Number 154, December 3, 1993 by Phillip F. Schewe and Ben Stein
THE MOST ACCURATE MEASUREMENT OF PARITY VIOLATION in an atom has been
made by researchers at the University of Washington (D.M. Meekhof et al.,
Physical Review Letters, 22 Nov.1993). The electromagnetic and strong nuclear
forces conserve parity; that is, they do not differentiate between left
and right. The weak force, in contrast, does not conserve parity, a fact
discovered first in the study of nuclear decays. One way for parity violation
to occur in atoms is when one of its electrons gets close enough to the
nucleus to experience the weak force. The Washington researchers (contact
Steve Lamoreaux, 206-543-2540) detected parity violation in atoms by measuring
a slight change (10**-7 radians, with an accuracy of 1%) in the polarization
angle of light passing through a vapor of lead atoms. However, because
of uncertainties in the atomic theory of that atom, the theoretical predictions
for parity violation in lead is uncertain to an 8% level, so an exacting
test of electroweak theory is not yet possible. Future plans would involve
measuring the effects in different lead isotopes, which would cancel out
the uncertainties. The group plans to use their techniques to make precision
measurements of parity violation in thallium, which has much smaller theoretical
uncertainties than lead. In addition, the researchers are hoping to detect
parity violation that occurs within the nucleus. Specifically, they are
looking to make the first measurements of the "anapole" moment,
the internal electromagnetic moment in the nucleus which comes about because
of the weak force.
TABLE-TOP TERAWATT LASERS provide 10**12 watts of light energy in nanosecond
or picosecond bursts. Until recently producing this sort of power required
a room-sized labyrinthine system which splits the beam into several branches
which are amplified in parallel (so as not to damage the laser rods) and
then recombined at a target. A newer approach stretches the beam pulse
with diffraction gratings, amplifies it, and then compresses it again in
a process called "chirping." By working in the time domain rather
than in space (farming out the beam to bulky ancillary laser amplifiers)
this approach greatly lowers the size and expense of high-power laser systems.
Alternatively, the chirped pulse amplification (CPA) technique can be coupled
to existing lasers. For example, CPA helped boost the power of the VULCAN
laser at Britain's Rutherford Appleton Lab to 10 Terawatts. Similar systems
are used at Livermore, Rochester, and Saclay. The high electric fields
in such light beams (higher than the fields that hold the hydrogen atom
together, 5 x 10**11 V/m) can be sent through nonlinear crystals to produce
higher harmonic waves, including x rays. It may also be possible to use
such high fields to create waves (Langmuir waves) in a column of plasma
which in turn can accelerate electron beams to higher energies than with
present technology. (New Scientist, 20 Nov. 1993.)
EARTH'S MAGNETIC FIELD fluctuates in strength over time. For example,
2000 years ago the field was 40% stronger than it is today and it continues
to decrease at a rate of 7% per century. A plot of geomagnetic intensity
over the past four million years, made using measurements of sedimentary
samples brought up from the ocean floor, exhibits an asymmetrical sawtooth
pattern with a time constant of about a half million years. Scientists
at the Institute of Earth Physics in Paris found that during a period of
stable polarity the intensity fell slowly but that after a polarity change
the intensity regenerated rapidly. (Jean-Pierre Valet and Laure Meynadier,
Nature, 18 Nov. 1993.)
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