The search for proton decay

Techniques used to search for evidence of proton or neutron decay (by one of the 32 theoretically possible modes) are examined, and some preliminary results are summarized, in a general review. The physical principles of proton decay are reviewed; the operation of water-Cerenkov and layered tracking detectors is explained and illustrated with diagrams; the 8000-ton water detector (equipped with 2048 5-inch photomultipliers on a 1-m grid) in operation in a salt mine at Paynesville, Ohio, at a depth of 600 m is described in detail; and the lower limits placed on the lifetime of a proton or neutron by the negative or ambiguous results obtained so far are indicated in a table. It is pointed out that these limits are already greater than the theoretically determined limits in many cases.