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

The collaboration
CERN-EP-2022-279, 2023.

Abstract (data abstract)
The direct searches for lepton flavour violation in Higgs boson decays, $H\to e\tau$ and $H\to \mu\tau$, 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$^{-1}$. Leptonic ($\tau \to \ell \nu_\ell \nu_\tau$) and hadronic ($\tau \to \text{hadrons}$ $\nu_\tau$) decays of the tau lepton are considered. Two background estimation techniques are exploited$\text{:}$ 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 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 at 95% confidence level on the branching ratios of $\mathcal{B}(H\to e\tau)<0.20\%$ ($0.12\%$) and $\mathcal{B}(H\to \mu\tau)<0.18\%$ ($0.09\%$) are obtained with the MC-template method from the simultaneous measurement of the $H\to e\tau$ and $H\to \mu\tau$ 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 decays to leptons, is $(0.25 \pm 0.10)\%$, compatible with a value of zero within $2.5\sigma$.

• #### Table 1a

Figure 13(left) in the paper

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Fit results of the simultaneous measurements of the $H\to e\tau$ and $H\to \mu\tau$ signals (2POI) showing upper limits at 95%...

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Fit results of the simultaneous measurements of the $H\to e\tau$ and $H\to \mu\tau$ signals (2POI) showing best-fit values of the...

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Figure 13(right) in the paper

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Fit results of the simultaneous measurements of the $H\to e\tau$ and $H\to \mu\tau$ signals (2POI) showing upper limits at 95%...

• #### Table 2b

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Fit results of the simultaneous measurements of the $H\to e\tau$ and $H\to \mu\tau$ signals (2POI) showing best-fit values of the...

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Figure 11(left) in the paper

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Fit results of the independent searches (1 POI) showing upper limits at 95% C.L. on the LFV branching ratios of...

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Fit results of the independent searches (1 POI) showing best-fit values of the LFV branching ratios of the Higgs boson...

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Figure 11(right) in the paper

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Fit results of the independent searches (1 POI) showing upper limits at 95% C.L. on the LFV branching ratios of...

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Fit results of the independent searches (1 POI) showing best-fit values of the LFV branching ratios of the Higgs boson...

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Figure 8(a) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 8(b) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 8(c) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 8(d) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 9(a) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 9(b) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 9(c) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 9(d) in the paper

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BDT score distribution, after the simultaneous fit of the $H\to e\tau$ and $H\to \mu\tau$ signals, obtained by fitting the data...

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Figure 10(b) in the paper

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NN score distribution, after an independent fit of the $H\to \mu\tau$ signal, obtained by fitting the data of the Symmetry...

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Figure 10(d) in the paper

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NN score distribution, after an independent fit of the $H\to \mu\tau$ signal, obtained by fitting the data of the Symmetry...

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Figure 16 in the paper

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Best-fit $\mathcal{B}(H\to \mu\tau)- \mathcal{B}(H\to e\tau)$ values, given in %, obtained from the standalone fits of the $\ell\tau_\ell'$ final state with...

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Figure 14 in the paper

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Best-fit value of the branching ratios $\mathcal{B}(H\to e\tau)$ and $\mathcal{B}(H\to \mu\tau)$, given in %, and likelihood contours at 68% and...

Version 2 modifications: Updated abstract and formatting of figures to match those of the final paper