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| CMS-EXO-16-004 ; CERN-EP-2017-330 | ||
| Search for decays of stopped exotic long-lived particles produced in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 31 December 2017 | ||
| JHEP 05 (2018) 127 | ||
| Abstract: A search is presented for the decays of heavy exotic long-lived particles (LLPs) that are produced in proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC and come to rest in the CMS detector. Their decays would be visible during periods of time well separated from proton-proton collisions. Two decay scenarios of stopped LLPs are explored: a hadronic decay detected in the calorimeter and a decay into muons detected in the muon system. The calorimeter (muon) search covers a period of sensitivity totaling 721 (744) hours in 38.6 (39.0) fb$^{-1}$ of data collected by the CMS detector in 2015 and 2016. The results are interpreted in several scenarios that predict LLPs. Production cross section limits are set as a function of the mean proper lifetime and the mass of the LLPs, for lifetimes between 100 ns and 10 days. These are the most stringent limits to date on the mass of hadronically decaying stopped LLPs, and this is the first search at the LHC for stopped LLPs that decay to muons. | ||
| Links: e-print arXiv:1801.00359 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $\Delta t_{\text {DT}}$ (left) and $\Delta t_{\text {RPC}}$ (right) distributions for 2016 data, MC simulated cosmic ray muon, 1000 GeV gluino signal, and 600 GeV MCHAMP signal events, for the muon search. The events plotted pass a subset of the full analysis selection that is designed to select good-quality DSA muon tracks but does not reject the cosmic ray muon background. The number of cosmic ray muon background events is greatly reduced when the full selection is applied, as we require $\Delta t_{\text {DT}} > -20$ ns and $\Delta t_{\text {RPC}} > -7.5$ ns. The gray bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 1-a:
The $\Delta t_{\text {DT}}$ distribution for 2016 data, MC simulated cosmic ray muon, 1000 GeV gluino signal, and 600 GeV MCHAMP signal events, for the muon search. The events plotted pass a subset of the full analysis selection that is designed to select good-quality DSA muon tracks but does not reject the cosmic ray muon background. The number of cosmic ray muon background events is greatly reduced when the full selection is applied, as we require $\Delta t_{\text {DT}} > -20$ ns and $\Delta t_{\text {RPC}} > -7.5$ ns. The gray bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 1-b:
The $\Delta t_{\text {RPC}}$ distribution for 2016 data, MC simulated cosmic ray muon, 1000 GeV gluino signal, and 600 GeV MCHAMP signal events, for the muon search. The events plotted pass a subset of the full analysis selection that is designed to select good-quality DSA muon tracks but does not reject the cosmic ray muon background. The number of cosmic ray muon background events is greatly reduced when the full selection is applied, as we require $\Delta t_{\text {DT}} > -20$ ns and $\Delta t_{\text {RPC}} > -7.5$ ns. The gray bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 2:
The $\varepsilon _{\text {reco}}$ values as a function of $E_{{\mathrm {g}}}$ or $E_{{\mathrm {t}}}$ (left), and $m_{\tilde{\mathrm{g}}} - m_{\tilde{\chi}^0_2}$ (right), for $ \tilde{\mathrm{g}}$ and $ \tilde{\mathrm{t}} $ R-hadrons that stop in the EB or HB, in the MC simulation, for the calorimeter search. The $\varepsilon _{\text {reco}}$ values are plotted for the two-body gluino and top squark decays (left) and for the three-body gluino decay (right). The shaded bands correspond to the systematic uncertainties, which are described in Section 7. |
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Figure 2-a:
The $\varepsilon _{\text {reco}}$ values as a function of $E_{{\mathrm {g}}}$ or $E_{{\mathrm {t}}}$ for $ \tilde{\mathrm{g}}$ and $ \tilde{\mathrm{t}} $ R-hadrons that stop in the EB or HB, in the MC simulation, for the calorimeter search. The $\varepsilon _{\text {reco}}$ values are plotted for the two-body gluino and top squark decays. The shaded bands correspond to the systematic uncertainties, which are described in Section 7. |
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Figure 2-b:
The $\varepsilon _{\text {reco}}$ values as a function of $m_{\tilde{\mathrm{g}}} - m_{\tilde{\chi}^0_2}$ for $ \tilde{\mathrm{g}}$ and $ \tilde{\mathrm{t}} $ R-hadrons that stop in the EB or HB, in the MC simulation, for the calorimeter search. The $\varepsilon _{\text {reco}}$ values are plotted for the three-body gluino decay. The shaded bands correspond to the systematic uncertainties, which are described in Section 7. |
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Figure 3:
The background extrapolation for the muon search. The integral of the fit function to $\Delta t_{\text {DT}}$ with the sum of two Gaussian distributions and a Crystal Ball function, for $\Delta t_{\text {DT}} > -20$ ns, is plotted as a function of the lower $\Delta t_{\text {RPC}}$ selection, for 2015 (red squares) and 2016 (black circles) data. The points are fitted with an error function and used to extrapolate to the signal region, which is defined as $\Delta t_{\text {RPC}} > -7.5$ ns. |
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Figure 4:
