Light Scalars at FASER
Felix Kling (Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, Hamburg, 22607, Germany); Shuailong Li (Department of Physics, University of Arizona, Tucson, AZ, 85721, USA); Huayang Song (CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing, 100190, China); Shufang Su (Department of Physics, University of Arizona, Tucson, AZ, 85721, USA); Wei Su (School of Science, Shenzhen Campus of Sun Yat-sen University, No. 66, Gongchang Road, Guangming District, Shenzhen, Guangdong, 518107, P.R. China)
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 β 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, 105) meters. Both FASER and FASER 2 can probe a large part of the parameter space in the large tan β region up to 107, extending beyond the constraints of the other existing experiments.
