Leptogenesis and Neutrino Mass Generation in a Leptoquark Model: A Simultaneous Solution

Core Concepts

This research paper demonstrates that a specific leptoquark model can simultaneously explain the observed baryon asymmetry of the universe and the smallness of neutrino masses through a mechanism called leptogenesis.

Abstract

• Bibliographic Information: Fridell, K. (2024). Leptogenesis and neutrino mass with scalar leptoquarks. arXiv preprint arXiv:2411.03282v1.
• Research Objective: This study investigates whether a model featuring scalar leptoquarks can simultaneously generate the observed baryon asymmetry of the universe (BAU) and the neutrino mass scale, while remaining consistent with existing experimental constraints.
• Methodology: The authors employ a theoretical framework based on Boltzmann equations to model the evolution of particle densities in the early universe. They consider a model with three scalar leptoquarks and analyze their decay and scattering processes, focusing on CP-violating interactions that can lead to leptogenesis. The neutrino mass generation mechanism is also incorporated into the model.
• Key Findings: The research demonstrates that there exist regions in the model's parameter space where both the observed BAU and neutrino mass scale can be generated simultaneously. This occurs through the out-of-equilibrium decay of a specific leptoquark (S1) into a Higgs boson and another leptoquark (R2), with CP-violation arising from interference between tree-level and loop-level decay diagrams. The study highlights the crucial role of ΔL = 2 washout processes, finding that their rate is proportional to the neutrino mass.
• Main Conclusions: The paper concludes that leptoquark models can provide a viable mechanism for simultaneously explaining the BAU and neutrino masses. The authors emphasize the interplay between these two phenomena, showing that a large neutrino mass would lead to significant washout effects, potentially erasing the generated baryon asymmetry.
• Significance: This research contributes to the ongoing effort in particle physics to understand the origin of matter-antimatter asymmetry and the nature of neutrino masses. It suggests that leptoquarks, hypothetical particles that mediate interactions between quarks and leptons, could play a crucial role in the early universe.
• Limitations and Future Research: The study acknowledges the simplification of neglecting thermal effects in the leptogenesis mechanism and suggests further investigation into their potential impact. Additionally, the authors propose exploring similar mechanisms in other radiative neutrino mass models and investigating the potential role of right-handed neutrinos in this context.

Stats

ηobs
B
= (6.20 ± 0.15) × 10−10 (observed baryon asymmetry)
mν ∼ Tr( ˆmν) (neutrino mass scale)
m1 → 0 (smallest neutrino mass scale in the minimum case)
qP
i |Uei|2 ˆm2
i < 0.45 eV (constraint from the KATRIN experiment)
mS3 > mS1 ≫ m ˜R2 ≫ ΛEWSB (mass hierarchy of leptoquarks)
mLQ > 1460 GeV (constraint from ATLAS leptoquark search)
(C1111
¯dLQLH1)−1/3 < 2.4 × 105 GeV (constraint from KamLAND-Zen experiment)
(C1111
¯dLQLH2)−1/3 < 1.4 × 105 GeV (constraint from KamLAND-Zen experiment)
(C2r1t
¯dLQLH1)−1/3 < 2.2 × 104 GeV (constraint from NA62 experiment)
τp→π0e+ > 1.6 × 1034 years (constraint on proton lifetime from Super-Kamiokande)
g′11
1 ≲ 0.9 × 10−9 (constraint on leptoquark coupling from proton decay)
τp→π+ν > 3.9 × 1032 years (constraint on proton lifetime from Super-Kamiokande)
BR(µN → eN)Au < 7 × 10−13 (constraint from SINDRUM II experiment)
g11
2 g21
2 /m2
˜R2 < (500 TeV)2 (constraint from muon-to-electron conversion)

Quotes

Key Insights Distilled From

Leptogenesis and neutrino mass with scalar leptoquarks

by Kåre... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.03282.pdf

Deeper Inquiries

How could the inclusion of thermal effects in the leptogenesis mechanism potentially modify the results and conclusions of this study?

Incorporating thermal effects introduces several modifications to the leptogenesis mechanism, potentially impacting the study's results and conclusions:

Modified CP-violation parameter (ε): Thermal effects can alter the CP-violation parameter (ε) through several mechanisms:

Thermal masses: Particles propagating in a hot, dense medium acquire thermal masses, shifting their dispersion relations and potentially modifying the interference between tree-level and loop-level diagrams contributing to CP-violation.
Modified decay and scattering rates: Thermal corrections to decay and scattering rates can alter the relative populations of particles involved in the leptogenesis process, indirectly impacting ε.
Thermal distributions: Instead of assuming particles follow equilibrium distributions, using thermal distribution functions (like Fermi-Dirac or Bose-Einstein) can lead to more accurate calculations of interaction rates and CP-asymmetries.

