CMS logoCMS event Hgg
Compact Muon Solenoid
LHC, CERN

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
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.
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.
References
1 J. J. Aubert et al. Experimental observation of a heavy particle $ J $ PRL 33 (1974) 1404
2 J. E. Augustin et al. Discovery of a narrow resonance in $ {e}^{+}{e}^{-} $ annihilation PRL 33 (1974) 1406
3 S. W. Herb et al. Observation of a dimuon resonance at 9.5 GeV in 400-GeV proton-nucleus collisions PRL 39 (1977) 252
4 UA1 Collaboration Experimental observation of lepton pairs of invariant mass around 95 GeV/c$ ^2 $ at the CERN SPS collider PLB 126 (1983) 398
5 ATLAS Collaboration Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC PLB 716 (2012) 1 1207.7214
6 CMS Collaboration Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC PLB 716 (2012) 30 CMS-HIG-12-028
1207.7235
7 ATLAS Collaboration Search for high-mass dilepton resonances using 139 fb$ ^{-1} $ of $ pp $ collision data collected at $ \sqrt{s}= $13 TeV with the ATLAS detector PLB 796 (2019) 68 1903.06248
8 CMS Collaboration Search for resonant and nonresonant new phenomena in high-mass dilepton final states at $ \sqrt{s} = $ 13 TeV JHEP 07 (2021) CMS-EXO-19-019
2103.02708
9 CMS Collaboration Search for high mass dijet resonances with a new background prediction method in proton-proton collisions at $ \sqrt{s} = $ 13 TeV JHEP 05 (2020) 033 CMS-EXO-19-012
1911.03947
10 CMS Collaboration Search for physics beyond the standard model in high-mass diphoton events from proton-proton collisions at $ \sqrt{s} = $ 13 TeV PRD 98 (2018) 092001 CMS-EXO-17-017
1809.00327
11 N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer, and N. Weiner A theory of dark matter PRD 79 (2009) 0810.0713
12 BaBar Collaboration Search for a dark photon in $ e^+e^- $ collisions at BaBar PRL 113 (2014) 201801 1406.2980
13 LHCb Collaboration Search for dark photons produced in 13 TeV $ pp $ collisions PRL 120 (2018) 061801 1710.02867
14 LHCb Collaboration Search for $ A'\to\mu^+\mu^- $ decays PRL 124 (2020) 041801 1910.06926
15 CMS Collaboration Search for a narrow resonance lighter than 200 GeV decaying to a pair of muons in proton-proton collisions at $ \sqrt{s} = $ 13 TeV PRL 124 (2020) 131802 CMS-EXO-19-018
1912.04776
16 P. Galison and A. Manohar Two Z's or not two Z's? PLB 136 (1984) 279
17 B. Holdom Two U(1)'s and $ \epsilon $ charge shifts PLB 166 (1986) 196
18 D. Curtin, R. Essig, S. Gori, and J. Shelton Illuminating dark photons with high-energy colliders JHEP 02 (2015) 157 1412.0018
19 U. Haisch, J. F. Kamenik, A. Malinauskas, and M. Spira Collider constraints on light pseudoscalars JHEP 03 (2018)
20 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004
21 CMS Collaboration Search for narrow resonances in dijet final states at $ \sqrt{s}= $8 TeV with the novel CMS technique of data scouting PRL 117 (2016) 031802
22 CMS Collaboration Search for dijet resonances in proton-proton collisions at $ \sqrt{s} = $ 13 TeV and constraints on dark matter and other models Physics Letters B 769 (2017) 520
23 S. Mukherjee Data Scouting: A New Trigger Paradigm for the CMS Collaboration, in 5th Large Hadron Collider Physics Conference, 2017 1708.06925
24 T. Sjöstrand et al. An introduction to PYTHIA 8.2 Comput. Phys. Commun. 191 (2015) 159 1410.3012
25 S. Gopalakrishna, S. Jung, and J. D. Wells Higgs boson decays to four fermions through an abelian hidden sector PRD 78 (2008) 055002 0801.3456
26 D. Curtin et al. Exotic decays of the 125 GeV Higgs boson PRD 90 (2014) 075004
27 J. Alwall et al. The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations JHEP 07 (2014)
28 NNPDF Collaboration Parton distributions for the LHC Run II JHEP 04 (2015) 040 1410.8849
29 CMS Collaboration Event generator tunes obtained from underlying event and multiparton scattering measurements EPJC 76 (2016) 155 CMS-GEN-14-001
1512.00815
30 GEANT4 Collaboration GEANT4--a simulation toolkit NIM A 506 (2003) 250
31 B. P. Roe et al. Boosted decision trees, an alternative to artificial neural networks NIM A 543 (2005) 577 physics/0408124
32 CMS Collaboration Performance of CMS muon reconstruction in pp collision events at $ \sqrt{s} = $ 7 TeV JINST 7 (2012) P10002
33 J. E. Gaiser Charmonium Spectroscopy from Radiative Decays of the J/$ \Psi $ and $ \Psi^{\prime} $ PhD thesis, Stanford University, SLAC Report SLAC-R-255, 1982
link
34 S. Bernstein Démonstration du théor\ème de Weierstrass fondée sur le calcul des probabilitiés Comm. Soc. Math. Kharkov 13 (1912) 1
35 R. A. Fisher On the interpretation of $ \chi^{2} $ from contingency tables, and the calculation of P J. R. Stat. Soc. 85 (1922) 87
36 P. D. Dauncey, M. Kenzie, N. Wardle, and G. J. Davies Handling uncertainties in background shapes: the discrete profiling method JINST 10 (2015) P04015 1408.6865
37 CMS Collaboration CMS luminosity measurement for the 2017 data-taking period at $ \sqrt{s}= $ 13 TeV CMS Physics Analysis Summary, 2018
link
CMS-PAS-LUM-17-001
38 CMS Collaboration CMS luminosity measurement for the 2018 data-taking period at $ \sqrt{s}= $ 13 TeV CMS Physics Analysis Summary, 2019
link
CMS-PAS-LUM-18-002
39 A. L. Read Presentation of search results: The CL(s) technique JPG 28 (2002) 2693
40 G. Cowan, K. Cranmer, E. Gross, and O. Vitells Asymptotic formulae for likelihood-based tests of new physics EPJC 71 (2011) 1554 1007.1727
41 T. Junk Confidence level computation for combining searches with small statistics NIM A 434 (1999) 435 hep-ex/9902006
42 LHCb Collaboration Searches for low-mass dimuon resonances JHEP 10 (2020) 156 2007.03923
43 K. R. Dienes, C. F. Kolda, and J. March-Russell Kinetic mixing and the supersymmetric gauge hierarchy NPB 492 (1997) 104 hep-ph/9610479
44 S. Camarda et al. Dyturbo: Fast predictions for drell-yan processes EPJC 80 (2020) 251 1910.07049
45 P. Ilten, Y. Soreq, M. Williams, and W. Xue Serendipity in dark photon searches JHEP 06 (2018) 004 1801.04847
Compact Muon Solenoid
LHC, CERN