Measurement of the Branching Fraction For the Semi-Leptonic Decays $D^{0(+)}\to \pi^{-(0)}\mu^+\nu_\mu$ and Test of Lepton Flavor Universality

TL;DR

This paper measures the branching fractions of specific semi-leptonic D meson decays involving muons, compares them to electron modes, and tests lepton flavor universality with high precision using BESIII data.

Contribution

It provides the most precise measurement of D0 to pi- mu+ nu_mu decay and first measurement of D+ to pi0 mu+ nu_mu, testing lepton flavor universality.

Findings

Measured branching fractions with improved precision.

Branching ratio ratios compatible with lepton flavor universality.

No significant deviations from Standard Model predictions.

Abstract

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.