Search for Invisible Decays of a Dark Photon Produced in e+e- Collisions at BaBar

TL;DR

Limits on the coupling strength of A^{'} to e^{+}e^{-} in the mass range m_{A^{'}}≤8  GeV are set, which exclude the values of the A^' coupling suggested by thedark-photon interpretation of the muon (g-2)_{μ} anomaly, as well as a broad range of parameters for the dark-sector models.

Abstract

We search for single-photon events in 53 fb^-1 of e+e- collision data collected with the BaBar detector at the PEP-II B-factory. We look for events with a single high-energy photon and a large missing momentum and energy, consistent with production of a spin-1 particle A' through the process e+e->gamma A', A'->invisible. Such particles, referred to as "dark photons", are motivated by theories applying a U(1) gauge symmetry to dark matter. We find no evidence for such processes and set 90% confidence level upper limits on the coupling strength of A' to e+e- in the mass range m_A'<=8 GeV. In particular, our limits exclude the values of the A' coupling suggested by the dark-photon interpretation of the muon (g-2) anomaly, as well as a broad range of parameters for the dark-sector models.

Figures (7)

• Figure 1: Measured maximum-likelihood values of the $A'$ mixing strength squared $\varepsilon^2$ as a function of the mass $m_{A'}$.
• Figure 2: Signal significance $\mathcal{S}$ as a function of the mass $m_{A'}$.
• Figure 3: Bottom: signal fit for $m_{A'}=6.21$ GeV to a combination of $焇{(2S)}$ and $焇{(3S)}$ datasets, shown for illustration purposes. The signal peak (red) corresponds to the local significance $\mathcal{S}=3.1$ (global significance of $2.6\sigma$). Blue solid line shows the full PDF, while the magenta dashed line corresponds to the background contribution. Top: distribution of the normalized fit residuals (pulls).
• Figure 4: Upper limits at 90% CL on $A'$ mixing strength squared $\varepsilon^2$ as a function of $m_{A'}$. Shown are the Bayesian limit computed with a uniform prior for $\varepsilon^2>0$ (solid red line) and the profile-likelihood limit (blue dashed line).
• Figure 5: Regions of the $A'$ parameter space ($\varepsilon$ vs $m_{A'}$) excluded by this work (green area) compared to the previous constraints Essig:2013lkaE787E949NA64 as well as the region preferred by the $(g-2)_\mu$ anomaly ref:g-2.