Abstract
Gamma ray spectral features are of interest for indirect searches of dark matter (DM). Following Barger et al. we consider 3 simple scenarios of DM that annihilates into Standard Model (SM) fermion pairs. Scenario 1 is a Majorana DM candidate coupled to a charged scalar, scenario 2 is a Majorana DM coupled to a charged gauge boson and scenario 3 is a real scalar DM coupled a charged vector-like fermion. As shown by Barger et al., these 3 scenarios share precisely the same internal Bremsstrahlung spectral signature into gamma rays. Their phenomenology is however distinct. In particular for annihilation into light SM fermions, in the chiral limit, the 2-body annihilation cross section is p-wave suppressed for the Majorana candidates while it is d-wave suppressed for the real scalar. In the present work we study the annihilation into 2 gammas, showing that these three scenarios have distinct, and so potentially distinguishable, spectral signatures into gamma rays. In the case of the real scalar candidate we provide a new calculation of the amplitude for annihilation into 2 gammas.
Highlights
In [9], we have shown that the 2-body annihilation of the scalar DM candidate is d-wave suppressed in the chiral limit, and that the Bremsstrahlung signal is parametrically larger
The spectrum of gamma rays from say, π0, being featureless we focus on gamma ray lines
It complements the phenomenological studies initiated in [8] and [9] in which it has been shown that a real scalar DM candidate S interacting with light Standard Model (SM) fermions could give a strong Bremsstrahlung signal
Summary
Majorana DM candidate χ and charged scalar E The couplings with SM fermions take the form. The collider constraints on the mass of heavy charged fermions (scalars or others) are weaker than those on particles that carry colour but on the other hand interactions like that of (2.1) are constrained by non-observation of lepton flavour violating processes, so one may have to compromise (see for instance [21]). That the annihilation cross section is p-wave in the chiral limit is well known [7] and may be stated as follows. A pair of non-relativistic Majorana DM particles in a s-wave corresponds the state 1S0(O−+) in the 2S+1LJ (JCP ) spectroscopic notation, which, in terms of bi-linear operators, is represented by χγ5χ. In a CP conserving theory, the final state fermion pair is represented by the operator ψlγ5ψl, which involves a chirality flip, and is mass suppressed. In the chiral limit, the annihilation cross section is p-wave
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