Search for the exclusive $W$ boson hadronic decays $W^{\pm}\to\pi^{\pm}\gamma$, $W^{\pm}\to K^{\pm}\gamma$ and $W^{\pm}\to\rho^{\pm}\gamma$ with the ATLAS detector

TL;DR

This paper reports a search for rare exclusive hadronic decays of the W boson into specific mesons and a photon, setting the most stringent upper limits on their branching fractions to date.

Contribution

It provides the first stringent upper limits on the branching fractions of W boson decays into $ ext{π}^ ext{±} ext{γ}$, $ ext{K}^ ext{±} ext{γ}$, and $ ext{ρ}^ ext{±} ext{γ}$ using ATLAS data, testing QCD predictions.

Findings

Set upper limits on $ ext{W}^ ext{±} o ext{π}^ ext{±} ext{γ}$, $ ext{K}^ ext{±} ext{γ}$, and $ ext{ρ}^ ext{±} ext{γ}$ branching fractions.

Most stringent limits at 95% confidence level to date.

Provides constraints on W boson couplings and QCD factorization models.

Abstract

A search for the exclusive hadronic decays $W^{\pm}\to \pi^{\pm}\gamma$, $W^{\pm}\to K^{\pm}\gamma$ and $W^{\pm}\to \rho^{\pm}\gamma$ is performed using up to $140\, \text{fb}^{-1}$ of proton-proton collisions recorded with the ATLAS detector at a center-of-mass energy of $\sqrt{s}=13\,\text{TeV}$. These rare processes provide a test bench for the quantum chromodynamics factorization formalism used to calculate cross sections at colliders, as well as a probe of $W$ boson coupling to quarks and a new way to measure the $W$ boson mass through fully reconstructed decay products. The search results in the most stringent upper limit to date on the branching fractions $\mathcal{B}(W^{\pm}\to \pi^{\pm}\gamma) < 1.9\times10^{-6}$, $\mathcal{B}(W^{\pm}\to K^{\pm}\gamma) < 1.7\times10^{-6}$, $\mathcal{B}(W^{\pm}\to \rho^{\pm}\gamma) < 5.2\times10^{-6}$ at 95% confidence level.