Search for supersymmetry in multijet events with missing transverse momentum in proton-proton collisions at 13 TeV

A search for supersymmetry is presented based on multijet events with large missing transverse momentum produced in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$. The data, corresponding to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$, were collected with the CMS detector at the CERN LHC in 2016. The analysis utilizes four-dimensional exclusive search regions defined in terms of the number of jets, the number of tagged bottom quark jets, the scalar sum of jet transverse momenta, and the magnitude of the vector sum of jet transverse momenta. No evidence for a significant excess of events is observed relative to the expectation from the standard model. Limits on the cross sections for the pair production of gluinos and squarks are derived in the context of simplified models. Assuming the lightest supersymmetric particle to be a weakly interacting neutralino, 95% confidence level lower limits on the gluino mass as large as 1800 to 1960 GeV are derived, and on the squark mass as large as 960 to 1390 GeV, depending on the production and decay scenario.

Figure 1

Example Feynman diagrams for the simplified model signal scenarios considered in this study: the (upper left) T1tttt, (upper right) T1tbtb, (lower left) T5qqqqVV, and (lower right) T2tt scenarios. In the T5qqqqVV model, the flavors of the quark $q$ and antiquark $\overline{q}$ differ from each other if the gluino $\tilde{g}$ decays as $\tilde{g}\to q\overline{q}{\tilde{\chi }}_1^{+}$, where ${\tilde{\chi }}_1^{+}$ is the lightest chargino.

Figure 2

Schematic illustration of the ten kinematic search intervals in the $H_{\mathrm{T}}^{\mathrm{miss}}$ versus $H_{\mathrm{T}}$ plane. Intervals 1 and 4 are discarded for $N_{\text{jet}}\ge 7$. The intervals labeled C1, C2, and C3 are control regions used to evaluate the QCD background. The rightmost and topmost bins are unbounded, extending to $H_{\mathrm{T}}=\infty$ and $H_{\mathrm{T}}^{\mathrm{miss}}=\infty$, respectively.

Figure 3

The lost-lepton background in the 174 search regions of the analysis as determined directly from $t\overline{t}$, single top quark, $W+\text{jets}$, diboson, and rare-event simulation (points, with statistical uncertainties) and as predicted by applying the lost-lepton background determination procedure to simulated electron and muon control samples (histograms, with statistical uncertainties). The results in the lower panel are obtained through bin-by-bin division of the results in the upper panel, including the uncertainties, by the central values of the “predicted” results. The ten results (eight results for $N_{\text{jet}}\ge 7$) within each region delineated by vertical dashed lines correspond sequentially to the ten (eight) kinematic intervals of $H_{\mathrm{T}}$ and $H_{\mathrm{T}}^{\mathrm{miss}}$ indicated in Table 1 and Fig. 2.

Figure 4

The background from hadronically decaying $\tau$ leptons in the 174 search regions of the analysis as determined directly from $t\overline{t}$, single top quark, and $W+\text{jets}$ simulation (points, with statistical uncertainties) and as predicted by applying the hadronically decaying $\tau$ lepton background determination procedure to a simulated muon control sample (histograms, with statistical uncertainties). The results in the lower panel are obtained through bin-by-bin division of the results in the upper panel, including the uncertainties, by the central values of the “predicted” results. The labeling of the bin numbers is the same as in Fig. 3.

Figure 5

The $Z\to \nu \overline{\nu }$ background in the 174 search regions of the analysis as determined directly from $Z(\to \nu \overline{\nu })+\text{jets}$ simulation (points, with statistical uncertainties), and as predicted by applying the $Z\to \nu \overline{\nu }$ background determination procedure to statistically independent $Z(\to {\ell }^{+}{\ell }^{-})+\text{jets}$ simulated event samples (histogram, with shaded regions indicating the quadrature sum of the systematic uncertainty associated with the assumption that ${\mathcal{F}}_{j,b}$ is independent of $H_{\mathrm{T}}$ and $H_{\mathrm{T}}^{\mathrm{miss}}$, and the statistical uncertainty). For bins corresponding to $N_{b\text{−}\text{jet}}=0$, the agreement is exact by construction. The results in the lower panel are obtained through bin-by-bin division of the results in the upper panel, including the uncertainties, by the central values of the “predicted” results. The labeling of the bin numbers is the same as in Fig. 3.

Figure 6

The QCD background in the low-$\mathrm{\Delta }\varphi$ CR as predicted by the R&S method (histograms, with statistical and systematic uncertainties added in quadrature), compared to the corresponding data from which the expected contributions of top quark, $W+\text{jets}$, and $Z+\text{jets}$ events have been subtracted (points, with statistical uncertainties). The lower panel shows the ratio of the measured to the predicted results and its propagated uncertainty. The labeling of the bin numbers is the same as in Fig. 3.

