While the identity and underlying properties of the dark matter (DM) in our Universe remain mysterious, its energy density has been precisely inferred by a series of satellite missions studying the Cosmic Microwave Background (CMB). Any theoretical description of DM must therefore include a DM production mechanism which leads to the observed DM relic abundance of Ω_DM×h² ≃ 0.12.
A particularly appealing framework for the genesis of DM, minimizing the dependence on initial conditions, is its creation out of a thermal bath. The most commonly adopted paradigm falling into this category is “thermal freeze-out” from the primordial plasma of Standard Model (SM) particles in the early Universe. However, given the increasingly strong constraints on this setup, as well as the natural productivity of DM phenomenologists, a plethora of alternate production scenarios with DM initially in thermal equilibrium have recently been proposed, including hidden sector freeze-out, Forbidden DM, Cannibal DM, Zombie DM etc etc. (But, all these are not our today’s topic. So, we will discuss them in upcoming articles.)
Another possibility is that DM never entered thermal equilibrium at all, in which case it can be produced via a ‘leakage’ out of a thermal bath, often referred to as freeze-in. While a large number of variants of the freeze-out paradigm have been suggested, less model building has been performed around the freeze-in idea.
Now, inspired by coronavirus outbreak, Bringmann and colleagues proposed a novel and generic DM production scenario between these two polarities which they referred to as “Pandemic DM”.
“This brings a new perspective to the widely studied freeze-in mechanism, which can trigger a subsequent transmission phase in a rather natural way.’— told Bringmann, first author of the study.
The name derives from the observation that a DM particle (χ) can ‘infect’ a heat bath particle (ψ) to convert it into another DM particle (χ), cf. Fig. 1. They referred this type of interaction process as transmission. For a small initial abundance (nχ) it results in an exponential growth of the DM abundance and therefore strongly resembles the epidemic curves (epi curves) encountered in the case of a spreading pathogen.
“The DM abundance grows exponentially with time, like the epi curve describing the spread of a pathogen, and in stark contrast to models of freeze-in where the abundance grows only as a power law. Indeed, we have established V -duality, which relates the Boltzmann equation for the DM number density to the SIR model for the transmission of pathogens. V-duality has the surprising implication that studies of epidemiology may help us to understand the origin of DM, and vice versa.”— told Schmidt-Hoberg, one of the author of the study.
To be in accord with observations this mechanism requires the exponential growth to shut off before the DM particle χ is fully thermalized, so one can also think of this mechanism as a ‘failed thermalization’. Interestingly, the exponential growth of (nχ) comes to an end naturally, e.g. due to the Boltzmann suppression of the heat bath particle ψ, so that the observed DM relic abundance is readily obtained by them in their framework.
“Such highly suppressed decays may potentially be visible in late-time observables related to cosmological structure formation or cosmic rays, making corresponding scenarios even more attractive. We leave a detailed exploration of phenomenological consequences of Pandemic DM for future work.”— concluded authors of the study.
Finally, they stressed out that the proposed mechanism works for a large range of different particle masses, not just the specific choice displayed in the epi curve examples, mχ = 10 GeV and mψ = 20 GeV.
Reference: Torsten Bringmann, Paul Frederik Depta, Marco Hufnagel, Joshua T. Ruderman, Kai Schmidt-Hoberg, “Pandemic Dark Matter”, ArXiv, pp. 1-7, 30 Mar 2021. https://arxiv.org/abs/2103.16572
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