CMS-PAS-EXO-21-005

CMS-PAS-EXO-21-005
Search for prompt production of a GeV scale resonance decaying to a pair of muons in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
24 March 2023

Abstract: A search for prompt low-mass dimuon resonances is performed using the 13 TeV proton-proton collision data collected by the Compact Muon Solenoid (CMS) detector during the 2017--2018 operation of the CERN's Large Hadron Collider and corresponding to an integrated luminosity of 96.6 fb $^{-1}$ . The search exploits a dedicated scouting trigger stream allowing CMS to record events with two muons with transverse momenta as low as 3 GeV by trading off recording the full event information. The search is performed by looking for narrow resonance peaks in the dimuon mass continuum in the ranges from 1.1-2.6 and from 4.2-7.9 GeV. The largest excess is observed in the boosted dimuon category at a dimuon mass of 2.41 GeV with the local significance of 3.2 standard deviations, with the global significance of the excess being 1.3 standard deviations. Model-independent limits on production rates of dimuon resonances within the experimental fiducial acceptance are set. Limits on parameters for two specific models, a dark photon model and a two Higgs doublet model with an extra scalar, are also set. This document has been revised with respect to the version dated March 11th, 2023.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 12 (2023) 070. The superseded preliminary plots can be found here.

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: The 2017 (left) and 2018 (right) measured efficiencies of the dimuon scouting trigger and logical OR of all L1 triggers using 2017 data. The value of each cell shows the probability that a valid pair of muons which satisfy the trigger requirements will cause the dimuon scouting trigger to fire. The $x$ -axis shows the dimuon mass and includes the entire relevant range for this analysis. The $y$ -axis shows the angular separation, $\Delta R$ , between the two muons.
png pdf	Figure 1-a: The 2017 (left) and 2018 (right) measured efficiencies of the dimuon scouting trigger and logical OR of all L1 triggers using 2017 data. The value of each cell shows the probability that a valid pair of muons which satisfy the trigger requirements will cause the dimuon scouting trigger to fire. The $x$ -axis shows the dimuon mass and includes the entire relevant range for this analysis. The $y$ -axis shows the angular separation, $\Delta R$ , between the two muons.
png pdf	Figure 1-b: The 2017 (left) and 2018 (right) measured efficiencies of the dimuon scouting trigger and logical OR of all L1 triggers using 2017 data. The value of each cell shows the probability that a valid pair of muons which satisfy the trigger requirements will cause the dimuon scouting trigger to fire. The $x$ -axis shows the dimuon mass and includes the entire relevant range for this analysis. The $y$ -axis shows the angular separation, $\Delta R$ , between the two muons.
png pdf	Figure 2: The $m_{\mu\mu}$ distribution obtained with the scouting data collected during 2017 and 2018 with two sets of selections: the $\Upsilon{\textrm{(1S)}}$ -trained muon identification MVA with the transverse displacement of vertex less than 0.015 cm (blue), and the $\mathrm{J}/\psi$ -trained muon MVA identification with the vertex transverse displacement of less than 3.5 standard deviations (red).
png pdf	Figure 3: The signal acceptance and reconstruction efficiency are extracted from DY (purple) and pseudoscalar (cyan) simulations. The occluded region at 3.5--4.5 GeV indicates the transition between the $\mathrm{J}/\psi$ -trained and $\Upsilon{\textrm{(1S)}}$ -trained muon MVA identifications.
png pdf	Figure 4: Left: Expected and observed model independent upper limits at 95% CL on the product of the signal cross section ( $\sigma$ ) times branching fraction to a pair of muons for the inclusive dimuon selection. Right: The model independent limits for the high- $p_{\mathrm{T}}$ selection. The mass region dominated by the $\mathrm{J}/\psi$ and $\psi^{\prime}$ resonances is not considered in the fit.
png pdf	Figure 4-a: Left: Expected and observed model independent upper limits at 95% CL on the product of the signal cross section ( $\sigma$ ) times branching fraction to a pair of muons for the inclusive dimuon selection. Right: The model independent limits for the high- $p_{\mathrm{T}}$ selection. The mass region dominated by the $\mathrm{J}/\psi$ and $\psi^{\prime}$ resonances is not considered in the fit.
png pdf	Figure 4-b: Left: Expected and observed model independent upper limits at 95% CL on the product of the signal cross section ( $\sigma$ ) times branching fraction to a pair of muons for the inclusive dimuon selection. Right: The model independent limits for the high- $p_{\mathrm{T}}$ selection. The mass region dominated by the $\mathrm{J}/\psi$ and $\psi^{\prime}$ resonances is not considered in the fit.
png pdf	Figure 5: Observed upper limits at 90% CL on the square of the kinetic mixing coefficient $\epsilon$ in the minimal model of a dark photon from the CMS search in the mass ranges 1.1--2.6 to 4.2--7.9 GeV (pink). The CMS limits are compared with the existing limits at 90% CL provided by the LHCb experiment [14] (blue) and BaBar experiment [12] (gray).
png pdf	Figure 6: Observed upper limits at 90% CL on the mixing angle $\theta_{\rm H}$ for the 2HDM+S scenario from the CMS search in the mass ranges 1.1-2.6 to 4.2-7.9 GeV (pink). The CMS limits are compared with the existing limits at 90% CL provided by the LHCb experiment [42] (blue) and BaBar experiment [12] (gray).

