Nuclear and Particle Physics at the 50-GeV PS

In nuclear and particle physics, one of current interests is to study the origin of mass. One is related to the mass of matter. It is known that over 99% mass of the matter is carried by atomic nuclei. A nucleus is an assembly of protons and neutrons. Each proton or neutron is made of three quarks. One puzzle which has not been solved quantitatively until now is that the mass of proton (or neutron) is ~1 GeV/c2, whereas the constituent quark mass is less than 1/100 of the proton mass. It is believed that the creation of a large proton mass is due to the symmetry breaking (called the chiral symmetry breaking), while a quantitative nature of this symmetry breaking has not been studied well. Theoretically, it is expected that the quantitative aspect of the symmetry breaking can be studied by inserting a meson (which is made of quark and anti-quark) or a baryon (which is made of three quarks) in the interior of extreme conditions and by studying the change of its mass.

One approach is to implant mesons or baryons in the interior of nuclear matter. Restoration of symmetry breaking is expected theoretically, so that the mass of meson or baryon would be reduced in these extreme conditions.

The other is related to the mass of neutrino. From the most fundamental principle, there are no reasons to prohibit a neutrino from having a mass, although it has been believed for many years that the neutrino has zero mass. In a recent experiment at SuperKamiokande, it was demonstrated that muon neutrinos (νµ) from the sky (which is called the atmospheric neutrinos) might be converted to another type of neutrinos called tau neutrinos (ντ) while traversing through the Earth.