The 95% CL upper limits on $\mathcal {B}\sigma $ for gluino and top squark pair production, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos that undergo a two-body decay (upper left), top squarks that undergo a two-body decay (upper right), and gluinos that undergo a three-body decay (lower). The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 4-a:
The 95% CL upper limits on $\mathcal {B}\sigma $ for gluino and top squark pair production, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos that undergo a two-body decay. The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 4-b:
The 95% CL upper limits on $\mathcal {B}\sigma $ for gluino and top squark pair production, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show top squarks that undergo a two-body decay. The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 4-c:
The 95% CL upper limits on $\mathcal {B}\sigma $ for gluino and top squark pair production, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos that undergo a three-body decay. The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 5:
The 95% CL upper limits on the gluino and top squark mass, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos and top squarks that undergo a two-body decay (left) and gluinos that undergo a three-body decay (right). The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. |
png pdf |
Figure 5-a:
The 95% CL upper limits on the gluino and top squark mass, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos and top squarks that undergo a two-body decay. The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. |
png pdf |
Figure 5-b:
The 95% CL upper limits on the gluino and top squark mass, using the cloud model of R-hadron interactions, as a function of lifetime, for combined 2015 and 2016 data for the calorimeter search. We show gluinos that undergo a three-body decay. The discontinuous structure observed between $10^{-7}$ and $10^{-5}$ s is due to the increase of the number of observed events in the search window as the lifetime increases. |
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Figure 6:
The 95% CL upper limits in the neutralino mass vs. gluino (top squark) mass plane, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the calorimeter search. The color map indicates the 95% CL upper limits on $\mathcal {B}\sigma $. The mostly triangular region defined by the black solid (dashed) line shows the excluded observed (expected) region. We show gluinos that undergo a two-body decay (upper left), top squarks that undergo a two-body decay (upper right), and gluinos that undergo a three-body decay (lower). |
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Figure 6-a:
The 95% CL upper limits in the neutralino mass vs. gluino (top squark) mass plane, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the calorimeter search. The color map indicates the 95% CL upper limits on $\mathcal {B}\sigma $. The mostly triangular region defined by the black solid (dashed) line shows the excluded observed (expected) region. We show gluinos that undergo a two-body decay. |
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Figure 6-b:
The 95% CL upper limits in the neutralino mass vs. gluino (top squark) mass plane, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the calorimeter search. The color map indicates the 95% CL upper limits on $\mathcal {B}\sigma $. The mostly triangular region defined by the black solid (dashed) line shows the excluded observed (expected) region. We show top squarks that undergo a two-body decay. |
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Figure 6-c:
The 95% CL upper limits in the neutralino mass vs. gluino (top squark) mass plane, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the calorimeter search. The color map indicates the 95% CL upper limits on $\mathcal {B}\sigma $. The mostly triangular region defined by the black solid (dashed) line shows the excluded observed (expected) region. We show gluinos that undergo a three-body decay. |
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Figure 7:
The 95% CL upper limits on $\mathcal {B}\sigma $ for 1000 GeV gluino (left) and 400 GeV MCHAMP (right) pair production as a function of lifetime, for combined 2015 and 2016 data for the muon search. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 7-a:
The 95% CL upper limits on $\mathcal {B}\sigma $ for 1000 GeV gluino pair production as a function of lifetime, for combined 2015 and 2016 data for the muon search. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 7-b:
The 95% CL upper limits on $\mathcal {B}\sigma $ for 400 GeV MCHAMP pair production as a function of lifetime, for combined 2015 and 2016 data for the muon search. The theory lines assume $\mathcal {B}= $ 100%. |
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Figure 8:
95% CL upper limits on $\mathcal {B}\sigma $ for gluino (left) and MCHAMP (right) pair production as a function of mass, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the muon search. The theory curves assume $\mathcal {B}= $ 100%. |
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Figure 8-a:
95% CL upper limits on $\mathcal {B}\sigma $ for gluino pair production as a function of mass, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the muon search. The theory curves assume $\mathcal {B}= $ 100%. |
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Figure 8-b:
95% CL upper limits on $\mathcal {B}\sigma $ for MCHAMP pair production as a function of mass, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data for the muon search. The theory curves assume $\mathcal {B}= $ 100%. |
