[Paper Review] Cosmological Constraints on B-L Violation

Sacha Davidson|ArXiv.org|Aug 26, 1998

Cosmology and Gravitation Theories1 references4 citations

TL;DR

This paper reviews cosmological constraints on B-L violation in the early universe, showing that if the baryon asymmetry was present before the electroweak phase transition, B+L-violating processes would erase it unless at least one B/3 - L_i interaction is strongly suppressed. The argument can be evaded via non-standard cosmology, late generation of asymmetry, or flavor-dependent mass effects.

ABSTRACT

This is a review of the cosmological bounds on $B-L$ violating interactions, and the loopholes in the argument that gives these constraints. If one assumes that the baryon asymmetry we observe today was present above the electroweak phase transition in equilibrium with the non-perturbative $B+L$ violating processes, then interactions that violate all three of ${B/3 - L_i}$ cannot simultaneously be in equilibrium. Otherwise the baryon asymmetry would be washed out. Therefore violation of at least one of the $B/3 - L_i$ must be small. This argument can be evaded by not having the observed baryon asymmetry present in the thermal bath (for instance, make it at the electroweak phase transition), by using a non-standard cosmological model, or possibly by some mass effects in models where the difference between lepton flavour asymmetries is conserved.

Motivation & Objective

• To examine cosmological constraints on B-L violating interactions in the early universe.
• To analyze the conditions under which the observed baryon asymmetry could survive electroweak-scale B+L violating processes.
• To identify loopholes in the standard argument that B+L violation would wash out the baryon asymmetry.
• To explore how non-standard cosmological models or flavor-dependent mass effects might evade the washout constraint.

Proposed method

• Analyzes the equilibrium dynamics of B+L-violating processes in the thermal plasma above the electroweak phase transition.
• Applies the requirement that the observed baryon asymmetry must not be erased by non-perturbative sphaleron processes.
• Derives constraints on the strength of individual B/3 - L_i interactions to prevent washout of the asymmetry.
• Considers alternative scenarios such as the asymmetry being generated at the electroweak phase transition rather than being pre-existing.
• Evaluates the impact of non-standard cosmological models on the washout mechanism.
• Examines the role of lepton flavor asymmetries and mass effects in potentially preserving asymmetries despite B+L violation.

Experimental results

Research questions

• RQ1 Under what conditions can the observed baryon asymmetry survive B+L-violating processes in thermal equilibrium?
• RQ2 How does the assumption of pre-existing asymmetry affect constraints on B-L violation?
• RQ3 What cosmological or model-specific loopholes can prevent washout of the baryon asymmetry?
• RQ4 Can flavor-dependent mass effects in lepton sector preserve asymmetries despite B+L violation?
• RQ5 How do non-standard cosmological models alter the standard washout bounds on B-L violation?

Key findings

• If the baryon asymmetry was present in thermal equilibrium above the electroweak phase transition, B+L-violating processes would erase it unless at least one B/3 - L_i interaction is strongly suppressed.
• The standard washout argument fails if the baryon asymmetry is not in thermal equilibrium with B+L-violating processes, such as if it is generated at the electroweak phase transition.
• Non-standard cosmological models can evade the standard washout constraints by altering the expansion rate or particle production history.
• Mass effects in models with conserved differences between lepton flavor asymmetries may allow partial preservation of the baryon asymmetry despite B+L violation.
• The paper identifies that the B/3 - L_i interactions must be sufficiently weak to avoid washout, placing bounds on their coupling strengths.
• The analysis shows that the observed baryon asymmetry implies a lower bound on the suppression of at least one B/3 - L_i interaction.