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| CMS-EXO-25-010 ; CERN-EP-2025-301 | ||
| Search for heavy long-lived charged particles with level-1 trigger scouting data from proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV | ||
| CMS Collaboration | ||
| 27 January 2026 | ||
| Submitted to Physics Letters B | ||
| Abstract: A search for heavy long-lived charged particles at the LHC is presented. Particles interacting with the CMS muon detector across several bunch crossings are searched for using a data sample of proton-proton collisions at $ \sqrt{s}= $ 13.6 TeV collected with the CMS detector in 2024, corresponding to an integrated luminosity of 3.7 fb$ ^{-1} $. This is the first search relying on the novel level-1 trigger scouting data set collected without any trigger selection, allowing correlations between bunch crossings to be analyzed. The results are interpreted as upper limits on the cross sections of several benchmark processes with pair production of heavy long-lived charged particles. Upper limits on the fiducial cross section of a heavy long-lived charged particle with $ p_{\mathrm{T}} > $ 500 GeV and $ |\eta| < $ 0.83 are also set in different ranges of $ \beta=v/c $. This analysis is a crucial proof of concept for the level-1 trigger data scouting system and complements existing searches for heavy long-lived charged particles by extending the sensitivity to lower $ \beta $ values. | ||
| Links: e-print arXiv:2601.20063 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The $ R $--$ z $ projection of a quarter of the CMS barrel muon system, where $ R $ is the radial distance from the beamline. |
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Figure 2:
Generated $ \beta $ distributions of the fourth-generation $ \tau^{'} $ leptons from nonresonant DY production, for various $ \tau^{'} $ masses $ m $. The histograms are normalized to unity. |
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Figure 3:
Distributions of the dimuon invariant mass (left) and per-track stub multiplicity (right) for events with two opposite-sign modified kBMTF tracks reconstructed from individual stubs in the same or different BXs. The invariant mass is required to be greater than 70 GeV in the stub multiplicity distributions to increase the purity. The muons are selected with $ p_{\mathrm{T}} > $ 15 GeV and $ |\eta| < $ 0.83. The nonprompt-background contribution is estimated by rescaling the data with two same-sign modified kBMTF tracks. The statistical uncertainty is indicated in the gray shaded area. The agreement between simulation and data is good by construction since the efficiency, energy scale, and energy smearing corrections derived from the same events have been applied. |
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Figure 3-a:
Distributions of the dimuon invariant mass (left) and per-track stub multiplicity (right) for events with two opposite-sign modified kBMTF tracks reconstructed from individual stubs in the same or different BXs. The invariant mass is required to be greater than 70 GeV in the stub multiplicity distributions to increase the purity. The muons are selected with $ p_{\mathrm{T}} > $ 15 GeV and $ |\eta| < $ 0.83. The nonprompt-background contribution is estimated by rescaling the data with two same-sign modified kBMTF tracks. The statistical uncertainty is indicated in the gray shaded area. The agreement between simulation and data is good by construction since the efficiency, energy scale, and energy smearing corrections derived from the same events have been applied. |
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Figure 3-b:
Distributions of the dimuon invariant mass (left) and per-track stub multiplicity (right) for events with two opposite-sign modified kBMTF tracks reconstructed from individual stubs in the same or different BXs. The invariant mass is required to be greater than 70 GeV in the stub multiplicity distributions to increase the purity. The muons are selected with $ p_{\mathrm{T}} > $ 15 GeV and $ |\eta| < $ 0.83. The nonprompt-background contribution is estimated by rescaling the data with two same-sign modified kBMTF tracks. The statistical uncertainty is indicated in the gray shaded area. The agreement between simulation and data is good by construction since the efficiency, energy scale, and energy smearing corrections derived from the same events have been applied. |
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Figure 4:
Fractions of the BX signatures of tracks reconstructed with the modified kBMTF algorithm as functions of the generated particle $ \beta $. This figure corresponds to the nonresonant fourth-generation lepton signal model with a mixture of HSCP masses between 1 and 6 TeV. The filled histograms are stacked to sum to unity. The blue histograms correspond to tracks reconstructed in a single BX. Different shades represent different BXs with respect to the collision BX ($ \mathrm{BX} = i $), as indicated in parentheses in the legend. The orange histograms correspond to tracks reconstructed over 2 BXs, with different shades corresponding to different BXs with respect to the collision BX. Different shades of the same color are indistinguishable experimentally since the production BX cannot be determined. The red and purple histograms represent tracks reconstructed over 3 or at least 4 BXs, respectively. The dashed line indicates the efficiency for the reconstructed tracks to satisfy the single-muon HLT selection. For $ \beta < $ 0.15, the efficiency goes to zero because the particles get stopped in the detector. For $ \beta > $ 0.8, the particles are increasingly reconstructed in the same BX and the HLT selection is fully efficient. |
