Number 286, September 13, 1996 by Phillip F. Schewe and Ben Stein
PARTICLE-LIKE LOCALIZED EXCITATIONS IN A BED OF SAND can form into molecules
and even crystals. University of Texas physicist Harry Swinney shakes an
evacuated container of sand (actually a lake of tiny bronze balls ) up
and down. At a certain frequency the energy put into the system manifests
itself as small isolated heaps of sand (about thirty grains in diameter)
which also bob up and down. These heaps, which Swinney calls "oscillons,"
are stable (holding together for thousands of shakings) and able to slowly
drift across the sand bed. And just as with electrical charges, when it
comes to oscillons opposites attract. As long as their centers are within
1.4 diameters of each other, oscillons of opposite phase (one at its peak
height and one at its shallowest depth) can enter into a dipole state.
These peak-crater pairs in turn were observed to form chains and other
configurations including extended lattices. Swinney and his colleagues
have no definite answer as to how and why the oscillons form and interact,
but he feels that such localized structures may exist in other dissipative
systems (systems which steadily lose energy), and not just in granular
materials (see Update 264). (Paul. B. Umbanhowar, Francisco Melo, and Harry
L. Swinney, Nature, 29 August 1996. Some associated figures can be viewed
on our Physics News Graphics website: /png/)
THE STANDARD MODEL OF PARTICLE PHYSICS seems more irreproachable than
ever. In numerous experiments physicists have sought to find a discrepancy
between experimental findings and predictions based on the model in which
the quarks and leptons interact via the exchange of photons, gluons, and
the W and Z bosons. (Gravity is usually left out of these studies.) A crack
in this model was seemingly uncovered recently at an experiment at CERN
in which the decay of Z bosons into particles containing a bottom quark
was unexpectedly high. But at the International Conference on High Energy
Physics in Warsaw in July, CERN scientists reported that the reanalyzed
data was now back in line with theory. (Nature, 22 August 1996.) Another
experimental result, the observation earlier this year of an excess of
high-energy particle jets in proton-antiproton collisions at Fermilab's
Tevatron, was interpreted by some to be possible evidence for the existence
of structure inside quarks, and as such to represent a departure from the
orthodox standard model. But here again, a further look at the data has
made it more compatible with the consensus theory. One of the cornerstones
of this theory is the Higgs boson, the particle that supposedly endows
the Z and W bosons with a heavy mass and which plays a role in making the
weak force (responsible for radioactivity) and the electromagnetic force
very different in the way that they act in the universe. Scientists would
like actually to detect the Higgs, but they don't know its mass. To get
an approximate mass value, and to determine whether present or future accelerators
would be able to find the Higgs, theorists need first to pin down the masses
of the top quark and the W boson. At a divisional meeting of the APS in
Minneapolis in August the two big Tevatron collaborations helped out: the
CDF group announced (on the basis of an inventory of 100 top-quark events,
from CDF and D0) a new top mass of 176.8 (with an uncertainty of 6.5) GeV;
meanwhile the D0 group reported a new W mass of 80.35 (uncertainty of 0.15)
GeV, based on data from D0 and CDF. (Science, 23 August 1996.)
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