Testing $WW\gamma$ vertex in radiative muon decay

TL;DR

This paper proposes a novel asymmetry in radiative muon decay to test the $WW\gamma$ vertex, revealing a new zero in the decay rate that allows sensitive probing of physics beyond the Standard Model.

Contribution

It introduces a new zero in the normalized differential decay rate of radiative muon decay, enabling a novel method to test the $WW\gamma$ vertex beyond the Standard Model.

Findings

A new zero in the decay rate asymmetry related to the $WW\gamma$ vertex.

Potential to probe the $WW\gamma$ vertex at the 1% level.

Method applicable at facilities producing large muon samples.

Abstract

Large numbers of muons will be produced at facilities developed to probe lepton flavor violating process $\mu\to e\gamma$. We show that by constructing a suitable asymmetry, radiative muon decay $\mu\to e \gamma\nu_\mu\bar{\nu}_e$ can also be used to test the $WW\gamma$ vertex at such facilities. The process has two missing neutrinos in the final state and on integrating their momenta, the partial differential decay rate shows no radiation-amplitude-zero. We establish, however, that an easily separable part of the normalized differential decay rate, odd under the exchange of photon and electron energies, does have a zero in the case of standard model (SM). This "new type of zero" has hitherto not been studied in literature. A suitably constructed asymmetry using this fact, enables a sensitive probe for the $WW\gamma$ vertex beyond the SM. With a simplistic analysis, we find that the $C$ and $P$ conserving dimension four $WW\gamma$ vertex can be probed at ${\cal O}(10^{-2})$ with satisfactory significance level.