Measurement of the lifetime of antimuons μ+
1 March 2011
The MuLan Collaboration at the Paul Scherrer Institute (Switzerland) has
performed the most accurate to date measurements of the lifetime τμ+
of antimuons μ+. These particles were produced in the decay of π+
mesons and were stopped in the target material. Ferromagnetic foil and quartz
disk were used as targets. In quartz μ+ formed hydrogen-like atoms of
muonium (a bound pair of antimuon and electron). Positrons produced in the
decays of μ+ were recorded using 170 pairs of triangular scintillator
detectors. On the whole the experiment recorded 2 × 1012 such decays
and it was possible to improve the precision of measuring τμ+ by a
factor of 15 in comparison with the earlier results. The value obtained,
τμ+ = 2196980,3 ± 2,2 ps, agrees to within experimental errors with
the earlier results for τμ+ for free μ+ but is 2,5 σ below
the current value of the Particle Data Group. The new measurements have also
allowed the calculation of the Fermi constant GF with record accuracy.
Source: Phys. Rev. Lett. 106 041803 (2011)
Thermal Casimir effect
1 March 2011
S.K. Lamoreaux (Yale University, USA) and his colleagues have measured for the
first time the force of attraction between two macroscopic bodies caused by
thermal fluctuations of the electromagnetic field. This phenomenon, a variation
of the Casimir effect, was predicted by E.M. Lifshitz in 1955 and was
previously observed only in the case of the atom-surface interaction (the
Casimir – Polder force). The ordinary Casimir effect, in contrast to the thermal
effect, stems from zero-point quantum fluctuations and has previously been
investigated in many experiments. S.K. Lamoreaux et al measured the force
exerted on the metal plate by an approaching metal sphere. Both bodies were
fixed on hinges and could transmit force to capacitor plates. The force was
measured by the potential difference across the capacitor needed to compensate
for the attraction of bodies investigated. The effect was observed at 300 K at
distances between the plate and the sphere of d = 0,7-7 µm when the
contribution of the zero-point quantum fluctuations (ordinary Casimir effect) is
insignificant. Technical difficulties had to be overcome in preparation for the
experiment: electrostatic forces, surface irregularities, vibrations, etc. In
recent years, there was a debate about which method should be preferable for
taking into account the dielectric constant of a body at low frequencies in
calculations of the thermal Casimir force. While transverse electric mode with
ω = 0 does not contribute to the attractive force in the Drude
free-electron model, it becomes important in the plasma model and increases the
force by a factor of two. The measurements of S.K. Lamoreaux et al are in
excellent agreement with the Drude model, so much so that the plasma model in
this case may be regarded as rejected.
Sources: Nature Physics 7 230 (2011)
Scale invariance of phase transitions in two-dimensional Bose gases
1 March 2011
As a rule, the properties of systems near phase transition points behave in a
universal manner which weakly depends on the microscopic structure of systems.
L.P. Pitaevskii and A. Rosch theoretically predicted in 1997 a universal
scaling behavior of the degenerate two-dimensional Bose gas of particles in a
wide fluctuation region near the point of the Berezinskii – Kosterlitz – Thouless
phase transition in which long-range order is destroyed by thermal fluctuations.
This versatility means a similarity of the properties of systems which have some
identical specific combinations of parameters, composed of the effective
constant of interaction between atoms, the system scale and other variables.
However, there were no reliable experimental data obtained until recently on the
presence of this scaling in 2D-systems. C.-L. Hung and his colleagues have
demonstrated for the first time a universal behavior in the experiment with
two-dimensional gas of cesium atoms 133Cs at different temperatures and
different sizes of the system and with variable strength of interaction between
atoms. A gas cloud consisting of 2 × 104 atoms was placed in a
quasi-two-dimensional optical trap. The scattering length of atoms and
correspondingly the interaction force were controlled by the magnetic field by
using the Feshbach resonance. The gas was observed through a microscope and
through a CCD-camera using the absorption technique. It was found that the shape
of the thermodynamic functions of the gas has a universal form, depending only
on certain combinations of parameters, which corresponds to the theoretically
predicted scaling invariance.
Source: Nature 470 236 (2011)
Light-induced superconducting transition
1 March 2011
A group of researchers led by A. Cavalleri (the University of Oxford and the
University of Hamburg) discovered the effect of transition of matter from normal
to superconducting state when exposed to femtosecond infrared pulses of a
powerful laser. The substances studied were cuprates
La1.675Eu0.2Sr0.125CuO4 at a temperature of about 10 K.
Superconductivity in a thin layer 10 µm deep under the sample surface was
recorded by Josephson plasma resonances in the spectrum of reflected radiation
in the THz range. The transitions occured very quickly, over not more than 1-2 ps.
The relaxation back to insulator state was not fixed over time intervals up
to 100 ps which could be observed in this experiment. This is indicative of
sufficient stability of the new phase created by pulse illumination. The cause
of the superconducting phase transition is likely to be light-induced
displacements of oxygen atoms from their seats in the crystal lattice to
quasistable new positions. Earlier experiments have observed transitions to the
superconducting state of matter after exposure to light but in those cases
transitions occured after the relaxation of heated charge carriers. In this
experiment, light pulses were the direct cause of transitions.
Source: Science 331 189 (2011)
Galaxy at z ≈ 10,3
1 March 2011
It is possible that the Hubble Space Telescope discovered a galaxy with record
high redshift z ≈ 10,3, visible in the epoch when the age of the
Universe was ≈ 480 million years. The mass of this galaxy is
approximately 1/100 of the mass of the Milky Way and is characterized by 10 times
slower rate of star formation than found in galaxies with z ≈ 8, i.e.
only about 170 million years older. Therefore, z ≈ 10 appears to
constitute the beginning of the epoch of active star formation in the Universe.
Stars could begin to be born through mergers of protogalaxies which thus reached
the average mass favorable for certain gas-dynamic processes. Difficulties in
observing such distant objects are caused by absorption of radiation by neutral
hydrogen at frequencies below Ly - α (taking into account the cosmological
redshift). The conclusion on the large z of the discovered galaxy was made by
using the photometric method based on the color of the galaxy but so far there
is no spectroscopic confirmation on the basis of spectral lines. For an
alternative model whose probability level is estimated at 20%, the paper
suggests reddening of a nearby galaxy due to matter present along the line of
sight. At present the highest spectroscopically confirmed redshift of a galaxy
stands at z ≈ 8,6.
Source: Nature 469 504 (2011)
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.