CMSEXO22016 ; CERNEP2023122  
Search for a highmass dimuon resonance produced in association with b quark jets at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
14 July 2023  
JHEP 10 (2023) 043  
Abstract: A search for highmass dimuon resonance production in association with one or more b quark jets is presented. The study uses protonproton collision data collected with the CMS detector at the LHC corresponding to an integrated luminosity of 138 fb$^{1}$ at a centerofmass energy of 13 TeV. Modelindependent limits are derived on the number of signal events with exactly one or more than one b quark jet. Results are also interpreted in a leptonflavoruniversal model with Z' boson couplings to a $ \mathrm{b}\mathrm{b} $ quark pair ($ g_{\mathrm{b}} $), an $ \mathrm{s}\mathrm{b} $ quark pair ($ g_{\mathrm{b}}\delta_{\mathrm{b}\mathrm{s}} $), and any sameflavor charged lepton ($ g_{\ell} $) or neutrino pair ($ g_{\nu} $), with $ g_{\nu}=g_{\ell} $. For a Z' boson with a mass $ m_{\mathrm{Z}' }= $ 350 GeV (2 TeV) and $ \delta_{\mathrm{b}\mathrm{s}} < $ 0.25, the majority of the parameter space with 0.0057 $ < g_{\ell} < $ 0.35 (0.25 $ < g_{\ell} < $ 0.43) and 0.0079 $ < g_{\mathrm{b}} < $ 0.46 (0.34 $ < g_{\mathrm{b}} < $ 0.57) is excluded at 95% confidence level. Finally, constraints are set on a Z' model with parameters consistent with lowenergy $ \mathrm{b}\to\mathrm{s}\ell\ell $ measurements. In this scenario, most of the allowed parameter space is excluded for a Z' boson with 350 $ < m_{\mathrm{Z}' } < $ 500 GeV, while the constraints are less stringent for higher $ m_{\mathrm{Z}' } $ hypotheses. This is the first dedicated search at the LHC for a highmass dimuon resonance produced in association with multiple b quark jets, and the constraints obtained on models with this signature are the most stringent to date.  
Links: eprint arXiv:2307.08708 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures  
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Figure 1:
Feynman diagrams of $ \mathrm{Z}' \to \mu^{} \mu^{+} $ with a Z' boson produced via $ \mathrm{b} \overline{\mathrm{b}} \to \mathrm{Z}' $ or $ \mathrm{s} \overline{\mathrm{b}} \to \mathrm{Z}' $, with at least one b quark in the final state. While a $ \mathrm{Z}' \mathrm{b}\mathrm{b} $ coupling may be present in any generic model, a $ \mathrm{Z}' \mathrm{s}\mathrm{b} $ coupling could arise through flavor mixing between the second and thirdgeneration quarks. 
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Figure 1a:
Feynman diagram of $ \mathrm{Z}' \to \mu^{} \mu^{+} $ with a Z' boson produced via $ \mathrm{b} \overline{\mathrm{b}} \to \mathrm{Z}' $, with one b quark in the final state. 
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Figure 1b:
Feynman diagram of $ \mathrm{Z}' \to \mu^{} \mu^{+} $ with a Z' boson produced via $ \mathrm{s} \overline{\mathrm{b}} \to \mathrm{Z}' $, with one b quark in the final state. While a $ \mathrm{Z}' \mathrm{b}\mathrm{b} $ coupling may be present in any generic model, a $ \mathrm{Z}' \mathrm{s}\mathrm{b} $ coupling could arise through flavor mixing between the second and thirdgeneration quarks. 
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Figure 1c:
Feynman diagram of $ \mathrm{Z}' \to \mu^{} \mu^{+} $ with a Z' boson produced via $ \mathrm{b} \overline{\mathrm{b}} \to \mathrm{Z}' $, with two b quark in the final state. 
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Figure 1d:
Feynman diagram of $ \mathrm{Z}' \to \mu^{} \mu^{+} $ with a Z' boson produced via $ \mathrm{s} \overline{\mathrm{b}} \to \mathrm{Z}' $, with one b quark in the final state. While a $ \mathrm{Z}' \mathrm{b}\mathrm{b} $ coupling may be present in any generic model, a $ \mathrm{Z}' \mathrm{s}\mathrm{b} $ coupling could arise through flavor mixing between the second and thirdgeneration quarks. 
