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Proton magnetic moment and s-quarks
1 June 2006
The structure and composition of the proton have been
determined with the highest precision yet in a new experiment conducted
at the Thomas Jefferson National Accelerator Facility (USA) on electron scattering by protons.
Along with the three valence uud quarks and the gluons that
bind them together, the proton also contains virtual quarks,
called sea quarks. One property of gluons is their ability to
briefly fluctuate into ss pairs which affect the charge
distribution in and the magnetic moment of the proton (or
affect the proton's form factor, to use the language of
scattering experiments). Based on previous calculations and
experiments (see, for example, Phys. Usp. 48 759 (2005)), s-quarks were believed to
contribute up to 10% of the photon's magnetic moment.
Now HAPPEx measurements at Jefferson Lab have reduced
this figure to or below 4%. In this experiment, a polarized
3-GeV electron beam was collided elastically with liquid-hydrogen
or liquid-helium targets, and by measuring parity-violating
asymmetries in electron-proton scattering, the
contribution from electromagnetic and neutral weak forces
to the interaction involved was found and the value of the
form factor determined.
Source: Physics News Update, Number 776
Testing special relativity
1 June 2006
A University of Washington team has conducted an experiment
to search for superweak vector fields that violate
Lorentz invariance by interacting with the spins of elementary
particles. The team worked with a torsion-balance
apparatus - a rotating pendulum made of blocks whose
material had its spin magnetism almost fully compensated by
the magnetic moments of orbital electrons in atoms. An
additional anisotropic interaction with spins, if present,
could violate this compensation as the pendulum rotated
about the direction of the hypothetical field. Although the
experiment did not uncover any new interactions, it placed
the tightest bound yet on the possible fields: their energy scale
cannot exceed 10-21eV according to the team. This effect is
comparable to what a magnetic field of 10-15G might
produce. The new result is a hundredfold improvement over
previous bounds.
Source: Physics News Update, Number 775
Fundamental constants may vary with time
1 June 2006
Although some of the Grand Unification theories allow for
the possibility that the fundamental constants are slowly
varying with time, the experimental searches for this variability
have so far yielded no decisive results. For example, no
evidence has been found to confirm the cosmological-time
dependence of the fine-structure constant a (see
Phys. Usp. 44 980 (2001). According
to some theoretical predictions, the proton-to-electron mass
ratio mp/me varies faster than a, making variations in
mp/me
more likely to be found. Now, by comparing the hydrogen
spectrum obtained in the laboratory and the position of
absorption lines in the spectra of distant quasars, W.Ubachs
of the Vrije Universiteit in Amsterdam (the Netherlands) and
his colleagues have concluded that over the last 12 billion
years the ratio mp/me has indeed decreased by the relative
amount of 2x10-5. The team relied on new, ultrahigh-resolution
laboratory measurements of hydrogen spectra
in the extreme-UV range, carried out under the direction
of Ubachs, and used the high-quality VLT spectra of a pair of
quasars, recorded with the ESO in Chile. Hydrogen lines in
the spectra of the quasars are due to light passing through
intergalactic gas clouds along the line-of-sight, and the
relative positions of the lines depend on the value of mp/me.
The positive result obtained has a statistical confidence at the
level of 3.5 standard deviations and is in need of independent
verification.
Source: Phys. Rev. Lett. 96 151101 (2006)
Optical spin-to-orbital angular momentumconversion in media
1 June 2006
L.Marrucci, C.Manzo, and D.Paparo (University of Naples,
Italy) have conducted an experiment in which the circular
polarization of a laser beam was reversed, being accompanied
by changes in photon spin from h to -h and transfer of the
excess spin angular momentum 2h to the orbital angular
momentum of the light wave. The optical conversion took
place when the laser beam in question was passed through a
liquid-crystal half-wave plate with specially fabricated axial
nonuniformity and with anisotropically oriented molecules.
The new technique can find application in quantum-communications
and information-processing devices.
Source: Phys. Rev. Lett. 96 163905 (2006)
Closest pair of supermassive black holes
1 June 2006
The closest pair of supermassive black holes ever discovered
in the Universe has been found at the center of the elliptical
galaxy 0402+379 using the
VLBA radio telescope. The separation between the holes is as
small as 24 light years (more than 100 times closer than any
pair found before), and their combined mass is 1.5x108
times the mass of the Sun. The presence of a double radio
source in the core of 0402+379 galaxy was detected earlier by
the same telescope, but it is the new and more detailed
observations, at radio frequencies of 22 and 43GHz, which
identified them as consisting of black holes. It is believed that
these black holes resided earlier at the centers of two separate
galaxies which later coalesced into one galaxy, 0402+379.
Galaxy collisions and coalescences were commonplace in the
early stages of galaxy formation and evolution, and virtually
every galaxy we see today has undergone one or more
coalescences in its history. According to some theoretical
calculations, after such a coalescence black holes gradually
approach each other and can ultimately coalesce into one
black hole. This black hole coalescence may have already
happened in many galaxies by now, giving rise to enormous
bursts of gravitational radiation which can be detected by
gravitational-wave detectors now under construction.
Clearly, the chances for such detection are higher now that a
closest pair of black holes has been discovered at the center of
galaxy 0402+379.
Source: http://www.nrao.edu/pr/2006/binarybh/
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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources.
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