CMS-PAS-EXO-24-004

CMS-PAS-EXO-24-004
Search for b hadron decays to long-lived particles in the CMS endcap muon detectors
CMS Collaboration
25 March 2025

Abstract: A search for long-lived particles originating from the decay of b hadrons produced by the LHC is presented. The long-lived particles interact with the CMS endcap muon system creating hadronic showers, which are used to construct clusters. The analysis is performed on a data set recorded in 2018 that is referred to as the B parking data set. The data set correspond to an integrated luminosity of 41.6 fb $^{-1}$ . Selected events contain at least one high-multiplicity cluster of hits in the muon endcaps and require the presence of a displaced muon. Stringent upper limits on $\mathcal{B}(\mathrm{B}\to \mathrm{K}\Phi)$ , when the long-lived particle, $\Phi$ , decays to a pair of oppositely charged pions are obtained for $\Phi$ masses of 0.3--3.0 GeV and $\Phi$ mean proper lifetimes in the range of 10--5000 mm.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example Feynman diagram for the production of a pair of b hadrons, where one b hadron decays to a muon, neutrino, and another hadron labeled $\mathrm{X}$ , while the other b hadron decays to a kaon and an LLP $\Phi$ (left). Penguin diagram displaying the flavor-changing interaction that produces $\Phi$ (right).
png pdf	Figure 1-a: Example Feynman diagram for the production of a pair of b hadrons, where one b hadron decays to a muon, neutrino, and another hadron labeled $\mathrm{X}$ , while the other b hadron decays to a kaon and an LLP $\Phi$ (left). Penguin diagram displaying the flavor-changing interaction that produces $\Phi$ (right).
png pdf	Figure 1-b: Example Feynman diagram for the production of a pair of b hadrons, where one b hadron decays to a muon, neutrino, and another hadron labeled $\mathrm{X}$ , while the other b hadron decays to a kaon and an LLP $\Phi$ (left). Penguin diagram displaying the flavor-changing interaction that produces $\Phi$ (right).
png pdf	Figure 2: Distribution of $t_\text{cluster}$ for the signal and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 3: Distributions of the CSC cluster size $N_\text{hits}$ (left) and $\Delta\phi\mathrm{(cluster, }\: \mu_{\text{trigger}})$ (right), shown for multiple signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 3-a: Distributions of the CSC cluster size $N_\text{hits}$ (left) and $\Delta\phi\mathrm{(cluster, }\: \mu_{\text{trigger}})$ (right), shown for multiple signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 3-b: Distributions of the CSC cluster size $N_\text{hits}$ (left) and $\Delta\phi\mathrm{(cluster, }\: \mu_{\text{trigger}})$ (right), shown for multiple signal samples and the background-enriched data. The distributions are normalized to unity and overflows are not included.
png pdf	Figure 4: The 2D distribution of the $N_\text{hits}$ versus $\Delta\phi\mathrm{(cluster, }\: \mu_{\text{trigger}})$ for data in the OOT region.
png pdf	Figure 5: 95% CL exclusion limits on the $\mathcal{B}({\mathrm{B}}\to \mathrm{K}\Phi)$ , where the scalar LLP, $\Phi$ , decays to a pair of pions. Limits are presented for scalar masses $m_\Phi =$ 0.3, 1.0, 2.0, and 3.0 GeV as a function of the $\Phi$ mean proper lifetime
png pdf	Figure 6: 95% CL exclusion limits on the $\mathcal{B}({\mathrm{B}} \to \mathrm{K}\Phi)$ , where the scalar LLP, $\Phi$ , decays to pions for different masses and decay lifetime.

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 the b hadron and decaying to a pair of charged pions 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 CMS 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 data-driven strategy was devised to separately estimate the uncorrelated background originating from prompt SM particles that produce a shower in the muon system and the jet-induced background from long-lived SM particles decaying to jets. The branching fraction

$\mathcal{B}({\mathrm{B}}\to \mathrm{K}\Phi)$ is constrained for four low-mass LLP signal hypotheses. The peak sensitivity for these models is generally in the range of 10--1000 mm for the

$\Phi$ mean proper decay length

$c\tau$ . The most sensitive point for each signal hypothesis moves towards larger values of

$c\tau$ as a function of the LLP mass. However, the peak sensitivity is similar between each signal model, constraining the

$\mathcal{B}({\mathrm{B}}\to \mathrm{K}\Phi)$ below

$10^{-5}$ at 95% confidence level depending on the value of

$c\tau$ for each of the signal models studied. This result yields stringent limits on the decay of a long-lived scalar particle to a pair of pions when the LLP is produced by a b hadron.

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