Search for $A'\!\to\!\mu^+\mu^-$ decays

TL;DR

This study searches for dark photons decaying into muon pairs in proton-proton collisions at 13 TeV, setting new limits on their properties and lifetimes, but finds no evidence of such particles.

Contribution

First to set stringent limits on dark photon production and decay properties in a broad mass range using LHCb data at 13 TeV.

Findings

No evidence for dark photon signals was observed.

Established the most restrictive limits on dark photon kinetic mixing for certain mass ranges.

Provided world-leading constraints on low-mass dark photons with short lifetimes.

Abstract

Searches are performed for both prompt-like and long-lived dark photons, $A'$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for $A'\!\to\!\mu^+\mu^-$ decays using a data sample corresponding to an integrated luminosity of 5.5/fb collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the $\gamma$-$A'$ kinetic-mixing strength. The prompt-like $A'$ search explores the mass region from near the dimuon threshold up to 70 GeV, and places the most stringent constraints to date on dark photons with $214 < m(A') \lesssim 740$ MeV and $10.6 < m(A') \lesssim 30$ GeV. The search for long-lived $A'\!\to\!\mu^+\mu^-$ decays places world-leading constraints on low-mass dark photons with lifetimes $\mathcal{O}(1)$ ps.