CMS-PAS-SUS-16-029

CMS-PAS-SUS-16-029
Search for direct top squark pair production in the fully hadronic final state in proton-proton collisions at $\sqrt{s} =$ 13 TeV corresponding to an integrated luminosity of 12.9 fb $^{-1}$
CMS Collaboration
August 2016

Abstract: A search for direct production of top squark pairs in events with jets and large transverse momentum imbalance is presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 12.9 fb $^{-1}$ . Two analyses are performed, a ``low $\Delta m$ " analysis that targets scenarios with a very small difference in mass between the top squark and the neutralino, and a ``high $\Delta m$ " analysis that targets topologies typical for larger mass splittings. No significant excess of events above the expected background from standard model processes is observed. Exclusion limits are set in the context of simplified models of top squark pair production under various decay hypotheses, ranging up to 860 GeV in the case of the high $\Delta m$ analysis and up to 450 GeV in the case of the low $\Delta m$ analysis.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1-a: Diagrams representing the pair production of top squarks and their subsequent decay modes that are studied in this document.
png pdf	Figure 1-b: Diagrams representing the pair production of top squarks and their subsequent decay modes that are studied in this document.
png pdf	Figure 1-c: Diagrams representing the pair production of top squarks and their subsequent decay modes that are studied in this document.
png pdf	Figure 2: $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )$ distribution after the high ${\Delta m}$ baseline selection for simulated events, normalized to an integrated luminosity of 12.9 fb $^{-1}$ . The expected signal yields are scaled by a factor of ten to facilitate a comparison with the expected SM backgrounds. Events surviving the high ${\Delta m}$ baseline selection are separated into categories defined by $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} ) <$ 175 GeV and $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )\geq$ 175 GeV.
png pdf	Figure 3: ${ {p_{\mathrm {T}}} ( {\mathrm {b}}_{12})}$ distribution after the low ${\Delta m}$ baseline selection in the two b-tag category for simulated events, normalized to an integrated luminosity of 12.9 fb $^{-1}$ . The expected signal yields are scaled by a factor of ten to facilitate a comparison with the expected SM backgrounds. Events entering the two b-tag category of the low ${\Delta m}$ analysis are subdivided into categories defined by ${ {p_{\mathrm {T}}} ( {\mathrm {b}}_{12})}\in [40, 100), [100, 160)$ GeV.
png pdf	Figure 4-a: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )<$ 175 GeV (a,b) and $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV, $N_{ {\mathrm {t}}}=$ 0, $N_{ {\mathrm {W}}}=$ 0 (c,d), for events with $N_{ {\mathrm {b}}}=$ 1 (a,c) and $N_{ {\mathrm {b}}}\geq$ 2 (b,d). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 4-b: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )<$ 175 GeV (a,b) and $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV, $N_{ {\mathrm {t}}}=$ 0, $N_{ {\mathrm {W}}}=$ 0 (c,d), for events with $N_{ {\mathrm {b}}}=$ 1 (a,c) and $N_{ {\mathrm {b}}}\geq$ 2 (b,d). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 4-c: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )<$ 175 GeV (a,b) and $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV, $N_{ {\mathrm {t}}}=$ 0, $N_{ {\mathrm {W}}}=$ 0 (c,d), for events with $N_{ {\mathrm {b}}}=$ 1 (a,c) and $N_{ {\mathrm {b}}}\geq$ 2 (b,d). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 4-d: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )<$ 175 GeV (a,b) and $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV, $N_{ {\mathrm {t}}}=$ 0, $N_{ {\mathrm {W}}}=$ 0 (c,d), for events with $N_{ {\mathrm {b}}}=$ 1 (a,c) and $N_{ {\mathrm {b}}}\geq$ 2 (b,d). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 5-a: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV , $N_{\text {j}}\geq$ 5, and at least one top- or W-tagged candidate for events with $N_{ {\mathrm {b}}}=$ 1 (a) and $N_{ {\mathrm {b}}}\geq$ 2 (b). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 5-b: Observed events and SM estimates for the high ${\Delta m}$ search regions with $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV , $N_{\text {j}}\geq$ 5, and at least one top- or W-tagged candidate for events with $N_{ {\mathrm {b}}}=$ 1 (a) and $N_{ {\mathrm {b}}}\geq$ 2 (b). The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 6-a: Observed events and SM estimates for the low ${\Delta m}$ search regions. The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) is shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction. Top: for $N_{ {\mathrm {b}}}=$ 0; Middle: for $N_{ {\mathrm {b}}}=$ 1; Bottom: for $N_{ {\mathrm {b}}}\geq$ 2.
png pdf	Figure 6-b: Observed events and SM estimates for the low ${\Delta m}$ search regions. The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) is shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction. Top: for $N_{ {\mathrm {b}}}=$ 0; Middle: for $N_{ {\mathrm {b}}}=$ 1; Bottom: for $N_{ {\mathrm {b}}}\geq$ 2.
png pdf	Figure 6-c: Observed events and SM estimates for the low ${\Delta m}$ search regions. The SM background predictions shown do not include the effects of the maximum likelihood fit to the data. The ratio of the observed data to the SM prediction (black points, with error bars corresponding to the data statistical uncertainty) is shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction. Top: for $N_{ {\mathrm {b}}}=$ 0; Middle: for $N_{ {\mathrm {b}}}=$ 1; Bottom: for $N_{ {\mathrm {b}}}\geq$ 2.
png pdf	Figure 7: Exclusion limits at 95% CL for simplified models of top squark pair production in the pure ${\tilde{t}} _{1}\to {\mathrm {t}} {\tilde{\chi}^{0}_{1}}$ (``T2tt") decay scenario. The solid black curves represent the observed exclusion contours with respect to NLO+NLL cross section calculations [22] and the corresponding $\pm$ 1 standard deviations. The dashed red curves indicate the expected exclusion contour and the $\pm$ 1 standard deviations with experimental uncertainties.
png pdf	Figure 8: Exclusion limits at 95% CL for simplified models of top squark pair production in the pure ${\tilde{t}} _{1}\to {\mathrm {b}} {\tilde{\chi}^\pm _{1}} \rightarrow {\mathrm {b}} {\mathrm {W}}^{\pm (*)} {\tilde{\chi}^{0}_{1}}$ (``T2bW") decay scenario. The solid black curves represent the observed exclusion contours with respect to NLO+NLL cross section calculations [22] and the corresponding $\pm$ 1 standard deviations. The dashed red curves indicate the expected exclusion contour and the $\pm$ 1 standard deviations with experimental uncertainties.
png pdf	Figure 9: Exclusion limits at 95% CL for simplified models of top squark pair production in the ${\tilde{t}} _{1} \to {\mathrm {b}}\mathrm {f} \tilde{\mathrm {f}} {\tilde{\chi}^{0}_{1}}$ (``T2fbd") four-body decay scenario. The solid black curves represent the observed exclusion contours with respect to NLO+NLL cross section calculations [22] and the corresponding $\pm$ 1 standard deviations. The dashed red curves indicate the expected exclusion contour and the $\pm$ 1 standard deviations with experimental uncertainties.
png pdf	Figure 10-a: Observed events and SM estimates for the aggregate search regions defined for the high ${\Delta m}$ (a) and low ${\Delta m}$ (b) analyses. The ratios of the observed data to the SM prediction derived from control regions (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.
png pdf	Figure 10-b: Observed events and SM estimates for the aggregate search regions defined for the high ${\Delta m}$ (a) and low ${\Delta m}$ (b) analyses. The ratios of the observed data to the SM prediction derived from control regions (black points, with error bars corresponding to the data statistical uncertainty) are shown in the ratio plots. The shaded blue band represents the statistical and systematic uncertainty on the background prediction.

