Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

Abstract

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: [Formula: see text], [Formula: see text], and [Formula: see text]. The searches are based on the data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 [Formula: see text] at a centre-of-mass energy of [Formula: see text] TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95[Formula: see text] confidence level: Br[Formula: see text], Br[Formula: see text], and Br[Formula: see text].

Fig. 1

Two-dimensional distributions of the transverse mass of the e-$E_{\text{T}}^{\text{miss}}$ system, $m_{\mathrm{T}}^{e,E_{\text{T}}^{\text{miss}}}$, and that of the ${\mathit{\tau }}_{\mathrm{had}}$-$E_{\text{T}}^{\text{miss}}$ system, $m_{\mathrm{T}}^{{\mathit{\tau }}_{\mathrm{had}},E_{\text{T}}^{\text{miss}}}$, in simulated $Z/{\mathit{\gamma }}^{\ast }\to \mathit{\tau }\mathit{\tau }$ (top left plot), W+jets (top right plot), $H\to e\mathit{\tau }$ signal (bottom left plot) and data (bottom right plot) events. Magenta, red and yellow boxes on the bottom right plot illustrate SR1, SR2, and WCR, respectively. All events used for these distributions are required to have a well-identified electron and ${\mathit{\tau }}_{\mathrm{had}}$ (as described in text) of opposite charge with $p_{\text{T}}({\mathit{\tau }}_{\mathrm{had}})>20$ GeV and $E_{\text{T}}(e)>26$ GeV

Fig. 2

Distributions of the mass reconstructed by the Missing Mass Calculator, $m_{e\mathit{\tau }}^{\mathrm{MMC}}$, in SR1 (left) and SR2 (right). The background distributions are determined in a global fit (described in Sect. 4.4). The signal distribution corresponds to Br($H\to e\mathit{\tau }$) = 25%. The bottom panel of each sub-figure shows the ratio of the observed data to the estimated background. Very small backgrounds due to single top, $t\overline{t}$, VV, $Z\to ee(\text{jet}\to {\mathit{\tau }}_{\mathrm{had}}^{\mathrm{misid}})$ and $H\to \mathit{\tau }\mathit{\tau }$ events are combined in a single background component labelled as “Other Backgrounds”. The grey band for the ratio illustrates post-fit systematic uncertainties in the background prediction. The statistical uncertainties in the background predictions and data are added in quadrature for the ratios. The last bin in each distribution contains events with $m_{e\mathit{\tau }}^{\mathrm{MMC}}>$ 250 GeV

Fig. 3

Post-fit combined $m_{e\mathit{\tau }}^{\mathrm{MMC}}$ distribution obtained by adding individual distributions in SR1 and SR2. In the lower part of the figure, the data are shown after subtraction of the estimated backgrounds. The grey band in the bottom panel illustrates the post-fit systematic uncertainties in the background prediction. The statistical uncertainties for data and background predictions are added in quadrature in the bottom part of the figure. The signal is shown assuming Br$(H\to e\mathit{\tau })=1.0\%$. Very small backgrounds due to single top, $t\overline{t}$, VV, $Z\to ee(jet\to {\mathit{\tau }}_{\mathrm{had}}^{\mathrm{misid}})$ and $H\to \mathit{\tau }\mathit{\tau }$ events are combined in a single background component labelled as “Other Backgrounds”. The last bin of the distribution contains events with $m_{e\mathit{\tau }}^{\mathrm{MMC}}>$250 GeV

Fig. 4

Collinear mass distributions in the ${\mathit{\tau }}_{\mathrm{lep}}$ channel: background estimate compared to the events observed in the data in the ${\mathrm{SR}}_{\mathrm{noJets}}$ (top) and ${\mathrm{SR}}_{\mathrm{withJets}}$ (bottom). Left $e\mathit{\mu }$ channel. Right $\mathit{\mu }e$ channel. In these plots, events from the three $f\left(p_{\mathrm{T}}^{\ell {}_2}\right)$ bins are combined, although the fit parameters are different in each $f\left(p_{\mathrm{T}}^{\ell {}_2}\right)$ bin. The signal expected for a $\text{Br}(H\to \mathit{\mu }\mathit{\tau })=1\%$ is shown in the $\mathit{\mu }e$ channel

Fig. 5

Upper limits on LFV decays of the Higgs boson in the $H\to e\mathit{\tau }$ hypothesis (left) and $H\to \mathit{\mu }\mathit{\tau }$ hypothesis (right). The limits are computed under the assumption that either Br($H\to \mathit{\mu }\mathit{\tau }$)=0 or Br($H\to e\mathit{\tau }$)=0. The $\mathit{\mu }{\mathit{\tau }}_{\mathrm{had}}$ channel is from Ref. [22]

Fig. 6

Distributions of the mass reconstructed by the Missing Mass Calculator, $m_{\mathit{\mu }\mathit{\tau }}^{\mathrm{MMC}}$, in $Z\to \mathit{\mu }\mathit{\tau }$ SR1 (left) and SR2 (right). The background distributions are determined in a global fit. The signal distributions are scaled to a branching ratio of Br($Z\to \mathit{\mu }\mathit{\tau }$) = ${10}^{-3}$ to make them visible. The bottom panel of each subfigure shows the ratio of the observed data to the estimated background. The hatched band for the ratio illustrates post-fit systematic uncertainties in the background prediction. The statistical uncertainties for data and background predictions are added in quadrature for the ratios. The last bin of the distribution contains events with $m_{\mathit{\mu }\mathit{\tau }}^{\mathrm{MMC}}>200$ GeV