Modified Washout Processes: Thermal effects can significantly impact washout processes:

New channels: New washout channels can open up at high temperatures, potentially involving particles not considered in the zero-temperature analysis.
Modified rates: Existing washout rates can be modified due to thermal masses and altered phase space.

Impact on Boltzmann Equations: The Boltzmann equations themselves need modification:

Thermal potential: A thermal potential term arises, reflecting the free energy difference between particles and antiparticles in the thermal bath. This term can influence the evolution of asymmetries.
Modified Hubble rate: The Hubble rate (H) depends on the relativistic degrees of freedom, which can change at high temperatures due to the presence of additional particle species.

Bound State Effects: For particles charged under SU(3)c, bound state formation can become relevant at finite temperature, potentially impacting both the asymmetry generation and washout processes.

Overall Impact:
Including thermal effects can lead to both suppression and enhancement of the final baryon asymmetry. A comprehensive analysis incorporating these effects is crucial for a more accurate determination of the viable parameter space for successful leptogenesis.

Could there be alternative explanations for the observed baryon asymmetry that do not rely on leptoquarks or similar extensions to the Standard Model?

Yes, several alternative explanations for the observed baryon asymmetry exist that don't rely on leptoquarks:

Electroweak Baryogenesis: This scenario utilizes the electroweak phase transition in the early universe. If the transition is strongly first-order, it can generate out-of-equilibrium conditions and CP-violation necessary for baryogenesis. However, the Standard Model alone cannot accommodate a sufficiently strong first-order transition. Extensions like supersymmetry or additional Higgs bosons are required.

GUT Baryogenesis: Grand Unified Theories (GUTs) unify the electroweak and strong forces at high energies. These theories often predict new heavy gauge bosons and Higgs-like particles whose decays in the early universe can violate baryon number and potentially generate the observed asymmetry.

Affleck-Dine Baryogenesis: This mechanism relies on scalar fields carrying baryon or lepton number. In the early universe, these fields can acquire large vacuum expectation values, and their subsequent evolution and decay can generate a baryon asymmetry.

Baryogenesis via CPT Violation: CPT symmetry (charge conjugation, parity, and time reversal) is a fundamental principle in quantum field theory. However, some theories allow for small CPT-violating effects. If present in the early universe, these violations could potentially lead to a baryon asymmetry.

Spontaneous Baryogenesis: This scenario involves a scalar field coupled to baryon number. If this field undergoes a phase transition in the early universe, its time-evolving vacuum expectation value can generate a baryon asymmetry even under thermal equilibrium conditions.

These are just a few examples, and the search for the origin of the baryon asymmetry remains an active area of research in cosmology and particle physics.

If the smallness of neutrino masses is indeed a consequence of the need to preserve the baryon asymmetry generated through leptogenesis, what are the implications for our understanding of the fundamental constants of nature and the possibility of other universes with different values for these constants?

If the smallness of neutrino masses is directly linked to preserving the baryon asymmetry via leptogenesis, it suggests an intriguing connection between seemingly disparate phenomena. This connection could point towards:

Interdependence of Fundamental Constants: The observed values of fundamental constants, like Yukawa couplings and the leptoquark mass scale, might not be arbitrary but rather constrained by the requirement of successful baryogenesis. This interdependence could hint at underlying symmetries or relationships between these parameters.

Anthropic Selection: The observed universe might be just one among a vast landscape of possible universes, each with different values for fundamental constants. In this multiverse scenario, only universes with specific combinations of constants allowing for both baryogenesis and sufficiently small neutrino masses (to avoid overclosure of the universe) could support the formation of structures and life as we know it. Our existence could then be seen as a selection effect within this landscape.

New Physics at High Scales: The connection between neutrino masses and baryogenesis might necessitate new physics beyond the Standard Model at energy scales currently inaccessible to experiments. This new physics could involve additional particles, interactions, or symmetries that govern the interplay between these phenomena.

Implications:

Deeper Understanding of Fundamental Physics: This connection could provide valuable clues for constructing more fundamental theories beyond the Standard Model, potentially unifying different forces and particles.
Exploring the Multiverse: If the anthropic principle plays a role, it raises profound questions about the nature of reality and the possibility of other universes with different physical laws.
Cosmological Implications: The interplay between neutrino masses and baryogenesis could have significant implications for the evolution of the universe, affecting structure formation, the cosmic microwave background radiation, and the abundance of light elements.
Further research, both theoretical and experimental, is crucial to unravel the full implications of this potential connection and its impact on our understanding of the universe.