Scalar Dark Matter Models with Significant Internal Bremsstrahlung

TL;DR

This paper investigates scalar dark matter models interacting via heavy fermions, highlighting how internal bremsstrahlung can produce sharp gamma-ray signals that are significantly enhanced compared to traditional Majorana models.

Contribution

It introduces a scalar dark matter model with a vector-like portal, demonstrating enhanced gamma-ray signals from internal bremsstrahlung compared to Majorana models.

Findings

Internal bremsstrahlung significantly boosts gamma-ray signals.

Scalar models can achieve large gamma-ray cross sections.

Comparison shows up to 100-fold enhancement over Majorana cases.

Abstract

There has been interest recently on particle physics models that may give rise to sharp gamma ray spectral features from dark matter annihilation. Because dark matter is supposed to be electrically neutral, it is challenging to build weakly interacting massive particle models that may accommodate both a large cross section into gamma rays at, say, the Galactic center, and the right dark matter abundance. In this work, we consider the gamma ray signatures of a class of scalar dark matter models that interact with Standard Model dominantly through heavy vector-like fermions (the vector-like portal). We focus on a real scalar singlet S annihilating into lepton-antilepton pairs. Because this two-body final-state annihilation channel is d-wave suppressed in the chiral limit, we show that virtual internal bremsstrahlung emission of a gamma ray gives a large correction, both today and at the time of freeze-out. For the sake of comparison, we confront this scenario to the familiar case of a Majorana singlet annihilating into light lepton-antilepton pairs, and show that the virtual internal bremsstrahlung signal may be enhanced by a factor of (up to) two orders of magnitude. We discuss the scope and possible generalizations of the model.