There has been a lot said about accelerator experiments and about neutrino beam experiments but now I want to put in a word for another important particle experiment. Super-Kamiokande started in 1996 and was an upgraded version of the original Kamiokande experiment started in 1985. It consists of a large tank of very pure water surrounded by a large number of photomultipliers to detect Cherenkov radiation from any interactions or decays in the tank.
Super-Kamiokande has had some spectacular success in detecting solar neutrinos and demonstrating neutrino oscillations, but its original purpose was to observe nucleon decays which are predicted by most grand unified theories. This type of experiment is unique in that it has a possibility to observe effects at the grand unified scale in non-cosmological context. Accelerators can not get anywhere near the energies required to probe this scale.
There were a couple of talks on Saturday at ICHEP about Super-Kamiokande: “Recent results on atomospheric neutrino oscillation from Super-Kamiokande” by Yoshihisa Obayashi and “Search for Nucleon Decays in Super-Kamiokand” by Makoto Miura. it will also be mentioned this afternoon in “Beyond the Standard Model searches” by Pavel Murat
It is well-known that Super-K has had negative results so far in its search for nucleon decay. This does not sound good but in fact this has been one of the most powerful results for theorists looking at particle models beyond the standard model. If ever a Nobel Prize was deserved for a negative result this would be it.
Super-K looks for two main decay channels for protons. The first is decay to electron plus pion, The second is to a Kaon and an anti-neutrino. Conventional GUT theories predict mostly the first mode, but supersymmetric models favour the second.
The first GUT theories used an SU(5) gauge group to unify the electroweak gauge theory with QCD. This predicted proton decay with a lifetime of between 3 x 1028 and 3 x 1031 years in the first mode. Super-K has set a lower bound of 7 x 1033 years, so this theory is long since dead.
SO(10) GUT can accommodate 1030 to 1040 seconds so its parameter space is much reduced but it still lives. SUSY versions of SU(5) are also ruled out by a 3 x 1033 limit on the second channel, while SUGRA SU(5) and SUSY SO(10) still cling to life.
In 2001 Super-K suffered a spectacular accident when a photomultipier tube imploded and stated a chain reaction of implosions spread by shock waves through the tank. Many expensive tubes were lost. initially the remaining tubes were redistributed and the experiment limped on as Super-K2. In 2006 it was fully restored and now it continues at full strength as Super-K3.
The real hope is that Super-K will see some positive results for nucleon decay. The parameters of the decays could then be studied in more detail to understand the GUT scale. Even if this does not transpire the possibility of ruling out more models makes Super-K a very worthwhile experiment.