Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino

summary

The gist

The gist: This model discusses leptogenesis in a framework featuring a GeV-scale quasi-Dirac neutrino, showing that Higgs-decay leptogenesis can naturally arise from spontaneous symmetry breaking and

In short

The model explores leptogenesis using a GeV-scale quasi-Dirac neutral lepton, showing that Higgs decay can generate the observed baryon asymmetry through thermal effects. This mechanism arises from spontaneous symmetry breaking and provides a viable parameter space accessible to future experiments like SHiP and FCC-ee. The framework also predicts observable gravitational waves.

Key concepts

Quasi-Dirac Neutrino: This is a type of neutral lepton that is nearly its own antiparticle, existing as two quasi-degenerate states (N1 and N2). They have very similar masses, with a tiny mass splitting that is crucial for the resonant enhancement needed to generate the CP asymmetry required for leptogenesis.
Higgs-Decay Leptogenesis: This mechanism generates the baryon asymmetry by having Higgs bosons decay into these quasi-Dirac neutrinos. The CP asymmetry arises from the interplay between how these neutrino masses are split in a vacuum and how they are affected by finite temperature effects during the early universe.
Resonant Enhancement: For successful leptogenesis at low scales, the CP asymmetry is maximized when there is a resonance between the mass splitting of the neutrinos and thermal corrections ($\Delta M_N \sim \Gamma_T$). This resonance allows for a significant generation of the asymmetry even with relatively small mass differences.
Stochastic Gravitational-Wave Background: The same symmetry breaking sector that drives leptogenesis also produces unstable domain walls. The annihilation of these walls in the early universe creates a background of gravitational waves. This provides an independent, complementary way to probe the underlying physics of the model.

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This episode discusses

The paper

Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino · Read on arXiv

Stephen F. King, *Soumen Kumar Manna, †Rishav Roshan, ↵Arunansu Sil

School of Physics and Astronomy, University of Southampton · Department of Physics, Indian Institute of Technology Bombay · Department of Physics, Indian Institute of Technology Guwahati

We discuss leptogenesis in a model with a GeV-scale quasi-Dirac neutral lepton, comprising mainly sterile states, which is accessible to future experiments. In particular, we show that Higgs-decay leptogenesis can naturally arise from the type-I seesaw mechanism with spontaneously broken global lepton number together with a discrete Z 2 symmetry, yielding two quasi-degenerate right-handed neutrinos (RHNs) which, to excellent approximation, form a single Dirac neutrino, with a tiny mass splitting as required for resonant enhancement generated by higher-dimensional lepton-number-violating operators. Incorporating finite-temperature effects, we show that the observed baryon asymmetry can be successfully generated through thermal Higgs decays for 0.1-10 GeV mass quasi-Dirac neutrinos, with the CP asymmetry arising from the interplay between the vacuum and thermal contributions to the mass splitting. In the parameter region of successful leptogenesis, the quasi-Dirac neutrino is accessible to future heavy neutral lepton searches at SHiP, MATHUSLA and FCC-ee. In addition, the same symmetry-breaking sector gives rise to unstable domain walls whose annihilation can generate a stochastic gravitational-wave background, providing a complementary probe of the framework.

Transcript

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

Vera: We're looking at the paper: "Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino".

Jocelyn: The gist: This model discusses leptogenesis in a framework featuring a GeV-scale quasi-Dirac neutrino,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: To recap, this paper explores leptogenesis using a GeV-scale quasi-Dirac neutral lepton model. The main idea is that Higgs-decay leptogenesis can arise naturally from the type-I seesaw mechanism when you have spontaneously broken global lepton number with a discrete Z2 symmetry.

Jocelyn: They show that this setup yields two quasi-degenerate right-handed neutrinos, N1 and N2, which essentially act like a single Dirac neutrino with just a tiny mass splitting.

Subrahmanyan: The crucial part is that this small splitting isn't arbitrary; it emerges naturally from the underlying symmetry structure through dimension-five operators that explicitly break lepton number while keeping the Z2 symmetry intact.

Vera: They then incorporate finite-temperature effects, showing that the observed baryon asymmetry can be successfully generated through thermal Higgs decays for neutrino masses in the range of zero point one to ten GeV <ref:2610.20729#pg1,that the observed baryon asymmetry can be successfully generated through thermal Higgs>.

Jocelyn: The CP asymmetry they find is intrinsically thermal, arising from the interaction between the symmetry-induced vacuum mass splitting and those temperature corrections to the RHN masses.

Subrahmanyan: This means that even at these low energy scales, we can generate a net baryon number if those resonance conditions are met in the thermal environment.

Vera: The paper also looks at experimental reach, showing that the viable parameter space is accessible to future searches at facilities like SHiP, MATHUSLA, and FCC-ee.

Jocelyn: And they add that this symmetry breaking sector also generates unstable domain walls which can create a stochastic gravitational-wave background as a complementary probe.

Conclusion: Vera: So, the authors, Stephen F. King, Soumen Kumar Manna, Rishav Roshan, and Arunansu Sil3, have put together a model that connects the requirements for generating the baryon asymmetry to the discoverability of these new particles.

Jocelyn: It boils down to this: if we can find evidence for active-sterile neutrino mixing in specific ways, it could point toward this type of GeV-scale quasi-Dirac neutrino scenario being responsible for our universe's matter density.

Subrahmanyan: The implication is that the model offers a concrete way to link particle physics structure directly to cosmological observations like the observed baryon asymmetry.

Vera: It suggests that testing for these specific mixing patterns might be a more direct path than just waiting for some massive collider to find these particles.

Jocelyn: It really frames the search not just as hunting for a specific mass, but as probing the fundamental symmetries that govern how matter behaves in the early universe.

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