Super-Kamiokande and nucleon decay (ICHEP)

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.

3 Responses to Super-Kamiokande and nucleon decay (ICHEP)

  1. Sad that one can always tune the parameters so that the conclusion that baryon and quark numbers are separately conserved cannot be made on experimental basis alone. I have also deep personal interests here here since the basic prediction of TGD is that B and L are separately conserved and correspond to different chiralities of M^4xCP_2 spinors. This is possible because color correspond to spinor partial waves in M^4xCP_2 rather than being spin like quantum number.

    If TGD option is correct then last 30 years of theoretical particle physics from GUTs to F theory models has been on wrong track.

  2. Bill K says:

    Will the dark-matter detectors now being built (particularly the liquid xenon ones) be competitive in the search for proton decay? Conversely, can Super-Kamiokande look for dark matter?

  3. philipgibbs says:

    Bill K, that is an interesting question and I am not expert enough to give a definitive answer. However, my understanding is that detectors such as Super-K can be used for dark matter searches but they are more limited by background. Dark matter detectors are typically much smaller so they do not have enough mass to be sensitive to nucleon decays given current known limits on lifetimes. For example, Super-K is using 3000 tonnes of water compared to just 150 Kg mass in Xenon100.

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