NEUTRINO BEAMS NOW GO TO KAMIOKA FROM KEK - THE FIRST LONG BASELINE NEUTRINO OSCILLATION EXPERIMENT COMMENCED -
KENZO NAKAMURA
One of the most important issues in today's elementary particle physics is whether or not neutrinos have mass. The standard theory of the elementary particles postulates that neutrinos are massless. However, there is no solid basis for this postulation, and most of elementary particle physicists now think it is natural that neutrinos have tiny mass.
It is a well-known fact that there exists three types of neutrinos. If individual neutrino types have different masses and also they mix just like quarks, a phenomenon called "neutrino oscillation" takes place. Conversely, if neutrino oscillation is established, that implies neutrino has mass. Neutrino oscillation means that a neutrino that is born as one type will, with the passage of time, change to a different type with a certain probability. It is one of the peculiar phenomena of quantum mechanics that governs the micro world.
In 1998, evidence for neutrino oscillation was announced by Super-Kamiokande, a gigantic underground neutrino detector in Japan, by the observation of atmospheric neutrinos that are produced by cosmic rays colliding with atmospheric nuclei. The data show muon neutrinos oscillate, most probably, to tau neutrinos.
A confirmation of the Super-Kamiokande results should be done using accelerator-produced muon neutrino beams with a flight distance of several hundred kilometers. Such neutrino beams have well-controlled properties, and this makes the experiment reliable. This kind of experiment is called a "long baseline neutrino oscillation experiment."
The first accelerator long baseline neutrino oscillation experiment has commenced in Japan this year. At KEK, a neutrino beamline was constracted to shoot the neutrino beams at 250-km distant Super-Kamiokande. After commissioning of the beamline, the experiment known as K2K (KEK-to-Kamioka) started in April. Now neutrino beams go to Kamioka from KEK. The experiment will continue for at least three years to establish, hopefully, neutrino oscillation.