Synthetic neutrinos appear to disappear

Announced at the recent Neutrino 2000 meeting in Sudbury, Canada, were the
first results from the K2K long-distance neutrino beam experiment in Japan.
For the first time, synthetic neutrinos made in a physics laboratory are seen to
disappear.

In the K2K study, neutrinos (of the muon-like variety)
generated at the Japanese KEK laboratory are directed towards the
Superkamiokande underground detector 250 km away. In the detector, 22.5
kT of water are monitored by sensitive photomultipliers to pick up the tiny
flashes of light produced by particle interactions.

The experiment, which
began running last year (October 1999 p5), was able to announce at Sudbury
that 17 neutrino counts had been picked up. The pulses of parent protons at
the source accelerator can be used to clock the arrival of the neutrinos, so the
results are essentially free of spurious background.

About 29 neutrino
counts were expected, assuming the neutrinos despatched from the KEK
laboratory arrived unscathed at Superkamiokande. Such a deficiency, if it
continues to be seen, implies that something happens to the particles along their
250 km flight path.

This is not a surprise. In 1998, initial results from
Superkamiokande on muon signals generated by neutrinos produced via
cosmic-ray collisions in the atmosphere showed that the signal from muon-like
neutrinos arriving from the atmosphere directly above the detector was very
different from the signal arriving from below.

This is not a result of
absorption in the Earth – 99.9999…% of neutrinos pass through the Earth as
though it were not there. The effect was interpreted as neutrino metamorphosis
– “oscillations” – as the particles passed through the planet.

Neutrinos
come in three varieties – electron, muon and tau – according to the particles
that they are associated with. For a long time, physicists thought that these
neutrino allegiances were immutable – a neutrino produced in an electron
environment could remain an electron-like neutrino for ever.

However,
this is only so if the neutrinos have no mass and travel at the speed of light. If
the particles do have a tiny mass, they can, in principle, switch their
electron/muon/tau allegiance en route. The 1998 atmospheric neutrino effects
seen in Superkamiokande provided the first firm evidence for such neutrino
oscillations. These Superkamiokande data have now been consolidated, while
other evidence has also appeared.

With neutrino oscillations, neutrinos of
a certain type are more likely to change into neutrinos of another type than to
interact with matter. If the viewing detector is sensitive only to neutrinos of a
certain type, then some neutrinos disappear from view.

Such neutrino
disappearance also correlates with the long-observed dearth of neutrino signals
from the Sun, where measurements using a variety of detectors (including
Superkamiokande) give only a fraction of the number of expected
electron-type solar neutrinos (July p17).

The Superkamiokande detector
is sensitive to electron-like and muon-like neutrinos. However, if the parent
KEK muon-like neutrinos change into tau-like neutrinos, Superkamiokande
would not see them. This is the interpretation of the initial deficiency logged by
the experiment in Japan. However, these are only the first results to appear
from the K2K experiment, and it usually takes a long time to assemble reliable
neutrino data.

The transmutation of muon-like into tau-like neutrinos is
good news for long-distance neutrino experiments now under construction.
The MINOS experiment in the US will send neutrinos 730 km from Fermilab
to a detector in the Soudan mine, Minnesota, while neutrinos from CERN will
be despatched towards new detectors in the Italian Gran Sasso underground
laboratory, also 730 km distant (January p5). The detectors in these projects
hope to pick up signs of tau-like neutrinos not present when the beams left the
parent laboratories.

Because of the scant affinity of neutrinos for matter,
intercepting them in a detector is always a challenge. The DONUT experiment
at Fermilab recently presented possible evidence for the production of tau-like
neutrinos in particle interactions (see p6).

The K2K experiment is a
collaboration involving Japan, Korea and the US.