Search for squarks and gluinos in final states with hadronically decaying $\tau$-leptons, jets, and missing transverse momentum using $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector

A search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying $\tau$-lepton is presented. Two exclusive final states with either exactly one or at least two $\tau$-leptons are considered. The analysis is based on proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$ delivered by the Large Hadron Collider and recorded by the ATLAS detector in 2015 and 2016. No significant excess is observed over the Standard Model expectation. At 95% confidence level, model-independent upper limits on the cross section are set and exclusion limits are provided for two signal scenarios: a simplified model of gluino pair production with $\tau$-rich cascade decays, and a model with gauge-mediated supersymmetry breaking (GMSB). In the simplified model, gluino masses up to 2000 GeV are excluded for low values of the mass of the lightest supersymmetric particle (LSP), while LSP masses up to 1000 GeV are excluded for gluino masses around 1400 GeV. In the GMSB model, values of the supersymmetry-breaking scale are excluded below 110 TeV for all values of $\tan \beta$ in the range $2\le \tan \beta \le 60$, and below 120 TeV for $\tan \beta >30$.

Figure 1

Example processes of (a) the GMSB model and (b) the simplified model of gluino pair production leading to final states with $\tau$-leptons, jets, and missing transverse momentum.

Figure 2

Distributions of (a) the $\tau$-lepton transverse mass $m_{\mathrm{T}}^{\tau }$ in the $1\tau$ channel and (b) the sum of $\tau$-lepton transverse masses $m_{\mathrm{T}}^{{\tau }_1}+m_{\mathrm{T}}^{{\tau }_2}$ in the $2\tau$ channel after the preselection, after applying data-driven normalization factors to the main backgrounds. The last bin includes overflow events. The total uncertainty in the background prediction is shown as a shaded band. The contribution labeled as “Other” includes multijet events and the $V+\text{jets}$ processes not explicitly listed in the legend. Signal predictions are overlaid for several benchmark model points. For the simplified model, LM, MM, and HM refer to low, medium, and high mass-splitting scenarios, with $(m_{\tilde{g}},m_{{\tilde{\chi }}_1^0})$ set to (1065,825) GeV, (1625,905) GeV, and (1705,345) GeV, respectively. The GMSB benchmark model corresponds to $\mathrm{\Lambda }=120\mathrm{TeV}$ and $\tan \beta =40$.

Figure 3

(a) Scalar sum of transverse momenta of $\tau$-leptons and jets $H_{\mathrm{T}}$ in the top true-$\tau$ CR, (b) missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$ in the $W$ fake-$\tau$ CR, (c) $H_{\mathrm{T}}$ in the $W$ kinematic CR, (d) sum of $\tau$-lepton transverse masses $m_{\mathrm{T}}^{{\tau }_1}+m_{\mathrm{T}}^{{\tau }_2}$ in the $Z(\tau \tau )$ CR, (e) $H_{\mathrm{T}}$ in the $Z(\nu \nu )$ CR, and (f) $E_{\mathrm{T}}^{\mathrm{miss}}$ in the multijet CR, illustrating the background modeling in the CRs after the fit. The contribution labeled as “Other” includes multijet events (except for the multijet CR) and the $V+\text{jets}$ processes not explicitly listed in the legend. The last bin of each distribution includes overflow events. The total uncertainty in the background prediction is shown as a shaded band.

Figure 4

Distributions of (a) $\tau$-lepton transverse mass $m_{\mathrm{T}}^{\tau }$ in the compressed $m_{\mathrm{T}}^{\tau }$ VR, (b) missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$ in the compressed $E_{\mathrm{T}}^{\mathrm{miss}}$ VR, (c) $m_{\mathrm{T}}^{\tau }$ in the medium-mass $m_{\mathrm{T}}^{\tau }$ VR, (d) $E_{\mathrm{T}}^{\mathrm{miss}}$ in the medium-mass $E_{\mathrm{T}}^{\mathrm{miss}}$ VR, and (e) scalar sum of $\tau$-lepton and jet transverse momenta $H_{\mathrm{T}}$ in the medium-mass $H_{\mathrm{T}}$ VR, illustrating the background modeling in the VRs of the $1\tau$ channel after the fit. The normalization factors obtained in the CRs are applied. The contribution labeled as “Other” includes multijet events and the $V+\text{jets}$ processes not explicitly listed in the legend. The last bin of each distribution includes overflow events. The total uncertainty in the background prediction is shown as a shaded band.

Figure 5

(a) Sum of $\tau$-lepton transverse masses $m_{\mathrm{T}}^{{\tau }_1}+m_{\mathrm{T}}^{{\tau }_2}$ in the top VR, (b) scalar sum of $\tau$-lepton and jet transverse momenta $H_{\mathrm{T}}$ in the $W$ VR, and (c) $m_{\mathrm{T}}^{{\tau }_1}+m_{\mathrm{T}}^{{\tau }_2}$ in the $Z$ VR, illustrating the background modeling in the VRs of the $2\tau$ channel after the fit. The normalization factors obtained in the CRs are applied. The contribution labeled as “Other” includes multijet events and the $V+\text{jets}$ processes not explicitly listed in the legend. The last bin of each distribution includes overflow events. The total uncertainty in the background prediction is shown as a shaded band.

