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| CMS-EXO-18-007 ; CERN-EP-2018-289 | ||
| Search for long-lived particles decaying into displaced jets in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 19 November 2018 | ||
| Phys. Rev. D 99 (2019) 032011 | ||
| Abstract: A search for long-lived particles decaying into jets is presented. Data were collected with the CMS detector at the LHC from proton-proton collisions at a center-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$ . The search examines the distinctive topology of displaced tracks and secondary vertices. The selected events are found to be consistent with standard model predictions. For a simplified model in which long-lived neutral particles are pair produced and decay to two jets, pair production cross sections larger than 0.2 fb are excluded at 95% confidence level for a long-lived particle mass larger than 1000 GeV and proper decay lengths between 3 and 130 mm. Several supersymmetry models with gauge-mediated supersymmetry breaking or $R$-parity violation, where pair-produced long-lived gluinos or top squarks decay to several final-state topologies containing displaced jets, are also tested. For these models, in the mass ranges above 200 GeV, gluino masses up to 2300-2400 GeV and top squark masses up to 1350-1600 GeV are excluded for proper decay lengths approximately between 10 and 100 mm. These are the most restrictive limits to date on these models. | ||
| Links: e-print arXiv:1811.07991 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The distributions of vertex track multiplicity (upper left), vertex $L_{\mathrm {xy}}$ significance (upper right), cluster RMS (lower left), and likelihood discriminant (lower right), for data, simulated QCD multijet events, and simulated signal events. The lower panel of each plot shows the ratio between the data and the simulated QCD multijet events. Data and simulated events are selected with the displaced-jet trigger. The offline $ {H_{\mathrm {T}}} $ is required to be larger than 400 GeV, and the jets are required to have $ {p_{\mathrm {T}}} > $ 50 GeV and $ | \eta | < 2.0$. The error bars and bands represent the statistical uncertainties of each distribution. Three benchmark signal distributions are shown (dashed lines) for the Jet-Jet model with $m_{X} = $ 300 GeV and varying lifetimes. For visualization each signal process is given a cross section, $\sigma $, such that $\sigma \times$ 35.9 fb$^{-1}$ $= 1\times 10^{6}$. |
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Figure 1-a:
The distribution of vertex track multiplicity for data, simulated QCD multijet events, and simulated signal events. The lower panel shows the ratio between the data and the simulated QCD multijet events. Data and simulated events are selected with the displaced-jet trigger. The offline $ {H_{\mathrm {T}}} $ is required to be larger than 400 GeV, and the jets are required to have $ {p_{\mathrm {T}}} > $ 50 GeV and $ | \eta | < 2.0$. The error bars and bands represent the statistical uncertainties. Three benchmark signal distributions are shown (dashed lines) for the Jet-Jet model with $m_{X} = $ 300 GeV and varying lifetimes. For visualization each signal process is given a cross section, $\sigma $, such that $\sigma \times$ 35.9 fb$^{-1}$ $= 1\times 10^{6}$. |
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Figure 1-b:
The distribution of vertex $L_{\mathrm {xy}}$ significance for data, simulated QCD multijet events, and simulated signal events. The lower panel shows the ratio between the data and the simulated QCD multijet events. Data and simulated events are selected with the displaced-jet trigger. The offline $ {H_{\mathrm {T}}} $ is required to be larger than 400 GeV, and the jets are required to have $ {p_{\mathrm {T}}} > $ 50 GeV and $ | \eta | < 2.0$. The error bars and bands represent the statistical uncertainties. Three benchmark signal distributions are shown (dashed lines) for the Jet-Jet model with $m_{X} = $ 300 GeV and varying lifetimes. For visualization each signal process is given a cross section, $\sigma $, such that $\sigma \times$ 35.9 fb$^{-1}$ $= 1\times 10^{6}$. |
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Figure 1-c:
The distribution of cluster RMS for data, simulated QCD multijet events, and simulated signal events. The lower panel shows the ratio between the data and the simulated QCD multijet events. Data and simulated events are selected with the displaced-jet trigger. The offline $ {H_{\mathrm {T}}} $ is required to be larger than 400 GeV, and the jets are required to have $ {p_{\mathrm {T}}} > $ 50 GeV and $ | \eta | < 2.0$. The error bars and bands represent the statistical uncertainties. Three benchmark signal distributions are shown (dashed lines) for the Jet-Jet model with $m_{X} = $ 300 GeV and varying lifetimes. For visualization each signal process is given a cross section, $\sigma $, such that $\sigma \times$ 35.9 fb$^{-1}$ $= 1\times 10^{6}$. |
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Figure 1-d:
The distribution of likelihood discriminant for data, simulated QCD multijet events, and simulated signal events. The lower panel shows the ratio between the data and the simulated QCD multijet events. Data and simulated events are selected with the displaced-jet trigger. The offline $ {H_{\mathrm {T}}} $ is required to be larger than 400 GeV, and the jets are required to have $ {p_{\mathrm {T}}} > $ 50 GeV and $ | \eta | < 2.0$. The error bars and bands represent the statistical uncertainties. Three benchmark signal distributions are shown (dashed lines) for the Jet-Jet model with $m_{X} = $ 300 GeV and varying lifetimes. For visualization each signal process is given a cross section, $\sigma $, such that $\sigma \times$ 35.9 fb$^{-1}$ $= 1\times 10^{6}$. |
