Search for bottom-squark pair production in $pp$ collision events at $\sqrt{s}=13\mathrm{TeV}$ with hadronically decaying $\tau$-leptons, $b$-jets, and missing transverse momentum using the ATLAS detector

A search for pair production of bottom squarks in events with hadronically decaying $\tau$-leptons, $b$-tagged jets, and large missing transverse momentum is presented. The analyzed dataset is based on proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ delivered by the Large Hadron Collider and recorded by the ATLAS detector from 2015 to 2018, and corresponds to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. The observed data are compatible with the expected Standard Model background. Results are interpreted in a simplified model where each bottom squark is assumed to decay into the second-lightest neutralino ${\tilde{\chi }}_2^0$ and a bottom quark, with ${\tilde{\chi }}_2^0$ decaying into a Higgs boson and the lightest neutralino ${\tilde{\chi }}_1^0$. The search focuses on final states where at least one Higgs boson decays into a pair of hadronically decaying $\tau$-leptons. This allows the acceptance and thus the sensitivity to be significantly improved relative to the previous results at low masses of the ${\tilde{\chi }}_2^0$, where bottom-squark masses up to 850 GeV are excluded at the 95% confidence level, assuming a mass difference of 130 GeV between ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^0$. Model-independent upper limits are also set on the cross section of processes beyond the Standard Model.

Figure 1

Simplified model of bottom-squark pair production and the decay chain targeted by this analysis.

Figure 2

Kinematic distributions of data and SM background for events that pass the preselection and have at least two hadronically decaying $\tau$-leptons. Predictions from three signal models are also shown, where the masses $m(\tilde{b})$ and $m({\tilde{\chi }}_2^0)$ are given in GeV in the legend. Distributions are displayed for the (a) $H_{\mathrm{T}}$, (b) $m({\tau }_1,{\tau }_2)$, and (c) $m_{\mathrm{T}2}$ variables. The hatched band indicates the total statistical and systematic uncertainty of the SM background. The “Other” contribution includes all the backgrounds not explicitly listed in the legend [$V+\text{jets}$ except $Z(\tau \tau )+\text{jets}$, diboson/triboson, multijet]. The top-quark and $Z(\tau \tau )$ background contributions are scaled with the normalization factors obtained from the background-only fit described in Sec. 6. The rightmost bin includes the overflow. The bottom panel shows the ratio of the observed data and the expected Standard Model background.

Figure 3

Schematic representation of the control region setup that is used to constrain the normalization of the (a) top-quark and (b) $Z(\tau \tau )+\mathrm{jets}$ backgrounds in the signal regions. The arrows represent the transfer factors associated with the replacement of muons with true $\tau$-leptons, which correct for acceptance. For the top-quark background, the sketch illustrates the normalization strategy for the ${\tau }_{\mathrm{true}}{\tau }_{\mathrm{true}}$ contribution. A similar strategy is employed for the ${\tau }_{\mathrm{true}}{\tau }_{\mathrm{fake}}$ contribution, where the ${\tau }_{\mathrm{fake}}$ can originate from a jet from a hadronically decaying $W$ boson, a $b\text{-jet}$, or a jet from initial-state radiation.

Figure 4

Kinematic distributions from the four control regions associated with the top-quark background, showing (a) $E_{\mathrm{T}}^{\mathrm{miss}}$ in $\mathrm{CR}\_\mathrm{Top}\_\mu {\tau }_{\mathrm{true}}$, (b) $p_{\mathrm{T}}(\tau )$ in $\mathrm{CR}\_\mathrm{Top}\_\mu {\tau }_{\text{fake}}$, (c) $p_{\mathrm{T}}({\text{jet}}_1)$ in $\mathrm{CR}\_\mathrm{Top}\_\mu$, and (d) $m_{\mathrm{T}}^{\tau }$ in $\mathrm{CR}\_\mathrm{Top}\_{\tau }_{\mathrm{true}}$. The hatched band indicates the total statistical and systematic uncertainty of the SM background. The top-quark and $Z(\tau \tau )$ background contributions are scaled with the normalization factors obtained from the background-only fit. The “Other” contribution includes all the backgrounds not explicitly listed in the legend ($V+\text{jets}$, $t\overline{t}X$, diboson/triboson, multijet). The rightmost bin includes the overflow. The bottom panel shows the ratio of the observed data and the expected Standard Model background.

