Using a data sample corresponding to an integrated luminosity of $2.93\,\rm fb^{ -1}$ taken at a center-of-mass energy of 3.773\,GeV with the BESIII detector operated at the BEPCII collider, we perform an analysis of the semi-leptonic decays $D^{ 0(+)}\to \pi^{ -(0)}\mu^+\nu_\mu$. The branching fractions of $D^0\to \pi^-\mu^+\nu_\mu$ and $D^+\to \pi^0\mu^+\nu_\mu$ are measured to be $(0.272 \pm 0.008_{ \rm stat.} \pm 0.006_{ \rm syst.})\%$ and $(0.350 \pm 0.011_{ \rm stat.} \pm 0.010_{ \rm syst.})\%$, respectively, where the former is of much improved precision compared to previous results and the latter is determined for the first time. Using these results along with previous BESIII measurements of $D^{ 0(+)}\to \pi^{ -(0)}e^+\nu_e$, we calculate the branching fraction ratios to be ${ \mathcal R}^0\equiv { \mathcal B}_{ D^{ 0}\to \pi^{ -}\mu^+\nu_\mu}/{ \mathcal B}_{ D^{ 0}\to \pi^{ -}e^+\nu_e}=0.922\pm 0.030_{ \rm stat.}\pm0.022_{ \rm syst.}$ and ${ \mathcal R}^+\equiv { \mathcal B}_{ D^{ +}\to \pi^{ 0}\mu^+\nu_\mu}/{ \mathcal B}_{ D^{ +}\to \pi^{ 0}e^+\nu_e}=0.964\pm 0.037_{ \rm stat.}\pm0.026_{ \rm syst.}$, which are compatible with the theoretical expectation of lepton flavor universality within $1.7\sigma$ and $0.5\sigma$, respectively. We also examine the branching fraction ratios in different four-momentum transfer square regions, and find no significant deviations from the standard model predictions.
Categories: hep-ex