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Figure 2:
Numbers of predicted and observed background events for the nominal background method and the three cross checks in the control region. Shown are the comparisons for likelihood discriminant thresholds of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 (left); and for thresholds of 0.95, 0.96, 0.97, 0.98, 0.99, and 0.9993 (right). The error bars represent the statistical uncertainties of the predictions and the observations. The data points at different likelihood discriminant thresholds are correlated, since the events passing higher likelihood discriminant thresholds also pass lower likelihood discriminant thresholds. |
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Figure 2-a:
Numbers of predicted and observed background events for the nominal background method and the three cross checks in the control region. Shown are the comparisons for likelihood discriminant thresholds of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9. The error bars represent the statistical uncertainties of the predictions and the observations. The data points at different likelihood discriminant thresholds are correlated, since the events passing higher likelihood discriminant thresholds also pass lower likelihood discriminant thresholds. |
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Figure 2-b:
Numbers of predicted and observed background events for the nominal background method and the three cross checks in the control region. Shown are the comparisons for likelihood discriminant thresholds of 0.95, 0.96, 0.97, 0.98, 0.99, and 0.9993. The error bars represent the statistical uncertainties of the predictions and the observations. The data points at different likelihood discriminant thresholds are correlated, since the events passing higher likelihood discriminant thresholds also pass lower likelihood discriminant thresholds. |
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Figure 3:
The expected and observed 95% CL upper limits on the pair production cross section of the long-lived particle X, assuming a 100% branching fraction for X to decay to a quark-antiquark pair, shown at different particle X masses and proper decay lengths for the Jet-Jet model. The solid (dashed) lines represent the observed (median expected) limits. The shaded bands represent the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 4:
Left : the expected and observed 95% CL upper limits on the pair production cross section of the long-lived gluino, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {g}} {\tilde{\mathrm {G}}} $ decays. The horizontal lines indicate the NLO+NLL gluino pair production cross sections for $m_{{\tilde{\mathrm {g}}}} = $ 2400 GeV and $m_{{\tilde{\mathrm {g}}}} = $ 1600 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. Right : the expected and observed 95% CL limits for the long-lived gluino model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {g}} {\tilde{\mathrm {G}}} $ decays, based on the NLO+NLL calculation of the gluino pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distribution of the expected limits under the background-only hypothesis. |
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Figure 4-a:
The expected and observed 95% CL upper limits on the pair production cross section of the long-lived gluino, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {g}} {\tilde{\mathrm {G}}} $ decays. The horizontal lines indicate the NLO+NLL gluino pair production cross sections for $m_{{\tilde{\mathrm {g}}}} = $ 2400 GeV and $m_{{\tilde{\mathrm {g}}}} = $ 1600 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 4-b:
The expected and observed 95% CL limits for the long-lived gluino model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {g}} {\tilde{\mathrm {G}}} $ decays, based on the NLO+NLL calculation of the gluino pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distribution of the expected limits under the background-only hypothesis. |
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Figure 5:
Left : the expected and observed 95% CL upper limits on the pair production cross section of the long-lived gluino, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ decays. The horizontal lines indicate the NLO+NLL gluino pair production cross sections for $m_{{\tilde{\mathrm {g}}}} = $ 2400 GeV and $m_{{\tilde{\mathrm {g}}}} = $ 1600 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. Right : the expected and observed 95% CL limits for the long-lived gluino model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ decays, based on the NLO+NLL calculation of the gluino pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 5-a:
The expected and observed 95% CL upper limits on the pair production cross section of the long-lived gluino, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ decays. The horizontal lines indicate the NLO+NLL gluino pair production cross sections for $m_{{\tilde{\mathrm {g}}}} = $ 2400 GeV and $m_{{\tilde{\mathrm {g}}}} = $ 1600 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 5-b:
