A long-standing puzzle of particle physics and cosmology is the origin of dark matter (DM). The DM is known to be (very) weakly coupled to the standard model (SM) particles, stable, cold, and abundant in the present Universe. However the mass, the interactions, and the production mechanism are still not clear.
The DM mass may be small, in which case the stability (or longevity) is easily explained due to the kinematics (or suppressed decay width). From the point of view of DM direct detections, if the mass is small enough, the recoil energy of a nucleon via the DM-nucleon scattering is highly suppressed, which is consistent with the null result of the direct detection experiments for WIMP. Nevertheless a light DM can be searched for from different approaches in the near future.
A difficulty of the light DM is the production in the early Universe. If it were produced thermally, like the WIMP, the number density cannot be larger than that of the SM photons, and may be too small unless the mass is larger than eV. Moreover, it may be too hot to be consistent with structure formation. For instance a thermal relic of sterile neutrino has to be heavier than 2-5 keV from Lyman-α forest data. If it is produced from freeze-in the bound is even severer.
The production of light DM has been discussed in many studies. Like the one in which an axion/axion-like particle (ALP) can be produced via the misalignment mechanism. A light hidden photon DM can be produced gravitationally. (Gravitational effect during inflation is also important for the axion/ALP DM). The hidden photon DM can be produced from a parametric/tachyonic resonance by coupling to the QCD axion. In addition, the axion (or ALP) production from the parametric resonance of another scalar field was also discussed by Mazumdar et al. & Harigaya et al. in their paper. A light DM may be produced by the decays of heavy particles at an early epoch.
Now, Takeo Moroi and Wen Yina in their recent paper, explore new possibility of light DM production. They proposed that light DM is produced from the decay of an inflaton (or more generically, an oscillating scalar field) which reheats the Universe.
They have showed that if the inflaton coupling to the SM sector has a sizable strength, the reheating temperature (Tr) due to the inflaton decay can be comparable to or higher than the inflaton mass (mφ). If Tr ≥ mφ, then the dark matter produced by the inflaton has a momentum smaller than those of particles in the thermal bath (consisting of the standard model particles), assuming that the interaction of the DM with the SM particles or itself is negligibly weak. It can help the DM produced by the decay to be cold enough to be consistent with the Lyman-α bound on warm DMs even if the DM mass is smaller than O(1) keV.
They have also showed that, if the DM is bosonic, the production of the DM from the inflaton decay can be enhanced due to the effect of the stimulated emission, like the LASER. The mechanism, called DASER (i.e, the DM amplification by stimulated emotion of radiation), can significantly enhance the DM abundance and can make light bosonic DM scenarios viable. In addition, if the DM is fermionic, the DM produced by the decay may form Dirac sea. The DASER and Dirac sea DMs produced by the inflaton decay may be searched for by future observations of the 21cm lines. The DMs produced by the above mentioned mechanism (i.e. from DASER) have null isocurvature perturbations.
Finally, they comment that the DMs produced in the present scenarios may have very special momentum distribution, and the information about the production mechanism discussed in their paper may be embedded in the momentum distribution of the DM. For example, the momentum of the bosonic DM produced by the DASER mechanism would have sharp peak at the IR mode. In addition, momentum distribution of the modes produced during the preheating depends on the thermal history during the preheating. One may probe the reheating phase or preheating phase if the information about the momentum distribution of the DM becomes available. For this purpose, they concluded that further study of the structure formation with non-standard momentum distribution of the DM is needed.
References: Takeo Moroi, Wen Yin, “Light Dark Matter from Inflaton Decay”, ArXiv, pp. 1-31, 2020. https://arxiv.org/abs/2011.09475v1
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