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Figure 5:
Low-track-quality validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 5-a:
Low-track-quality validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 5-b:
Low-track-quality validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 5-c:
Low-track-quality validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 6:
Noncolliding validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 6-a:
Noncolliding validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 6-b:
Noncolliding validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 6-c:
Noncolliding validation region distributions for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 7:
The $ p_{\mathrm{T}} $ distributions in the signal region for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 7-a:
The $ p_{\mathrm{T}} $ distributions in the signal region for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 7-b:
The $ p_{\mathrm{T}} $ distributions in the signal region for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 7-c:
The $ p_{\mathrm{T}} $ distributions in the signal region for the categories with tracks across $ > $2 ( = $ $2, = $ $2) BXs without (with, without) additional track requirement are shown in the upper (center, lower) part of the figure. The expected background distributions are the result of the maximum likelihood fit. The uncertainty bands account for all sources of background uncertainty, systematic as well as statistical, after the maximum likelihood fit. The lower panels show the observed to expected ratio. The signal is shown for a few mass hypotheses of the nonresonant fourth-generation lepton model, using a production cross section of 1\unitpb. The last $ p_{\mathrm{T}} $ bins include the overflow. |
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Figure 8:
Observed (solid line with markers) and expected (dashed black line) upper limits at 95% CL on the production cross section of heavy fourth-generation leptons through nonresonant DY production (left) and of a gluino $ R $-hadron pair with $ f= $ 1 (right). The inner (yellow) and the outer (blue) bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The expected limits for the combinations of 3-stub (dashed purple line) and 4-stub (dashed-red line) categories are also shown. |
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Figure 8-a:
Observed (solid line with markers) and expected (dashed black line) upper limits at 95% CL on the production cross section of heavy fourth-generation leptons through nonresonant DY production (left) and of a gluino $ R $-hadron pair with $ f= $ 1 (right). The inner (yellow) and the outer (blue) bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The expected limits for the combinations of 3-stub (dashed purple line) and 4-stub (dashed-red line) categories are also shown. |
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Figure 8-b:
Observed (solid line with markers) and expected (dashed black line) upper limits at 95% CL on the production cross section of heavy fourth-generation leptons through nonresonant DY production (left) and of a gluino $ R $-hadron pair with $ f= $ 1 (right). The inner (yellow) and the outer (blue) bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The expected limits for the combinations of 3-stub (dashed purple line) and 4-stub (dashed-red line) categories are also shown. |
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Figure 9:
Observed upper limits at 95% CL on the production cross section of heavy fourth-generation leptons through $ \mathrm{Z}^{'} $ boson decays. Lines indicate different values of the ratio of $ \mathrm{Z}^{'} $ to $ \tau^{'} $ masses, for which $ \beta $ values are typically similar. Signal hypotheses with 2 $ m_{\tau^{'}} < m_{\mathrm{Z}^{'}} < 3m_{\tau^{'}} $ are probed. For $ m_{\mathrm{Z}^{'}} > 3m_{\tau^{'}} $, the HSCPs typically have high $ \beta $ and are reconstructed in the same BX. The lowest limits are obtained for $ m_{\mathrm{Z}^{'}}/m_{\tau^{'}}\approx $ 2.15 (2.60), for which HSCPs are dominantly produced with $ \beta\approx $ 0.2 (0.6). The higher limits for $ m_{\mathrm{Z}^{'}}/m_{\tau^{'}}\approx $ 2.30 are related to the low analysis acceptance for HSCPs produced with $ \beta\approx $ 0.5 and largely reconstructed in a single BX. |
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Figure 10:
Observed and expected upper limits at 95% CL on the fiducial cross section of a heavy particle leaving a signature in the muon detector with $ |\eta| < $ 0.83, $ p_{\mathrm{T}} > $ 500 GeV, in bins of $ \beta $. The inner (yellow) and the outer (blue) bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The expected limits for groups of categories corresponding to a different number of muon detector layers crossed per BX are also shown with colored dashed lines. The limited sensitivity around $ \beta= $ 0.5 corresponds to a loss of acceptance from particles reconstructed entirely in the BX that followed the collision. |
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Figure 11:
Schematic definition of the 3-stub categories with tracks across $ > $2 BXs. |
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Figure 11-a:
Schematic definition of the 3-stub categories with tracks across $ > $2 BXs. |
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Figure 11-b:
Schematic definition of the 3-stub categories with tracks across $ > $2 BXs. |
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Figure 11-c:
Schematic definition of the 3-stub categories with tracks across $ > $2 BXs. |