The expected and observed 95% CL limits for the long-lived gluino model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ decays, based on the NLO+NLL calculation of the gluino pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 6:
Left : the expected and observed 95% CL upper limits on the pair production cross section of the long-lived top squark, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\mathrm {b}}\ell $ decays. The horizontal lines indicate the NLO+NLL top squark pair production cross sections for $m_{{\tilde{\mathrm {t}}}} = $ 1600 GeV and $m_{{\tilde{\mathrm {t}}}} = $ 1000 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. Right : the expected and observed 95% limits for the long-lived top squark model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\mathrm {b}}\ell $ decays, based on the NLO+NLL calculation of the top squark pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 6-a:
The expected and observed 95% CL upper limits on the pair production cross section of the long-lived top squark, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\mathrm {b}}\ell $ decays. The horizontal lines indicate the NLO+NLL top squark pair production cross sections for $m_{{\tilde{\mathrm {t}}}} = $ 1600 GeV and $m_{{\tilde{\mathrm {t}}}} = $ 1000 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 6-b:
The expected and observed 95% limits for the long-lived top squark model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\mathrm {b}}\ell $ decays, based on the NLO+NLL calculation of the top squark pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 7:
Left : the expected and observed 95% CL upper limits on the the pair production cross section of the long-lived top squark, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ decays. The horizontal lines indicate the NLO+NLL top squark pair production cross sections for $m_{{\tilde{\mathrm {t}}}} = $ 1600 GeV and $m_{{\tilde{\mathrm {t}}}} = $ 1000 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. Right : the expected and observed 95% limits for the long-lived top squark model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ decays, based on an NLO+NLL calculation of the top squark pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distribution of the expected limits under the background-only hypothesis. |
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Figure 7-a:
The expected and observed 95% CL upper limits on the the pair production cross section of the long-lived top squark, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ decays. The horizontal lines indicate the NLO+NLL top squark pair production cross sections for $m_{{\tilde{\mathrm {t}}}} = $ 1600 GeV and $m_{{\tilde{\mathrm {t}}}} = $ 1000 GeV, as well as their variations due to the uncertainties in the choices of renormalization scales, factorization scales, and PDF sets. The solid (dashed) lines represent the observed (median expected) limits, the bands show the regions containing 68% of the distributions of the expected limits under the background-only hypothesis. |
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Figure 7-b:
The expected and observed 95% limits for the long-lived top squark model in the mass-lifetime plane, assuming a 100% branching fraction for $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ decays, based on an NLO+NLL calculation of the top squark pair production cross section at $\sqrt {s} = $ 13 TeV. The thick solid black (dashed red) line represents the observed (median expected) limits at 95% CL . The thin black lines represent the change in the observed limit due to the variation of the signal cross sections within their theoretical uncertainties. The thin red lines indicate the region containing 68% of the distribution of the expected limits under the background-only hypothesis. |
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Figure 8:
Comparison with search for displaced vertices in multijet events at $\sqrt {s} = $ 13 TeV with the CMS detector [36] (referred to as the CMS DV search) for $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ (left) and $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ (right) models. The CMS DV search looks for a pair of displaced vertices within the beam pipe. The observed limits obtained by the CMS DV search (purple curves) are overlaid with the limits obtained by the search presented in this paper in the mass-lifetime plane, and are good complements for proper decay length $c\tau _{0} < 10 mm $ in these two signal models. |
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Figure 8-a:
Comparison with search for displaced vertices in multijet events at $\sqrt {s} = $ 13 TeV with the CMS detector [36] (referred to as the CMS DV search) for the $ {\tilde{\mathrm {g}}} \to {\mathrm {t}} {\mathrm {b}} {\mathrm {s}}$ model. The CMS DV search looks for a pair of displaced vertices within the beam pipe. The observed limits obtained by the CMS DV search (purple curves) are overlaid with the limits obtained by the search presented in this paper in the mass-lifetime plane, and are good complements for proper decay length $c\tau _{0} < 10 mm $ in this model. |
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Figure 8-b:
Comparison with search for displaced vertices in multijet events at $\sqrt {s} = $ 13 TeV with the CMS detector [36] (referred to as the CMS DV search) for the $ {\tilde{\mathrm {t}}} \to {\overline {\mathrm {d}}} {\overline {\mathrm {d}}}$ model. The CMS DV search looks for a pair of displaced vertices within the beam pipe. The observed limits obtained by the CMS DV search (purple curves) are overlaid with the limits obtained by the search presented in this paper in the mass-lifetime plane, and are good complements for proper decay length $c\tau _{0} < 10 mm $ in this model. |
