jats:p>
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1029-8479