Dark sectors charged under a new Abelian interaction have recently received much attention in the context of dark matter models. These models introduce a light new mediator, the so-called dark photon ($A^{\prime }$), connecting the dark sector to the standard model. We present a search for a dark photon in the reaction $e^{+}{e}^{-}\to \gamma A^{\prime }$, $A^{\prime }\to e^{+}{e}^{-}$, ${\mu }^{+}{\mu }^{-}$ using $514{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector. We observe no statistically significant deviations from the standard model predictions, 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 $0.02–10.2\mathrm{GeV}$. 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.

Searches for lepton-flavor-violating decays of a $\tau$ lepton to a lighter mass lepton and a photon have been performed with the entire data set of $(963\pm 7)\times {10}^6\tau$ decays collected by the BABAR detector near the ${\rm Y}(4S)$, ${\rm Y}(3S)$ and ${\rm Y}(2S)$ resonances. The searches yield no evidence of signals and we set upper limits on the branching fractions of $\mathcal{B}({\tau }^{\pm }\to e^{\pm }\gamma )<3.3\times {10}^{-8}$ and $\mathcal{B}({\tau }^{\pm }\to {\mu }^{\pm }\gamma )<4.4\times {10}^{-8}$ at 90% confidence level.

We have observed a narrow state near $2.32\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ in the inclusive $D_s^{+}{\pi }^0$ invariant mass distribution from $e^{+}{e}^{-}$ annihilation data at energies near 10.6 GeV. The observed width is consistent with the experimental resolution. The small intrinsic width and the quantum numbers of the final state indicate that the decay violates isospin conservation. The state has natural spin-parity and the low mass suggests a $J^P=0^{+}$ assignment. The data sample corresponds to an integrated luminosity of $91{\mathrm{f}\mathrm{b}}^{-1}$ recorded by the BABAR detector at the SLAC PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring.

We search for single-photon events in $53{\mathrm{fb}}^{-1}$ 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 $A^{\prime }$ through the process $e^{+}{e}^{-}\to \gamma A^{\prime }$; $A^{\prime }\to \text{invisible}$. 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 $A^{\prime }$ to $e^{+}{e}^{-}$ in the mass range $m_{A^{\prime }}\le 8\mathrm{GeV}$. In particular, our limits exclude the values of the $A^{\prime }$ coupling suggested by the dark-photon interpretation of the muon $(g-2{)}_{\mu }$ anomaly, as well as a broad range of parameters for the dark-sector models.

We present an updated measurement of time-dependent $\mathrm{CP}$-violating asymmetries in neutral $B$ decays with the BABAR detector at the PEP-II asymmetric $B$ Factory at SLAC. This result uses an additional sample of $\Upsilon \left(4S\right)$ decays collected in 2001, bringing the data available to $32\times {10}^6$ $B\overline{B}$ pairs. We select events in which one neutral $B$ meson is fully reconstructed in a final state containing charmonium and the flavor of the other neutral $B$ meson is determined from its decay products. The amplitude of the $\mathrm{CP}$-violating asymmetry, which in the standard model is proportional to $\sin 2\beta$, is derived from the decay time distributions in such events. The result $\sin 2\beta =0.59\mathrm{}\pm 0.14\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)$ establishes $\mathrm{CP}$ violation in the $B^0$ meson system. We also determine $|\lambda |=0.93\mathrm{}\pm 0.09\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)$, consistent with no direct $\mathrm{CP}$ violation.