The final results of the search for the lepton flavour violating decay [equation]based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and...

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.

The analysis of a combined data set, totaling $3.6\times {10}^{14}$ stopped muons on target, in the search for the lepton flavor violating decay ${\mu }^{+}\to e^{+}\gamma$ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of $5.7\times {10}^{-13}$ (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

The MEG experiment, designed to search for the [equation] decay, completed data-taking in 2013 reaching a sensitivity level of [equation]for the branching ratio. In order to increase the sensitivity...

We study the reaction $e^{+}{e}^{-}\to e^{+}{e}^{-}{\pi }^0$ in the single tag mode and measure the differential cross section $d\sigma /d{Q}^2$ and the $\gamma {\gamma }^{*}\to {\pi }^0$ transition form factor in the momentum transfer range from 4 to $40{\mathrm{GeV}}^2$. At $Q^2>10{\mathrm{GeV}}^2$ the measured form factor exceeds the asymptotic limit predicted by perturbative QCD. The analysis is based on $442{\mathrm{fb}}^{-1}$ of integrated luminosity collected at PEP-II with the BABAR detector at $e^{+}{e}^{-}$ center-of-mass energies near 10.6 GeV.