We report on the most recent searches for unknown low-mass states performed with the data collected by the BaBar detector at the PEP-II e+e- collider.
The first search is based on a sample corresponding to 53 fb−1 of e+e− collision data collected with a special single-photon trigger. 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−→γA′, 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′→e+e− for a dark photon with a mass lower than 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.
Many models of New Physics postulate the existence of new gauge bosons mediating interactions between “dark sectors” and the Standard Model. We present a second search for a dark boson Z′ coupling only to the second and third generation of leptons in the reaction e+e−→μ+μ−Z′, Z′→μ+μ− with the full BaBar dataset. No significant signal is observed and limits improving upon bounds derived from neutrino experiments are set.