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Figure 2:
Distribution of $ \min(m_{\mu\mathrm{b}\!}) $ as obtained from simulation in events with $ N_{\mathrm{b}}\geq $ 1 passing all the other selection requirements. In this search, we require $ \min(m_{\mu\mathrm{b}\!}) > $ 175 GeV. The stacked histogram displays the expected distribution from the simulation of the SM backgrounds, while the overlaid open histograms illustrate the size and shape of the Z' contribution from the LFU model described in Eq. (1), for several Z' mass hypotheses. For illustrative purposes, we choose couplings $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0. The contribution of background processes other than DY and $ \mathrm{t} \overline{\mathrm{t}} $ is so small that it is only barely visible at the bottom of the stacked histogram. The hatched region indicates the statistical uncertainty arising from the limited size of the SM simulated samples (Section 3). Histograms are normalized to unit area. 
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Figure 3:
Distributions of $ m_{\mu\mu} $ in the $ N_{\mathrm{b}}= $ 1 (left) and $ N_{\mathrm{b}}\geq $ 2 (right) event categories. The stacked histogram displays the expected distribution from the SM background simulation. The overlaid open distributions illustrate the Z' contribution from the LFU model at $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0 for a variety of Z' mass hypotheses. The observed data are shown as black points with statistical error bars. The hatched region indicates the statistical uncertainty arising from the limited size of the SM simulated samples. The size of the bins increases as a function of $ m_{\mu\mu} $. In extracting the results of the search, the background is estimated directly from data, so the SM background simulation is only illustrative in these distributions. 
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Figure 3a:
Distribution of $ m_{\mu\mu} $ in the $ N_{\mathrm{b}}= $ 1 event category. The stacked histogram displays the expected distribution from the SM background simulation. The overlaid open distributions illustrate the Z' contribution from the LFU model at $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0 for a variety of Z' mass hypotheses. The observed data are shown as black points with statistical error bars. The hatched region indicates the statistical uncertainty arising from the limited size of the SM simulated samples. The size of the bins increases as a function of $ m_{\mu\mu} $. In extracting the results of the search, the background is estimated directly from data, so the SM background simulation is only illustrative in these distributions. 
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Figure 3b:
Distribution of $ m_{\mu\mu} $ in the $ N_{\mathrm{b}}\geq $ 2 event category. The stacked histogram displays the expected distribution from the SM background simulation. The overlaid open distributions illustrate the Z' contribution from the LFU model at $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0 for a variety of Z' mass hypotheses. The observed data are shown as black points with statistical error bars. The hatched region indicates the statistical uncertainty arising from the limited size of the SM simulated samples. The size of the bins increases as a function of $ m_{\mu\mu} $. In extracting the results of the search, the background is estimated directly from data, so the SM background simulation is only illustrative in these distributions. 
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Figure 4:
Invariant mass $ m_{\mu\mu} $ distributions in the $ N_{\mathrm{b}}= $ 1 (left) and $ N_{\mathrm{b}}\geq $ 2 (right) categories, shown together with the corresponding selected background functional forms used as input to the \textitdiscrete profiling method [49] when probing the $ m_{\mathrm{Z}' }= $ 500 GeV hypothesis. The expected signal distribution for the LFU model described in Eq. (1), with couplings $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0, is overlaid. The displayed mass range corresponds to the fit window used for this $ m_{\mathrm{Z}' } $ hypothesis, which is $ \pm $10$\sigma_{\text{mass}} $ around the probed $ m_{\mathrm{Z}' } $ value. While the likelihood fits are performed on unbinned data, here we present the data in binned histograms with binning chosen to reflect the size of $ \sigma_{\text{mass}} $. 
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Figure 4a:
Invariant mass $ m_{\mu\mu} $ distributions in the $ N_{\mathrm{b}}= $ 1 category, shown together with the corresponding selected background functional forms used as input to the discrete profiling method [49] when probing the $ m_{\mathrm{Z}' }= $ 500 GeV hypothesis. The expected signal distribution for the LFU model described in Eq. (1), with couplings $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0, is overlaid. The displayed mass range corresponds to the fit window used for this $ m_{\mathrm{Z}' } $ hypothesis, which is $ \pm $ 10$\sigma_{\text{mass}} $ around the probed $ m_{\mathrm{Z}' } $ value. While the likelihood fits are performed on unbinned data, here we present the data in binned histograms with binning chosen to reflect the size of $ \sigma_{\text{mass}} $. 
