Y. Suzuki: Update from Superkamiokande
Yoichiro Suzuki gave a report of the status of Superkamiokande (right, in a beautiful picture taken with the vessel almost empty for servicing).
Superkamiokande is an extension of the Kamiokande experiment, funded in 1982 with as objectives the study of proton decay, atmospheric neutrinos, and SN neutrino bursts. Solar neutrinos were not in the original agenda as the threshold for observing them was supposedly too low… We know how that story ended.
It should also be mentioned in passing that last month was the 30th anniversary of the detection of neutrinos from a SN burst – the 1987A supernova.
In 1988 an anomaly in atmospheric neutrinos was detected, and the following year the first solar neutrinos confirmed the deficit of these events. In 1991 SuperKamiokande was approved as a 5-year construction project.
One important recent result was obtained on the search for proton decay. The proton is supposedly a perfectly stable system in the standard model, but extensions embedding the model within grand-unification groups of symmetry allow it to decay, with an exceedingly rare process whereby the proton turns into a positron and a neutral pion. This process would give rise to a distinctive signature of three Cherenkov rings in the detector, as the neutral pion would decay into a pair of photons which then convert into electron-positron pairs.
Unfortunately the proton decay signature can be mimicked by the interaction of a neutrino with a proton in a Oxygen nucleus. The Superkamiokande collaboration managed to reduce this background by “tagging” the neutron emission with the delayed gamma ray emitted when the neutron creates a deuteron nucleus.
No signal events have been observed by the search, which considered a dataset corresponding to 300 kilotons per year. The result is that the proton lifetime has been determined to be larger than 1.6×10^34 years. You can read more information in the relevant article here.