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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 | |
|---|---|
| Preprint Link: | https://arxiv.org/abs/2204.03670v2 (pdf) |
| Date submitted: | 2022-05-06 10:33 |
| Submitted by: | Banerjee, Avik |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| 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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Anonymous Report 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).