1985 – The IMB experiment & Russian team reports measurement of non-zero neutrino mass

The IMB experiment, named after the sponsoring organizations: University of California, Irvine, University of Michigan, and the Brookhaven National Laboratory, was a large neutrino observatory consisting of a roughly 20 meter (60 foot) cubical tank full of ultrapure water and layered on the inner surface with 2,048 photomultiplier tubes. Located in a purpose-built cavern in the Morton Salt Mine 2000 feet underground at Fairport Harbor, Ohio and under Lake Erie, it became famous in 1987 for having detected 8 of the approximately 1058 neutrinos emitted by Supernova 1987a.

A Proposal To Test For Baryon Stability To A Lifetime of 1033 Years

We have studied the properties of, and the expected backgrounds in, a totally active, 10,000 ton water Cerenkov detector located deep underground and sensitive to many of the conjectured decay modes of the nucleons in it. Sensitivity to π, μ and γ secondaries, good energy resolution, and good angular resolution provide sufficient background rejection in the proposed device and will permit us to obtain significant information about several decay channels, should they be observed. If no events should be recorded in the device in one year, a lower limit of ~ 1033 years will thus be placed on the partial lifetime for the most distinct nucleon decay modes. Depending upon the decay channel, this is ~3 orders of magnitude longer than previous measurements, and is at or beyond the level suggested by many unifying theories. The sensitivity predicted for this instrument is within an order of magnitude of that achievable in an arbitrarily large detector of this general type, since known background from atmospheric neutrinos imposes an inherent limit.

At the same time, a Russian scientists Stanislav Mikheyev and Alexei Smirnov, expanding on the work of Lincoln Wolfenstein, noted that flavor oscillations can be modified when neutrinos propagate through matter. This was a critical observation, a first time measurement and proof of a non-zero neutrino mass. Unfortunately subsequent attempts to independently reproduce the experiment were unsuccessful.