Astrophysical Constraints on Singlet Scalars at LHC

TL;DR

This paper investigates the astrophysical and cosmological constraints on heavy singlet scalar particles proposed as signals at the LHC, showing that simple models are ruled out due to their cosmological relics and astrophysical effects.

Contribution

It demonstrates that minimal UV complete models with singlet scalars and associated heavy fermions are incompatible with astrophysical observations, challenging their viability as LHC signals.

Findings

Heavy fermions form long-lived heavy mesons in the universe

Surviving heavy states could cause detectable astrophysical phenomena

Simple models are ruled out by lack of observed consequences

Abstract

We consider the viability of new heavy gauge singlet scalar particles at colliders such as the LHC. Our original motivation for this study came from the possibility of a new heavy particle of mass ~ TeV decaying significantly into two photons at colliders, such as LHC, but our analysis applies more broadly. We show that there are significant constraints from astrophysics and cosmology on the simplest UV complete models that incorporate such new particles and its associated collider signal. The simplest and most obvious UV complete model that incorporates such signals is that it arises from a new singlet scalar (or pseudo-scalar) coupled to a new electrically charged and colored heavy fermion. Here we show that these new fermions (and anti-fermions) would be produced in the early universe, then form new color singlet heavy mesons with light quarks, obtain a non-negligible freeze-out abundance, and remain in kinetic equilibrium until decoupling. These heavy mesons possess interesting phenomenology, dependent on their charge, including forming new bound states with electrons and protons. We show that a significant number of these heavy states would survive for the age of the universe and an appreciable number would eventually be contained within the earth and solar system. We show that this leads to detectable consequences, including the production of highly energetic events from annihilations on earth, new spectral lines, and, spectacularly, the destabilization of stars. The lack of detection of these consequences rules out such simple UV completions, putting pressure on the viability of such new particles at LHC. To incorporate such a scalar would require either much more complicated UV completions or even further new physics that provides a decay channel for the associated fermion.