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Fingerprints of freeze-in dark matter in an early matter-dominated era
by Avik Banerjee, Debtosh Chowdhury
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Submission summary
| Authors (as registered SciPost users): | Avik Banerjee · Debtosh Chowdhury |
| Submission information | |
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| Preprint Link: | https://arxiv.org/abs/2204.03670v2 (pdf) |
| Date submitted: | May 6, 2022, 10:33 a.m. |
| Submitted by: | Banerjee, Avik |
| Submitted to: | SciPost Physics |
| Ontological classification | |
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| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Phenomenological |
Abstract
We study the impact of an alternate cosmological history with an early matter-dominated epoch on the freeze-in production of dark matter. Such early matter domination is triggered by a meta-stable matter field dissipating into radiation. In general, the dissipation rate has a non-trivial temperature and scale factor dependence. Compared to the usual case of dark matter production via the freeze-in mechanism in a radiation-dominated universe, in this scenario, orders of magnitude larger coupling between the visible and the dark sector can be accommodated. Finally, as a proof of principle, we consider a specific model where the dark matter is produced by a sub-GeV dark photon having a kinetic mixing with the Standard Model photon. We point out that the parameter space of this model can be probed by the experiments in the presence of an early matter-dominated era.
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Reports on this Submission
Report #1 by Anonymous (Referee 1) on 2022-6-1 (Invited Report)
- Cite as: Anonymous, Report on arXiv:2204.03670v2, delivered 2022-06-01, doi: 10.21468/SciPost.Report.5163
Report
This paper considers the freeze-in production of dark matter during an early matter-dominated (EMD) epoch. A common assumption in these scenarios is that the field responsible for EMD decays with a constant decay rate into SM particles (as would be the case for a particle with mass much larger than the temperature). The authors consider a complementary range of models where the decay rate instead is scale-factor and/or temperature dependent. This can occur if the mass of the decaying particle is comparable to the temperature (such that the backreaction from the final state particles is significant) or if the potential of the decaying particle is not quadratic (in models where this field is an oscillating scalar). This is an interesting and important exercise since it can parametrically change our expectations for the relationship between the DM mass and coupling to SM particles. As the authors point out, in some models this leads to significant improvements in the testability of freeze-in models, which are notoriusly difficult to probe due to the (usually) tiny couplings needed to saturate the relic abundance.
The paper is well written and mostly clear. I will recommend the preprint for publication after the authors have addressed the following minor issues: 1) In the first paragraph the authors claim that "Such a tiny interaction strength renders freeze-in dark matter invisible to the experiments". This is not true; in fact, freeze-in DM is one of the main benchmarks targeted by low mass direct detection experiments. See, for example, Fig. 21 in https://arxiv.org/pdf/1904.07915.pdf and the references in the figure caption.
2) The detectability of freeze-in in an EMD cosmology was also mentioned in https://arxiv.org/pdf/1807.01730.pdf in Sec. III.F in a dark photon model similar to the one considered by the authors (in much more generality).
3) While the authors provide an extensive set of references that describe various models for the dissipation behaviour of the EMD field, I think the paper could benefit from a more explicit mapping between benchmark models considered in Table I and these references or concrete physical scenarios. Specifically, I think Table I can be extended with one or two extra columns giving a reference where the parameters in column two appear and a brief description of the scalar potential or particle mass that gives rise to this behaviour (if it possible to describe them briefly).
4) The benchmark models in Table I and Fig. 1, as well as several equations are close to those of https://arxiv.org/pdf/2007.04328.pdf. While the authors do cite this paper, they should make the citation more prominent whenever their results are used or used as direct motivation for the authors' study; if the authors have reproduced analytic results from that work, they should comment on whether they find agreement or not.
5) On page 7, the authors say "it is evident that one would need a larger coupling between the DM and the SM particles to achieve the observed relic abundance in presence of matter domination" after saying that their chosen lambda gives rise to overabundance. This seems countintuitive since the production rate in Eq. 3.3 (and indeed the freeze-in abundance) is proportional to the coupling; smaller abundance should therefore correspond to smaller couplings, as typical for freeze-in. Please clarify this point.
6) In Fig. 3 the authors show a region in gray in which thermal equilibrium is attained and therefore freeze-in is not possible. From what I can tell, this comes from comparing the 2->2 rate SM SM -> chi chi with the Hubble rate. However there may be other processes that are important, such as inverse decays SM SM -> A', and the A' thermalizing amongst themselves and chi via U(1)_D gauge interactions. The authors should comment on whether these are parametrically similar to the constraint shown.
7) In Fig. 3 there may be additional relevant bounds coming from supernova, see https://arxiv.org/pdf/1901.08596.pdf (visibly decaying dark photons) and https://arxiv.org/pdf/1905.09284.pdf (dark photons decaying to dark matter).