Nu Approach to CP Violation

The measured abundance of helium in the universe (about 25% of all normal matter) suggests that there is about one proton for every 1010 photons. This in turn suggests that at some earlier phase of the universe an almost equal number of protons and anti-protons existed and gradually annihilated, but that because of some fundamental asymmetry (at the level of one part per ten billion) in the way that the weak nuclear force treats matter and antimatter, protons but not anti-protons survived to the present time.

The standard model of particle physics usually enshrines this asymmetry in the form of “CP violation,” a mathematical convention concerning the interaction of particles in which one imagines what happens when the charge of all the particles is reversed (charge conjugation, abbreviated as C) and the coordinates of all particles is reversed (the parity operation, or P).The standard model is successful in predicting how CP violation works out in the decay of K mesons or B mesons (see Update 600) but not so good at predicting where the abundance of baryons (protons plus neutrons) comes from.

Now physicists at Hiroshima University, Niigata University (Japan) and Seoul National University (Korea) have proposed an explanation in which the proton excess comes (at least in part) from the decay of hypothetical heavy neutrinos (in addition to the electron, muon, and tau neutrinos already known). One testable prediction of this theory is that there should be a slight preponderance of anti-neutrinos over neutrinos, a disparity that could be studied in the next round of neutrino oscillation experiments being planned. (Endoh et al., Physical Review Letters, 2 December 2002; contact Takuya Morozumi, Hiroshima University, morozumi@hiroshima-u.ac.jp, 81-824-24-7364.)