Year: 2026
Type: article
Abstract: A light pseudoscalar that couples to muons through a current violating weak isospin acquires a contact interaction with the W boson, which enhances W → μνa by m W 2 / m μ 2 relative to radiation off the muon line. We show that the energy fraction of the muon that recoils against the a → μ + μ − resonance, evaluated in the reconstructed W rest frame, separates the two production mechanisms, because their amplitudes barely interfere and their recoil spectra are soft and hard, respectively. Reinterpreting the ATLAS three- and four-muon searches, we obtain the first direct limit on the contact coupling, m μ /|Δ μ | > 1.7 TeV at m a = 20 GeV, together with limits on the pseudoscalar coupling. For couplings inside these limits, a projection for 3 ab − 1 at the HL-LHC with 200 pileup interactions, including the uncertainty in the background shape, finds discovery for signals above about 20% of the present limit and a 5 σ separation of pure radiation from pure contact production above about two thirds of it for m a = 15 –20 GeV. The recoil spectrum thus tests weak isospin at the per-mille level in |Δ μ / g μμ |, a property of the production vertex that the rate alone cannot reveal.
Source: Physics Letters B
Author: Renjie Wang
Institutions: Chinese Academy of Sciences, Institute of High Energy Physics
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Funder: Institute of High Energy Physics
OpenAlex ID: W7223923756