MORE GOOD NEWS!! THREE NEUTRINOS MORE OR LESS CONFIRMED!!!

ONE NEUTRINO ANOMALY HAS BEEN RESOLVED while another has sprung up.
A Fermilab experiment called MiniBooNE provides staunch new evidence
for the idea that only three low-mass neutrino species exist. These
results, reported over the past week at a Fermilab lecture and at
the American Physical Society (APS) meeting in Jacksonville,
Florida, seem to rule out two-way neutrino oscillations involving a
hypothetical fourth type of low-mass neutrino.
Several experiments have previously shown that neutrinos, very light
or even massless particles that only interact via gravity and the
weak nuclear force, lead a schizoid life, regularly transforming
from one species into another. These neutrino oscillations were
presumably taking place among the three known types recognized by
the standard model of particle physics: electron neutrinos, muon
neutrinos, and tau neutrinos. However, one experiment, the Liquid
Scintillator Neutrino Detector (LSND) experiment at Los Alamos,
provided a level of oscillation that implied the existence of a
fourth neutrino species, a *sterile neutrino,* so-called because
it would interact only through gravity, the weakest of physical
forces. (For background see Physics Today, August 1995 and
http://www.aip.org/pnu/1995/split/pnu239-1.htm and
http://www.aip.org/pnu/1996/split/pnu269-1.htm) From the start,
this result stood apart from other investigations, especially since
it suggested possible neutrino masses very different from those
inferred from the study of solar or atomospheric neutrinos or from
other accelerator-based neutrino experiments.

MiniBooNE (whose name is short for Booster Neutrino Experiment; the
*mini* refers to the fact that they use one detector rather than
the
originally proposed two) set out to resolve the mystery. The
experiment proceeds as follows: protons from Fermilab*s booster
accelerator are smashed into a fixed target, creating a swarm of
mesons which very quickly decay into secondary particles, among them
a lot of muon neutrinos. Five hundred meters away is the MiniBooNE
detector. Although muon neutrinos might well oscillate into
electron neutrinos, over the short run from the fixed target to the
detector one would expect very few oscillations to have occurred.
The Fermilab detector, and the LSND detector before it, looked for
electron neutrinos. Seeking to address directly the LSND
oscillation effect, Fermilab tried to approximate the same ratio of
source-detector distance to neutrino energy. This ratio sets the
amount of likely oscillation. The Los Alamos experiment used 30 MeV
neutrinos observed after a 30 m distance; the Fermilab experiment
used 500 MeV neutrinos detected after a distance of 500 m.
The trick of doing this kind of experiment is to discriminate
between the few rare events in which an electron neutrino strikes a
neutron in a huge bath of mineral oil, thereby creating a
characteristic electron plus a slow moving proton, and the much more
common event in which a muon neutrino strikes a proton to make a
muon and proton. LSND saw a small (but, they argued, statistically
significant) number of electron neutrino events. MiniBooNE, after
taking into account expected background events, sees none. Thus
they see no oscillation and therefore no evidence for a fourth
neutrino.

Actually it*s not exactly true that they see no electron neutrinos.
At low neutrino energy they do see events, and this tiny subset of
the data remains a mystery, to be explored in further data taking
now underway using a beam of anti-neutrinos. At the APS meeting,
MiniBooNE co-spokesperson Janet Conrad (Columbia Univ) said that the
low-energy data are robust (meaning that a shortage of statistical
evidence or systematic problems with the apparatus should not be
major factors) and that some new physical effect cannot be ruled
out. At the very least, the low-energy data do not undo the new
assertion that the earlier LSND results cannot be explained by the
existence of a fourth neutrino type. (Fermilab press release and
figures, http://www.fnal.gov/pub/presspass/images/BooNE-images.html)