Dark matter indirect signals with long-lived mediators

Abstract

Dark matter particles could annihilate into light and metastable mediators subsequently decaying far away from where they are produced. In this scenario, the indirect signatures of dark matter are altered with respect to the conventional situation where standard model particles are directly injected where annihilation happens. We revisit the long-lived particle proposal (Rothstein et al., 2009) and devise the tools to explore this new phenomenology.We calculate the effective dark matter distribution resulting from the smearing by mediator propagation. We derive general expressions for the fluxes of mediators and their decay particles. We study how the J-factor, which naturally appears in the calculation of the dark matter induced gamma ray signal, is modified in the presence of mediators. We also derive the anisotropy which the cosmic ray positron flux exhibits in this scenario. We finally comment upon a recent proposal based on long-lived mediators where the effective dark matter density at the Earth is increased such as to explain the cosmic ray positron anomaly.We conclude that this scenario is barely tenable as regards the very dense dark matter spike which it requires at the Galactic center. The associated positron anisotropy is very small and undetectable, except at high energies where it reaches a level of order 10 −4 to 10 −based on non-relativistic long-lived mediators, observations of the gamma ray emission from the GC no longer preclude the large values of the WIMP annihilation cross section required to explain the cosmic ray positron excess. The other difference lies in the small mass and high energy of the mediators. In general, these particles are relativistic at the time of annihilation and lead to anisotropic distributions of cosmic rays. In this paper, we set up the formalism for computing the fluxes and associated anisotropies of the prompt species -positrons and photons -produced in the decays of these long-lived mediators.The paper is organized as follows. In section II A, we show how the DM density profile is smeared by mediators and compute the production rate of SM particles yielded by mediator decay.The conventional situation is recovered if the DM density is replaced by an effective value which depends on the mediator properties. In the scenario proposed by [14] to explain the cosmic ray positron excess, the effective DM density at the Earth is exceedingly large with respect to its actual value. The price to pay for such an enhancement is the existence of a very dense DM spike at the GC. In section II B we ponder upon possible astrophysical mechanisms which can alter the Galactic density profiles and show that adiabatic contraction resulting from the formation of the GC black hole can marginally lead to the desired DM density. In section III, we build the formalism for computing the fluxes of mediators and prompt decay particles. The observable flux of gamma rays is then described by an effective J-factor, which, for very long-lived mediators, considerably differs from the canonical one. ...

Self-interacting dark matter without prejudice

“…Our analysis suggests a long-lived mediator, and thus the need for analyzing displaced vertices in the sky and smallscale anisotropies in cosmic rays from DM annihilation [146,160], instead of exclusively focusing on astrophysical objects that are DM dense. For instance, a MeV-mass mediator with a lifetime of 10 10 sec (ϵ ∼ 10 −14 ), produced in GeV-mass DM annihilation in the Galactic center, preferentially decays outside our Galaxy.…”

“…bound, we require ctðm χ Þ ≥ 1 kpc to put an upper bound on ϵ. For larger in-flight decay lengths, the morphology of the spectrum is distorted [146], and thus the above bound cannot be applied directly. Overall, the ensuing limits are only competitive in the region of high mass m χ ≳ 100 GeV and do not improve the diffuse gamma-ray bound above noticeably.…”

Snowmass2021 Cosmic Frontier White Paper: Puzzling Excesses in Dark Matter Searches and How to Resolve Them

“…Critical observational improvements include advancements in the experimental sensitivity to the positron anisotropy. Models indicate that a sensitivity to a dipole anisotropy at the level of 10 −4 − 10 −3 would be required to discriminate between models powered by nearby sources (e.g., pulsar and supernova models) and potential dark matter explanations [172]. Expected improvements in the sensitivity of existing water-Cherenkov telescopes (e.g., HAWC and LHAASO) and upcoming imaging atmospheric Cherenkov telescopes (e.g., CTA) will be capable of both constraining the population of TeV halos as well as placing improved γ-ray limits on dark matter indirect detection targets (e.g., dwarf spheroidal galaxies).…”

Simple Hidden Sector Dark Matter

“…For fermionic dark matter, the dominant annihilation channel over the vast majority of the 7 Scenarios where this is not the case can lead to interesting signatures, see e.g. [62][63][64][65].…”