Dark photons, are hypothetical low-mass spin-1 particles that couple to dark matter but have vanishing couplings with normal matter. Such a boson, which may be associated with a U(1) gauge symmetry in the dark sector and mix kinetically with the Standard Model photon, offers an explanation for puzzling astrophysical observations such as the positron abundance in cosmic rays reported by the PAMELA satellite. Dark photons have also been invoked as possible explanations to the muon g-2 anomaly.
Based on single-photon events in 53 fb−1 of e+e– collision data collected at the PEP-II B factory in SLAC, California, the BaBar collaboration has now completed a thorough search for these particles (Aʹ) via the process e+e– → γ Aʹ. The search was based on the assumption that the dark photon decays almost entirely to dark-matter particles and therefore that no energy would be deposited in the BaBar detector from its decay products. Finding no evidence for such processes, the analysis places 90% confidence-level upper limits on the coupling strength of Aʹ to e+e– for dark photons lighter than 8 GeV. In particular, the BaBar limits exclude values of the Aʹ coupling suggested by the dark-photon interpretation of the muon g-2 anomaly, as well as a broad range of parameters for dark-sector models (see figure).
“This paper is the final word from BaBar on a search where the dark photon decays invisibly,” says BaBar spokesperson Michael Roney. “But we are continuing to search for dark photons and other dark-sector particles that have visible decay modes.”
The BaBar result follows another direct search for sub-GeV dark photons carried out recently by CERN’s NA64 experiment, in which electrons incident on an active target probe the process e− Z → e− Z Aʹ. Again, no evidence for such decays was found, and NA64 was able to exclude dark photons with a mass less than around 0.1 GeV.
“The thing is, there are dark photons and dark photons,” says theorist Sean Carroll of Caltech, who has worked on dark-photon models. “In contrast to massless dark photons, which are analogous to ordinary photons, this experiment constrains a slightly different idea of dark force-carrying particles that are associated with a broken symmetry, which therefore get a mass and then can decay. They are more like ‘dark Z bosons’ than dark photons.”