We report the first observation of the semi-muonic decay $D^+\to \omega \mu^+\nu_\mu$ using an $e^+e^-$ collision data sample corresponding to an integrated luminosity of $2.93~\mathrm{ fb}^{ -1}$ collected with the BESIII detector at a center-of-mass energy of 3.773 GeV. The absolute branching fraction of the $D^+\to \omega \mu^+\nu_\mu$ decay is measured to be ${ \mathcal B}_{ D^+\to \omega\mu^+\nu_\mu}=(17.7\pm1.8_{ \rm stat}\pm1.1_{ \rm syst})\times 10^{ -4}$. Its ratio with the world average value of the branching fraction of the $D^+\to \omega e^+\nu_e$ decay probes lepton flavor universality and it is determined to be ${ \mathcal B}_{ D^+\to \omega \mu^+\nu_\mu}/{ \mathcal B}^{ \rm PDG}_{ D^+\to \omega e^+\nu_e}=1.05\pm0.14$, in agreement with the standard model expectation within one standard deviation.
Categories: hep-ex