Anatomy of RHN DM relic in the vanilla scotogenic neutrino mass model

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Hi, I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Anatomy of RHN DM relic in the vanilla scotogenic neutrino mass model".

Jocelyn: Here's the gist of the paper. The gist: The lightest right-handed neutrino (RHN) can be a viable dark matter candidate in the minimal scotogenic neutrino mass model within specific mass ranges and parameter spaces, while displaced vertex signatures offer complementary collider probes for this scenario.

Vera: The Model and Neutrino Mass Generation The scotogenic model generates light neutrino masses radiatively at the one-loop level by incorporating three right-handed neutrinos (N1,2,3) and a scalar doublet η which belong to a Z2-odd dark sector <ref:2601.00436#pg5>.

Jocelyn: The lightest particle running in the loop can naturally serve as a viable dark matter (DM) candidate <ref:2601.00436#pg4>.

Vera: The neutrino mass matrix is given by an expression involving the masses of the particles in the model, structured as (mν)αβ = X3 i=1 yαiy∗ iβ 16π2 MNi

M2 ηR M2 ηR − M2 Ni log (M2 ηR M2 Ni) − M2 ηI M2 ηI − M2 Ni log (M2 ηI M Ni): <ref:2601.00436#pg7>.

Title and authors: Vera: We’ve just gone over the setup of this paper, and now we need to unpack what they actually found about the model itself.

Jocelyn: This section summarizes the core findings regarding how they handle that neutrino mass generation mechanism and what it implies for the dark matter candidate.

Subrahmanyan: Essentially, it confirms that by using three right-handed neutrinos and a scalar doublet eta in a Z2 odd dark sector <ref:2601.00436#pg1>.

Vera: The paper focuses on how they manage the complexity of calculating the relic density, which involves multiple channels like self-annihilation, co-annihilation, and those conversion processes we talked about.

Jocelyn: They show that getting the correct thermal relic density is possible by including all these channels in coupled Boltzmann equations <ref:2601.00436#pg10>.

Subrahmanyan: And they also highlight how the dynamics of those abundance evolutions show that the inverse decay rate minus sixteen over Hubble expansion rate becomes comparable to when eta is in equilibrium, bringing the final DM relic abundance to a ballpark consistent with observations <ref:2601.00436#pg33>.

Vera: So, one major finding is that the correct thermal DM relic density can be achieved across a full dark matter mass range once you incorporate all those conversion-driven processes into the Boltzmann equations <ref:2601.00436#pg7>.

Jocelyn: But there are still specific regions where thermal equilibrium with the Standard Model isn't reached, and in those cases, we need to consider the freeze-in mechanism instead <ref:2601.00436#pg19>.

Subrahmanyan: And they point out that this thermal relic is viable for dark matter masses MDM greater than Mh over two <ref:2601.00436#pg18> but they also find regions, especially at low DM masses, where you need the freeze-in mechanism > Page twenty-six > <ref:2601.00436#pg2>.

Vera: They also link the viable mass strongly to the choice of lambda three, four, and five or equivalently lambda R > Page eighteen > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Jocelyn: And for those regions that are accessible at colliders, they point to displaced vertex signatures as a way to probe things when there are large lambda five, large MDM, and small lambda four > Page twenty-four > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Subrahmanyan: The direct detection searches don't impose new constraints because the relevant interactions only appear at the one-loop level <ref:2601.00436#pg22>.

Vera: They also discuss how the SM Higgs boson h can decay into the neutral components eta R and eta I of that inert doublet > Page twenty-six > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Jocelyn: And they note that a specific choice for lambda four, namely lambda four Meta R, satisfies the LEP constraint on charged scalars and avoids contributions to the invisible decay width of the Z boson provided that Meta R plus Meta I is greater than MZ > Page twenty-six > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Subrahmanyan: The constraints from Higgs invisible decay are shown in a parameter space defined by Meta R minus lambda R > Page twenty-seven > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Vera: They also discuss the effect of co-scattering on the relic density, showing that it’s subdominant compared to the decay and inverse-decay processes within that conversion framework <ref:2601.00436#pg30>.

