Inclusive Dark Photon Search at LHCb

TL;DR

Provides a fully data-driven inclusive search for dark photons A' decaying to μ+μ- at LHCb Run 3, using both prompt and displaced di-muon resonances. The signal rate is inferred from the off-shell γ* → μ+μ- background, enabling robust background control and sensitivity estimates. The study projects reach in the 210–520 MeV and 10–40 GeV mass ranges, outlines detailed selection criteria, and discusses extensions to Run 4/5 and other experiments, including SN bounds. Leveraging LHCb's excellent mass/vertex resolution and PID capabilities, the approach offers a simple, adaptable framework to probe previously unexplored regions of the m_A'–ε^2 parameter space.

Abstract

We propose an inclusive search for dark photons $A'$ at the LHCb experiment based on both prompt and displaced di-muon resonances. Because the couplings of the dark photon are inherited from the photon via kinetic mixing, the dark photon $A' \to μ^+ μ^-$ rate can be directly inferred from the off-shell photon $γ^* \to μ^+ μ^-$ rate, making this a fully data-driven search. For Run 3 of the LHC, we estimate that LHCb will have sensitivity to large regions of the unexplored dark-photon parameter space, especially in the 210-520 MeV and 10-40 GeV mass ranges. This search leverages the excellent invariant-mass and vertex resolution of LHCb, along with its unique particle-identification and real-time data-analysis capabilities.

Figures (4)

• Figure 1: Previous and planned experimental bounds on dark photons (adapted from Essig:2013lka) compared to the anticipated LHCb reach for inclusive $A'$ production in the di-muon channel (see the text for definitions of prompt, pre-module, and post-module). The red vertical bands indicate QCD resonances which would have to be masked in a complete analysis. The LHCb $D^*$ anticipated limit comes from Ilten:2015hya, and Belle-II comes from Hearty.
• Figure 2: Predicted reconstructed di-muon invariant mass spectrum with our prompt selection criteria applied after Run 3, including the isolation criteria for $m_{\mu\mu} > m_\phi$. "EM" denotes the sum of "Mesons" and "DY/FSR".
• Figure 3: Same as Fig. 1 of the main text, but scaling the $15~\text{fb}^{-1}$ baseline up to $50~\text{fb}^{-1}$ and $500~\text{fb}^{-1}$, for both the $D^* \to D^0 A'$ search Ilten:2015hya and the inclusive di-muon search (this work). For reference, the green dashed line shows where the $A'$ lifetime is the same as the LHCb di-muon lifetime resolution and the orange dashed line shows where the average $A'$ transverse displacement matches the distance at which the muons from $A'\to\mu^+\mu^-$ decays likely no longer have enough hits in the LHCb VELO.
• Figure 4: Same as Fig. 1 of the main text, but with an extended $\epsilon^2$ range and without the exclusion regions due to narrow resonances.