Abstract
A search for the exclusive hadronic decays W± → π±γ, W± → K±γ, and W± → ρ±γ is performed using
up to 140 fb−1 of proton-proton collisions recorded with the ATLAS detector at a center-of-mass energy of √s = 13 TeV. If observed, these rare processes would provide a unique test bench for the quantum
chromodynamics factorization formalism used to calculate cross sections at colliders. Additionally, at
future colliders, these decays could offer a new way to measure the W boson mass through fully
reconstructed decay products. The search results in the most stringent upper limits to date on the branching
fractions B(W± → π±γ) < 1.9 × 10−6, B(W± → K±γ) < 1.7 × 10−6, B(W± → ρ±γ) < 5.2 × 10−6 at
95% confidence level.