Jocelyn: The results show that co-scattering contributes non-negligibly to the final relic density only when there are deviations from unity in the rho parameter > Page twenty-five > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Subrahmanyan: To summarize, the paper "Anatomy of RHN DM relic in the vanilla scotogenic neutrino mass model" shows how to handle a complex dark matter candidate, mapping out thermal viability across different mass ranges and pointing towards experimental signatures for those regions where thermal equilibrium is not achieved <ref:2601.00436#pg18>.

Vera: It's a lot of parameter space being mapped out, from the initial model setup through all the relic density mechanisms and the experimental constraints they have to deal with.

Jocelyn: It gives us concrete ways to look for this particle at colliders, like those displaced vertex signatures that depend on mass splitting and DM mass > Page fifty-one > <ref:2601.00436#pg2>.

Subrahmanyan: For cosmology, it tells us that the viability of this dark matter candidate is strongly tied to specific choices in the parameters lambda three, four, and five > Page twenty-six > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

The paper's summary: Tom: So we're looking at how they suggest improving this paper now, and what that means for us later on.

Vera: They are suggesting adding more detail to the relic density calculation by explicitly showing how self-annihilation and co-annihilation fit into those coupled Boltzmann equations <ref:2601.00436#pg11>.

Jocelyn: That's good because it takes the conversion term, Gamma two into one, and breaks down exactly what co-scattering versus decay and inverse decay are doing to the DM abundance <ref:2601.00436#pg11>.

Subrahmanyan: It’s a necessary step because we can’t just assume those conversion processes happen perfectly; we need to see if they actually balance out the depletion of dark matter over time <ref:2601.00436#pg33>.

Vera: Then there's this focus on mapping out the parameter space, specifically MDM versus Delta MR, incorporating all those constraints from S and T parameters and flavor violation data <ref:2601.00436#pg18>.

Jocelyn: That helps us narrow down where this model actually lives in reality; it moves beyond just finding a number to actually drawing a map of the allowed physics <ref:2601.00436#pg26>.

Subrahmanyan: If we can map that space, then we know which combinations of those Yukawa couplings lambda three, four, and five are physically possible for this neutrino mass generation mechanism <ref:2601.00436#pg18>.

Vera: And they’re also looking at the Higgs invisible decay constraints more closely by exploring the M eta R minus lambda R parameter plane > Page twenty-seven > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Jocelyn: That gives us a clearer picture of how tightly constrained that scalar doublet is, especially concerning those LEP bounds on charged scalars and Z boson invisible decays <ref:2601.00436#pg26>.

Subrahmanyan: So, the implication here is we’re getting tighter constraints on the new physics scale dictated by these couplings > Page eighteen > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Vera: The real improvement for us is seeing how this refined parameter space interacts with what we observe from direct detection experiments that are looking for those loop-level interactions <ref:2601.00436#pg22>.

Jocelyn: And they’re also setting up the roadmap for collider searches, specifically showing how to predict the decay length c tau as a function of DM mass and mass splitting Delta M plus > Page fifty-one > <ref:2601.00436#pg2>.

Subrahmanyan: That prediction is useful because it tells us exactly what kind of displaced vertex signature we should look for at facilities like CMS or ATLAS when searching for this particle <ref:2601.00436#pg51>.

Vera: So, they’re taking a model that was complex and showing how to refine the math so we can actually test it against real data from the sky and the lab.

Jocelyn: It’s moving us from just theoretical possibility to concrete experimental targets based on specific mass splittings <ref:2601.00436#pg51>.

Subrahmanyan: This refinement connects cosmology directly to high-energy physics searches, which is exactly where we need to be looking right now <ref:2601.00436#pg51>.

The paper's improvements: Vera: So, to wrap up this look at "Anatomy of RHN DM relic in the vanilla scotogenic neutrino mass model," we’ve seen how they nail down viable regions for this right-handed neutrino dark matter candidate.

