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| Issue 12, 2024 |
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Extracts from the Internet | ||
Measuring the electron magnetic moment1 August 2006An 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 directly1 August 2006According 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 11Li1 August 2006T 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 accretion1 August 2006The 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. | |
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