Dark Vector Boson Bremsstrahlung: New Form Factors for a Broader Class of Models

Abstract: We explore the sensitivity of collider experiments to a broad class of GeV-scale dark vector models of new physics via production in proton and neutron bremsstrahlung and initial state radiation. This is achieved using a new physically motivated model for timelike vector form factors with generic charges for both protons and neutrons, which is fit to a variety of timelike and spacelike data with quantified uncertainties. The production model for both proton and neutron bremsstrahlung is applied to re-cast and extend the reach of existing FASER data to GeV-mass dark photons, $U(1)B$, $U(1){B-L}$, and photophobic vectors, as well as forecasts for millicharged particles at FORMOSA.