We study the process $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ at a center-of-mass energy of 4.260 GeV using a $525{\mathrm{pb}}^{-1}$ data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be $(62.9\pm 1.9\pm 3.7)\mathrm{pb}$, consistent with the production of the $Y(4260)$. We observe a structure at around $3.9\mathrm{GeV}/c^2$ in the ${\pi }^{\pm }J/\psi$ mass spectrum, which we refer to as the $Z_c(3900)$. If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the ${\pi }^{\pm }J/\psi$ invariant mass spectrum, neglecting interference, results in a mass of $(3899.0\pm 3.6\pm 4.9)\mathrm{MeV}/c^2$ and a width of $(46\pm 10\pm 20)\mathrm{MeV}$. Its production ratio is measured to be $R=(\sigma \mathbf{(}{e}^{+}{e}^{-}\to {\pi }^{\pm }{Z}_c(3900{)}^{\mp }\to {\pi }^{+}{\pi }^{-}J/\psi \mathbf{)}/\sigma (e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi ))=(21.5\pm 3.3\pm 7.5)\%$. In all measurements the first errors are statistical and the second are systematic.

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 study $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ at center-of-mass energies from 3.90 to 4.42 GeV by using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies and are found to be of the same order of magnitude as those of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ but with a different line shape. In the ${\pi }^{\pm }{h}_c$ mass spectrum, a distinct structure, referred to as $Z_c(4020)$, is observed at $4.02\mathrm{GeV}/c^2$. The $Z_c(4020)$ carries an electric charge and couples to charmonium. A fit to the ${\pi }^{\pm }{h}_c$ invariant mass spectrum, neglecting possible interferences, results in a mass of $(4022.9\pm 0.8\pm 2.7)\mathrm{MeV}/c^2$ and a width of $(7.9\pm 2.7\pm 2.6)\mathrm{MeV}$ for the $Z_c(4020)$, where the first errors are statistical and the second systematic. The difference between the parameters of this structure and the $Z_c(4025)$ observed in the $D^{*}{\overline{D}}^{*}$ final state is within $1.5\sigma$, but whether they are the same state needs further investigation. No significant $Z_c(3900)$ signal is observed, and upper limits on the $Z_c(3900)$ production cross sections in ${\pi }^{\pm }{h}_c$ at center-of-mass energies of 4.23 and 4.26 GeV are set.

We search for the flavor-changing neutral-current decays $B\to K^{(*)}\nu \overline{\nu }$, and the invisible decays $J/\psi \to \nu \overline{\nu }$ and $\psi (2S)\to \nu \overline{\nu }$ via $B\to K^{(*)}J/\psi$ and $B\to K^{(*)}\psi (2S)$, respectively, using a data sample of $471\times {10}^6$ $B\overline{B}$ pairs collected by the BABAR experiment. We fully reconstruct the hadronic decay of one of the $B$ mesons in the ${\rm Y}(4S)\to B\overline{B}$ decay, and search for the $B\to K^{(*)}\nu \overline{\nu }$ decay in the rest of the event. We observe no significant excess of signal decays over background and report branching fraction upper limits of $\mathcal{B}(B^{+}\to K^{+}\nu \overline{\nu })<3.7\times {10}^{-5}$, $\mathcal{B}(B^0\to K^0\nu \overline{\nu })<8.1\times {10}^{-5}$, $\mathcal{B}(B^{+}\to K^{*+}\nu \overline{\nu })<11.6\times {10}^{-5}$, $\mathcal{B}(B^0\to K^{*0}\nu \overline{\nu })<9.3\times {10}^{-5}$, and combined upper limits of $\mathcal{B}(B\to K\nu \overline{\nu })<3.2\times {10}^{-5}$ and $\mathcal{B}(B\to K^{*}\nu \overline{\nu })<7.9\times {10}^{-5}$, all at the 90% confidence level. For the invisible quarkonium decays, we report branching fraction upper limits of $\mathcal{B}(J/\psi \to \nu \overline{\nu })<3.9\times {10}^{-3}$ and $\mathcal{B}(\psi (2S)\to \nu \overline{\nu })<15.5\times {10}^{-3}$ at the 90% confidence level. Using the improved kinematic resolution achieved from hadronic reconstruction, we also provide partial branching fraction limits for the $B\to K^{(*)}\nu \overline{\nu }$ decays over the full kinematic spectrum.