Light Scalars at FASER

TL;DR

This paper explores FASER's potential to detect light CP-even and CP-odd scalars at the LHC, developing a model-independent formalism and analyzing the reach in the two Higgs doublet model, showing FASER's extended sensitivity.

Contribution

It introduces a general formalism for scalar production and decay at FASER, and evaluates the experiment's sensitivity to light scalars in the Type-I two Higgs doublet model.

Findings

FASER and FASER 2 can probe large regions of parameter space for light scalars.

FASER extends beyond existing experimental constraints.

Light scalar decay lengths can vary widely, affecting detectability.

Abstract

FASER, the ForwArd Search ExpeRiment, is a currently operating experiment at the Large Hadron Collider (LHC) that can detect light long-lived particles produced in the forward region of the LHC interacting point. In this paper, we study the prospect of detecting light CP-even and CP-odd scalars at FASER and FASER 2. Considering a model-independent framework describing the most general interactions between a CP-even or CP-odd scalar and SM particles using the notation of coupling modifiers in the effective Lagrangian, we develop the general formalism for the scalar production and decay. We then analyze the FASER and FASER 2 reaches of light scalars in the large $\tan\beta$ region of the Type-I two Higgs double model as a case study, in which light scalars with relatively long lifetime could be accommodated. In the two benchmark scenarios we considered, the light (pseudo)scalar decay length varies in $(10^{-8}, 10^5)$ meters. Both FASER and FASER 2 can probe a large part of the parameter space in the large $\tan\beta$ region up to $10^5$, extending beyond the constraints of the other existing experiments.