Probing an MeV-scale scalar boson in association with a TeV-Scale top-quark partner at the LHC

Abstract

Searches for new low-mass matter and mediator particles have actively been pursued at fixed target experiments and at e+e− colliders. It is challenging at the CERN LHC, but they have been searched for in Higgs boson decays and in B meson decays by the ATLAS and CMS Collaborations, as well as in a low transverse momentum phenomena from forward scattering processes (e.g., FASER). We propose a search for a new scalar particle in association with a heavy vector-like quark. We consider the scenario in which the top quark (t) couples to a light scalar ϕ′ and a heavy vector-like top quark T. We examine single and pair production of T in pp collisions, resulting in a final state with a top quark that decays purely hadronically, a T which decays semileptonically (T → W + b → ℓ ν b), and a ϕ′ that is very boosted and decays to a pair of collimated photons which can be identified as a merged photon system. The proposed search is expected to achieve a discovery reach with signal significance greater than 5σ (3σ) for m(T) as large as 1.8 (2) TeV and m(ϕ′) as small as 1 MeV, assuming an integrated luminosity of 3000 fb−1. This search can expand the reach of T, and demonstrates that the LHC can probe low-mass, MeV-scale particles.