Neutrino mass was detected at Super Kamiokande.

The XVIIIth International Conference on Neutrino Physics and Astrophysics
was held in Takayama near Kamioka, Japan on 4-9 June, 1998.

Super Kamiokande

Super Kamiokande is large water Cherenkov detector with 50000 metric tons of ultra-pure water viewed by 11146 20"-PMTs. The detector is located approximately 1000m under ground (2700m water equivalent) in the Mozumi mine, which lies in Kamioka town in Gifu Prefecture.
The Super Kamiokande group have studied the physics of proton decay, atomspheric neutrinos and solar neutrinos.

Atmospheric neutrino

Atmospheric neutrinos are decay products of pi and K mesons created in interactions of primary cosmic rays in the atmosphere. From relatively simple arguments based on knowledge of the dominant decay chain, the ratio of the number of muon-neutrinos to electron-neutrinos is expected to be approximately two.

The Super Kamiokande collaboration reported, using 25.5kton*yr of detector exposure, that the number of muon-like events had a significant deficit compared with more precise predictions based on theory and detailed simulations of the detector. ("25.5kton*yr exposure" means that the 22.5 kilotons of water "target" used in the analysis were observed for a little over a year.) To avoid any difficulties related to the prediction of the absolute total number of neutrinos expected from cosmic rays, the group presents the data as a ratio of muon-type to electron-type events. This ratio for data is compared to the expected ratio: Defining R(mu/e):
R(mu/e)Data/expected = (mu/e Data)/(mu/e expected) = 0.66+_0.06+_0.08,
where first error is statistical and second error is systematic. If the basic theory is correct, this ratio of ratios should be 1. Furthermore, the group reports that R depends on the direction of events. In particular, the value of R reported for events coming downward, where the neutrinos travel only a few tens of kilometers after production is lower than the value for upward going events initiated by neutrinos which have travelled across the whole earth. The deficit in muon type neutrinos, and especially the "zenith angle dependence" observed is evidence for neutrino oscillations.
This evidence will be tested in a controlled environment by the K2K experiment. If neutrino oscillations are in fact the cause of this anomoly, and if the parameters of the phenomena are as suggested by the Super Kamiokande result, this experiment will most likely see a large and direct signature for the effect.

At the Neutrino'98 Conference, the Super Kamiokande group announced to the world that this evidence of neutrino mass was detected.
The below image of a transparency shown at the conference shows details of the most impressive result of the atmospheric neutrino measurement. The shaded areas show expected distributions of certain event types, and the dots with crosses show the measurements of those types. One easily sees that even if the overall scale is assumed to be not well known, the data cannot match the prediction of atmospheric models with previous particle physics theory. Notes in the margin give quantitative statistical statements about how strong this disagreement is.

Atmospheric neutrino results from Super-Kamiokande & Kamiokande T.Kajita(ICRR)

  • NEUTRINO'98 home page is here.
  • Super Kamiokande home page is here.
  • K2K experiment home page is here.

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