Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in $\sqrt{s}=13\mathrm{TeV}$ $pp$ collisions with the ATLAS detector

The results of a search for the top squark, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the 2015 LHC $pp$ collision data at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ recorded by the ATLAS detector and corresponding to an integrated luminosity of $3.2{\mathrm{fb}}^{-1}$. The analysis targets two types of signal models: gluino-mediated pair production of top squarks with a nearly mass-degenerate top squark and neutralino and direct pair production of top squarks, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and top squark masses are set at 95% confidence level. The results extend the LHC run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low top squark mass region and add an excluded top squark mass region from 745 to 780 GeV for the direct top squark model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vectorlike top quarks.

Figure 1

Diagrams illustrating the two considered signal scenarios. Left: Gluino-mediated top squark pair production, where each top squark decays to a low momentum (“soft”) charm quark and the lightest neutralino. Right: Top squark pair production, where each top squark decays to the top quark and the lightest neutralino (${\tilde{\chi }}_1^0$). For simplicity, no distinction is made between particles and antiparticles.

Figure 2

A schematic diagram for the various event selections used to estimate and validate the background model and then search for top squark production. Solid lines indicate kinematic boundaries while dashed lines indicate that the events can extend beyond the boundary. CR, VR, and SR stand for control region, validation region, and signal region, respectively. T, ST, TZ, and W stand for $t\overline{t}$, single top, $t\overline{t}+Z$, and $W+\text{jets}$, respectively.

Figure 3

Comparison of data with estimated backgrounds in the $a{m}_{\mathrm{T}2}$ (top left), $b$-tagged jet multiplicity (top right), and $\mathrm{\Delta }R(b_1,b_2)$ (bottom) distributions with the STCR1 event selection except for the requirement (indicated by an arrow) on the variable shown. Furthermore, the $\mathrm{\Delta }R(b_1,b_2)$ requirement is dropped for the $b$-tagged jet multiplicity distribution. The predicted backgrounds are scaled with the NFs documented in Table 4. The uncertainty band includes statistical and all experimental systematic uncertainties. The last bin includes overflow. The middle panel shows the ratio of the data yield to the SM prediction, while the lower panel shows the ratio of the single-top yield to either the $t\overline{t}$ prediction (top left and bottom) or the $W+\text{jets}$ prediction (top right). The error bars in the lower panel include statistical uncertainties only.

Figure 4

Comparison of data with estimated backgrounds in the ${\tilde{E}}_{\mathrm{T}}^{\text{miss}}$ and ${\tilde{m}}_{\mathrm{T}}$ distributions with the TZCR1 event selection except for the requirement (indicated by an arrow) on the shown variable. The variables ${\tilde{E}}_{\mathrm{T}}^{\text{miss}}$ and ${\tilde{m}}_{\mathrm{T}}$ are constructed in the same way as $E_{\mathrm{T}}^{\text{miss}}$ and $m_{\mathrm{T}}$ but treating the leading photon transverse momentum as invisible. The predicted backgrounds are scaled with the NFs documented in Table 4. The uncertainty band includes statistical and all experimental systematic uncertainties. The last bin includes overflow.

Figure 5

Comparison of the observed data ($n_{\mathrm{obs}}$) with the predicted background ($n_{\exp }$) in the validation and signal regions. The background predictions are obtained using the background-only fit configuration. The bottom panel shows the significance of the difference between data and predicted background, where the significance is based on the total uncertainty (${\sigma }_{\mathrm{tot}}$).

Figure 6

Jet multiplicity distributions for events where exactly two signal leptons (left) or one lepton plus one $\tau$ candidate (right) are selected. No correction factors are included in the background normalizations. The uncertainty band includes statistical and all experimental systematic uncertainties. The last bin includes overflow.

Figure 7

The $E_{\mathrm{T}}^{\text{miss}}$ (left) and $m_{\mathrm{T}}$ (right) distributions in SR1. In each plot, the full event selection in the corresponding signal region is applied, except for the requirement (indicated by an arrow) that is imposed on the variable being plotted. The predicted backgrounds are scaled with the NFs documented in Table 4. The uncertainty band includes statistical and all experimental systematic uncertainties. The last bin contains the overflow. Benchmark signal models are overlaid for comparison.

Figure 8

Expected (black dashed curve) and observed (red solid curve) 95% excluded regions in the plane of $m_{\tilde{g}}$ versus $m_{{\tilde{t}}_1}$ for gluino-mediated top squark production (left) and in the plane of $m_{{\tilde{t}}_1}$ versus $m_{{\tilde{\chi }}_1^0}$ for direct top squark pair production (right). Both scenarios assume the SUSY decays shown on the plots, each with a branching ratio of 100%. The gray filled areas and gray dashed lines show the observed and expected exclusion limits, respectively, from ATLAS run-1 searches in the inclusive one-lepton SUSY search [131] (left) and the top squark search in the one-lepton channel [29] (right). The $m_{{\tilde{t}}_1}<323\mathrm{GeV}$ region for the gluino-mediated scenario (left) is excluded by the search described in Ref. [132]. The gap in the observed exclusion between about 600 and 750 GeV in the direct top squark model is due to a transition between signal regions and the excess observed in SR1. For any model point, the single signal region used for the observed exclusion is chosen to be the one with the best expected ${\mathrm{C}\mathrm{L}}_{\mathrm{s}}$ value.

Figure 9

The observed and expected upper limits on $T$ quark pair production times the squared branching ratio for $T\to tZ$ as a function of the $T$ quark mass. The theory cross section is shown assuming a 100% branching ratio for $T\to tZ$.