Proton structure and the s-quark
1 July 2005
The G0 experiment at Jefferson Lab has provided new insight into the way s-quarks contribute to the proton magnetic moment and charge distribution. Protons are made of two u-quarks and one d-quark held together by gluons. Gluons, however, can fluctuate into quark-antiquark - in particular, ss - pairs for a short time. The G0 team studied the interaction of a longitudinally polarized electron beam with a hydrogen target, using a toroidal spectrometer to measure the amount by which scattered electrons became polarization asymmetric as a result of the parity violation due to interference between the electromagnetic and neutral weak interactions. The G0 results agree well with data obtained previously in other laboratories. Source: nucl-ex/0506021
1 July 2005
The ground energy state of the kaonic hydrogen has been examined in the DEAR experiment at the INFN (Institito Nazionale di Fizica Nucleare) Laboratory in Italy. The kaonic hydrogen atom consists of a negatively charged K--meson (antikaon) which revolves around a proton much like the electron in the ordinary hydrogen atom does. Colliding accelerator beams of electrons and positrons produced pions, which decayed into K- and were directed at a hydrogen target, where some of the K-s replaced electrons in excited orbitals after losing their kinetic energy in collisions with hydrogen atoms. The DEAR team studied the spectrum of the X-ray radiation due to K- transitions to the ground state orbital. For inner orbitals close to the nucleus, strong K--proton interactions were important, leading to the energy levels being shifted and broadened. The X-ray detector of the DEAR experiment was a high-precision one capable of detecting transitions between different levels. The measurements have yielded limits on the parameters of chiral symmetry violation and provided fresh insight into the properties of strong interactions.
Source: Phys. Rev. Lett. 94 212302 (2005)
Vortices in a degenerate Fermi gas
1 July 2005
Evidence for the superfluidity of a degenerate Fermi gas was found by W Ketterle and his colleagues at MIT by observing quantized vortex arrays in a rotating gas - a definitive result finally after previous indirect evidences (see Phys. Usp. 47 963 (2004)). In this experiment, a gas of lithium atoms kept in a magnetic trap was cooled to a temperature of 50 nk. With increasing magnetic field, the fermionic atoms of # formed bosonic pairs and made a transition to the Bose-Einstein condensate state which, as the field increased still further, became a degenerate Fermi gas of strongly interacting atoms. Using a laser, an angular momentum was transferred to the gas cloud with the result that, like in other superfluid liquids, quantized hollow core filaments (i. e., vortices) developed in the gas, with the total angular momentum added up from those of the individual vortices. The vortices repelled one another and formed an ordered lattice structure. A vortex pattern like this provides unambiguous evidence for the superfluidity of degenerate Fermi gas. Interestingly, interaction between Fermi gas atoms can be controlled by changing the magnetic field close to the Feshbach resonance. In this way, the `crossover' between the molecular Bose- Einstein condensate and the BCS regime was investigated.
Source: Nature 435 1047 (2005)
Photon momentum in a dispersive medium
1 July 2005
Although it is known that in a medium with refractive index n the momentum of a photon is nh/l (where l is the wavelength), the following question has long been a matter of dispute. Suppose outside photons with initial momentum h/l fly into a bounded medium and are there absorbed by its atoms. What will be the atomic recoil momentum, h/l or nh/l? The first direct experimental study of this question has been made by G Campbell and her colleagues at MTI, who used the Kapitza-Dirac interferometer to measure the absorption of photons by Bose condensed rubidium atoms. The recoil momentum result was nh/l, implying that even when considering individual absorption events one must take into account the presence of other atoms in the absorbing atom's environment. These other atoms acquire a total momentum (n-1)h/l directed in the opposite direction.
Source: Physics News Update, Number 732
Gravitational waves from a binary system
1 July 2005
A Chandra X-ray spectrum has been obtained from RX J0806.3+1527, a system consisting of two white dwarf orbiting each other with a 5.4 min period as measured from the way the system's X-ray and optical radiation vary in time. A period this short implies a very small radius of the orbit and hence large energy losses due to gravitational wave radiation. The gravitational radiation causes the orbit to shrink, resulting in a smaller rotation rate. A rotation rate decrease of 1.2 mks per year has indeed been observed. That gravitational waves can act as an energy loss mechanism is strongly suggested by the rotation period changing at a uniform rate over several years of observations. Previously, the orbit-changing effect of gravitational radiation was measured on neutron star binaries (Hulse-Taylor and PSR J0737-3039 pulsars, see Phys. Usp. 47 102 (2004)). The white dwarf system RX J0806.3+1527 has a much shorter orbital period, and the relativistic effects it exhibits are more pronounced. The mechanism by which RX J0806.3+1527 generates X-ray radiation has not been firmly established. In the `electric star' model, plasma is heated by unipolar induction currents. The alternative hypothesis that RX J0806.3+1527 is a single accreting white dwarf faces difficulties due to the accretion rate being inconsistent with the rate at which the rotation period changes.
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.