| Tables | |
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Table 1:
Summary of the preselection criteria |
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Table 2:
The definition of the different regions used in the background estimation. |
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Table 3:
The predicted and observed background in the control region for different likelihood discriminant thresholds. The background predictions are shown together with their statistical (first) and systematic (second) uncertainties (the systematic uncertainties in the background predictions are described in Section xxxxx). The observed significances are also shown in terms of $Z$-values, and are smaller than 1.5 standard deviations. |
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Table 4:
Systematic uncertainties in the signal yields, for each signal model studied. The quoted values reflect the largest variations due to each source for each signal model, in the studied range of masses and proper decay lengths. |
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Table 5:
Summary of predicted and observed events in the signal region, for different $ {H_{\mathrm {T}}} $ and number of dijet candidates values. |
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Table 6:
Signal efficiencies (in%) for the Jet-Jet model at different proper decay lengths $c\tau _{0}$ and different masses $m_{\mathrm {X}}$. Selection requirements are cumulative from the first row to the last. Uncertainties are statistical only. |
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Table 7:
Signal efficiencies (in%) for pair produced long-lived gluinos decaying to a gluon and a gravitino at different proper decay lengths $c\tau _{0}$ and different gluino masses $m_{{\tilde{\mathrm {g}}}}$. Selection requirements are cumulative from the first row to the last. Uncertainties are statistical only. |
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Table 8:
Signal efficiencies (in%) for pair produced long-lived gluinos decaying to top, bottom and strange antiquarks at different proper decay lengths $c\tau _{0}$ and different gluino masses $m_{{\tilde{\mathrm {g}}}}$. Selection requirements are cumulative from the first row to the last. Uncertainties are statistical only. |
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Table 9:
Signal efficiencies (in%) for pair produced long-lived top squarks decaying to a bottom quark and a lepton at different proper decay lengths $c\tau _{0}$ and different top squark masses $m_{{\tilde{\mathrm {t}}}}$. Selection requirements are cumulative from the first row to the last. Uncertainties are statistical only. |
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Table 10:
Signal efficiencies (in%) for pair produced long-lived top squarks decaying to two down antiquarks at different proper decay lengths $c\tau _{0}$ and different top squark masses $m_{{\tilde{\mathrm {t}}}}$. Selection requirements are cumulative from the first row to the last. Uncertainties are statistical only. |
| Summary |
| A search for long-lived particles decaying to jets is presented, based on proton-proton collision data collected with the CMS experiment at a center-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The analysis utilizes a dedicated trigger to capture events with displaced-jet signatures, and exploits jet, track, and secondary vertex information to discriminate displaced-jet candidate events from those produced by the standard model and instrumental backgrounds. The observed yields in data are in agreement with the background predictions. For a variety of models, we set the best limits to date for long-lived particles with proper decay lengths approximately between 5 mm and 10 m. Upper limits are set at 95% confidence level on the pair production cross section of long-lived neutral particles decaying to two jets, for different masses and proper lifetimes, and are as low as 0.2 fb at high mass ($m_{\mathrm{X}} > $ 1000 GeV) for proper decay lengths between 3 and 130 mm. A supersymmetric (SUSY) model with gauge-mediated supersymmetry breaking (GMSB) is also tested, in which the long-lived gluino can decay to one jet and a lightest SUSY particle. Upper limits are set on the pair production cross section of the gluino with different masses and proper decay lengths $c\tau_{0}$. Pair-produced long-lived gluinos lighter than 2300 GeV are excluded for proper decay lengths between 20 and 110 mm. For an $R$-parity violating (RPV) SUSY model, where the long-lived gluino can decay to top, bottom, and strange antiquarks, pair-produced gluinos lighter than 2400 GeV are excluded for decay lengths between 10 and 250 mm. For a second RPV SUSY model, in which the long-lived top squark can decay to one bottom quark and a charged lepton, pair-produced long-lived top squarks lighter than 1350 GeV are excluded for decay lengths between 7 and 110 mm. For another RPV SUSY model where the long-lived top squark decays to two down antiquarks, pair-produced long-lived top squarks lighter than 1600 GeV are excluded for decay lengths between 10 and 110 mm. These are the most stringent limits to date on these models. |
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