Vertexing displaced diphoton decays with recoil photons at Belle II

Abstract

We propose a recoil-assisted strategy for vertexing displaced diphoton decays in $e^+e^-\toγX$, $X\toγγ$, using only one converted daughter photon. The initial state and recoil-photon momentum define the LLP flight line, whose closest approach to the converted-photon trajectory locates the decay vertex. This removes the need for a second conversion, making the conversion-related efficiency scale linearly rather than quadratically with the photon-conversion probability. For photophilic axionlike particles at Belle II, the existing $408$ fb$^{-1}$ data set could probe previously unconstrained parameter space near $m_a \simeq 100$ MeV and $g_{aγγ} \simeq 2 \times 10^{-4}$ GeV$^{-1}$, improving leading bounds by almost an order of magnitude and testing the mass region below the $π^0$ mass, which the existing Belle II three-photon searches cannot access. With $50$ ab$^{-1}$, the reach extends to $m_a \simeq 310$ MeV and $g_{aγγ}\simeq2\times10^{-5}$ GeV$^{-1}$.