Gamma-ray line constraints on coy dark matter

Abstract

Coy dark matter is an effective scheme in which a fermionic dark matter candidate interacts with the Standard Model fermions via a pseudoscalar mediator. This simple setup avoids the strong constraints posed by direct detection experiments in a natural way and explains, on top of the observed dark matter relic abundance, the spatially extended γ-ray excess recently detected at the Galactic Center. In this Letter we study the phenomenology of coy dark matter accounting for a novel signature of the model: the diphoton annihilation signal induced by the Standard Model fermions at the loop level. By challenging the model with the observations of spheroidal dwarf satellite galaxies and the results of γ-ray line searches obtained by the Fermi LAT experiment, we assess its compatibility with the measured dark matter relic abundance and the Galactic Center excesses. We show that despite the γ-ray line constraint rules out a significant fraction of the considered parameter space, the region connected to the observed Galactic Center excess remains currently viable. Nevertheless, we find that nextgeneration experiments such as DAMPE, HERD and GAMMA-400 have the potential to probe exhaustively this elusive scenario. [I] IntroductionThe matter content of our Universe is dominated by a component which, differently from ordinary matter, interacts at most very weakly with the photons of the Standard Model (SM) -the dark matter (DM). It is usually assumed that DM consists of stable and weaklyinteracting massive particles (WIMPs), which are thermal relics of dynamics once active in the hot early Universe. The reason behind the success of this picture is that particles with masses and annihilation cross sections set by the electroweak scale yield, in a natural way, DM relic densities of the order of the observed one. The basis of this remarkable coincidence lies in the freeze-out mechanism (for a review: [1, 2]), a natural consequence of the interplay between particle physics and an expanding Universe. For its simplicity and the appealing connection to frameworks like supersymmetry, the WIMP model became the paradigm of DM and shaped the dedicated long term experimental program. Direct detection experiments as XENON100 [3], PANDA [4] and LUX [5] investigate the possible elastic scattering of DM on SM particles mediated by the postulated weak-scale interactions. Similarly, indirect detection investigations scrutinise potential traces of DM annihilations proceeding in the Galaxy or on cosmological scales via the same interactions. Finally, the direct production of DM particles in weak-scale phenomena is studied at collider experiments such as the LHC at CERN [6,7].To date, in spite of the intense experimental effort, the nature and the properties of DM remain still a puzzle. Furthermore, the negative results of dedicated experiments accumulated so far have started to shake the belief of the community in the WIMP paradigm. In fact, the non-detection of supersymmetric partners of SM particles at collider experiments has impair...

Pseudoscalar mediators: a WIMP model at the neutrino floor

“…This correlation can be weakened by considering next-to-minimal scenarios (see e.g. [14][15][16]) or models that lead to suppressed scattering scatterings rates due to the low energy scale in the process [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] such as the one investigated here. The latter takes advantage of the different energy scales involved in the annihilation and scattering process to loosen the correlation.…”

Cornering pseudoscalar-mediated dark matter with the LHC and cosmology

“…We, however, do not attempt to fit the Galactic Center excess observed by FermiLAT [98,99], which has been considered in the context of pseudoscalar mediated simplified models in refs. [100][101][102] and EFT [103].…”

Modifying dark matter indirect detection signals by thermal effects at freeze-out

“…the LUX [2], PandaX [3] and XENON1T [4] detectors, are getting very stringent. While these constraints can be evaded, for example, by 'secluded' WIMPs [5,6], semiannihilation [7][8][9][10][11][12][13][14][15][16] or a pseudoscalar mediator [17][18][19][20][21], it is usual for WIMPs that the same processes which determine DM freeze-out usually also cause an indirect detection signal.…”