Editing Proton decay (section)

== Baryogenesis ==
{{Main| Baryogenesis}}
{{unsolved|physics|Do protons [[Radioactive decay|decay]]? If so, then what is the [[half-life]]? Can [[nuclear binding energy]] affect this?}}
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 &ndash; that is, there is more matter than antimatter. Since it is assumed in [[physical cosmology|cosmology]] that the particles we see 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. This has led to a number of proposed mechanisms for [[symmetry breaking]] that favour the creation of normal matter (as opposed to antimatter) under certain conditions. This imbalance would have been exceptionally small, on the order of 1 in every 10<sup>10</sup> particles a small fraction of a second after the Big Bang, but after most of the matter and antimatter annihilated, what was left over was all the baryonic matter in the current universe, along with a much greater number of [[boson]]s.

Most grand unified theories explicitly break the baryon number symmetry, which would account for this discrepancy, typically invoking reactions mediated by very massive [[X boson]]s {{nowrap|({{SubatomicParticle|X boson}})}} or massive [[Higgs boson]]s ({{SubatomicParticle|Higgs boson}}). The rate at which these events occur is governed largely by the mass of the intermediate {{SubatomicParticle|X boson}} or {{SubatomicParticle|Higgs boson}} 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.