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In physical cosmology, baryogenesis is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe.

One of the outstanding problems in modern physics is the predominance of matter over antimatter in the universe. The universe, as a whole, seems to have a nonzero positive baryon number density – that is, matter exists. Since it is assumed in cosmology that these particles were created using the same physics we measure today, it would normally be expected that the overall baryon number should be zero, as matter and antimatter should have been created in equal amounts. A number of proposed mechanisms are proposed to account for this discrepancy, namely identifying conditions that favour symmetry breaking and the creation of normal matter (as opposed to antimatter). This imbalance would have been exceptionally small, on the order of 1 in every 16300000000 (~2×1010) particles a small fraction of a second after the Big Bang. After most of the matter and antimatter was annihilated, what remained was all the baryonic matter in the current universe, along with a much greater number of bosons. Experiments reported in 2010 at Fermilab, however, seem to show that this imbalance is much greater than previously assumed. In an experiment involving a series of particle collisions, the amount of generated matter was approximately 1% larger than the amount of generated antimatter. The reason for this discrepancy is yet unknown.

Most grand unified theories explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive X bosons (

X

) or massive Higgs bosons (

H0

). The rate at which these events occur is governed largely by the mass of the intermediate

X

or

H0

particles, so by assuming these reactions are responsible for the majority of the baryon number seen today, a maximum mass can be calculated above which the rate would be too slow to explain the presence of matter today. These estimates predict that a large volume of material will occasionally exhibit a spontaneous proton decay, which has not been observed. Therefore, the imbalance between matter and antimatter remains a mystery.

Baryogenesis theories are based on different descriptions of the interaction between fundamental particles. Two main theories are electroweak baryogenesis (standard model), which would occur during the electroweak epoch, and the GUT baryogenesis, which would occur during or shortly after the grand unification epoch. Quantum field theory and statistical physics are used to describe such possible mechanisms.

Baryogenesis is followed by primordial nucleosynthesis, when atomic nuclei began to form.

Background
The majority of ordinary matter in the universe is found in atomic nuclei, which are made of neutrons and protons. These nucleons are made up of smaller particles called quarks, and antimatter equivalents are predicted to exist by the Dirac equation in 1928. Since then, each kind of antiquark has been experimentally verified. Hypotheses investigating the first few instants of the universe predict a composition with an almost equal number of quarks and antiquarks. Once the universe expanded and cooled to a critical temperature of approximately 2×1012 K, quarks combined into normal matter and antimatter and proceeded to annihilate up to the small initial asymmetry of about one part in five billion, leaving the matter around us. Free and separate individual quarks and antiquarks have never been observed in experiments—quarks and antiquarks are always found in groups of three (baryons) or bound in quark–antiquark pairs (mesons). Likewise, there is no experimental evidence that there are any significant concentrations of antimatter in the observable universe.

There are two main interpretations for this disparity: either the universe began with a small preference for matter (total baryonic number of the universe different from zero), or the universe was originally perfectly symmetric, but somehow a set of phenomena contributed to a small imbalance in favour of matter over time. The second point of view is preferred, although there is no clear experimental evidence indicating either of them to be the correct one.

Ties to Dark Matter
A possible explanation for the cause of baryogenesis is the decay reaction of B-Mesogenesis. This phenomena suggests that in the early universe, particles such as the B-meson decay into a visible Standard Model baryon as well as a dark antibaryon that is invisible to current observation techniques. The process begins by assuming a massive, long-lived scalar particle Φ that exists in the early universe before Big Bang Nucleosynthesis. The exact behavior of Φ is as yet unknown, but it is assumed to decay into b quarks and anti-quarks in conditions outside of thermal equilibrium (such as Baryogenesis), thus satisfying one Sakharov condition. These b quarks form into B-mesons, which immediately hadronize into oscillating CP-violating $$B^0_s-\bar{B}^0_s $$ states, thus satisfying another Sakharov condition. These oscillating mesons then decay down into the baryon-dark antibaryon pair previously mentioned, $$B \rightarrow \psi \mathcal{B} \mathcal{M}$$, where $$B$$ is the parent B-meson, $$\psi$$ is the dark antibaryon, $$\mathcal{B}$$ is the visible baryon, and $$\mathcal{M}$$ is any extra light mesons required to satisfy other conservation laws in this decay. If this process occurs fast enough, the CP-violation effect gets carried over dark matter sector. This contradicts the last Sakharov condition, since although there is a matter preference in the visible universe as expected, there is a contrary antimatter preference in the dark matter of the universe and the total baryon number is actually conserved.

B-Mesogenesis results in missing energy between the initial and final states of the decay process, which, if recorded, could provide experimental evidence for dark matter. Particle laboratories equipped with B-meson factories such as Belle and BaBar are extermely sensitive to B-meson decays involving missing energy and currently have the capability to detect the $$B \rightarrow \psi \mathcal{B} \mathcal{M}$$ channel. The LHC is also capable of searching for this interaction since it produces several orders of magnitude more B-mesons than Belle or BaBar, but there are more challenges in the lost control over the initial B-meson energy in the accelerator.