Electroweak baryogenesis

Electroweak baryogenesis
Name	Electroweak baryogenesis
Field	Particle physics, Cosmology
Introduced	1980s
Key people	Andrei Sakharov, Mikhail Shaposhnikov, Mark Trodden, Tomislav Prokopec, Michael Dine

Electroweak baryogenesis is a theoretical framework proposing that the observed matter–antimatter asymmetry of the Universe arose during the electroweak interaction epoch when the Electroweak phase transition occurred. It seeks to realize the Sakharov conditions in the context of the Standard Model and its extensions by exploiting dynamics of the Higgs boson, electroweak sphalerons, and sources of CP violation active at temperatures near the electroweak scale. Proposals connect particle physics, early Big Bang cosmology, and experimental probes from collider experiments to astrophysical observations.

Introduction

Electroweak baryogenesis situates baryon asymmetry generation at the electroweak epoch shortly after the Big Bang, leveraging nonperturbative processes in the SU(2) sector of the Standard Model. Early proposals built on work by Andrei Sakharov and investigations by researchers including Mikhail Shaposhnikov and Michael Dine, aiming to reconcile the baryon-to-photon ratio measured by Wilkinson Microwave Anisotropy Probe and Planck with dynamics accessible to accelerators such as the Large Hadron Collider. The scenario requires new physics beyond minimal Glashow–Weinberg–Salam model realizations to meet all necessary criteria.

Sakharov conditions and electroweak context

Sakharov's three conditions—baryon number violation, C and CP violation, and departure from thermal equilibrium—frame electroweak baryogenesis. Baryon number violation in this context arises from electroweak sphaleron transitions associated with anomalous B+L violation in the SU(2) gauge sector, studied using semiclassical techniques and lattice computations by groups associated with CERN, Fermilab, and university collaborations. C and CP violation in the Standard Model originate from the Cabibbo–Kobayashi–Maskawa matrix and the QCD theta term, but these sources are typically too small; proposals invoke extensions like the Minimal Supersymmetric Standard Model, two-Higgs-doublet model, or new interactions motivated by ideas from Grand Unified Theory research and model builders around institutions such as SLAC National Accelerator Laboratory and IPPP. Departure from equilibrium is realized via a first-order electroweak phase transition, a property sensitive to parameters measured by experimental collaborations including ATLAS and CMS.

Electroweak phase transition and dynamics

The order and dynamics of the phase transition determine whether expanding bubble walls of broken electroweak symmetry can drive out-of-equilibrium processes. Lattice gauge theory calculations pioneered by groups at Yale University and University of Oxford explored the finite-temperature Higgs potential, showing that with a 125 GeV Higgs boson as measured by ATLAS and CMS, the transition in the minimal Standard Model is a crossover rather than strongly first-order. This motivated model building invoking scalar extensions, singlet fields, and supersymmetric partners studied by teams at University of Chicago, Harvard University, and Princeton University, which can produce strong first-order transitions with bubble nucleation, wall velocities, and friction coefficients computed using methods from nonequilibrium field theory developed by researchers at Perimeter Institute and CERN Theory Department.

Sources of CP violation in the electroweak sector

Successful baryogenesis requires CP-violating interactions beyond the tiny phase in the Cabibbo–Kobayashi–Maskawa matrix. Proposed sources include complex phases in the supersymmetry soft-breaking terms of the Minimal Supersymmetric Standard Model favored in early work by Mark Trodden and Tomislav Prokopec, additional phases in two-Higgs-doublet model Yukawa couplings, and CP-violating terms from singlet-extended Higgs sectors studied by groups at KEK and DESY. Experimental limits from electric dipole moment searches performed at institutions like ARGUS-era collaborations, modern measurements at Stanford Linear Accelerator Center, and dedicated experiments such as those at Institut Laue–Langevin and PSI constrain many of these phases, motivating stealthy or flavor-aligned constructions examined by theorists at MIT and Caltech.

Mechanisms of baryon number generation and transport

Baryon number generation in electroweak baryogenesis proceeds via CP-violating interactions at bubble walls that bias chiral charge densities; these biases are converted to baryon number by sphaleron processes in the symmetric phase. Transport equations, including semiclassical force formalisms and quantum kinetic approaches developed by researchers at University of Barcelona and University of Hamburg, model diffusion, reflection, and transmission of fermions across bubble walls. Scenarios differ between local baryogenesis near the wall and nonlocal transport where chiral asymmetries diffuse ahead of the wall; implementations often reference computational methods used in studies at Los Alamos National Laboratory and the Max Planck Institute.

Experimental constraints and observational prospects

Collider measurements of the Higgs potential and searches for new scalars, supersymmetric partners, or vectorlike fermions at LHC experiments ATLAS and CMS constrain parameter space. Precision probes such as electric dipole moment experiments by collaborations at Imperial College London and University of Washington set stringent limits on CP-violating phases. Cosmological observations—primarily the baryon asymmetry inferred from big bang nucleosynthesis and anisotropies measured by Planck—provide targets for model predictions. Future facilities like the proposed Future Circular Collider and proposed EDM searches at institutions including TRIUMF and JILA could further test viable models, while gravitational wave observatories inspired by concepts from LIGO and planned missions such as LISA offer complementary probes of strong first-order phase transitions.

Theoretical extensions and alternative models

Because the minimal Standard Model appears insufficient, many extensions have been proposed: Supersymmetry-based realizations such as the MSSM and next-to-minimal variants, scalar-extended frameworks including two-Higgs-doublet model and singlet extensions, and scenarios inspired by Composite Higgs models developed at research centers like CERN and IPMU. Alternative baryogenesis paradigms—such as leptogenesis associated with seesaw mechanism neutrino mass generation, Affleck–Dine baryogenesis tied to supersymmetric flat directions, and baryogenesis via heavy particle decays in Grand Unified Theory frameworks—remain actively compared against electroweak mechanisms by communities at Institute for Advanced Study and major universities worldwide.

Category:Cosmology