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PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics
Neues von der AIP
 News Number 502/505 September 14, 2000
by Phillip F. Schewe and Ben Stein
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AN INTRIGUING HINT OF THE HIGGS BOSON in collider data at the LEP accelerator at CERN has prompted officials there to extend the running period of the Large Electron Positron (LEP) collider by at least a month, instead of turning it off now to make way for the building of the Large Hadron Collider (or LHC, a proton- colliding machine to be housed in the same deep tunnel as LEP). CERN decided today that the high energy electron-positron collisions at LEP will continue, the better to supplement the meager, but potentially crucial, evidence for the Higgs boson, the particle widely thought to be responsible for endowing other known particles with mass. What happens at LEP, in effect, is that a lot of energy squeezed into a very tiny volume almost instantly rematerializes in the form of new particles. Theorists have said that in some collisions a Higgs boson (h) might be produced back to back with a Z boson, one of the carriers of the weak force and itself the object of a dramatic particle hunt at CERN 20 years ago. In these rare events, both h and Z are expected to decay quickly into two sprays, or jets, of particles. One tactic then is to search 4-jet events for signs that the combined mass of two jets at a special energy seems to stand out above pedestrian "background" events in which no true exotic particle had been produced. What has caught LEP physicists' attention is just such an enhancement, at a mass around 114 GeV/c^2. The enhancement is not statistically significant enough for CERN to claim a discovery yet, even when all four detector groups combine their data, but sufficient to cause excitement since the Higgs is perhaps the most sought after particle in all of high energy physics. The LEP extension is not expected to cause much of a delay in LHC construction. Some websites:
http://press.web.cern.ch
http://opal.web.cern.ch/Opal/
http://alephwww.cern.ch/WWW/

TRILOBITE MOLECULES. New research predicts the existence of a giant two-atom molecule with an electron cloud resembling a trilobite, the ancient, hard-shelled creature which lived in the Earth's seas over 300 million years ago (see figure at www.aip.org/physnews/graphics). Made of two rubidium atoms spaced very far apart, the trilobite molecule could conceivably materialize in a Bose-Einstein condensate (BEC). This is because a BEC's ultracold, dense environment favors the creation of exotic species in addition to the condensate itself. The trilobite molecule has many remarkable properties in addition to its shape, according to the collaboration that predicts its existence (Chris Greene, University of Colorado and JILA, 303-492-4770, chris.greene@colorado.edu). For starters, it would be huge for something consisting of just two atoms: the cores of the Rb atoms are separated by anywhere between 50 nm and 5 microns. Rubidium molecules in BECs have been formed before (Update 471), but they have been much smaller (only 2-4 nm). The researchers believe the trilobite molecule can be created by manipulating a rubidium BEC with laser pulses or external electromagnetic fields. One of the rubidium atoms in the pair must first be converted into a Rydberg atom, which contains an electron in a very high orbit. Ultra-long-range molecules would then form from a weak attraction between the Rydberg atom's outermost electron and another Rb atom. Some of these molecules would have no permanent separation of electric charge, but ones with the trilobite-shaped electron cloud could possess a large permanent electric dipole moment. With dipole moments roughly 1,000 times larger than typical polar diatomic molecules, these would be the first-ever polar molecules made up of two atoms of the same element and isotope. While extremely fragile, their large dipole moments suggest that trilobite molecules could be accelerated, transported, cooled and decelerated using much smaller electric fields than those required for any other molecule. (Greene, Dickinson, and Sadeghpour, Physical Review Letters, 18 Sept 2000; Select Article.)

