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| CMS-EXO-19-013 ; CERN-EP-2021-052 | ||
| Search for long-lived particles decaying to jets with displaced vertices in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 28 April 2021 | ||
| Phys. Rev. D 104 (2021) 052011 | ||
| Abstract: A search is presented for long-lived particles produced in pairs in proton-proton collisions at the LHC operating at a center-of-mass energy of 13 TeV. The data were collected with the CMS detector during the period from 2015 through 2018, and correspond to a total integrated luminosity of 140 fb$^{-1}$. This search targets pairs of long-lived particles with mean proper decay lengths between 0.1 and 100 mm, each of which decays into at least two quarks that hadronize to jets, resulting in a final state with two displaced vertices. No significant excess of events with two displaced vertices is observed. In the context of $R$-parity violating supersymmetry models, the pair production of long-lived neutralinos, gluinos, and top squarks is excluded at 95% confidence level for cross sections larger than 0.08fb, masses between 800 and 3000 GeV, and mean proper decay lengths between 1 and 25 mm. | ||
| Links: e-print arXiv:2104.13474 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Diagrams of the multijet signal model (left) showing long-lived neutralinos ($\tilde{\chi}^0$) or gluinos (${\mathrm{\tilde{g}}}$) decaying into top, bottom, and strange quarks via virtual top squarks ($\tilde{\mathrm{t}}$), and the dijet signal model (right) showing long-lived top and anti-top squarks decaying into two down-type quarks. In both cases, the long-lived particles are the LSPs in their respective models. |
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Figure 1-a:
Diagram of the multijet signal model showing long-lived neutralinos ($\tilde{\chi}^0$) or gluinos (${\mathrm{\tilde{g}}}$) decaying into top, bottom, and strange quarks via virtual top squarks ($\tilde{\mathrm{t}}$). The long-lived particle is the LSP in the model. |
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Figure 1-b:
Diagram of the dijet signal model showing long-lived top and anti-top squarks decaying into two down-type quarks. The long-lived particle is the LSP in the model. |
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Figure 2:
Schematic diagram of an event with two signal vertices with the beam spot $B$ at the origin. The beam direction is perpendicular to the $x$-$y$ plane shown. The distance between the vertices is defined as ${d_{\mathrm {VV}}}$. The distance from the beam spot to the vertices is defined as ${d_{\mathrm {BV}}}$ and the angle between the vertex displacement vectors is defined as ${\Delta \phi _{\mathrm {VV}}}$. |
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Figure 3:
The distribution of distances between vertices in the $x$-$y$ plane, ${d_{\mathrm {VV}}}$, for three simulated multijet signals each with a mass of 1600 GeV, with the background template distribution overlaid. The production cross section for each signal model is assumed to be the lower limit excluded by Ref. [25], corresponding to values of 0.8, 0.25, and 0.15 fb for the samples with $c\tau = $ 0.3, 1.0, and 10 mm, respectively. The last bin includes the overflow events. The two vertical pink dashed lines separate the regions used in the fit. |
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Figure 4:
Multijet (left) and dijet (right) signal efficiencies as a function of the signal mass and lifetime for events satisfying all event and vertex requirements, with corrections based on systematic differences in the vertex reconstruction efficiency between data and simulation. |
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Figure 4-a:
Multijet signal efficiency as a function of the signal mass and lifetime for events satisfying all event and vertex requirements, with corrections based on systematic differences in the vertex reconstruction efficiency between data and simulation. |
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Figure 4-b:
Dijet signal efficiency as a function of the signal mass and lifetime for events satisfying all event and vertex requirements, with corrections based on systematic differences in the vertex reconstruction efficiency between data and simulation. |
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Figure 5:
The distribution of ${d_{\mathrm {BV}}}$ for $\geq $5-track one-vertex events in data and three simulated multijet signal samples each with a mass of 1600 GeV. The production cross section for each signal model is assumed to be the lower limit excluded by Ref. [25], corresponding to values of 0.8, 0.25, and 0.15 fb for the samples with $c\tau = $ 0.3, 1.0, and 10 mm, respectively. The last bin includes the overflow events. |
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Figure 6:
Distribution of the $x$-$y$ distances between vertices, ${d_{\mathrm {VV}}}$, for 2017 and 2018 data with a background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ constructed from one-vertex events in data normalized to the two-vertex data for events with 3-track vertices (upper left), events with exactly one 4-track vertex and one 3-track vertex (upper right), and events with 4-track vertices (lower left). The background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ for $\geq $5-track two-vertex events (lower right) is normalized using one-vertex event information as described in the text. The two vertical red dashed lines separate the regions used in the fit. |
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Figure 6-a:
Distribution of the $x$-$y$ distances between vertices, ${d_{\mathrm {VV}}}$, for 2017 and 2018 data with a background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ constructed from one-vertex events in data normalized to the two-vertex data for events with 3-track vertices. The two vertical red dashed lines separate the regions used in the fit. |
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Figure 6-b:
