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Freezing In with Lepton Flavored Fermions
by G. D'Ambrosio, Shiuli Chatterjee, Ranjan Laha and Sudhir K Vempati
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|Authors (as registered SciPost users):||Shiuli Chatterjee · Giancarlo D'Ambrosio · Ranjan Laha · Sudhir Kumar Vempati|
|Preprint Link:||scipost_202103_00013v1 (pdf)|
|Date submitted:||2021-03-10 14:05|
|Submitted by:||Chatterjee, Shiuli|
|Submitted to:||SciPost Physics|
Dark, chiral fermions carrying lepton flavor quantum numbers are natural candidates for freeze-in. Small couplings with the Standard Model fermions of the order of lepton Yukawas are `automatic' in the limit of Minimal Flavor Violation. In the absence of total lepton number violating interactions, particles with certain representations under the flavor group remain absolutely stable. For masses in the GeV-TeV range, the simplest model with three flavors, leads to signals at future direct detection experiments like DARWIN. Interestingly, freeze-in with a smaller flavor group such as $SU(2)$ is already being probed by XENON1T.
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- Cite as: Anonymous, Report on arXiv:scipost_202103_00013v1, delivered 2021-05-03, doi: 10.21468/SciPost.Report.2869
The authors study lepton flavoured fermions as freeze-in dark matter candidates. These models are attractive because dark matter stability can follow automatically from lepton number conservation. And freeze-in appears natural as a consequence of the smallness of the electron Yukawa coupling. The authors choose a minimal model and analyse the relic abundance constraint as well as direct detection constraints on the model. The analysis is solid and the results appear reasonable, the paper certainly meets the acceptance criteria of the journal. There are a few minor points that could be discussed in more detail or improved in my opinion:
1. I think it would be useful to show Lambda_MFV, e.g. in Fig. 2. This would give the readers a better idea how suppressed e.g. higher dimensional operators would be.
2. For the relic density computation, I wonder if there could be a relevant contribution from Z decays to DM pairs, while it appears that only 2->2 scatterings were taken into account. Also WW-> chi_1 chi_1 scatterings should become relevant at higher reheating temperatures.
3. What is the lifetime of the heavier DM components? These are typically more strongly coupled due to the larger Yukawas, so they might play a role in the cosmological history of the model.
4. The caption of Fig. 4 is a bit short. It should be explained more clearly what the T_RH contours mean (I assume these are the relic density contours for the given T_RH value?)
5. I would like to see a discussion of the dimension 6 Lagrangian. Some operators there might be allowed that could affect the relic density if Lambda_MFW is not too large. I wonder in particular if at dimension 6 one can write down operators which are invariant under G_LF that involve both leptons and dark matter particles, and which are not suppressed by insertions of Yukawa couplings.
6. Also a longer discussion of neutrino masses would be nice. If Lepton number is imposed as exact global symmetry, then the neutrinos are Dirac, and the RH neutrinos should also transform as multiplets of the Lepton flavour group. This could allow many more operators maybe even at the level of dimension 4 and 5. Furthermore additional constraints arise, since the RH neutrinos should not be thermalised, otherwise Neff constraints could be violated.