Probing an MeV-Scale Scalar Boson in Association with a TeV-Scale Top-Quark Partner at the LHC

TL;DR

This paper proposes a novel LHC search strategy for detecting a light scalar particle produced alongside a heavy vector-like top quark, extending the potential to discover MeV-scale particles up to TeV-scale masses.

Contribution

It introduces a new search method for MeV-scale scalars associated with TeV-scale top partners at the LHC, focusing on boosted decay signatures and expanding the mass reach.

Findings

Potential to discover top partner up to 1.8 TeV with >5σ significance.

Can detect scalar particles as light as 1 MeV.

Demonstrates LHC's capability to probe low-mass particles in new channels.

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 $\phi^\prime$ 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 ($T$ $\rightarrow$ $W$ + $b$ $\rightarrow$ $\ell$ $\nu$ $b$), and a $\phi^\prime$ 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$\sigma$ (3$\sigma$) for $m(T)$ as large as 1.8 (2) TeV and $m(\phi^\prime)$ 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.