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Figure 4b:
Invariant mass $ m_{\mu\mu} $ distributions in the $ N_{\mathrm{b}}\geq $ 2 category, shown together with the corresponding selected background functional forms used as input to the discrete profiling method [49] when probing the $ m_{\mathrm{Z}' }= $ 500 GeV hypothesis. The expected signal distribution for the LFU model described in Eq. (1), with couplings $ g_{\ell} = g_{\nu} = g_{\mathrm{b}} = $ 0.03 and $ \delta_{\mathrm{b}\mathrm{s}}= $ 0, is overlaid. The displayed mass range corresponds to the fit window used for this $ m_{\mathrm{Z}' } $ hypothesis, which is $ \pm $ 10$\sigma_{\text{mass}} $ around the probed $ m_{\mathrm{Z}' } $ value. While the likelihood fits are performed on unbinned data, here we present the data in binned histograms with binning chosen to reflect the size of $ \sigma_{\text{mass}} $. 
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Figure 5:
Exclusion limits at 95% CL on the number of selected BSM events with $ N_{\mathrm{b}} \geq $ 1 as functions of $ m_{\mathrm{Z}' } $ for the different representative values of $ f_{2\mathrm{b}}= $ 0 (upper left), 0.25 (upper right), 0.75 (lower left), and 1 (lower right). The quantity $ f_{2\mathrm{b}} $ is the fraction of BSM events passing the analysis selection that have at least two b quark jets. The solid black (dashed red) curve represents the observed (median expected) exclusion. The inner green (outer yellow) band indicates the region containing 68 (95)% of the distribution of limits expected under the backgroundonly hypothesis. 
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Figure 5a:
Exclusion limits at 95% CL on the number of selected BSM events with $ N_{\mathrm{b}} \geq $ 1 as functions of $ m_{\mathrm{Z}' } $ for the different representative values of $ f_{2\mathrm{b}}= $ 0. The quantity $ f_{2\mathrm{b}} $ is the fraction of BSM events passing the analysis selection that have at least two b quark jets. The solid black (dashed red) curve represents the observed (median expected) exclusion. The inner green (outer yellow) band indicates the region containing 68 (95)% of the distribution of limits expected under the backgroundonly hypothesis. 
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Figure 5b:
Exclusion limits at 95% CL on the number of selected BSM events with $ N_{\mathrm{b}} \geq $ 1 as functions of $ m_{\mathrm{Z}' } $ for the different representative values of $ f_{2\mathrm{b}}= $ 0.25. The quantity $ f_{2\mathrm{b}} $ is the fraction of BSM events passing the analysis selection that have at least two b quark jets. The solid black (dashed red) curve represents the observed (median expected) exclusion. The inner green (outer yellow) band indicates the region containing 68 (95)% of the distribution of limits expected under the backgroundonly hypothesis. 
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Figure 5c:
Exclusion limits at 95% CL on the number of selected BSM events with $ N_{\mathrm{b}} \geq $ 1 as functions of $ m_{\mathrm{Z}' } $ for the different representative values of $ f_{2\mathrm{b}}= $ 0.75. The quantity $ f_{2\mathrm{b}} $ is the fraction of BSM events passing the analysis selection that have at least two b quark jets. The solid black (dashed red) curve represents the observed (median expected) exclusion. The inner green (outer yellow) band indicates the region containing 68 (95)% of the distribution of limits expected under the backgroundonly hypothesis. 
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Figure 5d:
Exclusion limits at 95% CL on the number of selected BSM events with $ N_{\mathrm{b}} \geq $ 1 as functions of $ m_{\mathrm{Z}' } $ for the different representative values of $ f_{2\mathrm{b}}= $ 1. The quantity $ f_{2\mathrm{b}} $ is the fraction of BSM events passing the analysis selection that have at least two b quark jets. The solid black (dashed red) curve represents the observed (median expected) exclusion. The inner green (outer yellow) band indicates the region containing 68 (95)% of the distribution of limits expected under the backgroundonly hypothesis. 
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Figure 6:
Observed (solid) and median expected (dashed) exclusion limits at 95% CL in the $ g_{\mathrm{b}} $$ g_{\ell} $ plane for the LFU model. The scenarios considered have $ \delta_{\mathrm{b}\mathrm{s}} $ values of either 0 (left) or 0.25 (right). In all cases, we assume $ g_{\nu} = g_{\ell} $. The exclusion limits are given up to coupling values at which the Z' width is equal to half of the $ \mu\mu $ invariant mass resolution, marked by the dotted curves. Beyond these coupling values, the narrow width approximation intrinsic to the search strategy is not considered valid. The enclosed regions are excluded. 
