CMS-PAS-EXO-25-010

CMS-PAS-EXO-25-010
Search for heavy long-lived charged particles with Level-1 Scouting data from proton-proton collisions at $ \sqrt{s}= $ 13.6 TeV
CMS Collaboration
2025-09-23

Abstract: Heavy long-lived charged particles crossing the CMS muon chambers in several bunch crossings are searched for using 3.7 fb$ ^{-1} $ of data collected in 2024 at a center-of-mass energy of 13.6 TeV. The search relies, for the first time, on the Level-1 Scouting data set, collected without any trigger selection, allowing for correlations between bunch crossings to be analyzed. The results are interpreted as upper limits on the production cross section of several benchmark processes with pair-production of heavy long-lived charged particles. Fiducial upper limits on the cross section of a heavy long-lived charged particle with $ p_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 of the Level-1 Data Scouting system and complements existing searches for heavy long-lived charged particles by extending the sensitivity to lower $ \beta $ values.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: SR 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 background distributions are the result of 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 pb.
png pdf	Figure 1-a: SR 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 background distributions are the result of 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 pb.
png pdf	Figure 1-b: SR 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 background distributions are the result of 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 pb.
png pdf	Figure 1-c: SR 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 background distributions are the result of 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 pb.
png pdf	Figure 2: Observed (full 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) band and the outer (blue) band 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 three-stub (dashed purple line) and four-stub (dashed-red line) categories are also shown.
png pdf	Figure 2-a: Observed (full 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) band and the outer (blue) band 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 three-stub (dashed purple line) and four-stub (dashed-red line) categories are also shown.
png pdf	Figure 2-b: Observed (full 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) band and the outer (blue) band 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 three-stub (dashed purple line) and four-stub (dashed-red line) categories are also shown.
png pdf	Figure 3: Observed upper limits at 95% CL on the production cross section of heavy fourth-generation leptons through $ \mathrm{Z}' $ decays. Lines indicate different values of the ratio of $ \mathrm{Z}' $ to $ \tau' $ masses, for which $ \beta $ values are typically similar. For $ m_{\mathrm{Z}'} > $ 3.0 $m_{\tau'} $, the HSCPs typically have high $ \beta $ and are reconstructed in the same BX.
png pdf	Figure 4: Observed and expected upper limits at 95% CL on the fiducial cross section of a heavy particle leaving a signature in the muon stations with $ p_{\mathrm{T}} > $ 500 GeV, $ \|\eta\| < $ 0.83, and a given $ \beta $ range. The inner (yellow) band and the outer (blue) band 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 stations crossed per BX are also shown with colorful dashed lines.

Tables

png pdf

Table 1:
Categories based on the stub multiplicity of the track, the stations with stubs, and the speed expressed as the number of crossed stations divided by the number of BXs spent to cross them. BX1112 (1122, 1222) corresponds to three (two, one) stubs in the first BX, and one (two, three) stubs in the next BX, whereas BX112 (122) corresponds to two (one) stubs in the first BX and one (two) stubs in the next BX. For three-stub (four-stub) tracks, BX124/134 corresponds to three (four) stubs in three different BXs across a range of 4 BXs. Three-stub categories denoted "fast" have 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 not have stubs in both extreme stations.

Summary

In summary, a search for long-lived massive charged particles has been presented, exploiting a muon-like signature in the barrel muon chambers spread across several bunch crossings. For the first time, this analysis relies on the Level-1 scouting data set, for which no trigger selection is applied. A data-driven method 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 cross section production 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 $ 0.50, which cannot be triggered on, and to neutral particles that acquire a charge when crossing the detector, which do not leave an ionization-loss signature in the tracker. An upper limit as low as 3.5fb is set at 95% confidence level on the fiducial production cross section of lepton-like charged particles with 0.1875 $ < \beta < $ 0.2125, $ |\eta| < $ 0.83, and $ p_{\mathrm{T}} > $ 500 GeV.

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