Number 642, June 19, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
Intriguing Oddities in High-Energy Nuclear
Collisions
Missing debris in the smashup between gold nuclei going at close to
the speed of light suggests the creation of a highly unusual plasma
environment, researchers have announced at Brookhaven National Laboratory.
By smashing together gold ions at Brookhaven's Relativistic Heavy Ion
Collider (RHIC), scientists are attempting to make and study a state
of matter that existed only millionths of a second after the big bang.
Called a quark-gluon plasma (QGP), it is a hot, dense soup of individual
quarks and gluons. In today's universe, by contrast, quarks come in
groups of twos and threes, held together by gluons. This spring, Brookhaven
researchers performed a "control" experiment, in which they
collided a gold nucleus with a deuteron, a light nucleus consisting
of just a proton and neutron. In these and other kinds of nuclear collisions,
a pair of quarks from a proton or neutron occasionally gets ejected.
In turn each ejected quark produces a stream or "jet" of particles
in its wake. In some of the gold-deuteron collisions, the researchers
indeed observed pairs of jets flying in opposite directions. But in
head-to-head collisions between two gold nuclei, researchers observed
only one, rather than two, jets. This property, called jet quenching,
suggests that the particle jet traveling in the direction of the collision
region is getting absorbed by a hot, dense state of matter. Jet quenching
is predicted to occur in the correspondingly hot, dense environment
of a quark-gluon plasma, but RHIC experimentalists are not ready to
claim the QGP prize quite yet. To verify its presence and rule out rival
scenarios, they are planning numerous other experiments for finding
other signatures of a QGP. However, the new data has convinced Columbia
theorist Miklos
Gyulassy that the RHIC team is already seeing a QGP. The gold-gold
collisions, he and his colleagues calculate, produce an environment
100 times denser than ordinary nuclear matter and display properties
predicted in QGP models based on quantum chromodynamics (QCD), the theory
of the strong force which holds nuclei together. On June 18, three of
the four RHIC experimental groups have submitted papers on the new results
to Physical Review Letters and researchers discussed these new results
at a special Brookhaven colloquium today. (Brookhaven
press release, June 11)
Solar Flares and Global Warming
A recent study by researchers at Duke University and the Army Research
Office has found new evidence of a link between solar flare activity
and the earth's temperature. The work is another contribution to the
ongoing debate over global warming and its causes. A strong link between
solar flares and our climate, if it exists, could override the influence
humans have on the temperature of our environment. One of the challenges
of determining the connection between solar flare activity and the atmosphere
stems from the fact that the motion of the air that blankets our planet
is turbulent and complex. A sudden burst of solar activity would, in
effect, be smeared out by moving air and its interaction with the earth's
surface. Any temperature increase caused by a given period of solar
flare activity would be difficult to determine, at best. Rather than
focus on such challenging one-to-one correlations, the new study compares
the form of the statistical fluctuations in solar flare activity with
the form of the statistical fluctuations of the earth's temperature.
The researchers (contact: Bruce
J. West, 919-549-4257) explain that solar flare activity can be
characterized by a type of statistics described by a Levy distribution,
which is generated by a "Levy-walk." (Many natural phenomena,
from foraging patterns of spider monkeys to complex hydrodynamic flows,
are well described by Levy walks, although the coefficients in the relevant
equations typically vary from one phenomenon to another. See Update
510-3 for one example.) Analyses of global and local temperature
fluctuations are also well described by a Levy-walk. In fact, a comparison
of the mathematical coefficients that describe the fluctuations suggest
to the researchers that the atmosphere directly inherits its temperature
fluctuations from the variation in solar flare activity. Unless some
other underlying cause is responsible for the unlikely correspondence
between solar flares and the earth's temperature, the research suggests
that for the large part variations in global temperatures are beyond
our control and are instead at the mercy of the sun's activity. (Nicola
Scafetta and Bruce J. West, Physical Review Letters, 20 June
2003)
Star Out of Round
The Very Large Telescope Interferometer (VLTI), an array of 2 telescopes
which combine their light signals to achieve a higher angular resolution
than is possible with any one scope, has determined that the star Achernar
is the flattest star ever studied. The VLTI, which does not provide
an actual image of the star but can provide an accurate estimate of
the star's profile, has determined that Achernar's equatorial radius
is 50% larger than its polar radius. This is quite oblate compared to
most other celestial bodies, such as our Earth, whose equatorial radius
is only 0.3% larger than its polar radius. Theorists do not yet know
how to explain how a star like this could turn fast enough to adopt
with such a shape without flying apart. Achernar is about 145 light
years away from Earth in the southern sky and has a mass of about 6
solar masses. The telescopes used to make the interference map were
not the giant 8.2-m VLT telescopes, but more modest 40-cm reflectors
set at various configurations with separations as large as 140 m. (European
Southern Observatory press release, 11 June)
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