Physics News Update, Number 502

Number 502, September 14, 2000 by Phillip F. Schewe and Ben Stein

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/c². 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: press.web.cern.ch; opal.web.cern.ch/Opal/; alephwww.cern.ch/WWW/)

A PLANETESIMAL AGGREGATION EXPERIMENT has been carried out in the low gravity environment of the Space Shuttle to test notions of how our solar system developed from a primordial cloud of micron sized dust particles. The experiment is the first direct re-creation, under realistic solar-nebula conditions, of the proposition that protoplanetary dust accumulates through sticky collisions amid the random Brownian motion of the particles. A consortium of German and US scientists (contact Jurgen Blum, University of Jena, Germany, 011-49-364-194-7515, blum@astro.uni-jena.de) observed that the dust quickly aggregates. The data bears out the main theory of planetesimal formation, but there was one surprise: the structures were expected to be somewhat fractal in nature, with a fractal dimension d of about 1.8, meaning that the mass of the cluster should be proportional to the cluster size raised to the d power. Instead the dimensionality turned out to be about 1.3, meaning the structures were observed to be more linear and less sheetlike (see figure at Physics News Graphics). (Blum et al., Physical Review Letters, 18 September 2000; Select Article.)

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 Physics News 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.)