Figure 6

Number of observed events $n_{\mathrm{obs}}$ and predicted background yields in the validation regions $n_{\mathrm{pred}}$ of the $1\tau$ and $2\tau$ channels. The background predictions are scaled using normalization factors derived in the control regions. The total uncertainty in the background predictions ${\sigma }_{\mathrm{tot}}$ is shown as a shaded band. The lower panel displays the significance of the deviation of the observed from the expected yield.

Figure 7

Distributions of kinematic variables in extended SR selections of the $1\tau$ channel after the fit: (a) $\tau$-lepton transverse mass $m_{\mathrm{T}}^{\tau }$ in the compressed SR without the $m_{\mathrm{T}}^{\tau }>80\mathrm{GeV}$ requirement and (b) scalar sum of $\tau$-lepton and jet transverse momenta $H_{\mathrm{T}}$ in the medium-mass SR without the $H_{\mathrm{T}}>1000\mathrm{GeV}$ requirement. The contribution labeled as “Other” includes multijet events and the $V+\text{jets}$ processes not explicitly listed in the legend. The last bin of each distribution includes overflow events. The total uncertainty in the background prediction is shown as a shaded band. Arrows in the Data/SM ratio indicate bins where the entry is outside the plotted range. The signal region is indicated by the arrow in the upper pane. Signal predictions are overlaid for several benchmark models. For the simplified model, LM, MM and HM refer to low, medium, and high mass-splitting scenarios, with $(m_{\tilde{g}},m_{{\tilde{\chi }}_1^0})$ set to (1065,825) GeV, (1625,905) GeV, and (1705,345) GeV, respectively. The GMSB benchmark model corresponds to $\mathrm{\Lambda }=120\mathrm{TeV}$ and $\tan \beta =40$.

Figure 8

Distributions of kinematic variables in extended SR selections of the $2\tau$ channel after the fit: (a) sum of transverse masses of $\tau$-leptons and jets $m_{\mathrm{T}}^{\mathrm{sum}}$ in the compressed SR without the $m_{\mathrm{T}}^{\mathrm{sum}}>1600\mathrm{GeV}$ requirement, (b) scalar sum of transverse momenta of $\tau$-leptons and jets $H_{\mathrm{T}}$ in the high-mass SR without the $H_{\mathrm{T}}>1100\mathrm{GeV}$ requirement, (c) sum of transverse masses of $\tau$-leptons $m_{\mathrm{T}}^{{\tau }_1}+m_{\mathrm{T}}^{{\tau }_2}$ in the multibin SR, and (d) $H_{\mathrm{T}}$ in the GMSB SR without the $H_{\mathrm{T}}>1900\mathrm{GeV}$ requirement. The contribution labeled as “Other” includes multijet events and the $V+\text{jets}$ processes not explicitly listed in the legend. The last bin of each distribution includes overflow events. The total uncertainty in the background prediction is shown as a shaded band. Arrows in the Data/SM ratio indicate bins where the entry is outside the plotted range. The signal region is indicated by the arrow in the upper pane. Signal predictions are overlaid for several benchmark models. For the simplified model, LM, MM, and HM refer to low, medium, and high mass-splitting scenarios, with $(m_{\tilde{g}},m_{{\tilde{\chi }}_1^0})$ set to (1065,825) GeV, (1625,905) GeV, and (1705,345) GeV, respectively. The GMSB benchmark model corresponds to $\mathrm{\Lambda }=120\mathrm{TeV}$ and $\tan \beta =40$.

Figure 9

Exclusion contours at the 95% confidence level as a function of the LSP mass $m_{{\tilde{\chi }}_1^0}$ and gluino mass $m_{\tilde{g}}$ for the simplified model of gluino pair production. The solid line and the dashed line correspond to the observed and median expected limits, respectively, for the combination of the $1\tau$ and $2\tau$ channels. The band shows the one-standard-deviation spread of expected limits around the median. The effect of the signal cross-section uncertainty on the observed limits is shown as dotted lines. The inward fluctuation of the $-1\sigma$ line originates from the method employed to perform the combination. The previous ATLAS result [19] obtained with $3.2{\mathrm{fb}}^{-1}$ of 13 TeV data is shown as the filled area in the bottom left.

Figure 10

Exclusion contours at the 95% confidence level as a function of $\tan \beta$ and the SUSY-breaking mass scale $\mathrm{\Lambda }$ for the gauge-mediated supersymmetry-breaking model. The solid line and the dashed line correspond to the observed and median expected limits, respectively, for the combination of the $1\tau$ and $2\tau$ channels. The band shows the one-standard-deviation spread of expected limits around the median. The effect of the signal cross-section uncertainty on the observed limits is shown as dotted lines. The gray and orange dash-dotted lines indicate the masses of gluinos and mass-degenerate squarks, respectively. The previous ATLAS result [19] obtained with $3.2{\mathrm{fb}}^{-1}$ of 13 TeV data is shown as the filled area on the left.