Experiments on Baryon and Lepton Non-Conservation
The recent theories which try to unify electroweak and strong interactions imply in most cases non conservation of the baryon and lepton numbers. Baryon non conservation can be investigated in a model independent way by searching for radioactive remnants or for abnormal isotopic abundance consequent to the decay of a proton or a neutron inside the nucleus. Since the sensitivity of these methods is for the moment quite low (up to 1027 years as lower lifetime for nucleon decay) more emphasis is presently devoted to “direct” experiments where nucleon stability is searched with huge detectors placed deep underground to reduce strongly the background due to cosmic rays. Two experiments are at present in Operation where the detector is essentially a calorimeter made by plates of Iron interleaved with planes of detecting tubes (proportional and limited streamer tubes). In the former of these experiments, running since two years in southern India, three nucleon decay candidates have been found, corresponding to a nucleon decay lifetime of about 1031 years. In the latter, carried out by an Italian collaboration in the Mont Blanc tunnel, many events totally confined inside the detector have been observed. One of them, hard to be interpreted, unlike the others, as a neutrino interaction, could be due to a proton decay. Its lifetime would ränge between 2 and 3 × 1031 years. A very large detector based on the detection of the Cerenkov light emitted by the charged secondaries of nucleon decays has up to now provided no evidence for proton decay into a positron and a neutral pion. An alternative way to investigate nucleon stability is to search n-n oscillations with thermal, cold or ultracold neutrons from a reactor. The only running experiment, at the Grenoble reactor, shows no evidence for such oscillations and yields a limit of 106 sec for the free neutron oscillation time.