Proton decay: decay of the proton, the major component of all atomic nuclei.

The proton is the core of the hydrogen atom and the component of all other atomic nuclei in virtue of which (together with the neutron) they carry their mass and charge. Protons themselves are made of two up quarks and one down quark. Since protons are the basic components of all matter, the question of their stability becomes the fundamental question of the stability of the entire universe. As far as we know, protons are very stable; a process of proton decay has never been directly observed.

Nonetheless, some theories beyond the standard model of physics deny that the proton's life span is infinite. These theories predict a proton decay, and hence a decay of matter, after a very long (but nevertheless finite) period of 1031 to 1036 years. But how can we verify such a prediction, given the enormous span of time involved? On one hand, the decay time of protons depends on neutrino mass: if different kinds of neutrinos have different mass and are therefore able to change from one kind to another, then protons can decay as well. On the other hand, to predict proton decay it suffices to observe an adequate number of protons, for the life span indicated above is only the average value — the so-called ►half-life — after which half of all existing protons will have decayed. If we have a quantity of 1036 protons with a decay period of 1036 years, then on average one of these protons per year will decay.

To be sure, in order to ascertain the occurrence of such decay we would have to observe a considerable quantity of protons by means of a considerable quantity of detectors for a considerably long period of time. Since the launching of the Super-Kamiokande experiment in 1996, scientists have been monitoring 11,200 decay detectors in a tank with 50,000 tons of ultrapure water that was drilled a thousand meters into the rock underneath the Japanese city of Kamioka. The experiment was actually designed to search for neutrino mass, but it can also reveal facts about proton decay. To date, no unambiguous results have been obtained. But scientists are patient. With the exception of a brief but dramatic interruption in 2001, when thousands of the detectors exploded in a chain reaction and had to be replaced, the tank has been under observation for over a decade and will remain so for the foreseeable future.

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