Baryogenesis

Overview

Baryogenesis is the hypothetical physical process that took place during the early universe to produce baryonic asymmetry, i.e., the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe. Despite the fact that the Standard Model of particle physics suggests that the Big Bang should have produced equal amounts of matter and antimatter, observations indicate that the universe is composed almost entirely of matter. This discrepancy is addressed by the theory of baryogenesis.

Theoretical Background

The term "baryogenesis" was coined by Andrei Sakharov in 1967, who identified three necessary conditions for baryogenesis in a seminal paper. These are known as the Sakharov conditions, and they provide the theoretical underpinning for our understanding of this complex process.

Sakharov Conditions

The Sakharov conditions are as follows:

1. Baryon number violation: A process that can change the number of baryons. In the Standard Model, this is only possible through non-perturbative effects such as sphalerons.

2. C and CP violation: Both charge conjugation symmetry (C) and charge-parity symmetry (CP) must be violated. This means that the laws of physics should be different for particles and antiparticles.

3. Departure from thermal equilibrium: The universe must be in a non-equilibrium state. This is because in thermal equilibrium, the rate of baryon production would be equal to the rate of antibaryon production, leading to no net baryon number.

These conditions are necessary for any viable baryogenesis scenario.

Baryon Number Violation

In the Standard Model, baryon number violation can occur through processes known as sphaleron transitions. These are non-perturbative effects that can change the baryon number. However, these processes are not sufficient to explain the observed baryon asymmetry, as they are suppressed at low temperatures.

C and CP Violation

C and CP violation are necessary conditions for baryogenesis. In the Standard Model, CP violation occurs in the weak interactions through the phase of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. However, this source of CP violation is not sufficient to explain the observed baryon asymmetry.

Departure from Thermal Equilibrium

A departure from thermal equilibrium is necessary for baryogenesis. In the early universe, this could be achieved through a phase transition, such as the electroweak phase transition.

Models of Baryogenesis

There are several models of baryogenesis that have been proposed to explain the observed baryon asymmetry. These include electroweak baryogenesis, GUT baryogenesis, and leptogenesis.

Electroweak Baryogenesis

Electroweak baryogenesis is a scenario in which the baryon asymmetry is generated during the electroweak phase transition in the early universe. This scenario is attractive because it can be tested at high-energy particle accelerators.

GUT Baryogenesis

GUT baryogenesis is a scenario in which the baryon asymmetry is generated during a phase transition associated with the breaking of a Grand Unified Theory (GUT). This scenario is less testable than electroweak baryogenesis, as it involves energy scales far beyond the reach of current particle accelerators.

Leptogenesis

Leptogenesis is a scenario in which the baryon asymmetry is generated indirectly through a lepton asymmetry. This scenario is motivated by the seesaw mechanism for neutrino masses.

Experimental Tests

While direct experimental tests of baryogenesis are currently not possible due to the high energy scales involved, there are several indirect tests that can provide evidence for a particular baryogenesis scenario. These include the search for neutrinoless double beta decay, the measurement of the electric dipole moments of elementary particles, and the study of matter-antimatter asymmetry in meson decays.