Number 118, March 12, 1993 by Phillip F. Schewe and Ben Stein
MYSTERIOUS DI-PHOTON EVENTS , electron-positron collision events at the
LEP accelerator in which the final-state particles include a pair of leptons
and a pair of photons (the photon energy adding up to about 60 GeV) have
been reported by the L3 and the DELPHI collaborations, two of the four
detector groups at LEP. At a meeting at CERN in November 1992 Sam Ting
of L3 showed the particle-track diagrams for four such events and Ugo Amaldi
of DELPHI showed two events. The hypothetical Higgs particle could decay
into a pair of photons, but other decay paths should also have been evident
in the data but weren't. (CERN Courier, Jan./Feb. 1993.)
HELIUM CAN FORM DIATOMIC MOLECULES , but only very weakly and only at
millikelvin temperatures. Furthermore, the two helium atoms remain at arm's
length, 55 angstroms apart. Ronald Gentry of the University of Minnesota
creates the molecules by squirting compressed helium gas into a vacuum
chamber and then looking for dimers in a mass spectrometer. (Science News,
6 Mar. 1993.)
ARTIFICIAL ATOMS are manmade, essentially zero-dimensional systems---usually
involving specially tailored nanometer semiconductor structures---in which
the presence or movement of single electrons can be important. One prominent
example is the quantum dot, a pointlike quantum-well structure that can
be fashioned as a tiny stump on a substrate by selectively etching away
surrounding material or as a pointlike isolated region inside a semiconductor
sandwich by pinching off a small volume of the material with electric fields
from overlying metal electrodes. In such a system, as in atoms, quantum
mechanics dictates that particles confined in a small enough space (roughly
10 nm for electrons in semiconductors) can assume only discrete energies.
If quantum dots are artificial atoms, then a planar array of a million
dots constitutes a sort of artificial lattice. Scientists have created
such arrays but have not yet been able to control crystalline uniformity
and the placement of electrodes sufficiently for studying the energy band
structure in this system as one does for a "real" crystal. In
addition to spatial-confinement effects, charge-quantization effects can
also influence the behavior of dots. That is, dots can be made to accept
only a single electron or just a few, and their coming and going can be
monitored; as the voltage is turned up, new electrons are able to overcome
the efforts of the electrons already on the dot to exclude newcomers through
an electrostatic "Coulomb blockade." This dependence on individual
electrons may make possible a range of new devices. (Physics Today, Jan.
1993; Scientific American, Jan. 1993; Science News, 20 Feb. 1993; The Economist,
27 Feb.)
ICE-AGE CAVE ART can be dated much more carefully now with accelerator
mass spectrometry (AMS) techniques. For example, three painted bison on
the ceiling of the Altamira Cave in northern Spain were thought to be done
at the same time, about 14,000 years ago. French and Spanish scientists
have recently shown that the three were actually painted centuries apart,
13,570, 13,940, and 14,330 years ago. The greater precision in this case
comes from the use of AMS which, instead of just sampling current carbon-14
radioactivity, directly compares the carbon isotopes present in the sample
by ionizing, accelerating, and sorting them. (New Scientist, 27 Feb. 1993.)
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