| Tables | |
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Table 1:
Summary of the values of $\varepsilon _{\text {stopping}}$, $\varepsilon _{\text {CSCveto}}$, $\varepsilon _{\text {DTveto}}$, and the plateau value of $\varepsilon _{\text {reco}}$ for different signals, for the calorimeter search. The efficiency $\varepsilon _{\text {stopping}}$ is constant for the range of signal masses considered. The efficiency $\varepsilon _{\text {reco}}$ is given on the $E_{{\mathrm {g}}}$ or $E_{{\mathrm {t}}}$ plateau for each signal. |
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Table 2:
Gluino $\varepsilon _{\text {stopping}}$ and $\varepsilon _{\text {reco}}$, as well as the number of expected gluino events with lifetimes between 10 $\mu$s and 1000 s, assuming $\mathcal {B}(\tilde{\mathrm{g}}\to {\mathrm {q}} {\overline {\mathrm {q}}} \tilde{\chi}^0_2) \mathcal {B}(\tilde{\chi}^0_2 \to {{{\mu ^+}} {{\mu ^-}}} \tilde{\chi}^0_2)= $ 100%, for each mass point considered for the 2016 muon search. The efficiencies are constant for this range of lifetimes. |
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Table 3:
MCHAMP $\varepsilon _{\text {stopping}}$ and $\varepsilon _{\text {reco}}$, as well as the number of expected MCHAMP events with lifetimes between 10 $\mu$s and 1000 s, assuming $\mathcal {B}(\text {MCHAMP} \to \mu ^{\pm}\mu ^{\pm})= $ 100%, for each mass point considered for the 2016 muon search. The efficiencies are constant for this range of lifetimes. |
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Table 4:
The background prediction for the calorimeter search. The total background median value is listed in parentheses; this value corresponds directly to the median expected limits shown below. |
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Table 5:
Systematic uncertainties in the signal efficiency in the 2015 and 2016 calorimeter searches. |
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Table 6:
Systematic uncertainties in the signal efficiency for the 2015 and 2016 muon searches. |
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Table 7:
Counting experiment results for different lifetimes in the calorimeter search with 2016 data. |
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Table 8:
Counting experiment results for different lifetimes in the muon search with 2016 data. |
| Summary |
|
A search has been presented for long-lived particles that stopped in the CMS detector after being produced in proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC. The subsequent decays of these particles to produce calorimeter deposits or muon pairs were looked for during gaps between proton bunches in the LHC beams. In the calorimeter (muon) search, with collision data corresponding to an integrated luminosity of 2.7\,(2.8) fb$^{-1}$ in a period of sensitivity corresponding to 135\,(155) hours of trigger livetime in 2015 and to an integrated luminosity of 35.9\,(36.2) fb$^{-1}$ in a period of sensitivity of 586\,(589) hours of trigger livetime in 2016, no excess above the estimated background has been observed. Cross section ($\sigma$) and mass limits have been presented at 95% confidence level (CL) on gluino (${\mathrm{\widetilde{g}}}$), top squark ($\tilde{\mathrm{t}}$), and multiply charged massive particle (MCHAMP) production over 13 orders of magnitude in the mean proper lifetime of the stopped particle. In the calorimeter search, combining the results from the 2015 and 2016 analyses and assuming a branching fraction ($\mathcal{B}$) of 100% for ${\mathrm{\widetilde{g}}} \to \mathrm{g}\tilde{\chi}^0$ (${\mathrm{\widetilde{g}}} \to \mathrm{q}\mathrm{\bar{q}}\tilde{\chi}^0$), where $\tilde{\chi}^0$ is the lightest neutralino, gluinos with lifetimes from 10 $mu$s to 1000 s and $m_{{\mathrm{\widetilde{g}}}} < $ 1385 (1393) GeV have been excluded, for a cloud model of R-hadron interactions and for the daughter gluon energy $E_{\mathrm{g}} > $ 130 GeV ($m_{{\mathrm{\widetilde{g}}}}-m_{\tilde{\chi}^0} > $ 160 GeV). Under similar assumptions, for the daughter top quark energy $E_{\mathrm{q}t} > $ 170 GeV and $\mathcal{B}(\tilde{\mathrm{t}} \to \mathrm{q}t\tilde{\chi}^0) = $ 100%, long-lived top squarks with lifetimes from 10 $\mu$s to 1000 s and $m_{\tilde{\mathrm{t}}} < $ 744 GeV have been excluded. These are the first limits on stopped long-lived particles at 13 TeV and the strongest limits to date. In the muon search, 95% CL upper limits on $\mathcal{B}\sigma$ were set for combined 2015 and 2016 data. For lifetimes between 10 $\mu$s and 1000 s, limits were set between 1 and 0.01 pb for gluinos with masses between 400 and 1600 GeV and for MCHAMPs with masses between 100 and 800 GeV and charge $| Q | = $ 2e. For lifetimes between 10 $\mu$s and 1000 s, gluinos with masses between 400 and 980 GeV have been excluded, assuming $\mathcal{B}({\mathrm{\widetilde{g}}} \to \mathrm{q}\mathrm{\bar{q}}\tilde{\chi}^0_2) \mathcal{B}(\tilde{\chi}^0Dt \to \mu^+\mu^-\tilde{\chi}^0)= $ 100%, $m_{\tilde{\chi}^0} = 0.25m_{{\mathrm{\widetilde{g}}}}$, and $m_{\tilde{\chi}^0Dt} = 2.5m_{\tilde{\chi}^0}$, where $\tilde{\chi}^0_2$ is the next-to-lightest neutralino. Under the same lifetime hypothesis, MCHAMPs with masses between 100 and 440 GeV and $| Q | = $ 2e have been excluded, assuming $\mathcal{B}(\text{MCHAMP} \to \mu^{\pm}\mu^{\pm})= $ 100%. These are the first limits obtained at the LHC for stopped particles that decay to muons. |
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