Probing axion-like particles through associated gauge-boson recoil and polarization at the LHC

Abstract

Axion-like particles (ALPs) appear in many well-motivated extensions of the Standard Model (SM) and can exhibit characteristic interactions with electroweak gauge bosons ($V=W^{\pm }/Z$) through higher-dimensional operators. We explore the production of ALPs ($a$) in association with $V$ at the HL-LHC, with $\sqrt{s}=14\mathrm{TeV}$ and an integrated luminosity of $\mathcal{L}=3{\mathrm{ab}}^{-1}$. We study the process $pp\to Va$, with $a\to \gamma \gamma$, for ALP masses in the range $20\mathrm{GeV}\le m_a\le 200\mathrm{GeV}$. This analysis extends existing associated-production searches by exploiting characteristic recoil and polarization features induced by the momentum-dependent $\mathrm{V}-\mathrm{V}-\mathrm{a}$ interaction. Using both cut-based selections and an XGBoost multivariate analysis, we demonstrate that these observables provide robust discrimination against SM backgrounds beyond the diphoton mass peak, improving the sensitivity to electroweak ALP couplings to $C_{WW}\gtrsim 5.6\times 10^{-3}{\mathrm{TeV}}^{-1}$ and $C_{BB}\gtrsim 2.8\times 10^{-2}{\mathrm{TeV}}^{-1}$. We further show that, if a resonance peak is observed in data, the same observables can help distinguish an ALP-like signal from a Higgs-like spin--0 state of the same mass.

38 pages, 13 figures, 3 tables