Search for supersymmetry in the all-hadronic final state using top quark tagging in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is presented for supersymmetry in all-hadronic events with missing transverse momentum and tagged top quarks. The data sample was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of $2.3{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. Search regions are defined using the properties of reconstructed jets, the multiplicity of bottom and top quark candidates, and an imbalance in transverse momentum. With no statistically significant excess of events observed beyond the expected contributions from the standard model, we set exclusion limits at 95% confidence level on the masses of new particles in the context of simplified models of direct and gluino-mediated top squark production. For direct top squark production with decays to a top quark and a neutralino, top squark masses up to 740 GeV and neutralino masses up to 240 GeV are excluded. Gluino masses up to 1550 GeV and neutralino masses up to 900 GeV are excluded for a gluino-mediated production case, where each of the pair-produced gluinos decays to a top-antitop quark pair and a neutralino.

Figure 1

Diagrams representing two cases of the simplified models of direct top squark pair production and decay considered in this study: the T2tt model with top squark decay via a top quark (top), and the T2tb model with the top squark decaying either via a top quark or via an intermediate chargino (bottom).

Figure 2

Diagrams representing the simplified models of gluino-mediated top squark production considered in this study: the T1tttt model (top) where the gluino decays to top quarks and the LSP via an off-shell top squark, and the T5ttcc model (bottom) where the gluino decays to an on-shell top squark, which decays to a charm quark and the LSP.

Figure 3

The tagging efficiency of the top quark tagger as a function of the generator-level hadronically decaying top quark $p_{\mathrm{T}}$ (black points). The efficiency was computed using the T2tt signal model with $m_{\tilde{t}}=850\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=100\mathrm{GeV}$, and it is similar for $t\overline{t}$ events. The vertical bars depict the statistical uncertainty. The colored lines show the expected hadronically decaying top quark $p_{\mathrm{T}}$ distribution from $t\overline{t}$ (red solid line), the T2tt signal model with $m_{\tilde{t}}=500\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=325\mathrm{GeV}$ (blue short-dashed line), the T2tt signal model with $m_{\tilde{t}}=750\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=50\mathrm{GeV}$ (green long-dashed line), the T1tttt signal model with $m_{\tilde{g}}=1200\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=800\mathrm{GeV}$ (purple long-dash-dotted line), and the T1tttt signal model with $m_{\tilde{g}}=1500\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=100\mathrm{GeV}$ (orange short-dash-dotted line). The last bin contains the overflow entries and the top quark $p_{\mathrm{T}}$ distributions are normalized to unit area.

Figure 4

Comparison of the distributions in data (black points), simulated SM backgrounds (filled stacked histograms) and several signal models in $N_t$ (top left), $N_b$ (top right), $M_{\mathrm{T}2}$ (bottom left), and $E_{\mathrm{T}}^{\mathrm{miss}}$ (bottom right), after the preselection requirements have been applied. The T2tt signal model with $m_{\tilde{t}}=500(750)\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=325(50)\mathrm{GeV}$ is shown with a red short-dashed (long-dashed) line, and the T1tttt signal model with $m_{\tilde{g}}=1200(1500)\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=800(100)\mathrm{GeV}$ with a dark green short-dash-dotted (long-dash-dotted) line. The distributions for the signal events have been normalized to the same area as the total background distribution, and the last bin contains the overflow events.

Figure 5

Search region definitions for bin numbers 0–41 of the gluino-mediated production optimization. The highest $E_{\mathrm{T}}^{\mathrm{miss}}$ and $M_{\mathrm{T}2}$ bins are open-ended, e.g., bin 10 requires $E_{\mathrm{T}}^{\mathrm{miss}}>450\mathrm{GeV}$ and $M_{\mathrm{T}2}>400\mathrm{GeV}$. In addition to the search bins shown in this figure, there are three bins (42–44) with $N_t\ge 3$, one for each $N_b$ bin, that contain no further binning in $E_{\mathrm{T}}^{\mathrm{miss}}$ or $M_{\mathrm{T}2}$ beyond baseline selection requirements.

Figure 6

Search region definitions for bin numbers 0–36 for the direct top squark production optimization. The highest $E_{\mathrm{T}}^{\mathrm{miss}}$ and $M_{\mathrm{T}2}$ bins are open-ended, e.g., bin 10 requires $E_{\mathrm{T}}^{\mathrm{miss}}>450\mathrm{GeV}$ and $M_{\mathrm{T}2}>400\mathrm{GeV}$.

