|
Measuring the electron magnetic moment
1 August 2006
An experiment by G.Gabrielse, B.Odom, and David Hanneke at Harvard
University has provided the most accurate value yet for the
electron magnetic moment. Corrections to the value of the moment
come from the electron's virtual cloud described by quantum
electrodynamics, and the exact measurement of the moment is
important for verifying theoretical calculations in this field. The
Harvard experiment consisted in monitoring the motion of a single
electron in a trap over a period of several months. The trap acted
as a one-electron cyclotron and consisted of a positively-charged
central electrode and two negatively-charged electrodes, one above
and one below. Using a solenoid, a vertically directed magnetic
field was created. The motion of the electron in this situation
combined a closed circular motion in the horizontal plane and small
vertical vibrations. The circular motion was of quantum nature:
cooling the setup and isolating various types of noise brought the
electron down to the lowest cyclotron quantum state. The classical
vertical vibrations produced small changes in the electric
potential, which exerted a feedback on the circular motion. This
interplay allowed very accurate measurements of the trapped
electron's spin and cyclotron energy levels; and enabled the
electron magnetic moment to be measured to within 7.6x10-13, six times
better than previous experiments. The new value is 1.7 standard
deviations less than the previous one. Based on the electron
magnetic moment measurements, the accuracy of the fine structure
constant was improved by a factor of 10 over the previous value. An
important point to note is that, unlike accelerator measurements,
Harvard University's is a low energy experiment. Improving the
accuracy of the magnetic moment will lead, in particular, to new
limits on the size and hypothetical structure of the electron,
currently considered to be pointlike. Source: Physics News Update, Number 783
`Spin-charge separation' observed directly
1 August 2006
According to theoretical predictions, 1D structures can exhibit the
so-called `spin-charge separation' phenomenon, in which spinons (i.
e., quasiparticles carrying spin excitations) and electric charge
waves are spatially separated. Now this effect has been directly
and confidently observed for the first time at Berkeley Lab by
applying Angular Resolution Photoemission Spectroscopy (ARPES) to
a 1D sample of SrCuO2. On illuminating the sample by a high-power,
coherent, X-ray undulator beam, the electrons knocked out of the
sample showed two spectral peaks that corresponded to the spatially
separated excitations of spin and charge densities. While the 1D
structure is a feature of superthin `quantum wires', there are many
types of crystalline compounds that, even when in bulk, have their
electrons moving in one dimension. Given that the `spin-charge
separation' effect also underlies some high-temperature
superconductor models, its direct observation is important for the
understanding and further application of quantum wires as well as
for testing superconductivity models. Source: cond-mat/0606238
Coulomb dissociation of 11Li
1 August 2006
T Nakamura and his colleagues from Japan and the US used a new
experimental technique to gain more insight into the nucleus of 11Li
- a compact central core of 9Li surrounded by a halo of two
neutrons. One particular finding is that the nucleus has low-energy
excitations close to 0.6 MeV. The exact theoretical description of
the system of three bodies (here, the nucleus of 9Li and two
neutrons) is still lacking, nor have previous experiments produced
definitive and consistent results. The new experiment used a double
neutron counter which has a coincidence recognition system to
prevent counting one and the same neutron more than once. The team
irradiated a lead target with a beam of 11Li ions at an energy of
about 70MeV and measured the energy and angle distributions of
the collision-induced fragmentation products from the 11Li nuclei, i.
e., the nuclei of 9Li and neutron pairs. Using a gamma-ray detector,
it was verified that the destruction of the nuclei did not involve
emitting photons, suggesting that the 9Li nuclei remained in their
lowest energy state. What the experiment primarily implies is that
the structure of 11Li cannot be described correctly unless the mutual
interaction of the halo neutrons is taken into account. Source:
Phys. Rev. Lett. 96 252502 (2006)
Magnetic field and accretion
1 August 2006
The accretion of matter onto black holes is primarily controlled by
the magnetic field according to the Chandra data on the binary
system J1655-40, in which gas is transferred from the star onto the
accretion disk around a stellar-mass black hole. J1655-40 is
located in our Galaxy about 3 kpc from the Sun and belongs to the
class of microquasars, so called due to the
relativistic jets they feature. It is likely that the disk
accretion, jets, and radiation in microquasars are small-scale
analogues of processes in cosmologically distant quasars. Distant
quasars involve accretion onto supermassive black holes, and their
dynamic time scale is orders of magnitude larger than that for
stellar mass systems. As the gas in the accretion disk loses its
angular momentum because of viscosity, it slowly moves to the
center of the system and flows down onto the black hole.
Importantly, the observed X-ray spectrum of J1655-40 is close to
one obtained by simulating a magnetically turbulent accretion disk
- thus supporting the currently most accepted hypothesis that
viscosity is dominated by magnetic turbulence in this context. Source:
http://chandra.nasa.gov
|
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.
|