Distribution of the $x$-$y$ distances between vertices, ${d_{\mathrm {VV}}}$, for 2017 and 2018 data with a background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ constructed from one-vertex events in data normalized to the two-vertex data for events with exactly one 4-track vertex and one 3-track vertex. The two vertical red dashed lines separate the regions used in the fit. |
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Figure 6-c:
Distribution of the $x$-$y$ distances between vertices, ${d_{\mathrm {VV}}}$, for 2017 and 2018 data with a background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ constructed from one-vertex events in data normalized to the two-vertex data for events with 4-track vertices. The two vertical red dashed lines separate the regions used in the fit. |
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Figure 6-d:
Background distribution ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ for $\geq $5-track two-vertex events, normalized using one-vertex event information as described in the text. The two vertical red dashed lines separate the regions used in the fit. |
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Figure 7:
Observed 95% CL upper limits on the product of cross section and branching fraction squared for the multijet (left) and dijet (right) signals, as a function of mass and $c\tau $. The overlaid mass-lifetime exclusion curves assume pair-production cross sections for the neutralino (red) and gluino (purple) in multijet signals and top squark cross sections for the dijet signals with 100% branching fraction to each model's respective decay mode specified. The solid black (dashed colored) lines represent the observed (median expected) limits at 95% CL. The thin black lines represent the variation of the observed limit within theoretical uncertainties of the signal cross section. The thin dashed colored lines represent the region containing 68% of the expected limit distribution under the background-only hypothesis. The observed limits from the CMS displaced jets search [29] are also shown in teal for comparison. |
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Figure 7-a:
Observed 95% CL upper limits on the product of cross section and branching fraction squared for the multijet signal, as a function of mass and $c\tau $. The overlaid mass-lifetime exclusion curves assume pair-production cross sections for the neutralino (red) and gluino (purple) in multijet signals and top squark cross sections for the dijet signals with 100% branching fraction to each model's respective decay mode specified. The solid black (dashed colored) lines represent the observed (median expected) limits at 95% CL. The thin black lines represent the variation of the observed limit within theoretical uncertainties of the signal cross section. The thin dashed colored lines represent the region containing 68% of the expected limit distribution under the background-only hypothesis. The observed limits from the CMS displaced jets search [29] are also shown in teal for comparison. |
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Figure 7-b:
Observed 95% CL upper limits on the product of cross section and branching fraction squared for the dijet signal, as a function of mass and $c\tau $. The overlaid mass-lifetime exclusion curves assume pair-production cross sections for the neutralino (red) and gluino (purple) in multijet signals and top squark cross sections for the dijet signals with 100% branching fraction to each model's respective decay mode specified. The solid black (dashed colored) lines represent the observed (median expected) limits at 95% CL. The thin black lines represent the variation of the observed limit within theoretical uncertainties of the signal cross section. The thin dashed colored lines represent the region containing 68% of the expected limit distribution under the background-only hypothesis. The observed limits from the CMS displaced jets search [29] are also shown in teal for comparison. |
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Figure 8:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for multijet signals (left) and dijet signals (right), for a fixed $c\tau $ of 0.3 mm (upper), 1 mm (middle), and 10 mm (lower) in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-a:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for multijet signals, for a fixed $c\tau $ of 0.3 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-b:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for dijet signals, for a fixed $c\tau $ of 0.3 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-c:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for multijet signals, for a fixed $c\tau $ of 1 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-d:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for dijet signals, for a fixed $c\tau $ of 1 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-e:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for multijet signals, for a fixed $c\tau $ of 10 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 8-f:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of mass for dijet signals, for a fixed $c\tau $ of 10 mm in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. |
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Figure 9:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for multijet signals (left) and dijet signals (right), for a fixed mass of 800 GeV (upper), 1600 GeV (middle), and 2400 GeV (lower) in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-a:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for multijet signals, for a fixed mass of 800 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-b:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for dijet signals, for a fixed mass of 800 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-c:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for multijet signals, for a fixed mass of 1600 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-d:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for dijet signals, for a fixed mass of 1600 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-e:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for multijet signals, for a fixed mass of 2400 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
