Probing some photon portals to new physics at intensity frontier experiments

A number of extensions of the Standard Model predict the existence of new light, weakly-coupled particles that couple to the visible sector through higher-dimensional operators containing one or two photons, suppressed by a high new physics scale, and thus have long lifetimes. In this work, we study the prospects for detecting three $\sim\,$sub-GeV such long-lived particles (LLP) at intensity frontier experiments: a massive spin-2 mediator ($G$), a dark axion portal, and a light neutralino coupled to ALPino or gravitino. We consider the production and visible decays of these particles in several current and proposed beam dump experiments (CHARM, NuCal, SeaQuest, NA62, SHiP) as well as in the LHC detectors (FASER, FASER$\nu$, FLArE, MATHUSLA). In addition to the usual displaced vertex signature, we also examine the impact of electron scattering signature and the Primakoff-like process which leads to conversion of $G$ into a photon or to the secondary LLP production via upscattering of the lighter dark sector state on dense material put in front of the detector. In all cases, we find the SHiP experiment could provide the strongest constraints for the displaced vertex search, while FASER2/FPF could provide complementary coverage of the $\gamma c\tau \sim 1$ m decay length region of the parameter space.