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

Summary

In summary, we present a search for a prompt narrow resonance decaying to a pair of muons using proton-proton collision data recorded by the CMS experiment at

$\sqrt{s}=$ 13 TeV in 2017 and 2018. The search is performed in the dimuon mass region between 1.1-2.6 GeV and 4.2-7.9 GeV using data collected with high-rate dimuon triggers in a dedicated dimuon scouting stream, corresponding to an integrated luminosity of 96.6 fb

$^{-1}$ . Compared with the previous prompt resonance search for larger resonance masses [15], a dedicated multivariate analysis method is used to identify muons to achieve a higher sensitivity. No significant excess of events above the expectation from the standard model background is observed. Model-independent limits on production rates of dimuon resonances within the experimental fiducial acceptance are set. Competitive limits have been set both in the minimal dark photon and two Higgs doublet plus scalar models. The squared kinetic mixing coefficient

$\epsilon^2$ in the dark photon model above 10

$^{-6}$ is mostly excluded in the mass range of the search. In the two Higgs doublet plus scalar model, the mixing angle

$\sin(\theta_{\rm H})$ above 0.08 is mostly excluded in the mass range of the search with fixed

$\tan\beta=$ 0.5.

Additional Figures
png pdf	Additional Figure 1: Upper limits at 90% CL on the square of the kinetic mixing coefficient $\epsilon$ in the minimal model of a dark photon from the CMS search in the mass ranges 1.1-2.6 to 4.2-7.9 GeV using 2017 and 2018 scouting data which corresponds to an integrated luminosity of 96.6 fb $^{-1}$ . The theoretical uncertainty includes the variation of QCD scales when calculating the production cross section, as well as the variance in fiducial acceptance between dark photon signal events produced using two different generators; DYTurbo-1.2 and MADGRAPH5_aMC@NLO v3.4.1.
png pdf	Additional Figure 2: Upper limits at 90% CL on the mixing angle $\theta_{\rm H}$ for the 2HDM+S scenario the CMS search in the mass ranges 1.1-2.6 to 4.2-7.9 GeV using 2017 and 2018 scouting data which corresponds to an integrated luminosity of 96.6 fb $^{-1}$ . The theoretical uncertainty includes the variation of QCD scales when calculating the production cross section, as well as the variance in fiducial acceptance between scalar signal events produced using two different generators; PYTHIA 8.230 and MADGRAPH5_aMC@NLO v3.4.1.
png pdf	Additional Figure 3: Background only and signal plus background fits, for the mass window 2.25-2.56 GeV with 3.24 $\sigma$ excess in 2017 (top) and 2018 (bottom). This excess is observed only in the mass distribution with the dimuon high- $p_{\mathrm{T}}$ selection and therefore only affects the limit on the scalar model. The lower pads show the difference between the data and corresponding pdf, divided by the statistical uncertainty of the data in that bin.
png pdf	Additional Figure 3-a: Background only and signal plus background fits, for the mass window 2.25-2.56 GeV with 3.24 $\sigma$ excess in 2017 (top) and 2018 (bottom). This excess is observed only in the mass distribution with the dimuon high- $p_{\mathrm{T}}$ selection and therefore only affects the limit on the scalar model. The lower pads show the difference between the data and corresponding pdf, divided by the statistical uncertainty of the data in that bin.
