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CMS-HIG-18-003 ; CERN-EP-2018-288
A search for pair production of new light bosons decaying into muons in proton-proton collisions at 13 TeV
Phys. Lett. B 796 (2019) 131
Abstract: A search for new light bosons decaying into muon pairs is presented using a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy $\sqrt{s} = $ 13 TeV, collected with the CMS detector at the CERN LHC. The search is model independent, only requiring the pair production of a new light boson and its subsequent decay to a pair of muons. No significant deviation from the predicted background is observed. A model independent limit is set on the product of the production cross section times branching fraction to dimuons squared times acceptance as a function of new light boson mass. This limit varies between 0.15 and 0.39 fb over a range of new light boson masses from 0.25 to 8.5 GeV. It is then interpreted in the context of the next-to-minimal supersymmetric standard model and a dark supersymmetry model that allows for nonnegligible light boson lifetimes. In both cases, there is significant improvement over previously published limits.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Left: Distribution of the invariant masses $ {m_{({\mu} {\mu})_1}} $ vs. $ {m_{({\mu} {\mu})_2}} $ of the isolated dimuon systems; triangles represent data events passing all the selection criteria and falling in the SR $ {m_{({\mu} {\mu})_1}} \simeq {m_{({\mu} {\mu})_2}} $ (outlined by dashed lines); white bullets represent data events that pass all selection criteria but fall outside the SR. Right: The 95% CL upper limit set on ${\sigma ({\mathrm {p}} {\mathrm {p}}\to 2 {\mathrm {a}} + {\mathrm {X}})\mathcal {B}^2({\mathrm {a}} \to 2 {\mu}) {\alpha _\text {gen}}}$ over the range 0.25 $ < {m_{{\mathrm {a}}}} < $ 8.5 GeV.

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Figure 1-a:
Distribution of the invariant masses $ {m_{({\mu} {\mu})_1}} $ vs. $ {m_{({\mu} {\mu})_2}} $ of the isolated dimuon systems; triangles represent data events passing all the selection criteria and falling in the SR $ {m_{({\mu} {\mu})_1}} \simeq {m_{({\mu} {\mu})_2}} $ (outlined by dashed lines); white bullets represent data events that pass all selection criteria but fall outside the SR.

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Figure 1-b:
The 95% CL upper limit set on ${\sigma ({\mathrm {p}} {\mathrm {p}}\to 2 {\mathrm {a}} + {\mathrm {X}})\mathcal {B}^2({\mathrm {a}} \to 2 {\mu}) {\alpha _\text {gen}}}$ over the range 0.25 $ < {m_{{\mathrm {a}}}} < $ 8.5 GeV.

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Figure 2:
Left: The limits are compared to a representative model (solid curve) obtained using the simplified scenario described in the text. The figure is separated into two regions: $ {m_{{\mathrm {h}} _{i}}} = {m_{{\mathrm {h}} _{1}}} < $ 125 GeV with $ {m_{{\mathrm {h}} _{2}}} = $ 125 GeV, and $ {m_{{\mathrm {h}} _{1}}} = $ 125 GeV with $ {m_{{\mathrm {h}} _{i}}} = {m_{{\mathrm {h}} _{2}}} > $ 125 GeV. Right: These limits are compared to a representative model (solid curve) from the simplified scenario described in the text. The simplified scenario includes gg-fusion, VBF, and VH production modes.

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Figure 2-a:
The limits are compared to a representative model (solid curve) obtained using the simplified scenario described in the text. The figure is separated into two regions: $ {m_{{\mathrm {h}} _{i}}} = {m_{{\mathrm {h}} _{1}}} < $ 125 GeV with $ {m_{{\mathrm {h}} _{2}}} = $ 125 GeV, and $ {m_{{\mathrm {h}} _{1}}} = $ 125 GeV with $ {m_{{\mathrm {h}} _{i}}} = {m_{{\mathrm {h}} _{2}}} > $ 125 GeV.

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Figure 2-b:
These limits are compared to a representative model (solid curve) from the simplified scenario described in the text. The simplified scenario includes gg-fusion, VBF, and VH production modes.

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Figure 3:
The 90% CL upper limits (black solid curves) from this search as interpreted in the dark SUSY scenario, where the process is $ {\mathrm {p}} {\mathrm {p}}\to {\mathrm {h}} \to 2 {\text {n}_{1}} \to 2 {{{\gamma}}_{\mathrm {D}}} +2 {\text {n}_{\mathrm {D}}} \to 4 {\mu} + {\mathrm {X}} $, with $ {m_{{\text {n}}_{1}}} = $ 10 GeV, and $ {m_{{\text {n}}_{\mathrm {D}}}} = $ 1 GeV. The limits are presented in the plane of the parameters ($\varepsilon $ and ${m_{{{\gamma}}_{\mathrm {D}}}}$). Constraints from other experiments [22,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82] showing their 90% CL exclusion contours are also presented. The colored contours for the CMS and ATLAS limits represent different values of $ {\mathcal {B}({\mathrm {h}} \to 2 {{{\gamma}}_{\mathrm {D}}} + {\mathrm {X}})} $ that range from 0.1 to 40%.
Tables

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Table 1:
The full reconstruction efficiency over signal acceptance $ {{\epsilon _{\text {full}}} / {\alpha _\text {gen}}} $ in% for several representative signal NMSSM (upper) and dark SUSY benchmark models (lower). All uncertainties are statistical.
Summary
A search for pairs of new light bosons that subsequently decay to pairs of oppositely charged muons is presented. This search is developed in the context of a Higgs boson decay, $\mathrm{h}\to2\mathrm{a}+\mathrm{X}\to4\mu+\mathrm{X}$ and is performed on a data sample collected by the Compact Muon Solenoid experiment in 2016 that corresponds to an integrated luminosity of 35.9 fb$^{-1}$ proton-proton collisions at 13 TeV. This data set is larger and collected at a higher center-of-mass energy than the previous CMS search [15]. Additionally, both the mass range of the light boson a and the maximum possible displacement of its decay vertex are extended compared to the previous version of this analysis. Nine events are observed in the signal region (SR), with 7.95 $\pm$ 1.12 (stat) $\pm$ 1.45 (syst) events expected from the standard model (SM) backgrounds. The distribution of events in the SR is consistent with SM expectations. A model independent 95% confidence level upper limit on the product of the production cross section times branching fraction to dimuons squared times acceptance is set over the mass range 0.25 $ < {m_{\mathrm{a}}} < $ 8.5 GeV and is found to vary between 0.15 and 0.39 fb. This model independent limit is then interpreted in the context of dark supersymmetry (dark SUSY) with nonnegligible light boson lifetimes of up to ${c\tau_{{\gamma}_{\mathrm{D}}}} = $ 100 mm and in the context of the next-to-minimal supersymmetric standard model (NMSSM). For the dark SUSY interpretation, the upper bound of ${m_{{\gamma}_{\mathrm{D}}}} $ was increased from 2 to 8.5 GeV and the excluded $\varepsilon$ was improved by a factor of approximately 2.5. In the NMSSM, the 95% CL upper limit was improved by a factor of $\approx$ 1.5 (3) for ${m_{\mathrm{a}_{1}}} = $ 3.55 (0.25) GeV over previously published limits.
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