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Anomalous magnetic moment of the muon
1 February 2004
Earlier, a discrepancy was reported between the measured
and Standard Model predicted magnitudes of the anomalous
magnetic moment of the antimuon (Phys. Usp. 44 330 (2001);
Phys. Usp. 45 998 (2002)). Now, data from an
analogous experiment, but with negative muons
have been processed by the g-2 Collaboration at Brookhaven
National Laboratory. Within the scatter of the experiment,
the anomalous magnetic moments of muon and anti-muon are the
same, as they must be according to the CPT theorem. At
the same time, the calculated value of the anomalous moment
and that obtained from these three experiments differ at the
level of 2.8s. This is the strongest disagreement yet between
experiment and the Standard Model. Supersymmetry effects
or other phenomena beyond the Standard Model may
possibly provide an explanation of the discrepancy.
Sources: http://www.bnl.gov/bnlweb/pubaf/pr/2004/bnlpr010804.htm
Spins of quarks in the proton
1 February 2004
High-precision measurements of the spin direction distribu-
tion of quarks in neutrons and protons have been carried out
at T Jefferson Laboratory in the US under the leadership of
J P Chen and Z E Meziani. Nucleons consist of three valence
quarks as well as gluons and virtual quark-antiquark pairs
(sea quarks). The spin of a nucleon is not due to the spins of its
constituent particles alone, but also to the orbital angular
momenta the particles have when they move inside the
nucleon. The team examined the scattering of an electron
beam from a helium target. The electron energy was chosen to
be 5.7GeV, when electrons interact primarily with valence
quarks and to a lesser degree with sea quarks and gluons. The
results obtained agree well with the theoretical models
accounting for the orbital angular momenta of the particles.
It was found that two valence u-quarks in the proton have
their spins along the total proton spin, whereas the spin of the
valence d-quark may be directed differently.
Source:
http://www.jlab.org/news/articles/2003/nucleon.html
Superfluidity of solid helium
1 February 2004
In an experiment carried out at Pennsylvania State Uni-
versity, E Kim and M H W Chan may have first discovered
the superfluidity of solid 4He. Although superfluidity is
usually associated with quantum liquids, A F Andreev and
I MLifshits pointed out in 1969 that crystalline materials can
also possess superfluidity provided there are enough defects
and vacancies in them. Defects and vacancies can even appear
in a crystal at an absolute zero of temperature owing to
quantum fluctuations. To transfer helium into the solid state,
it is necessary not only to cool down the sample but also to
subject it to a considerable pressure. Kim and Chan filled a
disc made of porous glass with liquid helium-4. Helium
penetrated the pores, whose total volume was about 30% of
that of the disc, and was carried to the solid phase through
cooling and compressing. The disc was mounted on a torsion
pendulum, from whose period of swing the moment of inertia
of the disc could be measured. At a pressure of several dozen
atmospheres and a cooling temperature below 0.175mK, the
moment of inertia was observed to drop steeply, presumably
indicating the transformation of the solid 4He to the super-
fluid state. Numerous control experiments revealed no
systematic errors in the experimental methodology
employed. In a similar experiment with 3He, for example, no
decrease was observed in the moment of inertia.
Source: Nature
427 225 (2004)
Domain wall motion
1 February 2004
An experiment by K S Novoselov, A K Geim, E W Hill,
I V Grigor'eva (all from the University of Manchester), and
S V Dubonos (Institute of Microelectronics Technology,
Chernogolovka) detected domain wall displacements of as
little as 0.5A, one-hundredth that for the spatial resolution of
previous experiments. The domain walls studied were those
separating regions of different magnetization in a thin film of
the compound (YBi)3(FeGa)5O12.
The walls had a thickness
of about 11nm, which is only 6 times the crystal plane
separation. The basis for the displacement detector was a
two-dimensional electron gas, highly sensitive to even the
slightest changes in the magnetic flux. The motion of the
domain walls was induced by slowly changing the external
magnetic field. The team observed displacements over a
distance of 0.5A, although exactly how these displacements
take place is not yet clear. To describe the motion of the
domain walls, R Peierl's theory of magnetic potential
provides a useful framework.
Source: Nature 426
812 (2003)
A binary pulsar
1 February 2004
A previous issue of the journal (Phys. Usp. 47 102 (2004)) reported the discovery of the
pulsar J0737-3039 with a rotation period of 23 ms, forming a
close pair with another neutron star. Follow-up observations
using the 64-m Parkes radio telescope in Australia have
shown that the second neutron star is also a pulsar, marking
the first observation of a neutron star pair with both its
components being pulsars. The second pulsar has a mass of
1.25 solar masses and a period of rotation of 2.8s, and there is
an orbital modulation in its emission, presumably due to the
energy flow coming to its magnetosphere from the first
neutron star. The observation of a close pair of two pulsars
holds promise for the high-precision measurement of general
relativity effects. In particular, it is expected that relativistic
corrections of a higher order than v/c2
might be measured. In addition, a detailed insight into the pulsar magnetospheres
will be provided. The existence of binary pulsars has long
been predicted by the evolution theory of multiple stars.
Source: Nature 426 531
(2003)
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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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