Physics news update 614

November 20, 2002

Record-high magnetic fields in the lab, almost a Gigagauss in magnitude, have been achieved by aiming intense laser light at a dense plasma, expanding the possibilities for laboratory re-creations of astrophysical events.

At last week's APS Division of Plasma Physics Meeting in Orlando, researchers from Imperial College, London, and the Rutherford Appleton Lab in the UK announced evidence of super-strong magnetic fields that are hundreds of times more intense than any previous magnetic field created in an Earth laboratory and up to a billion times stronger than our planet's natural magnetic field. Such intense magnetic fields may soon enable researchers to recreate extreme astrophysical conditions, such as the atmospheres of neutron stars and white dwarfs, in their very own laboratories.

At the Rutherford Appleton Laboratory near Oxford in the UK, researchers at the VULCAN facility aimed intense laser pulses, lasting only picoseconds (trillionths of a second), at a dense plasma. The resulting magnetic fields in the plasma were on the order of 400 Megagauss.

To determine the magnitude of the fields, the researchers made polarization measurements of high-frequency light emitted during the experiment. Recent measurements presented at the APS/DPP conference suggested that the peak magnetic field in the densest region of the plasma approaches 1 Gigagauss.

Due to technological advances peak laser intensities are likely to increase still further and consequently even higher magnetic fields may soon be possible, making it possible to put models of extreme astrophysical conditions to the test. (Poster CP1.125, November 11, contact Karl Krushelnick, Imperial College, University of London, 011-44-20-7594-7635, kmkr@ic.ac.uk; for background see Tatarakis et al., Nature, 17 January 2002)

Megagauss in Picoseconds

The item above describes the creation of high fields; this item describes the rapid measurement of high fields. Physicists from the Tata Institute and the Institute for Plasma Research in India have recorded in detail, for the first time, the huge magnetic spike encountered by atoms in a sample bearing the brunt of an intense laser shot.

Fields as great as 27 megagauss, roughly 50 million times the strength of Earth's magnetic field, come about very quickly in the following way: the 1016-watt/cm2 pump laser beam strikes an aluminum target, the surface layer of atoms is quickly ionized, and a stream of very fast electrons is released into the body of the target, inducing the huge field.

Many high-power lasers around the world study the effects of intense light upon a solid sample. The chief achievement of the Indian researchers is to look at this process with unprecedented temporal precision, monitoring the rising magnetic field in femtosecond intervals by watching the polarization of a delayed secondary laser beam reflected from the particle plasma engulfing the sample.

Femtosecond knowledge of megagauss fields might have a bearing on designs for nuclear fusion reactions, and for studying other subjects where high magnetic fields are important--NMR, Hall effect, and perhaps even fast magnetic information storage and switching devices. (Sandhu et al., Physical Review Letters, 25 November 2002; contact G. Ravindra Kumar, Tata Institute, grk@tifr.res.in; 91-22-2152971 x 2381; http://www.tifr.res.in).

Nu Approach to CP Violation

The measured abundance of helium in the universe (about 25% of all normal matter) suggests that there is about one proton for every 1010 photons. This in turn suggests that at some earlier phase of the universe an almost equal number of protons and anti-protons existed and gradually annihilated, but that because of some fundamental asymmetry (at the level of one part per ten billion) in the way that the weak nuclear force treats matter and antimatter, protons but not anti-protons survived to the present time.

The standard model of particle physics usually enshrines this asymmetry in the form of "CP violation," a mathematical convention concerning the interaction of particles in which one imagines what happens when the charge of all the particles is reversed (charge conjugation, abbreviated as C) and the coordinates of all particles is reversed (the parity operation, or P).The standard model is successful in predicting how CP violation works out in the decay of K mesons or B mesons (see Update 600) but not so good at predicting where the abundance of baryons (protons plus neutrons) comes from.

Now physicists at Hiroshima University, Niigata University (Japan) and Seoul National University (Korea) have proposed an explanation in which the proton excess comes (at least in part) from the decay of hypothetical heavy neutrinos (in addition to the electron, muon, and tau neutrinos already known). One testable prediction of this theory is that there should be a slight preponderance of anti-neutrinos over neutrinos, a disparity that could be studied in the next round of neutrino oscillation experiments being planned. (Endoh et al., Physical Review Letters, 2 December 2002; contact Takuya Morozumi, Hiroshima University, morozumi@hiroshima-u.ac.jp, 81-824-24-7364.).
-end-
Physics News Update Number 614, November 20, 2002 by Phil Schewe, James Riordon, and Ben Stein

American Institute of Physics

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