PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 820 April 18, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
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)
GRAVITY PROBE B, the orbiting observatory devoted to testing the
general theory of relativity, has measured the geodetic effect-the
warping of spacetime in the vicinity of and caused by Earth-with a
precision of 1%. The basic approach to studying this subtle effect
is to monitor the precession of gyroscopes onboard the craft in a
polar orbit around the Earth. The observed precession rate, 6.6
arc-seconds per year, is close to that predicted by general
relativity. The geodetic effect can be measured in several ways,
including the use of clocks, the deflection of light, and the
perturbative influence of massive bodies on nearby gyroscopes. GP-B
is of the latter type, and its current precision is as good as or
better than previous measurements. And once certain unanticipated
torques on the gyroscopes are better understood, GP-B scientists
expect the precision of their geodetic measurement to improve to a
level of 0.01%.
These first GP-B results were reported at the APS meeting by Francis
Everitt (Stanford). The idea for using gyroscopes to observe the
warping of spacetime was proposed almost 50 years ago, and Everitt
has been an active proponent and then scientific overseer of the
project for much of that subsequent time.
A second major goal of GP-B is to measure frame dragging, a
phenomenon which arises from the fact that space is, in the context
of general relativity, a viscous fluid rather than the rigid
scaffolding Isaac Newton took it to be. When the Earth rotates it
partly takes spacetime around with it, and this imposes an
additional torque on the gyroscopes. Thus an extra precession,
perpendicular to and 170 times weaker than for the geodetic effect,
should be observed. Everitt said that GP-B saw *glimpses* of
frame
dragging in this early analysis of the data and expects to report an
actual detection with a precision at the 1% level by the time of the
final presentation of the data, now scheduled for December 2007.
(An indirect measurement of frame dragging at the 10-15% uncertainty
level was made earlier by the LAGEOS satellite.)
Some of the GP-B equipment is unprecedented. The onboard telescope
used to orient the gyroscopes (by sighting toward a specific star)
provided a star-tracking ability better by a factor of 1000 than
previous telescopes. The gyroscopes themselves-four of them for
redundancy-are the most nearly spherical things ever made: the
ping-pong-ball-sized objects are out of round by no more than 10
nm. They are electrostatically held in a small case, spun up to
speeds of 4000 rpm by puffs of gas. The gas is then removed,
creating a vacuum of 10^-12 torr. Covered with niobium and reposing
at a temperature of a few kelvin, the balls are rotating
superconductors, and as such they develop a tiny magnetic signature
which can be read out to fix the sphere*s instantaneous
orientation. (For more information see einstein.stanford.edu)
***********
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