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 ATLAS 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, corresponds to $0.25 \pm 0.10\%$.

  • Table 1a

    Figure 13(left) in the paper

    10.17182/hepdata.135719.v1/t1

    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 1b

    10.17182/hepdata.135719.v1/t2

    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...

  • Table 2a

    Figure 13(right) in the paper

    10.17182/hepdata.135719.v1/t3

    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

    10.17182/hepdata.135719.v1/t4

    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...

  • Table 3a

    Figure 11(left) in the paper

    10.17182/hepdata.135719.v1/t5

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

  • Table 3b

    10.17182/hepdata.135719.v1/t6

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

  • Table 4a

    Figure 11(right) in the paper

    10.17182/hepdata.135719.v1/t7

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

  • Table 4b

    10.17182/hepdata.135719.v1/t8

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

  • Figure 1a

    Figure 8(a) in the paper

    10.17182/hepdata.135719.v1/t9

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

  • Figure 1b

    Figure 8(b) in the paper

    10.17182/hepdata.135719.v1/t10

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

  • Figure 1c

    Figure 8(c) in the paper

    10.17182/hepdata.135719.v1/t11

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

  • Figure 1d

    Figure 8(d) in the paper

    10.17182/hepdata.135719.v1/t12

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

  • Figure 2a

    Figure 9(a) in the paper

    10.17182/hepdata.135719.v1/t13

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

  • Figure 2b

    Figure 9(b) in the paper

    10.17182/hepdata.135719.v1/t14

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

  • Figure 2c

    Figure 9(c) in the paper

    10.17182/hepdata.135719.v1/t15

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

  • Figure 2d

    Figure 9(d) in the paper

    10.17182/hepdata.135719.v1/t16

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

  • Figure 3a

    Figure 10(b) in the paper

    10.17182/hepdata.135719.v1/t17

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

  • Figure 3b

    Figure 10(d) in the paper

    10.17182/hepdata.135719.v1/t18

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

  • Figure 4

    Figure 16 in the paper

    10.17182/hepdata.135719.v1/t19

    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...

  • Figure 5

    Figure 14 in the paper

    10.17182/hepdata.135719.v1/t20

    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...

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