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22. Sept. 2001 © Schulphysik">email: Schulphysik
"The American Institute of Physics Bulletin of Physics News" AIP Auswahl Sept. 2001 by Phillip F. Schewe and Ben Stein
ANTIPROTON'S MASS AND CHARGE
have been measured to within 60 parts per billion, affording new tests of quantum mechanics. Paul Dirac's 1930 prediction of a whole shadow family of particles, antiparticle counterparts of the known particles, was quickly born out. In 1932 the anti-electron, the positron, was discovered and in 1955 antiprotons (pbar) were made artificially in an accelerator for the first time. Since that time physicists have sought to determine that antimatter plays by the same rules as ordinary matter. An excellent place for these studies is at the CERN Antiproton Decelerator in Geneva, where antiprotons are created in high energy collisions, then collected, cooled, decelerated, and directed toward a number of experimental setups. One such experiment, staffed by a Japanese-European collaboration, sends the antiprotons into a bottle of cold helium. About a million of the pbars at a time ingratiate themselves into helium atoms, essentially taking the place of an electron and, at least in principle, obeying all known laws of atomic physics, including the ability to make quantum jumps between energy states of this exotic "antiprotonic" helium atom. In fact the pbar intruder begins in a somewhat circular orbit but after about one microsecond undergoes a transition to a closer orbit. It does this again and again until the antiproton eventually annihilates with a proton or neutron in the helium nucleus. Before this happens, however, the CERN scientists have more than enough time to perform some crucial atomic physics, including the first-ever measurement of ultraviolet transitions in this kind of exotic atom. Not waiting for the transitions to occur, the researchers actually induce them with a beam of laser light. Knowing the laser frequency at which the transitions occur allows one to calculate a number proportional to the antiproton charge squared times the antiproton mass. When this number is combined with a separate measurement of the antiproton's motion in an atom trap (see Update 426), which supplies a value for the ratio of the antiproton's charge to its mass (a ratio measured with uncertainties of only 90 parts per trillion), then a separate value for the mass and charge of the antiproton can be determined. In this case the values agree with those of the proton (allowing for the opposite charge) to within 60 parts per billion.
Hori et al., Physical Review Letters, 27 August 2001; contact Masaki Hori, firstname.lastname@example.org, 41- 22-762-8306, or John Eades at CERN, email@example.com
IS ALPHA, LIKE
PI, A FUNDAMENTAL CONSTANT,
FOR A RE-IONIZATION ERA
in the early universe has been glimpsed in the form of a quasar spectrum exhibiting a paucity of radiation at UV and shorter wavelengths. Let's retrace some cosmological history. In the early years after the big bang conditions were too hot for neutral atoms to form; protons and electrons roamed independently in plasma form. Later, about 300,000 years after the big bang, things were cool enough for electrons and protons to form neutral hydrogen, making the universe transparent to visible light but opaque to higher-energy light which (if there were much of it about, and there wasn't) would be absorbed by these same H atoms. Later still the first stars and quasars started to pump out UV light. This radiation was avidly absorbed by surrounding reservoirs of neutral H, sometimes ionizing the atoms in the process. As time passed more stars/galaxies/quasars formed, more UV was produced, and more of the neutral H was being turned back into ions. At a certain point, the great majority of H would be re-ionized. Since bare electrons and protons cannot absorb light, UV photons could thereafter proceed largely unhindered through the cosmos.
A new study of quasars made with the Sloan Digital Survey telescope looks at this process happening; it shows that quasar spectra out to a redshift of about 6 feature UV emission, but that the furthest-out (earliest after the big bang) quasar yet glimpsed, at a redshift of 6.28, does not, suggesting that this quasar was active in an epoch when neutral H was still plentiful enough to choke off high energy radiation. Thus a re-ionization era would seem to have occurred around Z=6, at a time corresponding roughly to 800 million years after the big bang.
(Becker et al., Los Alamos preprint at http://arXiv.org/abs/astro-ph/?0108097 )
|ANOMALOUS ACOUSTOELECTRIC EFFECT
has been discovered by a Russia-Poland-Ukraine collaboration (A.V. Goltsev, Ioffe Physical Technical Institute, St. Petersburg, firstname.lastname@example.org). When an acoustic wave propagates through an electrically conducting surface, it can drag electric charge along with it, just as wind drags autumn leaves along a street. This "acoustic wind" is known more formally as the acoustoelectric (AE) effect. Studying the electric current produced by the AE effect can provide important information on how electrically charged particles interact with the crystal lattice of a conducting material. Such materials include "manganites," manganese-based compounds that can exhibit "colossal magnetoresistance," in which electrical conductivity becomes tremendously sensitive to external pressure and applied magnetic fields. Towards these ends, the researchers investigated the AE effect in a manganite thin film atop a lithium-niobium-oxygen (LNO) substrate. They observed an unusual effect: sending an acoustic wave in a certain direction produced a much weaker electric current than expected in that direction. The researchers discovered why: in addition to the ordinary acoustic wind, a countervailing wind was flowing in a direction opposite to the acoustic wave. The countervailing wind arose from the fact that the substrate was "piezodielectric," in which electric fields were generated in response to pressure. When the acoustic wave created an alternating pattern of compression and expansion in the substrate, the compressed regions produced electric fields pointing in the direction of the countervailing wind. These fields interacted with the electrons on the thin film. Since the manganites increase their conductivity dramatically when compressed, this encouraged a flow of electrons in the countervailing direction. While this anomalous AE effect is probably too weak for technological applications, measuring it could provide a new method for studying the effects of applied pressure on a conducting material. This could be useful in those cases when employing conventional methods for those measurements is difficult, as is the case for thin films or quantum wells, wires, or dots. (Ilisavskii et al., Physical Review Letters, 1 October 2001)