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Wikipedia says :

Recent experiments at the Super-Kamiokande water Cherenkov radiation detector in Japan gave lower limits for proton half-life, at 90% confidence level, of 6.6×10^33 years via antimuon decay and 8.2×1033 years via positron decay. Newer, preliminary results estimate a half-life of no less than 1.29×10^34 years via positron decay.

So how is the lower limit of the proton lifetime measured in this experiment ?

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I think they just take a large amount of material and look for protons decaying in it. There are a lot of protons, so in order for none of them to decay the half-life must be absurdly long.

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    $\begingroup$ Exactly, if you look at 100 protons for 1 second, it's the same as looking at 1 proton for 100 seconds. They look at a huge quantity of water, looking for a p->e+X or p->mu+X where the anti-electron or anti-muon will emit Cherenkov light. The rate of such event helps to set a limit on proton lifetime. $\endgroup$ – Michaël Ughetto Mar 15 '15 at 9:05
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    $\begingroup$ The devil is in the details, of course. These detector show events due to other physics (cosmic rays, cosmic neutrinos, solar neutrinos, radiological contamination, in the case of Super-K there are beam neutrinos from T2K) quite regularly, so sorting out one from another is the hard part. $\endgroup$ – dmckee --- ex-moderator kitten Mar 15 '15 at 15:59

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