Figure 7

Top: the lost-lepton background in the 45 search regions optimized for gluino-mediated production as determined directly from $t\overline{t}$, single top quark, and $W+\text{jets}$ simulation (points) and as predicted by applying the lost-lepton background determination procedure to the simulated muon control sample (histograms). The lower panel shows the same results after dividing by the predicted value. Bottom: the corresponding simulated results for the background from hadronically decaying $\tau$ leptons. For both plots, vertical lines indicate search regions with different $N_t$, $N_b$, and $M_{\mathrm{T}2}$ values. Within each ($N_t$, $N_b$, $M_{\mathrm{T}2}$) region, the bins indicate the different $E_{\mathrm{T}}^{\mathrm{miss}}$ selections, as defined in Fig. 5. Only statistical uncertainties are shown.

Figure 8

The $N_b$ (top) and $E_{\mathrm{T}}^{\mathrm{miss}}$ (bottom) distributions in data and simulation in the loose dimuon control region, after applying the $S_{\mathrm{DY}}(N_j)$ scale factor to the simulation. The lower panels show the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses in the legend indicate the integrated yield for each given process. The “rare” category includes background processes such as triboson and $t\overline{t}W$ production.

Figure 9

The QCD multijet background in the 45 search regions optimized for gluino-mediated production as determined directly from simulation (points) and as predicted by applying the QCD multijet background determination procedure to simulated event samples in the inverted-$\mathrm{\Delta }\varphi$ control region (histograms). The lower panel shows the same results after dividing by the predicted value. Only statistical uncertainties are shown. The labeling of the search regions is the same as in Fig. 7.

Figure 10

Observed event yields in data (black points) and predicted SM background (filled solid area) for the 37 search bins optimized for direct top squark production. The red and dark green lines indicate various signal models: the T2tt model with $m_{\tilde{t}}=500\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=325\mathrm{GeV}$ (red short-dashed line), the T2tt model with $m_{\tilde{t}}=750\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=50\mathrm{GeV}$ (red long-dashed line), and the T2tb model with $m_{\tilde{t}}=700\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=100\mathrm{GeV}$ (dark green dashed-dotted line). The lower panel shows the ratio of data over total background prediction in each search bin. For both panels, the error bars show the statistical uncertainty associated with the observed data counts, and the grey (blue) hatched bands indicate the statistical (systematic) uncertainties in the total predicted background.

Figure 11

Observed event yields in data (black points) and predicted SM background (filled solid area) for the 45 search bins optimized for gluino models. The red and dark green lines indicate various signal models: the T1tttt model with $m_{\tilde{g}}=1200\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=800\mathrm{GeV}$ (dark green short-dashed line), the T1tttt model with $m_{\tilde{g}}=1500\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=100\mathrm{GeV}$ (dark green long-dashed line), and the T5ttcc model with $m_{\tilde{g}}=1200\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}=800\mathrm{GeV}$ (red dashed-dotted line). The lower panel shows the ratio of data over total background prediction in each search bin. For both panels, the error bars show the statistical uncertainty associated with the observed data counts, and the grey (blue) hatched bands indicate the statistical (systematic) uncertainties in the total predicted background.

Figure 12

Exclusion limits at 95% CL for simplified models of top squark pair production in the T2tt (top) and T2tb (bottom) scenario, assuming a 50% branching fraction for each of the $\tilde{t}\to t{\tilde{\chi }}_1^0/\tilde{t}\to b{\tilde{\chi }}_1^{\pm }$ modes and a 5 GeV mass difference between the ${\tilde{\chi }}_1^{\pm }$ and ${\tilde{\chi }}_1^0$. The solid black curves represent the observed exclusion contour with respect to $\mathrm{NLO}+\mathrm{NLL}$ cross section calculations [88] and the corresponding $\pm 1$ standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm 1$ standard deviation uncertainties including experimental uncertainties. No interpretation is provided for signal models for which $|m_{\tilde{t}}-m_{{\tilde{\chi }}_1^0}-m_t|\le 25\mathrm{GeV}$ and $m_{\tilde{t}}\le 275\mathrm{GeV}$ because of significant differences between the fast simulation and the geant4 -based simulation for these low-$E_{\mathrm{T}}^{\mathrm{miss}}$ scenarios.

Figure 13

Exclusion limits at 95% CL for simplified models of top squarks produced via decays of gluino pairs in the T1tttt (top) and T5ttcc (bottom) scenarios. The solid black curves represent the observed exclusion contour with respect to $\mathrm{NLO}+\mathrm{NLL}$ cross section calculations [88] and the corresponding $\pm 1$ standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm 1$ standard deviation uncertainties including experimental uncertainties.