Science > Physics > PHYSICS NEWS UPDATE -- Number 776 May 2, 2006 by Phillip F. Schewe,Ben Stein and Davide Castelvecchi
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PHYSICS NEWS UPDATE -- Number 776 May 2, 2006 by Phillip F. Schewe,Ben Stein and Davide Castelvecchi |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 776 May 2, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu
THE SEA OF VIRTUAL QUARKS shimmering inside every proton inside
every atom has now been studied with exquisite precision in a new
experiment conducted at the Jefferson Lab. The surprising result is
that the quark-antiquark pairs bubbling irrepressibly into and out
of existence, especially those with a strange flavor, contribute so
little to the life of the proton, prompting theorists to puzzle even
more intently over the basic question: what is a proton? The simple
answer has been that the proton consists of three regular (valence)
quarks always present plus the effervescent "sea quarks" emerging
from the vacuum plus a fleet of force-carrying gluons. But if ever
the whole did not equal the sum of its parts, this is true for the
proton. Sum the charge of the valence quarks and you get the charge
of the proton. So far, so good. But sum the mass of the valence
quarks and you account for less than 1% of the proton's mass.
The Hall A Proton Parity Experiment (HAPPEx) at Jefferson Lab
scatters a 3-GeV beam of electrons from a slender thermos bottle of
liquid hydrogen, providing in effect a target full of protons, and
from a helium target, which provides both protons and neutrons.
Only those events in which the electron scatters elastically (they
lose none of their energy, but do deflect through an angle of 6
degrees) are chosen for analysis. One can think of the electron as
scattering from the proton by sending ahead a virtual photon
(carrying the electromagnetic force) or a virtual Z boson (carrying
the weak force) which probes the proton much as bright light sent
and scattered through a microscope probes a bacterium. In this case
the wavelength of the HAPPEx "microscope" is chosen with great
care (by fixing the energy of the electrons and the position of the
detector) to equal the size of the proton itself, namely one
femtometer, 10^-15 m. In this case the microscope is viewing the
whole proton all at once. It doesn't try to "image" the proton so
much as it attempts to determine what the proton is at the moment of
scattering. By controlling the polarization (spin orientation) of
the electrons, and by comparing the proton and helium scattering
data, one can determine separately the contributions from electric,
magnetic, and weak-force scattering. And from these, the degree to
which sea quarks are present in the proton (encapsulated in a
parameter called a form factor) can be deduced. The proton is
nominally made of two up quarks and one down quark, and so still
more up and down quarks from the "sea" contribute little of
interest. Therefore probing the sea is really a sort of referendum
on the status of the strange quark, the next heavier quark, inside
the proton. Previous theories, supported by some rough experimental
evidence, supported that the idea that strange quarks could account
for as much as 10% of the proton's magnetic moment.
One of the HAPPEx scientists, Paul Souder of Syracuse
(souder@physics.syr.edu), reported at last week's APS April Meeting
in Dallas that, with much greater precision, strange quarks can
account for about 1% of the proton's charge and no more than 4% of
its magnetic moment, and that owing to experimental uncertainties
both of these measured values might be consistent with zero. In
other words, the proton is a lot less strange than thought.
In addition to being the best expose of sea quarks, HAPPEx is
notable for these reasons: it constitutes the most controlled use of
a polarized electron beam; it provides the best measurement yet of
the asymmetry between the scattering of electrons with their spins
pointed along or against the line of movement, which in turn
provides a measure of the relative strength of the electromagnetic
and weak-force scattering, with a value of about 10^-7; and it
arrives at a rudimentary measurement 20 attometers for the average
separation between a sea quark and its antiquark twin inside the
proton.
LASER GAIN-WITHOUT-INVERSION, IN A SOLID. Early descriptions of
lasers emphasized that a majority of participating atoms in a laser
medium needed to have undergone a "population inversion." That
is, most of the atoms had to be in an excited state, the better to be
stimulated into emitting light and contributing to a growing pulse
of laser light. But this "gain" can be achieved without
inversion.
Experiments have shown that by coherently controlling the electrons
in ground-state atoms through a process called electromagnetically
induced transparency, the electrons could mostly be kept from
absorbing laser light being developed among the small number of
atoms in a sample actually in an excited state. This
gain-without-inversion (GWI) phenomenon has now been demonstrated in
a solid material for the first time. Speaking at last week's
Institute of Physics Condensed Matter and Materials Physics
Conference in Exeter (UK), Chris Phillips of Imperial College said
that his lab achieved GWI in an array of semiconductor
nanostructures---in effect, artificial atoms. Not only gain, but
slowing of light can be achieved in the Imperial College solid state
arrangement, making it possibly useful for future quantum
information applications. (See also Frogley et al., Nature
Materials, March 2006)
CORRECTION. We were off by 39 orders of magnitude: in PNU 775, the
RHIC peak energy density achieved in high energy collisions is 15
GeV per cubic-femtometer, not per cubic centimeter.
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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