Antiproton-to-electron mass ratio
1 September 2011
Ì. Hori (Max Planck Institute of Quantum Optics, Germany and the University of Tokyo, Japan)
and his colleagues have measured with record accuracy the ratio of masses of the antiproton
anti-p and the electron e- in a metastable antiprotonic helium atom anti-pHe+ which
is the nucleus of ordinary helium with an electron on the ground orbit and the antiproton anti-p on the high Rydberg orbit. The lifetime of anti-p within anti-pHe+ is large enough
for precise spectroscopic measurements, since the wave function of anti-p has almost no
overlap with the nucleus while e- shields anti-p from destructive interactions with other
atoms. The experiment was performed at CERN where the beams of anti-p to be trapped into
anti-pHe+ were produced in the «Antiproton Decelerator». Hori et al studied two-photon
transitions (n,l) → (n-2,l-2) as the gas of anti-pHe+ was irradiated by oppositely
directed UV laser beams. The two-photon spectroscopy has allowed Hori to achieve high precision
(2.3-5) × 10-9 in the measurement of spectral lines due to partial compensation of
Doppler broadening. To calculate the ratio màíòè-p/me using the spectral data,
theoretical QED calculations of the levels anti-pHe+ were used. The obtained
result màíòè-p/me = 1836.1526736(23) agrees very well, within experimental errors, with
the ratio of proton and electron masses which at present is known at a comparable accuracy.
According to the CPT theorem, these ratios must be exactly equal.
Source: Nature 475 484 (2011)
Quantum decoherence in photodetectors
1 September 2011
Decoherence of the quantum states of different systems has already been studied in a number of
experiments. V. D'Auria and her colleagues in the Laboratoire Kastler Brossel, Universite
Pierre et Marie Curie (Paris, France) performed a new original experiment in which they
investigated the decoherence not in a quantum state but in a detector through which the
observation was conducted. Light was sent into a detector (avalanche photodiode) as an
attenuated laser beam in which only a few photons were left in each pulse. External noise which
caused decoherence was simulated by a second continuously working laser. Statistics of detector
counts made it possible to find the evolution of the Wigner function which characterizes the
distribution of quantum probabilities. Negative values of the Wigner function at low noise
level were an indication of the quantum nature of the detector. When the noise level reached
approximately half of the quantum efficiency of the detector, the Wigner function grew positive
everywhere which corresponded to the decoherence of the detector and its transition to the
semi-classical state. This study is important for designing devices processing
quantum information as the decoherence of the detector may cause an undesirable
decoherence of quantum states at the subsequent stages of information processing.
Source: Phys. Rev. Lett. 107 050504 (2011)
Violation of the Wiedemann – Franz law in one-dimensional conductor
1 September 2011
N.E. Hussey (University of Bristol, UK) and his colleagues found that the ratio of the Hall
(transverse) thermal conductivity coefficient to that of the Hall electric conductivity in the
metallic phase of the compound Li0.9Mo6O17 (characterized by quasi-one-dimensional
crystal structure) increases with decreasing temperature; at 25 K the ratio
κxy/σxy exceeds the value typical of conventional metals by a factor of
105. Such behavior differs greatly from the Wiedemann – Franz law which states that
κ/σT≈ const; indeed, the heat and charge in the ordinary 3D-metals are
transferred by the same quasiparticles. The inapplicability of the Wiedemann – Franz law to
1D-systems occurs, according to the Tomonaga – Luttinger theory, because heat is transferred in
them by collective excitations both of spin (spinons) and of charge (holons) while charge is
transferred by holons only. The separation of fluxes of quasiparticles and more efficient heat
transfer are caused by much stronger scattering of holons by impurities in comparison with
spinons. Owing to this factor the transfer of holons in 1D-systems is hindered.
Source: Nature Communications 2 396 (2011)
Mosaic distribution of static charges
1 September 2011
It is usually assumed that the electrification by friction of two different dielectrics causes
their surfaces to acquire approximately uniform distribution of charges of opposite signs. Í.Ò.
Baytekin (Northwestern University, USA) and his colleagues studied charge distribution on the
surface of polymeric dielectrics (polycarbonates etc) and were able to show that in reality a
mosaic pattern forms on the surfaces of these materials in which oppositely charged areas
alternate in a random fashion. Measurements with an atomic force microscope in the surface
potential mode (the Kelvin method) showed that statistically the mosaic distribution can be
described by two random fields with mean fluctuations scales of 0.45 µm and 0.044 µm.
In the past the static charge was understood to be only the charge with mean surface density
≈ 0.2 nC cm-2 averaged over the scale > 0.45 µm. This averaging results in
compensation of charges of opposite signs; however, much larger alternating charges ≈ ± 1 µC cm-2 remain on the smaller scale. Experimenters noticed that such factors as
duration of friction, or pressure applied during friction, or method of friction application,
or inhomogeneity of the surfaces of dielectrics had no significant effect on the appearance of
the mosaic. No mosaic distribution of opposite charges was found on the surface of samples of
simple substances — silicon and aluminum — subjected to similar electrification. The
mechanism of electrification of dielectrics is not yet completely understood. Í.Ò. Baytekin et
al used confocal Raman and X-ray photoelectron spectroscopies to investigate the chemical
properties of polymer surfaces after electrification and found that friction changed the
configuration of the chemical bonds Ñ=O in molecules at the surface of the samples and that
there was an exchange of matter between chemically dissimilar friction surfaces. These
phenomena may be connected in some manner with the mosaic distribution of charges.
Sources: Science 333 308 (2011), elementy.ru
Antiprotons in the Earth's radiation belts
1 September 2011
Detailed theoretical calculations have shown that anti-p born in high-energy particle
collisions of cosmic rays with the atmosphere must be captured by the Earth's magnetic field
and form a wide antiproton radiation belt at an altitude of several hundred kilometers above
the Earth surface. The basic process of production of anti-p are decays of anti-n generated
by ðp collisions. The lifetime and the number of anti-p in the belt are constrained by their
annihilation and scattering by other particles. The predicted belt of anti-p with energies of
60-750 MeV was first detected by the PAMELA detector placed on board the Russian satellite
Resurs-DK1. At its orbit, PAMELA can observe the belt of atmospheric anti-p only when
crossing the South Atlantic Magnetic Anomaly in which radiation belts dip close to the Earth.
On the whole, 28 atmospheric anti-p were recorded during the observation period, which is
three orders of magnitude higher than the amount that could be produced by the flux of galactic
anti-p. The spectrum of galactic anti-p created outside the solar system in the collisions
of cosmic rays with interstellar matter was also measured earlier by the PAMELA detector.
Source: Astrophys. J. Lett. 737 L29 (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.