Search for a dark photon in e+e- collisions at BABAR

TL;DR

This study searches for a light dark photon A' produced in radiative e+e- collisions at BaBar, with A' decaying to e+e- or μ+μ- across 0.02–10.2 GeV. The analysis uses 514 fb^-1 of BaBar data, MC-driven signal shapes, detailed background modeling, and a mass-scan fit strategy to extract potential A' signals, incorporating neural-network suppression of photon-conversion background. No significant A' signal is observed, and 90% CL upper limits on the mixing strength ε reach the 10^-4–10^-3 range, improving prior constraints and excluding much of the parameter space that could explain the muon g-2 anomaly. The results constrain low-mass dark-sector scenarios and outline remaining regions (notably 15–30 MeV) to be probed by upcoming experiments.

Abstract

Dark sectors charged under a new Abelian force have recently received much attention in the context of dark matter models. These models introduce a light new mediator, the so-called dark photon (A'), connecting the dark sector to the Standard Model. We present a search for a dark photon in the reaction e+e- -> gamma A', A'-> e+e-, mu+mu- using 514 fb-1 of data collected with the BABAR detector. We do not observe a significant signal and we set 90% confidence level upper limits on the mixing strength between the photon and dark photon at the level of 10^-4 - 10^-3 for dark photon masses in the range 0.02 - 10.2 GeV. We further constrain the range of the parameter space favored by interpretations of the discrepancy between the calculated and measured anomalous magnetic moment of the muon.

Figures (4)

• Figure 1: Distribution of the final dielectron (top) and reduced dimuon invariant masses (bottom), together with the predictions of various simulated SM processes and ISR production of ${J / \psi }\xspace, \psi{(2S)}\xspace$, $焇{(1S)}\xspace$, and $焇{(2S)}\xspace$ resonances (collectively labeled as R). The fit to the ratio between data and simulated events is described in the text.
• Figure 2: The $e^+e^- \rightarrow \gamma A', A' \rightarrow e^+e^-$ (top) and $e^+e^- \rightarrow \gamma A', A' \rightarrow \mu^+\mu^-$ (bottom) cross-sections together with their respective statistical significance ($S_S$) as a function of the dark photon mass. The gray bands indicate the mass regions that are excluded from the analysis.
• Figure 3: Distribution of the statistical significance ($S_S$) from the fits to the dielectron (left) and dimuon (right) final states, together with the expected distribution for the null hypothesis (dashed line).
• Figure 4: Upper limit (90% CL) on the mixing strength $\epsilon$ as a function of the dark photon mass. The values required to explain the discrepancy between the calculated and measured anomalous magnetic moment of the muon Pospelov:2008zw are displayed as a red line.