Measurement of B(Ds -> munu)

TL;DR

This paper reports a precise measurement of the branching fraction of the decay D_s^+ to mu+ nu_mu and derives the decay constant using data from the Belle experiment, employing a full reconstruction method for high resolution.

Contribution

It introduces a high-resolution full reconstruction technique to measure the D_s^+ decay branching fraction and decay constant with improved accuracy.

Findings

Branching fraction B(D_s^+→μ^+ν_μ) = (6.44 ± 0.76 (stat) ± 0.57 (syst)) × 10^{-3}

Decay constant f_Ds = (275 ± 16 (stat) ± 12 (syst)) MeV

Method achieves background separation comparable to tau-charm factory experiments.

Abstract

We present a measurement of the branching fraction $B(D_s^+\to\mu^+\nu_\mu)$ using a 548 fb$^{-1}$ data sample collected by the Belle experiment at the KEKB $e^+e^-$ collider. The $D_s$ momentum is determined by reconstruction of the system recoiling against $DK\gamma X$ in events of the type $e^+e^-\to D_s^\ast DKX, D_s^\ast \to D_s\gamma$, where $X$ represents additional pions or photons from fragmentation. The full reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that reached by experiments at the tau-charm factories. We obtain the branching fraction $B(D_s^+\to\mu^+\nu_\mu) = (6.44 \pm 0.76({\rm stat}) \pm 0.57({\rm syst})) \cdot 10^{-3}$, implying a $D_s$ decay constant of $f_{D_s} = (275 \pm 16({\rm stat}) \pm 12({\rm syst}))$ MeV.