Neutrino Oscillations and Mass at Super-Kamiokande
Hank Sobel
Super-Kamiokande is a 50,000 ton water Cherenkov detector operating at
a depth of 1000 meters beneath the Japanese Alps. It was built by a
Japanese and American collaboration over a period of five years. The
detector is used to study neutrinos which originate in the Sun and in
the Earth's atmosphere and to search for nucleon decay. The neutrinos
are detected by observing the charged particles that the neutrinos
produce when they interact in the detector water. A charged particle
can go faster than the the velocity of light in water and will produce
a conical pattern of Cherenkov light which is detected by the 11,146
twenty inch diameter photomultiplier tubes which cover the walls of the
detector.
In June, 1998 the Super-Kamiokande collaboration announced the
observation of a significant, angle dependent deficit of upward-going
muon neutrinos relative to downward going ones. Since neutrinos are
produced isotropically in the Earth's atmosphere, this deficit has
been interpreted as a manifestation of a process known as "neutrino
oscillations". In this phenomenon, the observed electron, muon or tau
neutrino is thought to be composed of a mixture of definite mass
neutrino eigen-states. Since the masses of these eigen-states are
different, the mixture changes character as the neutrinos travel from
their point of production. As a consequence, a muon neutrino, for
example, can change into an electron or tau neutrino as a function of
distance and experimentally, the number of observed muon neutrinos
could change with distance from the production point in the
atmosphere.
Since this phenomenon can only occur if the neutrino has a mass, the
experimental observation is the first definitive demonstration of
neutrino mass. The theoretical implications are still being worked
out and more experimental information is necessary before we can fully
understand what is happening, but it is safe to say that these results
have opened a new chapter in the understanding of one of the
fundamental constituents of matter.
The UC Irvine team is composed of experiment co-spokesperson Hank
Sobel and research physicists Dave Casper, Wojtek Gajewski, Danuta
Kielczewska, Bill Kropp, LeRoy Price, and Mark Vagins; post-doctoral
fellows Michael Smy and Shin'uchi Mine and graduate student Tomasz
Barszczak.
In 1999 the UCI group is starting a long baseline neutrino oscillation
experiment pointing a neutrino beam from the KEK accelerator in the
direction of the SuperK detector which is 250 km away. This
experiment "K2K" is intended as a check of the atmospheric neutrino
results.
Links to Super-K Sites and to Press Coverage
Hank Sobel, hsobel@uci.edu