Soft Photon Problem in Leptonic B-decays

TL;DR

This paper highlights the significant impact of soft photon emissions in B --> mu nu decays, which could affect experimental measurements and suggests ways to mitigate this issue through improved photon detection and lattice QCD calculations.

Contribution

It identifies the structure-dependent soft photon emission as a key systematic uncertainty in leptonic B-decay measurements and proposes experimental and theoretical strategies to control it.

Findings

Soft photon emissions can dominate systematic uncertainties in B --> mu nu decay measurements.

Lowering the photon detection threshold improves control over soft photon effects.

Lattice QCD calculations of form factors can aid in quantifying soft photon contributions.

Abstract

We point out at the peculiarity of B --> mu nu decay, namely the enhancement of the soft photon events which originate from the structure dependent part of the B --> mu nu gamma amplitude. This may be a dominant source of systematic uncertainty and compromise the projected experimental uncertainty on Gamma(B --> mu nu). We show that the effect of these soft photons can be controlled if the experimental cut on identification of soft photons is lowered and especially if the better resolution in identifying the momentum of muon emerging from B --> mu nu, is made. A lattice QCD computation of the relevant form factors would be highly helpful for a better numerical control over the structure dependent soft photon emission.