Observation of the Decay $D^0\to \rho^-\mu^+\nu_\mu$

TL;DR

This paper reports the first measurement of the branching fraction of the decay $D^0 o ho^-\mu^+ u_$ using BESIII data, testing lepton flavor universality and isospin symmetry.

Contribution

First measurement of the $D^0 o ho^-\mu^+ u_$ decay branching fraction and related ratios, providing experimental tests of lepton flavor universality and isospin symmetry.

Findings

Branching fraction ${ m B}(D^0 o ho^-\mu^+ u_) = (1.35 \u00b1 0.09_{ m stat} \u00b1 0.09_{ m syst}) imes 10^{-3}$

Branching fraction ratio ${ m B}(D^0 o ho^-\mu^+ u_)/{ m B}(D^0 o ho^- ext{ extit{e}}^+ u_ ext{ extit{e}}) = 0.90 \u00b1 0.11$

Decay width ratio ${ m }^0 o ho^- ext{ extit{ extmu}} u_ ext{ extmu}/{2 m }^+ o ho^0 ext{ extit{ extmu}} u_ ext{ extmu} = 0.71 \u00b1 0.14$

Abstract

By analyzing an $e^+e^-$ annihilation data sample corresponding to an integrated luminosity of $2.93~\mathrm{fb}^{-1}$ collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, we measure the branching fraction of the $D^0\to \rho^- \mu^+\nu_\mu$ decay for the first time. We obtain ${\mathcal B}_{D^0\to \rho^- \mu^+\nu_\mu}=(1.35\pm0.09_{\rm stat}\pm0.09_{\rm syst})\times 10^{-3}$. Using the world average of ${\mathcal B}_{D^0\to \rho^- e^+\nu_e}$, we find a branching fraction ratio of ${\mathcal B}_{D^0\to \rho^- \mu^+\nu_\mu}/{\mathcal B}_{D^0\to \rho^- e^+\nu_e}=0.90\pm0.11$, which agrees with the theoretical expectation of lepton flavor universality within the uncertainty. Combining the world average of ${\mathcal B}_{D^+\to \rho^0 \mu^+ \nu_\mu}$ and the lifetimes of $D^{0(+)}$, we obtain a partial decay width ratio of ${\Gamma}_{D^0\to \rho^- \mu^+ \nu_{\mu}}/(2{\Gamma}_{D^+\to \rho^0 \mu^+ \nu_{\mu}}) = 0.71\pm0.14$, which is consistent with the isospin symmetry expectation of one within $2.1\sigma$. For the reported values of ${\mathcal B}_{D^0\to \rho^- \mu^+\nu_\mu}/{\mathcal B}_{D^0\to \rho^- e^+\nu_e}$ and ${\Gamma}_{D^0\to \rho^- \mu^+ \nu_{\mu}}/2{\Gamma}_{D^+\to \rho^0 \mu^+ \nu_{\mu}}$, the uncertainty is the quadratic sum of the statistical and systematic uncertainties.