Yoichiro Suzuki: Report from SuperKamiokande
The SK detector will be 17-years-old this April. Started in 1996, the experiment has a glorious past: it discovered atmospheric neutrino oscillation in 1998, and solar neutrino oscillations with SNO in 2001. An accident in 2001 caused the loss of a large part of the scintillators, but the facility was rebuilt and improved.
Suzuki discussed the updates to SK-IV in some detail. Then he discussed the landscape of neutrino oscillations, where SuperKamiokande gave enormous contributions. Now one of the open questions is that we must find the octant where the parameter theta_23 lies, and the mass hierarchy (the sign of dm_13^2).
For atmospheric neutrinos SuperKamiokande benefits from a wide range of baselines, from 10 to 13000 km (the latter refers to upward-going neutrinos, that cross the earth before reaching the detector). SK is sensitive to 5 orders of magnitude of energy range, from 0.1 to 10000 GeV. A variety of matter effects are also at play, and four kinds of neutrinos to study (nu_e, nu_mu, and their antiparticles). So with atmospheric neutrinos they can determine the _23 parameters very precisely. But to study the octant, they need to study the nu_e appearance in atmospheric neutrinos. They may reach 2-sigma results, and 2-3 experiments may suffice to clarify the situation.
For solar neutrinos, they have a dataset of 3904 days of operation: 57574.1 events (don’t ask me what the .1 means!!) They can reach down to 3 MeV in energy in the near future, for now this is at 3.5 MeV in SK-IV. In order to see a signal they limit the fiducial volume at low energy, to 8.8 kT (for 5 MeV threshold it is 22.5 ktons). Radon contamination is at the lower level of the water tank. They also need to avoid convection of the inner water, making a laminar flow. So the temperature control of the input water they flow in is important. Also, they may need to make an acrylic vessel to prevent Rn from sneaking into the fiducial volume.
For the flux measurement of solar neutrino, they got down to a total error of 1.7%. They measure the day/night asymmetry, due to regeneration of electron neutrinos in the earth, one expects a 2-3% effect. The spectrum variation depends on dm_12. Their result is consistent with the dm_12 value and with the model. They do not see an indication of the LMA solution as a rise at low energy of the flux. If they run for 10 more years, they should see a clear departure.
For supernova neutrinos, they of course keep doing burst searches. They expect 8000 neutrino events from a supernova bursting out at 10 kiloparsecs from us, or 20 million neutrinos for a star like Betelgeuse (which is at 640 light years distance from us). They have a watch in place, minimizing dead time and down time for calibration. They also have a early-warning system for optical signals: neutrinos would arrive 20-40 hours earlier from Betelgeuse, for instance.
The supernova relic neutrinos can be studied in the gap between solar and atmospheric neutrino energy spectra: it is a “window of opportunity” where the relic component can be sought. However, the signal is weak, and current limits are a factor of three above the predictions.
A future project is GadZOOKs, to detect neutrinos created by electron antineutrinos by a coincidence of positron and neutron signals. The neutron can be detected with a Gadolinium dopant: Gd is emitting a photon of 8 MeV when capturing the neutron, 20 microseconds after the original creation. This can be useful for supernova relic neutrino detection, because a reduction of five in backgrounds would allow the detection of 20-40 events in the 10 to 30 MeV range. Also, they could detect neutrinos from Silicon burning (which occurs for two days after the burst).
For proton decay searches, they got to 1.3E34 years in the positron-pizero decay mode, and several other limits in other final states.
In summary, after the discovery of nu oscillations SuperKamiokande made great progress in the study of neutrino phenomenology. After 17 years of operation, SK is stilll quite active, and will continue data taking for at least 10 more years, to study the mass hierarchy, CPV, relic neutrinos, and to look for neutrino bursts from supernovae and search for proton decay.