Search for a Narrow Resonance Lighter than 200 GeV Decaying to a Pair of Muons in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

A search is presented for a narrow resonance decaying to a pair of oppositely charged muons using $\sqrt{s}=13\mathrm{TeV}$ 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 and $96.6{\mathrm{fb}}^{-1}$ 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.

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 $Z_D$ 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.

Figure 2

Expected and observed upper limits at 95% C.L. 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.

Figure 3

Expected and observed upper limits at 90% C.L. on ${\epsilon }^2$, the square of the kinetic mixing coefficient, as a function of the $Z_D$ mass. Results obtained using the scouting (standard) triggers are to the left (right) of the vertical purple line. Limits at 90% C.L. from the search performed by the LHCb Collaboration [16] are shown in red, and constraints at 95% C.L. from the measurements of the electroweak observables are shown in light blue [9].