From: John P. Roberts (jpr602_at_mac.com)
Date: Wed Apr 21 2004 - 23:54:34 PDT
This is the most lucid explanation of the concept of parity (in
subatomic physics) that I've seen.
I have a B.S. in physics, but a deep understanding of parity is beyond
me. With a B.S. in physics, you have an understanding of physics
knowledge as it was in about 1930.
On Apr 21, 2004, at 1:54 PM, LARRY KLAES wrote:
> PHYSICS NEWS UPDATE
> The American Institute of Physics Bulletin of Physics News
> Number 682 April 21, 2004 by Phillip F. Schewe, Ben Stein
>
> PARITY VIOLATION IN ELECTRON-ELECTRON SCATTERING has been seen for
> the first time, adding to physicists' understanding of the elusive
> weak force. Parity is name for the proposition that if one viewed
> an interaction among particles in a special mirror that reflected in
> all three dimensions then physics would be the same in the ordinary
> and in the mirror world. Three of the four known physical
> forces---gravity, electromagnetic, and strong---respect (or
> "conserve") parity. The fourth force, the weak force, does not
> conserve parity, a fact established in the 1950s by watching the
> decays of cobalt nuclei. Since then parity violation has also been
> observed in other reactions, such as transitions between energy
> levels within atoms and electron-positron annihilations, but never
> before in low-angle, relatively low-energy electron-electron
> scattering. Electrons are non-nuclear particles; so why do they
> scatter via any kind of nuclear force, much less the weak nuclear
> force? Because the weak and electromagnetic forces, though normally
> very different in their attributes (the electromagnetic force keeps
> atoms together and governs light, while the weak force exerts itself
> only at very short range, within about the size of a proton, and is
> responsible for some kinds of radioactivity) the two forces are
> still, properly speaking, parts of a single underlying "electroweak"
> force. Therefore even though electrons interact chiefly through the
> electromagnetic force, there is enough admixture of weak-force to
> make itself felt, albeit only in an experiment of great delicacy.
> Researchers at SLAC scattered a high energy beam of polarized
> electrons off electrons in a liquid hydrogen target and measured the
> fractional difference in scattering rates when the intrinsic spin of
> the beam electrons were lined up with or against the direction of
> the beam. The observed asymmetry
> not only demonstrated that a bit of parity-violating force was
> present (in keeping with theoretical ideas about the weak force) but
> also provided a measure---in fact, the first quantitative
> measure---of the electrons' "weak charge," a commodity, analogous to
> electric charge, and indicative of the strength of the weak
> interaction between two electrons. One of the team members, Krishna
> Kumar of the University of Massachusetts (kkumar_at_physics.umass.edu),
> asserts that the statistical error of 30 parts per billion (ppb) is
> the most precise measurement of an asymmetry (the measured effect
> was 175 parts per billion) in a lepton scattering experiment (that
> is, one involving electrons, muons, or neutrinos). (Anthony et al.,
> Physical Review Letters, upcoming article)
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