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 $\ddot{I}{•}^{\prime }$ 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 ($T$ $\to$ $W$ + $b$ $\to$ $\ell$ $\hat{I}½$ $b$), and a $\ddot{I}{•}^{\prime }$ 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 5$\ddot{I}ƒ$ (3$\ddot{I}ƒ$) for $m(T)$ as large as 1.8 (2) TeV and $m(\ddot{I}{•}^{\prime })$ 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.

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