Effective theory of dark matter decay into monochromatic photons and its implications: Constraints from associated cosmic-ray emission

Abstract

We show that there exists only a quite limited number of higher dimensional operators which can naturally lead to a slow decay of dark matter particles into monochromatic photons. As each of these operators inevitably induces decays into particles other than photons, we show that the γ-lines it induces are always accompanied by a continuum flux of cosmic rays. Hence constraints on cosmic-ray fluxes imply constraints on the intensity of γ-lines and vice versa. A comparison with up to date observational bounds shows the possibilities to observe or exclude cosmic rays associated to γ-line emission, so that one could better determine the properties of the DM particle, possibly discriminating between some of the operators.

Can a millicharged dark matter particle emit an observable γ-ray line?

“…The methodology used to obtain these constraints is the same as in ref. [8]. By comparing the constraints from cosmic rays on n γ /n CR and the limits from direct searches for photon line spectral features, this plot shows which operator are compatible with a possible near future observation of a γ-ray line.…”

“…These operators come in addition to the effective operators which can lead to a γ-ray line in the case where DM would be exactly neutral, given and studied in ref. [8]. The former operators involve a covariant derivative of the millicharged field, whereas the latter ones can involve a photon only from the presence of a hypercharge or SU(2) L field strength F µν Y,L in the operator.…”

“…[8], there are a priori essentially two main ways to constraint the operators and thus to possibly discriminate between them, from γ-ray line spectral features and from the associated continuum of cosmic rays produced. The fact that the lifetime of the DM particle must be longer than the age of the Universe provides an additional constraint which is relevant in special cases.…”

“…[8]. The whole issue is that for a given operator, due to gauge invariance, there always is a continuum flux of cosmic rays associated to the production of a γ-ray line, especially if its energy is larger than the Z boson mass.…”

“…In this case, there is production of W bosons and a + b = 2 in eq. (4.3), meaning that 8) at the lowest order in or φ. Here, in order to obtain a conservative modelindependent upper bound, we made the hypothesis that the charged ψ components produced in two-body decays together with W bosons do not yield an important contribution to cosmic rays production.…”