png pdf	Additional Figure 3-b: Background only and signal plus background fits, for the mass window 2.25-2.56 GeV with 3.24 $\sigma$ excess in 2017 (top) and 2018 (bottom). This excess is observed only in the mass distribution with the dimuon high- $p_{\mathrm{T}}$ selection and therefore only affects the limit on the scalar model. The lower pads show the difference between the data and corresponding pdf, divided by the statistical uncertainty of the data in that bin.
png pdf	Additional Figure 4: The combined efficiency of the dimuon scouting trigger and the MVA muon selection, averaged between 2017 and 2018, weighted by the integrated luminosity of each year. The solid line shows the efficiency of the inclusive selection used for the limit on the dark photon model. The dashed line shows the efficiency of the boosted selection optimized for the scalar model.
png pdf	Additional Figure 5: Theory cross-section times branching fraction to muons times acceptance for the dark photon and 2HDM+S models. The dark photon theory cross section is calculated using MADGRAPH5_aMC@NLO v3.4.1 assuming $\epsilon =$ 0.02, and the acceptance is derived using DYTurbo-1.2. The 2HDM+S model theory cross section is calculated using HIGLU at NNLO assuming $\sin(\theta_{\rm H}) =$ 1, and the acceptance is derived from PYTHIA 8.230.
png pdf	Additional Figure 6: The contribution of $\mathrm{D}\to \mathrm{K}\mathrm{K}$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to \mathrm{K}\mathrm{K}$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 6-a: The contribution of $\mathrm{D}\to \mathrm{K}\mathrm{K}$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to \mathrm{K}\mathrm{K}$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 6-b: The contribution of $\mathrm{D}\to \mathrm{K}\mathrm{K}$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to \mathrm{K}\mathrm{K}$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 7: The contribution of $\mathrm{D}\to\mathrm{K}\pi$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to\mathrm{K}\pi$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 7-a: The contribution of $\mathrm{D}\to\mathrm{K}\pi$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to\mathrm{K}\pi$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 7-b: The contribution of $\mathrm{D}\to\mathrm{K}\pi$ in the simultaneous fit in signal region (left) and control regions (right) in 2018 data for the inclusive dimuon selection. The background contains non-peaking combinatorial background and $\mathrm{D}\to\mathrm{K}\pi$ background. In the bottom panel, the combinatorial background component in the signal plus background fit is subtracted from the observed data (``Data - Comb. Bkg.'').
png pdf	Additional Figure 8: The observed local p-value for the inclusive dimuon selection (left) and high- $p_{\mathrm{T}}$ selection (right). The mass region dominated by the J $/\psi$ and $\psi$ (2S) resonances is excluded from the search.
png pdf	Additional Figure 8-a: The observed local p-value for the inclusive dimuon selection (left) and high- $p_{\mathrm{T}}$ selection (right). The mass region dominated by the J $/\psi$ and $\psi$ (2S) resonances is excluded from the search.
png pdf	Additional Figure 8-b: The observed local p-value for the inclusive dimuon selection (left) and high- $p_{\mathrm{T}}$ selection (right). The mass region dominated by the J $/\psi$ and $\psi$ (2S) resonances is excluded from the search.
png pdf	Additional Figure 9: The ROC curves of J $/\psi$ -trained and $\Upsilon$ -trained muon MVA identification in comparison to the cut-based identification used in the previous CMS dark photon search.

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