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The mass of a top quark
1 July 2004
The most accurate measurement to date of the mass of the t-quark
has been made by the D0 collaboration at Fermilab. The result,
178.0 + - 4.3 GeV, was obtained by re-analyzing the data collected
by the Tevatron accelerator before its 1999 shutdown. Thus, the
average expected value of the t-quark mass is 5.3 GeV larger than
previously thought. Measuring the masses of the t-quark and of W-
bosons provides an estimate for the mass of the Higgs boson, a
so-far elusive particle that has been predicted by the Standard
Model of elementary particles. The problem of finding it is one
of most topical interest in high energy physics. Given the new
value of the t-quark mass, the expected mass of the Higgs boson
is 117 GeV (compared to the previous value of 96 GeV), the upper
limit on the mass increasing from 219 GeV to 251 GeV. It is
perhaps because of this large mass that the Higgs boson avoids
being detected with current accelerators. Due to the revised mass
of the t-quark, the well-known restrictions on the parameters of
supersymmetric models are also altered.
Source: Nature 429 638 (2004)
Bose-Einstein condensate in the Tonks-Girardeau regime
1 July 2004
B Paredes from the Max Planck Institute for Quantum Optics and
his colleagues from France and the Netherlands have for the first
time brought a Bose-Einstein condensate of rubidium-87 atoms into
the Tonks-Girardeau regime, in which repulsion forces partly
reminiscent of those existing between fermions act between
bosons. This property was achieved by localizing the condensate
on a two-dimensional optical lattice which was created by
interfering laser beams in such a way that the only way for atoms
to move was along the beams. To enhance the effect, an additional
optical lattice was created along each of the beams. The
calculated momentum distribution of the atoms is consistent with
the theoretical prediction for a Tonks-Girardeau gas.
Source: Nature 429 277 (2004)
Quantum teleportation of ions
1 July 2004
Experiments on the quantum teleportation of singly charged ions
have been conducted independently by two teams, one in Austria
and the other in the US. So far, such experiments have only been
done on photons. The Austrian experiment, led by R Blatt, and the
American experiment, conducted by Wineland and his colleagues,
involved calcium and beryllium ions, respectively. The
researchers used lasers to control the quantum states (i. e.,
spin directions) of the ions. The quantum state was teleported
over a distance of a few microns from one of the trapped ions to
another via a third ion, quantum-correlated with the first.
Unlike the Einstein-Podolsky-Rosen thought experiment, in a
teleportation experiment classical information on the measurement
conditions must first be transferred. For more details, see the
book by B B Kadomtsev, Dynamics and Information, Physics-
Uspekhi Publ., 1999.
Source: Nature 429 734 (2004)
Nanotube properties in a magnetic field
1 July 2004
The effects of a magnetic field on the conducting properties of
single-wall carbon nanotubes has been studied by J Cono and his
colleagues at Rice and Florida State Universities at fields of up
to 45 T using optical spectroscopy techniques. For originally
semiconducting nanotubes it is found that the valence-conduction
band gap decreases with increasing magnetic field. This is
directly opposite to what is observed in ordinary semiconductors,
in which the gap increases as the magnetic field is increased.
The researchers expect that as the field is further increased,
the gap will disappear, turning nanotubes into conductors.
Originally conducting multiwall carbon nanotubes were studied by
a team of researchers led by A Bezryadin of the University of
Illinois. It is found that increasing magnetic field first causes
the appearance of an energy gap and the transition of nanotubes
to the semiconducting phase, but as the field is further
increased, the gap decreases, turning nanotubes to conductors
again. Both observed phenomena had earlier been predicted in the
framework of the theory of the Aharonov-Bohm effect. These
experiments are the first to detect the influence of the
Aharonov-Bohm effect on the band structure of solids. The new
properties of carbon nanotubes hold promise for practical
applications in devices with magnetic-field-controlled material
characteristics.
Source: Science 304 1129 (2004),
Science 304 1132 (2004)
Dark energy
1 July 2004
Dark energy (in the form of a cosmological constant or
quintessence) fills the Universe, dominating in terms of mass
over other forms of substance, including matter. Earlier, the
existence of dark energy was inferred - and constraints on its
equation of state obtained - by observing remote `standard
candle' supernovas and measuring the anisotropy of the cosmic
microwave background radiation. Now an international team of
astronomers has developed a new method for studying dark energy
by observing X-radiation from galaxy clusters. X-ray emitting hot
gas and dark matter are present in similar amounts in almost all
clusters, making it possible to estimate cluster distances and to
calculate how the cosmological expansion has changed over the
last few billion years. The nature of expansion is determined by
the equation of state of the material that fills the Universe.
The Chandra X-Ray Space Telescope has studied 26 X-ray-bright
dynamically relaxed clusters in the redshift range from 0.07 to
0.9. When previous observations were taken into account, it was
found that dark energy accounts for about 75% of the mass of the
Universe, and that the dark energy equation-of-state parameter
is w=-1,20-0,28+0,24. Interestingly, the preferred value w<-1 corresponds to a
dark energy whose density increases with time. Matter with w<-1 is
usually called `phantom energy.' However, the case of a pure
cosmological constant, w=-1, is also compatible with the
observations.
Source: http://arxiv.org/abs/astro-ph/0405340
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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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