Dark vector boson bremsstrahlung: New form factors for a broader class of models

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 recast and extend the reach of existing FASER data to GeV-mass dark photons, U ( 1 ) B , U ( 1 ) B − L , and protophobic vectors, as well as forecasts for millicharged particles at FORMOSA.