(The XVIth edition of "Neutrino Telescopes" is going on in Venice this week. The writeup below is from a talk by M.Nakahata at the morning session today. For more on the conference and the results shown and discussed there, see the conference blog.)
SK is a collaboration of 120 people from 34 institutions in 7 countries. It is a 50kton water detector. Data taking started in 1996. At present it is in its fourth phase of raunning. It has higher than 99% efficiency above 4MeV for neutrinos.
Atmospheric neutrinos are produced by cosmic ray interactions. Neutrinos travel 10-10000km before detection. Both muon and electron neutrinos are detected, with a ratio of 2/1 at low energy. 30% of the final sample are antineutrinos. The flux spans many decades in energy from 100 MeV to 100 TeV.
48,000 events have been detected in 4581 days of running. 38000 of them are fully-contained events. Event topology subdivides the data in 19 analysis samples.
Most of the oscillation signal is muon to tau neutrino oscillations. One of the recent achievements of SK was the appearance signal of tau neutrinos. They searched for events consistent with the hadronic decay of tau leptons: multi-ring electron-like events with a visible energy above 1.3 GeV. They used a neural-network selection. Fitting the 2D distribution of cosine theta versus the NN output they obtained a signal intensity of 1.42+-0.35 times the predicted intensity. This corresponds to 180.1 events, a 3.8 standard deviation excess. It was published in Phys. Rev.Lett 110 (2013). The picture below shows the excess of tau appearance events as a function of zenith angle.
Nakahata then showed some quite cool oscillation probability maps for muon neutrinos as a function of cosine of zenith angle and energy (see eg. the picture on the right). In the detector, the effects of different theta_23 and delta_cp values cause observable variations in the flux ratios for the different kind of detectable events. The effects are of a few percent, but because of the large statistics of collected data they can be studied. The data allow to favor the normal hierarchy at a delta-chisquared of 0.9 units.
A comparison with T2K and Minos results show consistency in the plane of delta_m^2(32) versus sin^2 theta_23. SK's sensitivity can be improved using constraints from other experiments. The result is that the preference for a normal hierarchy increases to a delta-chisquared of 1.2.
A sterile neutrino oscillation search in atmospheric neutrinos was also carried out with SK data. Limits on |U(mu4)|^2 were set at 0.041 at 90%CL.
SK also performed a indirect WIMP search from the Sun. This implied fitting the data with a atmospheric neutrino simulation and a WIMP simulation, to search for neutrinos from WIMP annihilations in the Sun's core. A WIMP signal would create enhancements from the Sun's direction at sub-GeV energies. Using the absence of a signal, SK excludes some region of the WIMP cross section versus mass plane. The exclusion limit has some uncertainty due to solar models and velocity distribution uncertainties. The limits are the strongest to date for WIMP masses below 200 GeV, for the spin-dependent cross section. The limit in the spin-independent cross section covers a wide range of energies and excludes the Dama signal, as shown in the figure below.
Another measurement was that of Boron solar neutrinos. They have about 20 events per day from this source, so they can measure possible time variations of the flux, energy spectrum distortions due to solar matter effects, and day/night flux asymmetries due to Earth matter effects. If one studies the survival probability of electron neutrinos as a function of energy, SK can search the knee in the function. They have 70k signal events in the solar direction, extracting a very nice "image" of the sun, smeared by a point-spread function due to scattering effects. The observed 8B flux is 2.343+-0.044 x10^6 /cm^2/s. The observed flux has remained stable in all four phases of SK data so far analyzed.
One can study the 8B flux comparing with the Wolf number and the 11-year solar cycle. No effect is observed, as the data show no time modulation in phase with the solar activity, as shown in the figure below (black points are the sun spot numbers as a function of time, the red points are the neutrino fluxes).
- Tau neutrino appearance was observed at 3.8 sigma.
- The normal hierarchy is favoured with a delta-chisquared of 1.2 with SK+T2K data.
- Indirect DM search sets new limits for light WIMPS (with masses below 6 GeV) for spin-independent cross section.
- No significant correlation of solar neutrino fluxes with solar activity is seen.