[FPSPACE] FW: Physics News Update 820

Wed Apr 18 16:31:19 EDT 2007



>From: physnews at aip.org
>Reply-To: physnews at aip.org
>To: ljk4 at MSN.COM
>Subject: Physics News Update 820
>Date: Wed, 18 Apr 2007 10:00:22 -0400
>
>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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