Search for dark photons produced in 13 TeV $pp$ collisions

TL;DR

The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature and the constraints placed on promptlike dark photons are the most stringent to date for the mass range 10.6<m(A^{'})<70  GeV, and are comparable to the best existing limits.

Abstract

Searches are performed for both prompt-like and long-lived dark photons, $A^{\prime}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{\prime}\toμ^+μ^-$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the LHCb detector. The prompt-like $A^{\prime}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{\prime}$ search is restricted to the low-mass region $214<m(A^{\prime})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $γ$-$A^{\prime}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6 < m(A^{\prime}) < 70$ GeV, and are comparable to the best existing limits for $m(A^{\prime}) < 0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.

Figures (9)

• Figure 1: Prompt-like mass spectrum, where the categorization of the data as prompt $\mu^+\mu^-$, $\mu_Q\mu_Q$, and $hh+h\mu_Q$ is determined using the fits described in the text.
• Figure 2: Regions of the $[m(A^{\prime}\xspace)\xspace,\varepsilon^2]$ parameter space excluded at 90% CL by the prompt-like $A^{\prime}$ search compared to the best existing limits Lees:2014xhaKLOE:2016lwm.
• Figure 3: Ratio of the observed upper limit on $n_{\rm ob}^{A^{\prime}\xspace}[m(A^{\prime}\xspace)\xspace,\varepsilon^2]$ at 90% CL to its expected value, where regions less than unity are excluded. There are no constraints from previous experiments in this region.
• Figure 4: Three-dimensional distribution of $\chi^2_{\rm DF}$ versus $t$ versus $m(\mu^+\mu^-)$, which is fit to determine the long-lived signal yields. The data are consistent with being predominantly due to $b$-hadron decays at small $t$, and due to $K^0_{\mathrm{ S}}$ decays for large $t$ and $m(\mu^+\mu^-)\xspace \gtrsim 280\mathrm{\,Me V}\xspace$. The largest signal-like excess occurs at $m(A^{\prime}\xspace)\xspace = 239\mathrm{\,Me V}\xspace$ and $\tau(A^{\prime}\xspace)\xspace = 0.86\hbox{,ps}\xspace$.
• Figure 5: Efficiency ratio $\epsilon_{{}^{\gamma^*}}^{{}_{A^{\prime}\xspace}}[m(A^{\prime}\xspace)\xspace,\tau(A^{\prime}\xspace)\xspace]$ for long-lived dark photons, integrated over decay time. The sharp decrease at larger values of $\varepsilon^2$ is due to the stringent ${\rm min}[\chi^2_{\mathrm{IP}}(\mu^{\pm})]$ criterion applied in the 2016 trigger.