CMS-PAS-EXO-25-019

CMS-PAS-EXO-25-019
Search for low-mass resonances decaying to a pair of muons in proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV
CMS Collaboration
2026-05-16

Abstract: A search for prompt production of low-mass dimuon resonances is reported using proton-proton collision events at $ \sqrt{s} = $ 13.6 TeV collected by the CMS detector in 2022-2024, corresponding to an integrated luminosity of 175$ \mathrm{fb}^{-1} $. The search employs a newly developed inclusive dimuon trigger and probes the existence of GeV-scale dark photons. The search is performed by looking for narrow peaks in the dimuon mass spectrum in the ranges of 0.88-0.92$ \mathrm{GeV} $, 1.10-2.88$ \mathrm{GeV} $, and 4.00-7.95$ \mathrm{GeV} $. Model-independent upper limits are set for resonances within the fiducial acceptance. No obvious excess is observed. The most stringent constraints to date on minimal dark photon models are placed.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $ m_{\mu\mu} $ distribution obtained with the data collected during 2022--2024 after the full selection.
png pdf	Figure 2: Postfit distributions for the dark photon search for the dark photon search at 1.108 GeV (left) and 6.580 GeV (right) using data collected during 2022--2024. The lower panel shows the data and the fit result after subtracting the total background component of the fit.
png pdf	Figure 2-a: Postfit distributions for the dark photon search for the dark photon search at 1.108 GeV (left) and 6.580 GeV (right) using data collected during 2022--2024. The lower panel shows the data and the fit result after subtracting the total background component of the fit.
png pdf	Figure 2-b: Postfit distributions for the dark photon search for the dark photon search at 1.108 GeV (left) and 6.580 GeV (right) using data collected during 2022--2024. The lower panel shows the data and the fit result after subtracting the total background component of the fit.
png pdf	Figure 3: Expected and observed model-independent limits on the cross section times branching fraction of the signals to dimuons at 95% CL for data collected during 2022--2024. The intervals 0.92--1.10 and 2.9--4.0 GeV are excluded because of the presence of $ \phi $, $ \mathrm{J}/\psi $, and $ \PGyP{2S} $ resonances.
png pdf	Figure 4: Upper limits at 90% CL on the square of the kinetic mixing coefficient $ \varepsilon $ in the minimal model of a dark photon. Observed and expected values for this analysis are compared to the expected limits from the original Run 2 publication and those re-interpreted from Run 2 measurements using the current framework.
png pdf	Figure 5: Observed upper limits at 90% CL on the square of the kinetic mixing coefficient $ \varepsilon $ in the minimal model of a dark photon from the CMS search (pink). The results are compared with the existing limits at 90% CL provided by LHCb [5] (blue), BaBar [4] (gray), and CMS Run 2 measurement [7] (green).

Tables
png pdf	Table 1: Summary of the experimental systematic uncertainties in the model-independent search for a dimuon resonance.

Summary

In summary, a search for the prompt production of a narrow resonance decaying to a pair of muons is presented, using proton-proton collision data recorded by the CMS experiment at $\sqrt{s} = 13.6$ TeV corresponding to an integrated luminosity of 175$ \mathrm{fb}^{-1} $. The search is performed in the dimuon mass regions of 0.88--0.92 GeV, 1.10--2.88 GeV, and 4.00--7.95 GeV using a newly developed inclusive dimuon trigger using the parking data processing and storage model. This allowed an improvement in the selection relative to Run~2 due to the full offline reconstruction being available for these events. The direct production of dark photons decaying to dimuons is probed, and no significant excess above the fitted background is observed. This analysis achieves the most stringent constraints across the probed mass range, representing a significant improvement over the previous best results [5,7].

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