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Figure 11-d:
Schematic definition of the 3-stub categories with tracks across $ > $2 BXs. |
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Figure 12:
Schematic definition of the 4-stub categories with tracks across $ > $2 BXs. |
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Figure 12-a:
Schematic definition of the 4-stub categories with tracks across $ > $2 BXs. |
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Figure 12-b:
Schematic definition of the 4-stub categories with tracks across $ > $2 BXs. |
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Figure 12-c:
Schematic definition of the 4-stub categories with tracks across $ > $2 BXs. |
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Figure 13:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-a:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-b:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-c:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-d:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-e:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-f:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 13-g:
Schematic definition of the categories with tracks across 2 BXs. |
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Figure 14:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1234 category. The upper (lower) half includes orderings with the first (last) stub in MB1 (MB4). |
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Figure 14-a:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1234 category. The upper (lower) half includes orderings with the first (last) stub in MB1 (MB4). |
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Figure 14-b:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1234 category. The upper (lower) half includes orderings with the first (last) stub in MB1 (MB4). |
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Figure 15:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1112 category. The left (right) diagram shows the ordering where the stub detected in a different BX than the other three stubs is in MB2 (MB3). |
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Figure 15-a:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1112 category. The left (right) diagram shows the ordering where the stub detected in a different BX than the other three stubs is in MB2 (MB3). |
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Figure 15-b:
Schematic definition of the asynchronous orderings used to estimate the backgrounds in the BX1112 category. The left (right) diagram shows the ordering where the stub detected in a different BX than the other three stubs is in MB2 (MB3). |
| Tables | |
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Table 1:
Categories based on muon detector layers with stubs and speed expressed as the number of crossed layers divided by the number of BXs spent to cross them. The BX1234 category includes 4-stub tracks for which the 4 stubs are in 4 subsequent BXs. For 3-stub (4-stub) tracks, the BX123 categories correspond to 3 (4) stubs in 3 different BXs across a range of 3 BXs, whereas the BX124/134 categories correspond to 3 (4) stubs in 3 different BXs across a range of 4 BXs. The 4-stub category BX1112 (1122, 1222) corresponds to 3 (2, 1) stubs in the first BX, and 1 (2, 3) stubs in the next BX. Similarly, the 3-stub categories BX112 (122) correspond to 2 (1) stubs in the first BX and 1 (2) stub in the next BX. The 3-stub categories are further separated into fast and slow subcategories: those denoted ``fast" feature a track with the first stub in MB1 and the last one in MB4, corresponding to a longer distance crossed in the same amount of time as their ``slow" counterparts, which do have exactly one stub within the innermost and outermost detector layers. The symbol $ \oplus $ represents the exclusive ``or", whereas $ \land $ is the logical ``and". |
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Table 2:
Definition of asynchronous track orderings used to build the alternative $ p_{\mathrm{T}} $ distributions corresponding to a $ \pm $ 1 s.d. of the background systematic uncertainty. |
| Summary |
| A search for long-lived massive charged particles has been presented, exploiting a muon-like signature in the barrel muon detectors spread across several LHC proton bunch crossings. This is the first time that an analysis relies on the novel CMS level-1 trigger scouting data set, for which no trigger selection is applied. A method based on control samples in data is used to estimate the backgrounds, relying on tracks not compatible in time with the expectations from a slow particle exiting the detector. No significant excess of data above the predicted standard model backgrounds is observed. Upper limits are set on the production cross section of heavy stable charged particles in several models, and fiducial upper limits on the production cross section for different $ \beta=v/c $ ranges are also set for a model-independent interpretation. The analysis has unique sensitivity to particles with 0.15 $ \lesssim \beta \lesssim $ 0.5, which are challenging to trigger on, and to neutral particles that acquire a charge when crossing the detector, which do not leave an ionization-loss signature in the tracker. Upper limits as low as 3.5\unitfb at 95% confidence level are set on the fiducial production cross section of lepton-like charged particles with $ |\eta| < $ 0.83 and $ p_{\mathrm{T}} > $ 500 GeV, in several bins of $ \beta $, extending the reach of the existing searches to higher masses and lower values of $ \beta $. |
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