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Figure 9-f:
Observed and expected 95% CL upper limits on the product of cross section and branching fraction squared, as a function of $c\tau $ for dijet signals, for a fixed mass of 2400 GeV in the full Run-2 data set. The neutralino and gluino pair production cross sections are shown for the multijet signals, and the top squark pair-production cross section is shown for the dijet signals. For $m = $ 2400 GeV, the expected neutralino cross section is $\approx $8$\times 10^{-5}$ fb and is not shown. |
| Tables | |
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Table 1:
Event yields in the control regions in data. The "one-vertex'' events correspond to events containing exactly one vertex with the specified number of tracks. The "two-vertex'' events have two or more vertices containing the specified numbers of tracks. We seek the signal in the $\geq $5-track two-vertex sample. |
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Table 2:
Signal-related systematic uncertainties for dijet and multijet signal models. The total uncertainty is the sum in quadrature of the individual components. The ranges presented reflect differences among the various signal mass and lifetime hypotheses, as well as differences between the 2017 and 2018 data. |
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Table 3:
Systematic uncertainties in the background prediction in each ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ bin arising from varying the construction of the ${d_{\mathrm {VV}}^{\,\mathrm {C}}}$ template. The total systematic uncertainty in each bin is the sum in quadrature of the values, assuming no correlations among the sources. |
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Table 4:
Predicted yields for the background-only normalized template, predicted yields for three simulated multijet signals each with a mass of 1600 GeV, and the observed yield in each ${d_{\mathrm {VV}}}$ bin. The production cross section for each signal model is assumed to be the lower limit excluded by Ref. [25], corresponding to values of 0.8, 0.25, and 0.15 fb for samples with $c\tau = $ 0.3, 1.0, and 10 mm, respectively. The uncertainties in the signal yields and the systematic uncertainties in the background prediction reflect the systematic uncertainties given in Tables 2 and 4, respectively. |
| Summary |
|
A search for pair-produced long-lived particles decaying into multijet and dijet final states in proton-proton collisions collected with the CMS detector at a center-of-mass energy of 13 TeV has been described. No events in the signal region in the 2017 and 2018 data sets, and no excess yield beyond the standard model prediction in the full Run-2 data set, which corresponds to an integrated luminosity of 140 fb$^{-1}$, are observed. This analysis extends a previous CMS search that used the 2015 and 2016 data sets, with improvements in background rejection, background estimation techniques, and uncertainty estimation. At 95% confidence level, upper limits are set on an $R$-parity violating (RPV) supersymmetry (SUSY) model in which a long-lived neutralino or gluino decays into a multijet final state with top, bottom, and strange quarks. Signal pair-production cross sections larger than 0.08fb are excluded for long-lived neutralino, gluino, and top squark masses between 800 and 3000 GeV and mean proper decay lengths between 1 and 25 mm. For the range of mean proper decay lengths between 0.6 and 90 mm, the data exclude gluino masses up to 2500 GeV. For the case where the lightest SUSY particle is a neutralino, the data exclude neutralino masses up to 1100 GeV for mean proper decay lengths between 0.6 and 70 mm. Additionally, limits are placed for an RPV SUSY model in which a long-lived top squark decays into a dijet final state with two down-type quarks. The data exclude top squark masses up to 1600 GeV for mean proper decay lengths between 0.4 and 80 mm. These results, which supersede those in Ref. [25], are the most stringent bounds on these models for mean proper decay lengths between 0.1 and 15 mm for all masses considered, and complement the results of the CMS displaced jets search [29]. While the search directly constrains these two RPV SUSY models, the techniques and methodology are generic and, as described in the Appendix, the results are applicable to other models of pair-produced long-lived particles that decay into jets. |
| Additional Figures | |
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Additional Figure 1:
Distribution of the azimuthal angle between vertices, $\Delta \phi _{\mathrm {VV}}$, for 2017 and 2018 data. The background distribution (blue continuous line) is constructed from 3-track one-vertex events in data, and is normalized to the number of 3-track two-vertex events in data. |
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Additional Figure 2:
Distribution of the azimuthal angle between vertices, $\Delta \phi _{\mathrm {VV}}$, for 2017 and 2018 data. The background distribution (blue continuous line) is constructed from 4-track and 3-track one-vertex events in data, and is normalized to the number of two-vertex events in data which have exactly one 4-track vertex and one 3-track vertex. |
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Additional Figure 3:
Distribution of the azimuthal angle between vertices, $\Delta \phi _{\mathrm {VV}}$, for 2017 and 2018 data. The background distribution (blue continuous line) is constructed from 4-track one-vertex events in data, and is normalized to the number of 4-track two-vertex events in data. |
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Additional Figure 4:
Distribution of the azimuthal angle between vertices, $\Delta \phi _{\mathrm {VV}}$, for 2017 and 2018 data. The background distribution (blue continuous line) is constructed from $\geq $5-track one-vertex events in data, and is normalized using one-vertex event information. No $\geq $5-track two-vertex data events pass the selection. |
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