[EAS-I]FWD>Physics News Update #49

pjk pjk at design.eng.yale.edu
Thu Jul 20 19:13:47 EDT 2000

Mail*Link® SMTP               FWD>Physics News Update #495

Dear Colleagues -

I'm forwarding this issue of this weekly newsletter from the AIP,
because it is a worthwhile publication to be aware of, and because
both items were in the popular news recently, where they always
acquire strong science fiction overtones. No, space travel at
speeds faster than light is not imminent.

All best,  --PJK

Date: 7/20/00 12:54 PM
From: AIP listserver
PHYSICS NEWS UPDATE                         
The American Institute of Physics Bulletin of Physics News
Number 495  July 20, 2000   by Phillip F. Schewe and Ben Stein

DIRECT EVIDENCE FOR TAU NEUTRINOS will be reported tomorrow in a
seminar at Fermilab.  While the existence of neutrinos associated
with the tau lepton was not in doubt, actually observing the
particle interact had not occurred until now.   This rounds out the
program of experimental sightings of the truly fundamental building
blocks prescribed by the standard model of particle physics.  This
official alphabet consists of six quarks  known as up, down,
strange, charm, top, and bottom and six leptons electron, electron
neutrino, muon, muon neutrino, tau, and tau neutrino.  All matter,
according to the theory, should be made up from these most basic of
constituents.  Other particles, such as the anti-matter
counterparts of the quarks and leptons, the force-carrying bosons
(e.g., photons, gluons, etc.), and the Higgs boson (which confers
mass upon some of the other particles) also appear in the theory. 
(Still other candidates, such as the "supersymmetric" particles,
are not part of, but are expected to be compatible with, the
standard model.)  The evidence for the tau neutrino is slim but
impressive: five scattering events are being exhibited at the
seminar by Fermilab physicist Byron Lundgren, leader of Experiment
872, the Direct Observation of Nu Tau (or DONUT) collaboration
(http://fn872.fnal.gov/).  Their experiment proceeds in the
following manner.  Fermilab's 800-GeV proton beam (the highest beam
energy in the world) was steered onto a tungsten target, where some
of the prodigious incoming energy is turned into new particles. 
Some of these quickly decay into taus and tau neutrinos.  Next
comes an obstacle course of magnets (meant to deflect charged
particles away) and shielding material (meant to absorb most of the
other particles except for rarely interacting neutrinos).  Beyond
this lies a sequence of emulsion targets in which the neutrinos can
interact, leaving a characteristic signature.  Evidence for a tau
neutrino in the emulsion is the creation of a tau lepton, which
itself quickly decays (after traveling about 1 mm) into other
particles.  The E872 physicists estimate that about 10^14 tau
neutrinos entered the emulsion, of which perhaps 100 interacted
therein.  It is a carefully analyzed handful of such events that is
now being presented to the public in evidence.  The tau neutrino is
the third neutrino type to be detected.  The detection of the
electron neutrino by Clyde Cowan and Frederick Reines garnered
Reines the 1995 Nobel Prize for physics (Cowan had died some years
before).  For discovering the muon neutrino, Leon Lederman, Melvin
Schwartz, and Jack Steinberger won the Nobel Prize in 1988.   

Institute in Princeton have performed an experiment in which the
group velocity of a light pulse traveling through a special medium
appears to be faster than c, the speed of light in a vacuum,
without, however, violating the principle of  causality or the
theory of relativity.  Such experiments have been performed before
and have exploited the fact that a finite pulse of light is
necessarily the sum of an ensemble of waves at different
frequencies.  One therefore speaks of a "phase velocity" for
component waves and the "group velocity" for the pulse as a whole. 
When such an ensemble enters a medium with a frequency-dependent
index of refraction, interesting things start to happen.  In a
Harvard experiment last year, for example, the component light
waves of a pulse passing through a Bose-Einstein condensate were
affected in such a way as to yield a group velocity of only 17
m/sec (Hau et al., Nature, 18 February 1999).  Working in the other
direction, manipulating the component waves in order to achieve a
higher group velocity, is more difficult to establish since it
usually occurs when the index of refraction is varying rapidly in a
frequency range where the light is being absorbed by the medium;
hence the light pulse can be severely distorted or attenuated,
making it difficult to detect superluminal effects.  In the NEC
experiment, by contrast, the medium in question, a cell filled with
a gas of cesium atoms, does not absorb light at the crucial
frequencies but actually enhances the light through a type of laser
action; that is, the cesium atoms are promoted into an excited
state and contribute to the light pulse when it travels through. 
Consequently the pulse shape is largely preserved even as the
component waves interfere (through a process called anomalous
dispersion) in such a way as to shift the pulse forward in time by
a tiny amount, about 1.7% of the original pulse width, compared to
the situation in which the cell is not present.  According to the
NEC researchers, "the peak of the pulse appears to leave the cell
before entering it."  This superluminal behavior does not
contradict the principles of Einstein's relativity theory, but it
might well encourage further discussion among scientists about how
exactly to specify the onset of light signals.   (Wang et al.,
Nature, 20 July 2000.)

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