Baryon asymmetry

The baryon asymmetry problem in physics refers to the imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics, nor the theory of general relativity provides an obvious explanation for why this should be so, and it is a natural assumption that the universe be neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to have been the case, it is likely some physical laws must have acted differently or did not exist for matter and antimatter.

Several competing hypotheses exist to explain the imbalance of matter and antimatter that resulted in baryogenesis. However, there is as of yet no consensus theory to explain the phenomenon. As remarked in a 2012 research paper, "The origin of matter remains one of the great mysteries in physics."

One of the Sakharov conditions for generating baryon asymmetry is that a process is able to happen at a different rate to its antimatter counterpart. This is called CP violation. In the Standard Model, CP violation appears as a complex phase in the quark mixing matrix of the weak interaction. There may also be a non-zero CP-violating phase in the neutrino mixing matrix, but this is currently unmeasured. CP violation was first observed in the 1964 Fitch-Cronin experiment with neutral kaons, which resulted in the 1980 Nobel Prize in physics. It should be noted that, given the limits on baryon number violation, the amount of CP violation in the Standard Model is insufficient to account for the observed baryon asymmetry of the universe, hence beyond-Standard Model sources are needed.

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