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| CMS-PAS-EXO-17-004 | ||
| Search for long-lived particles that stop in the CMS detector and decay to muons at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| July 2017 | ||
| Abstract: A search for long-lived particles that are produced in proton-proton collisions at the CERN LHC, come to rest in the CMS detector, and decay to muons is presented. The decays of stopped particles could be observed during the intervals between LHC beam crossings, at times well separated from any proton-proton collisions. The analysis uses 39 fb$^{-1}$ of $\sqrt{s}= $ 13 TeV data collected by CMS in 2015 and 2016, during a search interval totaling 744 hours of trigger livetime. The results are interpreted with one model that predicts a long-lived gluino and another model that predicts a long-lived particle that has twice the electron charge and that behaves like a lepton. Cross section limits are set as a function of lifetime and as a function of mass, for lifetimes between 100 ns and 10 days. This is the first search for stopped particles that decay to muons at the LHC. | ||
| Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The stopping efficiency per particle as a function of mass for gluinos and mchamps. The mchamps have $|Q|=2e$. |
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Figure 2:
A diagram showing the typical direction and the corresponding $t_{\mathrm {DT}}$ values of muons coming from cosmic muon background (left) and signal (right). This is a simple schematic and does not necessarily reflect how close the muons would come to the beamspot. Furthermore, the muons from the signal are not necessarily back-to-back. |
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Figure 3:
DSA muon track $t_{\mathrm {DT}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {DT}}$ of the upper (left) and lower (right) hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The gluino and mchamp distributions are not exactly the same because the two muons from the mchamp decay are back-to-back, but the two muons from the gluino decay are not. The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 3-a:
DSA muon track $t_{\mathrm {DT}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {DT}}$ of the upper hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The gluino and mchamp distributions are not exactly the same because the two muons from the mchamp decay are back-to-back, but the two muons from the gluino decay are not. The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 3-b:
DSA muon track $t_{\mathrm {DT}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {DT}}$ of the lower hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The gluino and mchamp distributions are not exactly the same because the two muons from the mchamp decay are back-to-back, but the two muons from the gluino decay are not. The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 4:
A diagram showing the typical direction and thus, the sign of $\beta $ of muons coming from cosmic muon background (left) and signal (right). This is a simple schematic and does not necessarily reflect how close the muons would come to the beamspot. Furthermore, the muons from the signal are not necessarily back-to-back. |
png pdf |
Figure 5:
DSA muon track $\beta ^{-1}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. The $\beta ^{-1}$ of the upper (left) and lower (right) hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 5-a:
DSA muon track $\beta ^{-1}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. The $\beta ^{-1}$ of the upper hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 5-b:
DSA muon track $\beta ^{-1}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. The $\beta ^{-1}$ of the lower hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
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Figure 6:
A diagram showing the typical RPC BX assignments of muons coming from cosmic muon background (left) and signal (right). This is a simple schematic and does not necessarily reflect how close the muons would come to the beamspot. Furthermore, the muons from the signal are not necessarily back-to-back. |
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Figure 7:
DSA muon track $t_{\mathrm {RPC}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {RPC}}$ of the upper (left) and lower (right) hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 7-a:
DSA muon track $t_{\mathrm {RPC}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {RPC}}$ of the upper hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 7-b:
DSA muon track $t_{\mathrm {RPC}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. $t_{\mathrm {RPC}}$ of the lower hemisphere DSA tracks is plotted. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 8:
DSA muon track $\Delta t_{\mathrm {DT}}$ (left) and $\Delta t_{\mathrm {RPC}}$ (right) for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 8-a:
DSA muon track $\Delta t_{\mathrm {DT}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 8-b:
DSA muon track $\Delta t_{\mathrm {RPC}}$ for 2016 search sample data, cosmic muon MC simulation, 2000 GeV gluinos, and 600 GeV mchamps. 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 muon background (see Section 5). The grey bands indicate the statistical uncertainty in the simulation. The histograms are normalized to unit area. |
png pdf |
Figure 9:
The integral under the fit to $\Delta t_{\mathrm {DT}}$ with the sum of two Gaussian distributions and a Crystal Ball function, when $\Delta t_{\mathrm {DT}}>-20$ ns, as a function of the lower $\Delta t_{\mathrm {RPC}}$ 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 $\Delta t_{\mathrm {RPC}}>-7.5$ ns. |
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Figure 10:
95% CL cross section times BF limits on 1000 GeV gluino (left) and 1000 GeV mchamp (right) pair production as a function of lifetime, for combined 2015 and 2016 data. The observed limits are shown in the solid black line, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
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Figure 10-a:
95% CL cross section times BF limits on 1000 GeV gluino pair production as a function of lifetime, for combined 2015 and 2016 data. The observed limits are shown in the solid black line, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
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Figure 10-b:
95% CL cross section times BF limits on 1000 GeV mchamp pair production as a function of lifetime, for combined 2015 and 2016 data. The observed limits are shown in the solid black line, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
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Figure 11:
95% CL cross section times BF limits on 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. The observed limits are shown in the solid black points, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
png pdf |
Figure 11-a:
95% CL cross section times BF limits on gluino pair production as a function of mass, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data. The observed limits are shown in the solid black points, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
png pdf |
Figure 11-b:
95% CL cross section times BF limits on mchamp pair production as a function of mass, for lifetimes between 10 $\mu$s and 1000 s, for combined 2015 and 2016 data. The observed limits are shown in the solid black points, the expected limits are shown in the dotted black line, the expected 1$\sigma $ and 2$\sigma $ bands are shown in green and yellow, respectively, and the theoretical cross sections assuming 100% BF are shown in the red line. |
| Tables | |
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Table 1:
Systematic uncertainties in the signal efficiency for the 2015 and 2016 analyses. |
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Table 2:
Gluino stopping efficiency, reconstruction efficiency, and the number of expected gluino events with lifetimes between 10 $\mu$s and 1000 s assuming 100% BF, for each mass for the 2016 analysis. |
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Table 3:
Mchamp stopping efficiency, reconstruction efficiency, and the number of expected mchamp events with lifetimes between 10 $\mu$s and 1000 s assuming 100% BF, for each mass for the 2016 analysis. |
| Summary |
|
A search for delayed muons produced in pp collisions at $\sqrt{s}= $ 13 TeV has been performed at CMS with 2.8 fb$^{-1}$ and 36.2 fb$^{-1}$ data collected in 2015 and 2016. This search looked for new long-lived particles that stopped in the CMS detector and subsequently decayed to muons. These stopped particles were looked for when there were no collisions in the detector, namely, during gaps between LHC beam crossings. No evidence of signal was found and 95% confidence level cross section times branching fraction (BF) upper limits 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 of mass between 400 and 2600 GeV and for mchamps of mass between 100 and 2600 GeV. Assuming a 100% BF, then for lifetimes between 10 $\mu$s and 1000 s, gluinos with mass between 400 and 970 (970) GeV observed (expected) are excluded, while mchamps with mass between 100 and 410 (410) GeV observed (expected) are excluded. Cross section times BF limits were also set for each gluino and mchamp mass as a function of lifetime, for lifetimes between 100 ns and 10 days. These are the first limits for stopped particles that decay to muons at the LHC. |
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Compact Muon Solenoid LHC, CERN |
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