Radiative Signature of New Scalar Boson Decays in the $m_{\ell \ell \gamma}$ Spectrum at the LHC

Abstract

We investigate the radiative decay $S \to W^+W^-\gamma$ in the context of the multi-lepton anomalies and recent indications of a narrow scalar resonance near $m_S = 152 \pm 1~\text{GeV}$ in the $\gamma\gamma$, $Z\gamma$, and $W^+W^-$ channels at the Large Hadron Collider. These excesses arise in final states containing leptons, missing transverse momentum, and associated $b$-jets, and motivate a search for a corresponding localized excess in the invariant-mass spectrum of the dilepton--photon system, $m_{\ell\ell\gamma}$, in events with associated $b$-jets. We use recent CMS measurements of the ${t\bar{t}}\gamma$ differential cross sections~\cite{CMS:2025zbe} to study the $m_{\ell\ell\gamma}$ spectrum and perform a search for a scalar-resonance contribution. A localized excess is observed, compatible with the scalar-resonance hypothesis, with a global significance of $2.7\sigma$ at $m_S = 152~\text{GeV}$. This result provides additional support for the hypothesis of a narrow resonance. The ratio $\sigma(S \to W^+W^-\gamma)/\sigma(S \to W^+W^-) = (2.14 \pm 0.77)$\% is extracted. This value is compatible with an enhanced radiative contribution that could arise in scenarios beyond the Standard Model.