# Radiative decays of a singlet scalar boson through vector-like quarks

## TL;DR

This paper investigates the loop-induced radiative decays of a singlet scalar boson in a model with vector-like quarks, highlighting conditions for enhancement and current experimental constraints from the LHC.

## Contribution

It introduces a novel analysis of loop effects and polarization enhancement in singlet scalar decays mediated by vector-like quarks, with detailed phenomenological implications.

## Key findings

- Large mass differences among vector-like quarks enhance loop effects.
- The mixing angle with the Higgs must be less than about 0.1 for a 750 GeV scalar.
- Current LHC data strongly constrains the model parameters.

## Abstract

If the standard model Higgs boson were much heavier, it would appear as a broad resonance since its decay into a pair of longitudinally polarized gauge bosons is highly enhanced. We study whether the same enhancement happens at loop level in a simple extension of the standard model with a singlet scalar boson $S$. In order to focus on the loop effects, we assume that $S$ does not interact with the standard model particles at tree level. The singlet scalar $S$ is linked to the standard model world by vector-like quarks running in the loop. We introduce three vector-like quark multiplets, an $SU(2)_L$ doublet, an up-type singlet, and a down-type singlet. There are two kinds of loop effects in the $S$ phenomenology, the mixing with the Higgs boson and the radiative decays into $hh$, $WW$, $ZZ$, $gg$, and $\gamma\gamma$ through the triangle loops. We show that the crucial condition for enhancing loop effects including the longitudinal polarization enhancement is the large mass differences among vector-like quarks. The current LHC constraints on $S$ from the heavy scalar searches and the Higgs precision data are shown to be very significant: the mixing angle with the Higgs boson should be smaller than about 0.1 for $m_S= 750$ GeV.