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CMS-EXO-19-018 ; CERN-EP-2019-265
Search for a narrow resonance lighter than 200 GeV decaying to a pair of muons in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Rev. Lett. 124 (2020) 131802
Abstract: A search is presented for a narrow resonance decaying to a pair of oppositely charged muons using $\sqrt{s} = $ 13 TeV proton-proton collision data recorded at the LHC. In the 45-75 and 110-200 GeV resonance mass ranges, the search is based on conventional triggering and event reconstruction techniques. In the 11.5-45 GeV mass range, the search uses data collected with dimuon triggers with low transverse momentum thresholds, recorded at high rate by storing a reduced amount of trigger-level information. The data correspond to integrated luminosities of 137 fb$^{-1}$ and 96.6 fb$^{-1}$ for conventional and high-rate triggering, respectively. No significant resonant peaks are observed in the probed mass ranges. The search sets the most stringent constraints to date on a dark photon in the $\sim$ 30-75 and 110-200 GeV mass ranges.
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Figure 1:
The $m_{\mu\mu}$ distributions of events selected with the standard muon triggers (maroon, darker), and the scouting dimuon triggers (green, lighter). Events are required to pass all the selection requirements. The inset is restricted to events in the barrel category in the mass range 23.9-26.1 GeV. A function describing the background is fit to these data, and a 25 GeV $\mathrm{Z} _{\mathrm {D}}$ signal corresponding to $\epsilon ^{2} = 2 \times 10^{-5}$ is added. The bottom panel of the inset shows the bin-by-bin difference between the number of events in data and the prediction from the background fit, divided by the statistical uncertainty.

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Figure 2:
Expected and observed upper limits at 95% CL on the product of the signal cross section ($\sigma $) for a narrow resonance, branching fraction to a pair of muons ($\mathcal {B}$), and acceptance (A) as a function of the mass of a narrow resonance. Results obtained using the scouting (standard) triggers are to the left (right) of the vertical purple line.

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Figure 3:
Expected and observed upper limits at 90% CL on $\epsilon ^{2}$, the square of the kinetic mixing coefficient, as a function of the $\mathrm{Z} _{\mathrm {D}}$ mass. Results obtained using the scouting (standard) triggers are to the left (right) of the vertical purple line. Limits at 90% CL from the search performed by the LHCb Collaboration [16] are shown in red, and constraints at 95% CL from the measurements of the electroweak observables are shown in light blue [9].
In summary, a search has been presented for a narrow resonance decaying to a pair of muons using proton-proton collision data recorded by the CMS experiment at $\sqrt{s}=$ 13 TeV. The search in the 45--75 and 110--200 GeV resonance mass ranges uses fully reconstructed data containing a pair of muons with transverse momenta greater than 20 and 10 GeV, corresponding to an integrated luminosity of 137 fb$^{-1}$. The search in the resonance mass range of 11.5--45.0 GeV is performed using data collected with high-rate dimuon triggers, corresponding to an integrated luminosity of 96.6 fb$^{-1}$. This is the first search that uses data with reduced trigger-level muon information, collected with dimuon triggers that have transverse momentum thresholds of 3 GeV. The data are found to be consistent with the background prediction. The search sets the lowest upper limits to date on the kinetic mixing coefficient of a dark photon in the $\sim$30--75 and 110--200 GeV mass ranges.
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