Figure 7

The QCD background in the 174 search regions of the analysis as determined directly from QCD simulation (points, with statistical uncertainties) and as predicted by applying the low-$\mathrm{\Delta }\varphi$ extrapolation QCD background determination procedure to simulated event samples (histograms, with statistical and systematic uncertainties added in quadrature). Bins without a point have no simulated QCD events in the search region, while bins without a histogram have no simulated QCD events in the corresponding control region. The results in the lower panel are obtained through bin-by-bin division of the results in the upper panel, including the uncertainties, by the central values of the “predicted” results. No result is given in the lower panel if the value of the prediction is zero. The labeling of the bin numbers is the same as in Fig. 3.

Figure 8

Comparison between the predictions for the number of QCD events in the 174 search regions of the analysis as determined from the rebalance-and-smear (R&S, histograms) and low-$\mathrm{\Delta }\varphi$ extrapolation (points) methods. For both methods, the error bars indicate the combined statistical and systematic uncertainties. The lower panel shows the ratio of the low-$\mathrm{\Delta }\varphi$ extrapolation to the R&S results and its propagated uncertainty. The labeling of the bin numbers is the same as in Fig. 3.

Figure 9

The observed numbers of events and prefit SM background predictions in the 174 search regions of the analysis, where “prefit” means there is no constraint from the likelihood fit. Numerical values are given in Tables 6, 7, 8, 9, 10. The hatching indicates the total uncertainty in the background predictions. The lower panel displays the fractional differences between the data and SM predictions. The labeling of the bin numbers is the same as in Fig. 3.

Figure 10

The observed numbers of events and prefit SM background predictions in the 12 aggregate search regions, with fractional differences displayed in the lower panel, where “prefit” means there is no constraint from the likelihood fit. The hatching indicates the total uncertainty in the background predictions. The numerical values are given in Table 11.

Figure 11

The observed numbers of events and SM background predictions for regions in the search region parameter space particularly sensitive to the production of events in the (upper left) T1tttt, (upper right) T1bbbb, (middle left) T1qqqq, (middle right) T2tt, (lower left) T2bb, and (lower right) T2qq scenarios. The selection requirements are given in the figure legends. The hatched regions indicate the total uncertainties in the background predictions. The (unstacked) results for two example signal scenarios are shown in each instance, one with $\mathrm{\Delta }m\gg 0$ and the other with $\mathrm{\Delta }m\approx 0$, where $\mathrm{\Delta }m$ is the difference between the gluino or squark mass and the sum of the masses of the particles into which it decays.

Figure 12

The 95% C.L. upper limits on the production cross sections for the (upper left) T1tttt, (upper right) T1bbbb, (middle left) T1qqqq, (middle right) T5qqqqVV, and (lower left) T1tbtb simplified models as a function of the gluino and LSP masses $m_{\tilde{g}}$ and $m_{{\tilde{\chi }}_1^0}$. The thick solid (black) curves show the observed exclusion limits assuming the $\mathrm{NLO}+\mathrm{NLL}$ cross sections [60, 61, 62, 63, 64] and the thin solid (black) curves show the change in these limits due to variation of the signal cross sections within their theoretical uncertainties [79]. The thick dashed (red) curves present the expected limits under the background-only hypothesis, while the thin dotted (red) curves indicate the region containing 68% of the distribution of limits expected under this hypothesis. Lower right: The corresponding 95% $\mathrm{NLO}+\mathrm{NLL}$ exclusion curves for the mixed models of gluino decays to heavy squarks. For the T1tbtb model, the results are restricted to $m_{{\tilde{\chi }}_1^0}>25\mathrm{GeV}$ for the reason stated in the text.

Figure 13

(Left) The 95% C.L. upper limits on the production cross section for the (upper left) T2tt, (upper right) T2bb, and (lower) T2qq simplified models as a function of the squark and LSP masses $m_{\tilde{q}}$ and $m_{{\tilde{\chi }}_1^0}$. The diagonal dotted line shown for the T2tt model corresponds to $m_{\tilde{q}}-m_{{\tilde{\chi }}_1^0}=m_{\mathrm{top}}$. Note that for the T2tt model we do not present cross section upper limits in the unshaded diagonal region at low $m_{{\tilde{\chi }}_1^0}$ for the reasons discussed in the text, and that there is a small region corresponding to $m_{\tilde{t}}\lesssim 230\mathrm{GeV}$ and $m_{{\tilde{\chi }}_1^0}\lesssim 20\mathrm{GeV}$ that is not included in the $\mathrm{NLO}+\mathrm{NLL}$ exclusion region. The results labeled “one light $\tilde{q}$ ” for the T2qq model are discussed in the text. The meaning of the curves is described in the caption of Fig. 12.