CMS-EXO-24-004

CMS-EXO-24-004 ; CERN-EP-2025-166
Search for b hadron decays to long-lived particles in the CMS endcap muon detectors
CMS Collaboration
8 August 2025
Submitted to J. High Energy Phys.
Abstract: A search for long-lived particles originating from the decay of b hadrons produced in proton-proton collisions with a center-of-mass energy of 13 TeV at the LHC is presented. The analysis is performed on a data set recorded in 2018, corresponding to an integrated luminosity of 41.6 fb$ ^{-1} $. Interactions of the long-lived particles in the CMS endcap muon system would create hadronic or electromagnetic showers, producing clusters of detector hits. Selected events contain at least one such high-multiplicity cluster in the muon endcaps and require the presence of a displaced muon. The most stringent upper limits to date on the branching fraction $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $, where the long-lived particle $ \Phi $ decays to a pair of hadrons, are obtained for $ \Phi $ masses of 0.3-3.0 GeV and $ \Phi $ mean proper decay lengths in the range of 1-500 cm.
Links: e-print arXiv:2508.06363 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example Feynman diagram for the production of a pair of $ {\mathrm{B}} $ mesons, where one $ {\mathrm{B}} $ meson decays to a muon, neutrino, and another hadron labeled $ \mathrm{X} $, while the other $ {\mathrm{B}} $ meson decays to a kaon and the LLP $ \Phi $ (left). Penguin diagram displaying the flavor-changing interaction which changes the flavor of the b quark to a s quark, producing the $ \Phi $ particle (right). The interaction proceeds via a virtual top quark, denoted by the cross, which undergoes a chirality flip, denoted by the outward-pointing arrows on either side of the cross in the diagram.
png pdf	Figure 1-a: Example Feynman diagram for the production of a pair of $ {\mathrm{B}} $ mesons, where one $ {\mathrm{B}} $ meson decays to a muon, neutrino, and another hadron labeled $ \mathrm{X} $, while the other $ {\mathrm{B}} $ meson decays to a kaon and the LLP $ \Phi $ (left). Penguin diagram displaying the flavor-changing interaction which changes the flavor of the b quark to a s quark, producing the $ \Phi $ particle (right). The interaction proceeds via a virtual top quark, denoted by the cross, which undergoes a chirality flip, denoted by the outward-pointing arrows on either side of the cross in the diagram.
png pdf	Figure 1-b: Example Feynman diagram for the production of a pair of $ {\mathrm{B}} $ mesons, where one $ {\mathrm{B}} $ meson decays to a muon, neutrino, and another hadron labeled $ \mathrm{X} $, while the other $ {\mathrm{B}} $ meson decays to a kaon and the LLP $ \Phi $ (left). Penguin diagram displaying the flavor-changing interaction which changes the flavor of the b quark to a s quark, producing the $ \Phi $ particle (right). The interaction proceeds via a virtual top quark, denoted by the cross, which undergoes a chirality flip, denoted by the outward-pointing arrows on either side of the cross in the diagram.
png pdf	Figure 2: Display of a candidate signal event in the $ {\mathrm{B}} $ parking data set. The display shows the hits (magenta points) in the muon endcap system that form the MDS cluster opposite the muon (red line), which is used to select the event online. Muon system chambers that recorded hits from a muon are shown as red rectangles.
png pdf	Figure 3: Distribution of $ t_\text{cluster} $ for the signal and the background-enriched data. The distributions are normalized to unity and overflows are not included. To enhance the visibility of fluctuations away from the peak, the background-enriched data distribution has been scaled by a factor of 10.
png pdf	Figure 4: Distributions of the CSC cluster size $ N_\text{hits} $ (left) and $ \Delta\phi\text{(cluster, }\: \mu_{\text{trigger}}) $ (right), shown for various signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 4-a: Distributions of the CSC cluster size $ N_\text{hits} $ (left) and $ \Delta\phi\text{(cluster, }\: \mu_{\text{trigger}}) $ (right), shown for various signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 4-b: Distributions of the CSC cluster size $ N_\text{hits} $ (left) and $ \Delta\phi\text{(cluster, }\: \mu_{\text{trigger}}) $ (right), shown for various signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 5: The 2-dimensional distribution of the $ N_\text{hits} $ versus $ \Delta\phi\text{(cluster, }\: \mu_{\text{trigger}}) $ for data in the OOT region.
png pdf	Figure 6: The 95% CL exclusion limits on $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $, where the scalar LLP, $ \Phi $, decays to a pair of hadrons. Limits are presented for scalar masses $ m_{ \Phi } = $ 0.3, 0.5, 1.0, 2.0, and 3.0 GeV as a function of the $ \Phi $ mean proper decay length.
png pdf	Figure 7: The 95% CL exclusion limits on $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $, where the scalar LLP, $ \Phi $, decays to a pair of hadrons for different masses and mean proper decay lengths.

Tables
png pdf	Table 1: Summary of the systematic uncertainties in the signal yield expectations in the A, B, C, and D regions for CSC clusters.
png pdf	Table 2: Summary of the observed event yields and estimated uncorrelated and jet-induced backgrounds in the A (SR), B, C, and D regions. The corresponding statistical uncertainties in the estimates are shown.

Summary

A search for a long-lived particle (LLP) $ \Phi $ produced from the decay of a bottom hadron and decaying to a pair of hadrons in the CMS endcap muon system has been carried out. The search uses the $ {\mathrm{B}} $ parking data set of proton-proton collisions at $ \sqrt{s}= $ 13 TeV collected by the CMS experiment in 2018 and corresponding to an integrated luminosity of 41.6 fb$ ^{-1} $. The data set was collected using a set of high-rate triggers that require the presence of a single displaced muon. A strategy based on control samples in data has been developed to evaluate separately the uncorrelated background originating from prompt standard model particles that produce a shower in the muon system and the jet-induced background from long-lived standard model particles decaying to jets. The branching fraction $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $ is constrained for five low-mass LLP signal hypotheses. The best sensitivity for measuring a limit on $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $ for these models is observed in the range of 1-100 cm for the $ \Phi $ mean proper decay length $ c\tau $. The most sensitive $ c\tau $ value monotonically increases with LLP mass. However, the best sensitivity is similar across all models, constraining $ \mathcal{B}({\mathrm{B}}\to \mathrm{K} \Phi ) $ to be below $ 10^{-4} $ at 95% confidence level depending on the value of $ c\tau $ for each of the signal models studied. This result yields the most stringent limits to date on the decay of a long-lived scalar particle to a pair of hadrons when the LLP is produced by a bottom hadron.

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