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FIRST RESULTS FROM RHIC. Brookhaven's Relativistic Heavy Ion Collider (RHIC) had their first heavy-ion collisions back in June and since then extremely energetic smashups between gold atoms have been lighting up detectors in the four interaction halls, creating fireballs that approximate tiny pieces of the universe as it might been only microseconds after the big bang. One conspicuous goal at RHIC is to rip apart protons and neutrons inside the colliding nuclei in order to create novel new forms of nuclear matter, such as quark gluon plasma. The beam energies have been as high as 130 GeV per nucleon and the beam density is up to about 10% of its design value. In this first published RHIC paper, the PHOBOS collaboration (contact Gunther Roland, MIT, gunther.roland@cern.ch) describes the "pseudorapidity" (related to the velocity along the direction of the beams) of the myriad particles emerging from the collisions. The researchers pay special attention to particles emerging at right angles to the incoming beams. These particles emanate from the most violent of collisions, which on average create about 6000-7000 particles per event, more than have ever been seen in accelerator experiments before. The number of particles produced in turn is indicative of the energy density of the fireball produced at the moment of collision; this density, 70% higher than in previous heavy-ion experiments, carries the RHIC researchers into a new portion of the nuclear phase diagram. The data presented here help to constrain models of this high-density nuclear realm. (Back et al., Physical Review Letters, 9 Oct Select Articles.) All four RHIC detector groups (STAR, PHENIX, and BRAHMS are the three others) will be presenting their first scientific findings at the American Physical Society Division of Nuclear Physics Meeting in Williamsburg, VA on October 4-7
http://www.aps.org/meet/DNP00
While no announcement of a quark gluon plasma is expected, researchers plan to describe numerous impressive aspects of RHIC's early operation.

DIRECT PHOTONS ARE SEEN IN HEAVY-ION COLLISIONS. In high energy heavy-ion collisions heated nuclear matter, both the original protons and neutrons (or maybe even their constituent quarks) as well as additional particles created out of the excess energy, can be thought of as a hot gas. High energy gamma photons have been observed, as expected, from the decay of particles exiting the hot nuclear gas. But gammas are also expected to be emitted at a lower rate as a kind of thermal glow from the interactions of the particles in the gas. Such "direct photons" have now been seen for the first time in an experiment conducted at CERN, where lead ions smashed into a stationary lead target (contact Terry Awes, Oak Ridge Natl.Lab, 865-574- 4587, awes@mail.phy.ornl.gov). Some theorists believe that direct photons, like the suppression of psi mesons or an enhancement in the production of strange mesons, might constitute evidence for the production of quark gluon plasma. (Aggarwal et al., Physical Review Letters, 2 Oct; Select Articles.)

CONNECTING THE WAVE AND PARTICLE ASPECTS OF LIGHT by detecting a photon and then measuring the fluctuations of a closely associated electromagnetic field has been experimentally achieved for the first time. In most experiments, researchers focus upon either light's particle aspects (by counting photons, for instance) or wave aspects (by measuring an interference between electromagnetic fields, to cite a simple example). Now, researchers at SUNY-Stony Brook and the University of Oregon (Luis Orozco, Stony Brook, 631-632-8138, lorozco@notes.cc.sunysb.edu) have demonstrated an experimental setup, which they call a "Wave-Particle Correlator," for determining the relationships between both aspects of the light that comes from a single physical process. The "light source" in their experiment consists of a beam of rubidium atoms passing in between a highly reflecting pair of mirrors (a "cavity QED system"). In their setup, a laser aims light into the cavity through one of its mirrors. Acting as a sort of "artificial molecule," the cavity absorbs the light and re-emits it. A photon occasionally escapes through an output mirror, only to be detected as a particle by a photodiode. The photon detection sets up a subsequent measurement of wavelike properties, as the cavity occasionally gets rid of a second photon to relax to a stable state. The resulting electromagnetic field gets mixed with another known electromagnetic wave to produce an interference pattern. The pattern emerges only after averaging over many such "conditional" measurements triggered by photodiode detections. It reveals that the electromagnetic field inside the cavity after the first photon's departure contains, in effect, a tenth of a photon, since a second photon is only emitted about 10 percent of the time. Measuring such wave-particle correlations might bring about new microscopy techniques. (Foster et al., Physical Review Letters, 9 October 2000; Select Article).

 

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