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Figure 6a:
Observed (solid) and median expected (dashed) exclusion limits at 95% CL in the $ g_{\mathrm{b}} $$ g_{\ell} $ plane for the LFU model. The considered scenario has $ \delta_{\mathrm{b}\mathrm{s}} = $ 0. We assume $ g_{\nu} = g_{\ell} $. The exclusion limits are given up to coupling values at which the Z' width is equal to half of the $ \mu\mu $ invariant mass resolution, marked by the dotted curves. Beyond these coupling values, the narrow width approximation intrinsic to the search strategy is not considered valid. The enclosed regions are excluded. 
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Figure 6b:
Observed (solid) and median expected (dashed) exclusion limits at 95% CL in the $ g_{\mathrm{b}} $$ g_{\ell} $ plane for the LFU model. The considered scenario has $ \delta_{\mathrm{b}\mathrm{s}} = $ 0.25. We assume $ g_{\nu} = g_{\ell} $. The exclusion limits are given up to coupling values at which the Z' width is equal to half of the $ \mu\mu $ invariant mass resolution, marked by the dotted curves. Beyond these coupling values, the narrow width approximation intrinsic to the search strategy is not considered valid. The enclosed regions are excluded. 
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Figure 7:
Exclusion limits at 95% CL in the $ \theta_{23} $$ g_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9], for representative values of $ m_{\mathrm{Z}' }= $ 500 GeV (upper left), $ m_{\mathrm{Z}' }= $ 1 TeV (upper right), $ m_{\mathrm{Z}' }= $ 1.5 TeV (lower left), and $ m_{\mathrm{Z}' }= $ 2 TeV (lower right). The solid black (dashed red) curves represent the observed (median expected) exclusions. The dotted curves denote the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. For a given mass, the region enclosed between the solid black (dashed red) and dotted curves is (expected to be) excluded. The dotted curve for $ m_{\mathrm{Z}' } = $ 500 GeV lies beyond the displayed $ g_{\mathrm{Z}' } $ range and is, therefore, not shown. The shaded blue area represents the region preferred from the global fit in Ref. [9] at 95% CL. The region above the green dashdotted curve is incompatible at 95% CL with the measurement of the mass difference between the mass eigenstates of the neutral $ \mathrm{B}_{s} $ mesons [12]. 
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Figure 7a:
Exclusion limits at 95% CL in the $ \theta_{23} $$ g_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9], for representative values of $ m_{\mathrm{Z}' }= $ 500 GeV. The solid black (dashed red) curves represent the observed (median expected) exclusions. The dotted curves denote the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. For a given mass, the region enclosed between the solid black (dashed red) and dotted curves is (expected to be) excluded. The dotted curve for $ m_{\mathrm{Z}' } = $ 500 GeV lies beyond the displayed $ g_{\mathrm{Z}' } $ range and is, therefore, not shown. The shaded blue area represents the region preferred from the global fit in Ref. [9] at 95% CL. The region above the green dashdotted curve is incompatible at 95% CL with the measurement of the mass difference between the mass eigenstates of the neutral $ \mathrm{B}_{s} $ mesons [12]. 
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Figure 7b:
Exclusion limits at 95% CL in the $ \theta_{23} $$ g_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9], for representative values of $ m_{\mathrm{Z}' }= $ 1 TeV. The solid black (dashed red) curves represent the observed (median expected) exclusions. The dotted curves denote the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. For a given mass, the region enclosed between the solid black (dashed red) and dotted curves is (expected to be) excluded. The shaded blue area represents the region preferred from the global fit in Ref. [9] at 95% CL. The region above the green dashdotted curve is incompatible at 95% CL with the measurement of the mass difference between the mass eigenstates of the neutral $ \mathrm{B}_{s} $ mesons [12]. 
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Figure 7c:
Exclusion limits at 95% CL in the $ \theta_{23} $$ g_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9], for representative values of $ m_{\mathrm{Z}' }= $ 1.5 TeV. The solid black (dashed red) curves represent the observed (median expected) exclusions. The dotted curves denote the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. For a given mass, the region enclosed between the solid black (dashed red) and dotted curves is (expected to be) excluded. The shaded blue area represents the region preferred from the global fit in Ref. [9] at 95% CL. The region above the green dashdotted curve is incompatible at 95% CL with the measurement of the mass difference between the mass eigenstates of the neutral $ \mathrm{B}_{s} $ mesons [12]. 