Jocelyn: It’s clear that achieving the correct thermal relic density isn't just a theoretical exercise; it depends on these specific choices you have to make in the couplings, like lambda four and lambda five <ref:2601.00436#pg18>.

Subrahmanyan: And for me, what this means is that we need observational data that can constrain those coupling parameters because right now they are quite broad across the mass range <ref:2601.00436#pg18>.

Vera: Exactly; these constraints from flavor violation and the muon magnetic moment are really helping us narrow down where this whole model might be physically real > Page twenty-six > <ref:2601.00436#pg2> <ref:2601.00436#pg2>.

Jocelyn: And for those specific regions where thermal equilibrium doesn't happen, we’ve got the freeze-in mechanism as a way to generate the dark matter density instead of relying on standard annihilation <ref:2601.00436#pg19> > <ref:2601.00436#pg19>.

Subrahmanyan: That freeze-in scenario is intriguing because it opens up a whole new production channel that might be testable with very different experimental setups than what we usually run <ref:2601.00436#pg19> > <ref:2601.00436#pg19>.

Vera: And for us, the observational astronomer side, it points us toward looking for those displaced vertex signatures at colliders when lambda five is large and delta M plus is small enough to make sense > Page fifty-one > <ref:2601.00436#pg2>.

Jocelyn: I think that’s the big payoff because it gives us a clear signature to hunt for if we build those future searches correctly <ref:2601.00436#pg51> > <ref:2601.00436#pg51>.

Subrahmanyan: It really shows how cosmology and structure formation connect with these particle physics models through the relic density calculation of this RHN DM candidate <ref:2601.00436#pg51> > <ref:2601.00436#pg51>.

Vera: So, this paper gives us a solid framework for where to look in the parameter space and what kind of signals to expect from future experiments on both the ground and in space > Page fifty-one > <ref:2601.00436#pg2>.

Jocelyn: It's a good roadmap for us as we plan what follow-up observations are needed to settle these uncertainties about the DM abundance <ref:2601.00436#pg51> > <ref:2601.00436#pg51>.

Subrahmanyan: I'm still waiting for those direct detection results to come in, because that’s the ultimate confirmation or rejection of whether this model has any footing in reality <ref:2601.00436#pg51> > <ref:2601.00436#pg51>.

Conclusion: Vera: So we've covered the full scope of "Anatomy of RHN DM relic in the vanilla scotogenic neutrino mass model," focusing on how this right-handed neutrino acts as dark matter.

Jocelyn: It really boils down to how you get that correct thermal relic density, which depends heavily on those specific choices in the couplings like lambda four and lambda five.

Subrahmanyan: And for me, what this means is that we need observational data to constrain those coupling parameters because right now they are quite broad across the mass range for this fermionic candidate.

Vera: Exactly; these constraints from flavor violation and the muon magnetic moment are really helping us narrow down where this whole model might be physically real.

Jocelyn: And for those specific regions where thermal equilibrium doesn't happen, we’ve got the freeze-in mechanism as a way to generate the dark matter density instead of relying on standard annihilation.

Subrahmanyan: That freeze-in scenario is intriguing because it opens up a whole new production channel that might be testable with very different experimental setups than what we usually run.

Vera: And for us, the observational astronomer side, it points us toward looking for those displaced vertex signatures at colliders when lambda five is large and delta M plus is small enough to make sense.

Jocelyn: I think that’s the big payoff because it gives us a clear signature to hunt for if we build those future searches correctly.

Subrahmanyan: It really shows how cosmology and structure formation connect with these particle physics models through the relic density calculation of this RHN DM candidate.

Vera: So, this paper gives us a solid framework for where to look in the parameter space and what kind of signals to expect from future experiments on both the ground and in space.

Jocelyn: It's a good roadmap for us as we plan what follow-up observations are needed to settle these uncertainties about the DM abundance.

Subrahmanyan: I'm still waiting for those direct detection results to come in, because that’s the ultimate confirmation or rejection of whether this model has any footing in reality.