Thermodynamic evolution of secluded vector dark matter: conventional WIMPs and nonconventional WIMPs

Abstract

The secluded dark matter resides within a hidden sector and self-annihilates into lighter mediators which subsequently decay to the Standard Model (SM) particles. Depending on the coupling strength of the mediator to the SM, the hidden sector can be kinetically decoupled from the SM bath when the temperature drops below the mediator's mass, and the dark matter annihilation cross section at freeze-out is thus possible to be boosted above the conventional value of weak interacting massive particles. We present a comprehensive study on thermodynamic evolution of the hidden sector from the first principle, using the simplest secluded vector dark matter model. Motivated by the observation of Galactic center gamma-ray excess, we take two mass sets ∼ O(80 GeV) for the dark matter and mediator as examples to illustrate the thermodynamics. The coupled Boltzmann moment equations for number densities and temperature evolutions of the hidden sector are numerically solved.The formalism can be easily extended to a general secluded dark matter model. We show that a long-lived mediator can result in a boosted dark matter annihilation cross section to account for the relic abundance. We further show the parameter space which provides a good fit to the Galactic center excess data and is compatible with the current bounds and LUX-ZEPLIN projected sensitivity. We find that the future observations of dwarf spheroidal galaxies offer promising reach to probe the most relic allowed parameter space relevant to the boosted dark matter annihilation cross section.

A potentially detectable gamma-ray line in the Fermi Galactic center excess — in light of one-step cascade annihilations of secluded (vector) dark matter via the Higgs portal

“…As for a small mixing angle α 2 × 10 −6 , the hidden sector has been thermally decoupled from the bath before it becomes nonrelativistic, such that the resulting DM annihilation cross section that accounts for the correct relic density could be significantly boosted above the conventionally thermal WIMP value [17]. 3 Formulation of the gamma spectrum with prominent lines arising from one-step cascade DM annihilations…”

“…The thermodynamic evolution of the hidden sector (vector) dark matter interacting with the SM through the Higgs portal has been studied in ref. [17]. Here we will present the main properties, and refer readers to ref.…”

“…Instead, a paradigm of DM was proposed to suggest that (WIMP) dark matter is secluded within one of the hidden sectors, and is very weakly coupled to the visible sector via a metastable mediator which is lighter than the DM [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. As such, the secluded DM may become undetectable or hard to detect in colliders and underground direct searches, but can still produce viable signals in the indirect experiments [14,16,17].…”

“…In indirect searches, the qualitative fit of the gamma-ray spectrum is relevant to the decay channels of the mediator as well as the DM mass, which determines the initially kinetic energy of the mediator and thus the boost factor for the spectrum, while the DM annihilation cross section plays as an overall factor in the fit. For determination of the DM relic abundance today, the thermodynamic evolution of the hidden sector before freeze-out depends on the strength of couplings between the mediator and SM [17]. If the couplings are small enough, the hidden sector can be kinetically decoupled from the bath before it becomes nonrelativistic.…”

“…If the couplings are small enough, the hidden sector can be kinetically decoupled from the bath before it becomes nonrelativistic. As such the freeze-out DM annihilation cross section required to give a correct relic density could be boosted above the conventional WIMP value [17,[35][36][37].…”