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Figure 7d:
Exclusion limits at 95% CL in the $ \theta_{23} $$ g_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9], for representative values of $ m_{\mathrm{Z}' }= $ 2 TeV. The solid black (dashed red) curves represent the observed (median expected) exclusions. The dotted curves denote the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. For a given mass, the region enclosed between the solid black (dashed red) and dotted curves is (expected to be) excluded. The shaded blue area represents the region preferred from the global fit in Ref. [9] at 95% CL. The region above the green dashdotted curve is incompatible at 95% CL with the measurement of the mass difference between the mass eigenstates of the neutral $ \mathrm{B}_{s} $ mesons [12]. 
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Figure 8:
Exclusion limits at 95% CL in the $ g_{\mathrm{Z}' } $$ m_{\mathrm{Z}' } $ plane for the $ B_{3}\!\!L_{2} $ model [9] for a fixed value of $ \theta_{23}= $ 0. The solid black (dashed red) curve represents the observed (median expected) exclusion. The dotted curve denotes the coupling values at which the Z' width equals one half of the $ \mu\mu $ invariant mass resolution. For larger values of the couplings, the narrow width approximation intrinsic to the search strategy is not considered valid. The region enclosed between the solid black (dashed red) and the dotted curves is (expected to be) excluded. The shaded blue area represents the $ g_{\mathrm{Z}' } $ range preferred from the global fit in Ref. [9] at 95% CL. 
Tables  
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Table 1:
Summary of signal uncertainties. The uncertainties are grouped based on whether they affect the normalization or the shape of the signal, and any variations for the two categories of $ N_{\mathrm{b}} $ are shown. The fit parameter $ \overline{m}_{\mu\mu} $ corresponds to the position of the maximum of the reconstructed $ m_{\mu\mu} $ distribution, and $ \overline{\sigma}_{\text{mass}} $ is the resolution parameter used in the fit, distinguished from the values of $ \sigma_{\text{mass}} $ extracted from simulation. 
Summary 
A search for highmass dimuon resonance production in association with one or more b quark jets has been presented, using data collected with the CMS experiment at the LHC that correspond to an integrated luminosity of 138 fb$^{1}$ at a centerofmass energy of 13 TeV. The limits are derived on the total number of signal events with $ N_{\mathrm{b}}= $ 1 and $ \geq $2, where $ N_{\mathrm{b}} $ denotes the multiplicity of b quark jets, and these limits are model independent to the extent that the signal arises from the direct production and subsequent decay of a narrow dimuon resonance. The relative fraction of events with $ N_{\mathrm{b}}\geq $ 2 is varied to probe a range of hypotheses of signal production in association with b quarks. The limits are presented as a function of the analyzed dimuon resonance mass values. Results are also interpreted in terms of a leptonflavoruniversal model that involves Z' boson couplings to b quarks ($ g_{\mathrm{b}} $) and muons, where the Z' boson couplings to all neutrinos ($ g_{\nu} $) and to all charged leptons ($ g_{\ell} $) are assumed to be equal, the $ g_{\mathrm{b}} $ coupling scales both $ \mathrm{Z}' \mathrm{b}\mathrm{b} $ and $ \mathrm{Z}' \mathrm{s}\mathrm{b} $ interactions, and the separate $ \delta_{\mathrm{b}\mathrm{s}} $ coupling solely scales the $ \mathrm{Z}' \mathrm{s}\mathrm{b} $ interaction. The exclusions in this model are presented in terms of the coupling strengths $ g_{\ell} $ and $ g_{\mathrm{b}} $, and the mass of the {\mathrm{Z}' } boson ($ m_{\mathrm{Z}' } $). For a Z' boson with $ m_{\mathrm{Z}' }= $ 350 GeV (2 TeV) and $ \delta_{\mathrm{b}\mathrm{s}} < $ 0.25, the majority of the parameter space with 0.0057 $ < g_{\ell} < $ 0.35 (0.25 $ < g_{\ell} < $ 0.43) and 0.0079 $ < g_{\mathrm{b}} < $ 0.46 (0.34 $ < g_{\mathrm{b}} < $ 0.57) is excluded at 95% confidence level. Constraints are also set on a specific Z' model ( $ B_3\!\!L_2 $), constructed to accommodate possible contributions to $ \mathrm{b} \to \mathrm{s} \ell^{} \ell^{+} $ transitions beyond the standard model. In this scenario, most of the allowed parameter space is excluded for a Z' boson with 350 $ < m_{\mathrm{Z}' } < $ 500 GeV, while the constraints are less stringent for higher $ m_{\mathrm{Z}' } $ hypotheses. This is the first dedicated search at the LHC for a highmass, narrow dimuon resonance produced in association with multiple b quark jets, and the constraints obtained on models with this signature are the most stringent to date. 
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