Dark photon pair production via off-shell dark Higgs at FASER

TL;DR

This paper investigates the potential of FASER to detect dark photons produced via off-shell decay of a new scalar boson, extending the experimental sensitivity beyond previous limits and exploring uncharted parameter space.

Contribution

It introduces a novel analysis of dark photon production through off-shell scalar decay at FASER, expanding the sensitivity region and discussing perturbative validity.

Findings

FASER can probe dark photon parameter space beyond on-shell decay thresholds.

Off-shell production extends the sensitivity to previously inaccessible regions.

Perturbative calculations are valid within certain parameter space for unitarity.

Abstract

We consider a dark photon model in which the dark U(1) gauge symmetry is spontaneously broken by a vacuum expectation value of a new scalar boson. We focus on the ForwArd Search ExpeRiment (FASER) and calculate its sensitivity to the dark photon produced from the off-shell decay of the new scalar boson. It is found that the off-shell production extends the sensitivity region beyond the kinematical threshold of the on-shell decay of the scalar boson, and that the sensitivity region can be spanned to unexplored region. We also show the parameter space in which perturbative calculation is valid for the unitarity of an S matrix.