Baryogenesis

The Baryogenese is the theory for the dynamic creation (Greek. Genesis ) of the baryon d. H. the imbalance of matter ( baryons ) and antimatter (antibaryons) in the universe . Baryogenesis was followed by the much better understood primordial nucleosynthesis .

Question

Our visible universe consists predominantly of matter and only a small fraction of antimatter. Both types of matter annihilate when they meet, releasing energy in an annihilation reaction .

On the one hand, there is the possibility of assuming that this asymmetry represents a (random) initial condition of the universe.
On the other hand, it would be more natural to assume that matter and antimatter were present in equal quantities at the beginning of the universe and that the asymmetry only emerged dynamically during the development of the universe up to the present time. Theoretical models that accomplish this are summarized under the term baryogenesis.

Sakharov's conditions for baryon asymmetry

Andrei Sakharov was the first to recognize in 1967 the conditions necessary for the appearance of asymmetry. Independently of him, the Russian researcher Vadim Alexejewitsch Kuzmin found these conditions in 1970 .

Sakharov's work was not known in the West for a long time. Before it became known, Leonard Susskind published his own theories of baryogenesis.

The different theories differ in the way in which these conditions are theoretically met in detail.

Failure to maintain the baryon number

The number of baryons must not be constant, but must change in the course of the Big Bang . ${\ displaystyle B \! \,}$

Fulfillment: In GUT (Great Unified Theory) there are two ways a heavy X boson could decay. Both series of decays occur with different probabilities , which changes the number of baryons:

51%: X boson → up quark + up quark

49%: X boson → anti down quark + positron

{\ displaystyle \ Rightarrow B \! \,}

= 0.51 × (+2/3) + 0.49 × (−1/3) = +0.177

The antiparticle of the X boson, on the other hand, decays as follows:

49%: Anti X boson → anti up quark + anti up quark

51%: Anti X boson → down quark + electron

{\ displaystyle \ Rightarrow B \! \,}

= 0.49 × (-2/3) + 0.51 × (+1/3) = -0.157

This violation of the number of baryons can be explained by the sphaleron interaction between quarks and leptons .

Symmetry breaking

Two symmetries between particle and antiparticle have to be broken.

Fulfillment: At the moment only very weak CP injuries are known that cannot explain the existence of the baryon asymmetry. The CP symmetry (C for charge charge ; P for parity parity ) describes a course in which a particle is transformed into an antiparticle (or vice versa) and the parity (the spatial orientation) changes. If this course cannot be clearly reversed, one speaks of a symmetry breaking .

Thermal imbalance

There must be a loss of thermal equilibrium .

Fulfillment: The universe cools through its expansion from. As a result, not all reactions can take place with the same probability, and two quarks no longer have the necessary energy to generate an X boson.

Leptogenesis

Newer theories on the origin of baryon asymmetry favor leptogenesis . Here the asymmetry is first created between leptons and antileptons and then converted into baryon asymmetry by Sphaleron processes.

literature

Spectrum of Science, James M. Cline, Nov. 2004, 32-41
Electroweak baryogenesis by ME Shaposhnikov , Contemporary Physics, 1998, Volume 39, Page 177