On the decay of a light spinless particle into two photons

TL;DR

Problem: predict the two-photon decay of a light spinless φ within a general CP-violating EFT framework. Approach: develop a general electroweak-scale dim-5 EFT, perform threshold matching and running to GeV scales, and employ chiral perturbation theory to incorporate φ–meson mixing and η–η' effects to compute the one-loop photon couplings. Key contributions: explicit photon couplings up to order 1/Λ, inclusion of isospin breaking and η–η' mixing, and systematic matching to Higgs portal, dilaton, and ALP scenarios with internal consistency checks. Significance: provides model-agnostic, precise predictions for the φ→γγ rate, enabling robust interpretation of experimental searches and astrophysical bounds for light scalars.

Abstract

We analyze the effective couplings of a light, spinless, gauge-singlet particle $φ$ to on-shell photons. Starting from the most general theory at the electroweak scale, which allows for CP-violating interactions suppressed by inverse powers of an ultraviolet scale $Λ$, we derive the corresponding low-energy effective theory valid below the GeV scale. Within this framework, we systematically expand the effective couplings of $φ$ to on-shell photons in powers of small parameters. Working at the one-loop level, we retain terms at first order in $1/Λ$. We incorporate both isospin-breaking effects and $η$-$η'$ mixing, and provide explicit expressions for the couplings up to first order in $m_φ^2/m_η^2$ and $m_π^2/m_η^2$. As applications, we compute the decay rate of $φ$ into two photons and illustrate our results in several physically motivated scenarios.

Figures (1)

• Figure 1: One-loop diagrams contributing to $C_\gamma^\text{eff}(m_\phi)$. The tree-level diagram proportional to $d-4$ vanishes in $d=4$ dimensions, but produces a finite contribution when combined with the external-leg divergences induced by the last two diagrams.