Figure 5

The upper panel shows the expected number of SM background events and the number of events observed in data for each of the four validation regions. In the lower panel, the significance of the deviation of the observed yield from the expected yield is shown. The top-quark, $Z(\tau \tau )$, and $Z(\mu \mu )$ background contributions are scaled with the normalization factors obtained from the background-only fit described in Sec. 6. The hatched band indicates the total statistical and systematic uncertainty of the SM background. The “Other” contribution includes all the backgrounds not explicitly listed in the legend [$V+\text{jets}$ except $Z(\mu \mu )+\text{jets}$, $t\overline{t}X$, diboson/triboson, multijet].

Figure 6

Kinematic distributions from the four validation regions, showing (a) ${\mathrm{\Theta }}_{\min }$ in $\mathrm{VR}\_\mathrm{Top}\_\tau \tau$, (b) $m_{\mathrm{T}2}$ in $\mathrm{VR}\_\mathrm{Top}\_\mu {\tau }_{\text{fake}}$, (c) $m_{\mathrm{T}2}$ in $\mathrm{VR}\_\mathrm{Top}\_\mu {\tau }_{\mathrm{true}}$, (d) ${\mathrm{\Theta }}_{\min }$ in $\mathrm{VR}\_\mathrm{Top}\_\mu {\tau }_{\mathrm{true}}$, (e) $p_{\mathrm{T}}({\mu }_1,{\mu }_2)$ in $\mathrm{VR}\_\mathrm{Z}\_\mu \mu 2\mathrm{b}$, (f) $H_{\mathrm{T}}$ in $\mathrm{VR}\_\mathrm{Z}\_\mu \mu 2\mathrm{b}$. The hatched band indicates the total statistical and systematic uncertainty of the SM background. The top-quark and $Z(\tau \tau )$ background contributions are scaled with the normalization factors obtained from the background-only fit. The “Other” contribution includes all the backgrounds not explicitly listed in the legend ($V+\text{jets}$, $t\overline{t}X$, diboson/triboson, multijet). The rightmost bin includes the overflow. The bottom panel shows the ratio of the observed data and the expected Standard Model background.

Figure 7

Comparison of the expected and observed event yields in the signal regions defined in Table 2. The top-quarkand $Z(\tau \tau )$ background contributions are scaled with the normalization factors obtained from the background-only fit. The “Other” contribution includes all the backgrounds not explicitly listed in the legend [$V+\text{jets}$ except $Z(\tau \tau )+\mathrm{jets}$, diboson/triboson, multijet]. The hatched band indicates the total statistical and systematic uncertainty of the SM background. The contributions from three signal models to the signal regions are also displayed, where the masses $m(\tilde{b})$ and $m({\tilde{\chi }}_2^0)$ are given in GeV in the legend. The lower panel shows the significance of the deviation of the observed yield from the expected background yield.

Figure 8

Exclusion contours at the 95% C.L. as a function of $m(\tilde{b})$ and $m({\tilde{\chi }}_2^0)$, assuming $\mathrm{\Delta }m({\tilde{\chi }}_2^0,{\tilde{\chi }}_1^0)=130\mathrm{GeV}$. Observed and expected limits are shown for the present search that requires hadronically decaying $\tau$-leptons, $b\text{-jets}$, and $E_{\mathrm{T}}^{\mathrm{miss}}$ in the final state. The observed exclusion limit from a previous ATLAS search [22] that requires $b\text{-jets}$ and $E_{\mathrm{T}}^{\mathrm{miss}}$ in the final state is also displayed. The region $m(\tilde{b})<400\mathrm{GeV}$ is excluded by a previous search from CMS [23].