Tables
png pdf	Table 1: Summary of the 60 disjoint search regions used in the high ${\Delta m}$ analysis.
png pdf	Table 2: Summary of the 40 search regions used in the low ${\Delta m}$ analysis.
png pdf	Table 3: The $R_{Z}$ factors obtained from ${ {\mathrm {Z}}\rightarrow \ell \ell }$ +jets events that are used to normalize the ${ {\mathrm {Z}}\rightarrow {\nu } {\nu }}$ simulation sample. The factors are obtained using 12.9 fb $^{-1}$ of data and calculated for different $N_{ {\mathrm {b}}}$ selections in order to account for differences in heavy flavor production between data and simulation.
png pdf	Table 4: Predicted yields for each background with uncertainties in the $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )<$ 175 GeV regions of the high ${\Delta m}$ search. The number of events observed in data is given in the last column.
png pdf	Table 5: Predicted yields for each background with uncertainties in the $M_{\mathrm {T}}( {\mathrm {b}}_{1,2}$ , ${E_{\mathrm {T}}^{\text {miss}}} )>$ 175 GeV regions of the high ${\Delta m}$ search. The number of events observed in data is given in the last column.
png pdf	Table 6: Predicted yields for each background with uncertainties in the low ${\Delta m}$ search regions. The number of events observed in data is given in the last column.
png pdf	Table 7: Summary of the 13 disjoint aggregate search regions defined for the high ${\Delta m}$ analysis.
png pdf	Table 8: Summary of the 12 disjoint aggregate search regions defined for the low ${\Delta m}$ analysis.
png pdf	Table 9: Observed events and SM estimates for the aggregate search regions of the high ${\Delta m}$ analysis.
png pdf	Table 10: Observed events and SM estimates for the aggregate search regions of the low ${\Delta m}$ analysis.

Summary

The results of a search for direct production of top squark pairs in the fully-hadronic final state have been presented, based on data collected in 2016 by the CMS detector in proton-proton collisions at a center-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 12.9 fb

$^{-1}$ . No significant excess of events beyond the expected contribution from standard model processes is observed, and exclusion limits are set in the context of simplified models of top squark production. Top squark masses up to 450 GeV are probed for a neutralino mass of 430 GeV in the scenario of a very compressed mass spectrum between the

$\tilde{\mathrm{t}}_{1}$ and

$\tilde{\chi}^0_1$ where the

$\tilde{\mathrm{t}}_{1}$ decays via a four body decay. In the scenario of larger mass differences between the

$\tilde{\mathrm{t}}_{1}$ and

$\tilde{\chi}^0_1$ when the top squark decays to an on-shell top quark and a neutralino, top squark masses up to 860 GeV, and

$\tilde{\chi}^0_1$ masses up to 320 GeV are probed. When the top squarks decay to a bottom quark and a

$\tilde{\chi}^{\pm}_1$ , top squark masses up to 740 GeV, and

$\tilde{\chi}^0_1$ masses up to 260 GeV are probed.

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