CERN Accelerating science

ATLAS Note
Report number	ATL-PHYS-PROC-2024-106
Title	Searches for lepton-flavour-violating decays of the Higgs boson into $e\tau$ and $\mu\tau$ in $\sqrt(s) = 13$ TeV pp collisions with the ATLAS detector
Author(s)	Chu, Michael Kwok Lam (Weizmann Institute of Science (IL))
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2025
Imprint	19 Nov 2024
Number of pages	4
In:	PoS LHCP2024 (2025) 245
In:	12th Edition of the Large Hadron Collider Physics Conference (LHCP2024), Boston, United States, 3 - 7 Jun 2024, pp.245
DOI	10.22323/1.478.0245
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	HIGGS
Abstract	This document presents direct searches for lepton-flavour-violating for lepton flavour violation in Higgs boson decays, $H\rightarrow e\tau$ and $H\rightarrow\mu\tau$, 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 $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Both leptonic ($\tau \to \ell \nu_\ell \nu_\tau$) and hadronic ($\tau \to \text{hadrons } \nu_\tau$) decays of the $\tau$-lepton are considered and two background estimation techniques are employed: the MC-template method, based on data-corrected simulation samples, and the data-driven Symmetry method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. The observed (expected) upper limits set on the branching ratios at 95% confidence level, $\mathcal{B}(H\to e\tau)<0.20\%$ ($0.12\%$) and $\mathcal{B}(H\to \mu\tau)<0.18\%$ ($0.09\%$), are obtained with the MC-template method from a simultaneous measurement of potential $H \rightarrow e\tau$ and $H \rightarrow\mu\tau$ signals. The best-fit branching ratio difference, $\mathcal{B}(H\to \mu\tau)- \mathcal{B}(H\to e\tau)$, measured with the Symmetry method in the channel where the $\tau$-lepton decays to leptons, is $(0.25 \pm 0.10)\%$, compatible with a value of zero within $2.5\sigma$.

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