Heavy Fermion Bound States for Diphoton Excess at 750GeV $\sim$ Collider and Cosmological Constraints $\sim$

TL;DR

This paper investigates whether bound states of a hypothetical colored fermion could explain the 750 GeV diphoton excess observed at the LHC, considering collider and cosmological constraints.

Contribution

It proposes a specific SU(2)$_L$ singlet colored fermion with hypercharge 4/3 as a candidate, analyzing its decay channels, collider signatures, and relic density implications.

Findings

Best-fit cross section for the bound state matches the observed excess.

The scenario remains viable with a mass difference between the fermion and a stable particle above 30 GeV.

Relic density constraints are satisfied for mass differences below 40 GeV.

Abstract

A colored heavy particle with sufficiently small width may form non-relativistic bound states when they are produced at the large hadron collider\,(LHC), and they can annihilate into a diphoton final state. The invariant mass of the diphoton would be around twice of the colored particle mass. In this paper, we study if such bound state can be responsible for the 750 GeV diphoton excess reported by ATLAS and CMS. We found that the best-fit signal cross section is obtained for the SU(2)$_L$ singlet colored fermion $X$ with $Y_X=4/3$. Having such an exotic hypercharge, the particle is expected to decay through some higher dimensional operators, consistent with the small width assumption. The decay of $X$ may involve a stable particle $\chi$, if both $X$ and $\chi$ are odd under some conserved $Z_2$ symmetry. In that case, the particle $X$ suffers from the constraints of jets + missing $E_T$ searches by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still survives if the mass difference between $X$ and $\chi$ is above $\sim$ 30 GeV for $m_X \sim 375$ GeV. Even assuming pair annihilation of $\chi$ is small, the relic density of $\chi$ is small enough if the mass difference between $X$ and $\chi$ is smaller than $\sim$ 40 GeV.