Proton Decay Session
The longevity of the proton is at the heart of our own existence. While current experiments show that its lifetime exceeeds about 1033 years, its ultimate stability has been questioned since the early 1970's in the context of theoretical attempts to arrive at a unified picture of the fundamental particles - the quarks and leptons - and of their three forces: the strong, electromagnetic and weak. These attempts of unification, commonly referred to as "Grand Unification" , have turned out to be supported empirically by the dramatic meeting of the strengths of the three forces, that is found to occur at high energies in the context of so-called "supersymmetry", as well as by the kind of neutrino- masses that is suggested by the discovery of atmospheric and solar neutrino oscillations. One of the most crucial and generic predictions of grand unification ,however, is that proton must ultimately decay into leptonic matter(such as positron+meson), revealing quark-lepton unity.

Certain early versions of grand unification based on the so-called SU(5) and minimal supersymmetric SU(5) models predict relatively short lifetimes for the proton ranging from 1028 to 1032 years, which are however excluded by IMB/Kamiokande and SuperKamiokande experiments. A class of well-motivated theories of grand unification, based on the symmetry SO(10) and supersymmetry, which have the virtue that they successfully describe the masses and mixings of all quarks and leptons including neutrinos, and which also explain the origin of an excess of matter over antimatter through a process called "leptogenesis", provide a conservative (theoretical) upper limit on proton lifetime which is within a factor of ten higher than its current empirical lower limit.This makes the discovery potential for proton decay in a next-generation detector rather high.

From a broader viewpoint, proton decay, if found, would provide us with a unique window to view physics at truly short distances- less than 10-30 cm., corresponding to energies greater than 1016 GeV - a feature that can not be achieved by any other means. It would provide the missing link of grand unification. Last but not least, it would help ascertain our ideas about the origin of an excess of matter over antimatter (mentioned above) that is crucial to the origin of life itself. In this sense, and given that the predictions of a well-motivated class of grand unified theories for proton lifetime are not far above the current empirical limit, the need for an improved search for proton decay through a next-generation detector seems compelling. The session would include theoretical and experimental discussions along these lines and expose a spectrum of current thinking in the field.

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