Science 10/25/02
by Charles Seife
EAST LANSING, MICHIGAN--Physicists' quest for a new state of matter has taken a bewildering turn. At a meeting here last week,* researchers from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, announced results that, so far, nobody can explain. By slamming gold atoms together at nearly the speed of light, the physicists hoped to make gold nuclei melt into a novel phase of matter called a quark-gluon plasma. But although the experiment produced encouraging evidence that they had succeeded, it also left them struggling to account for the behavior of the particles that shoot away from the tremendously energetic smashups.
"The more I think about it, the more I think it's not completely wacko," William Zajc of Columbia University, spokesperson for one of the four particle detectors at RHIC, said privately at the conference. Zajc ruminated for a few moments and then corrected himself. "Well, it is completely wacko," he said. "We don't get it. I really don't know--on a fundamental level."
The confusion comes from PHENIX, one of the four detectors, which probed the
differences between "hard" and "soft" nuclear
collisions. Nuclei are collections of protons and neutrons, and at low energies,
they behave like hard objects. Smash one nucleus into another, and the components
scatter like billiard balls. But scientists think they behave differently in
very high-energy
collisions. Neutrons and protons are made up of particles known as quarks and
gluons, and at very high temperatures and pressures these particles should burst
their bindings and roam free, forming a state of matter known as a quark-gluon
plasma. In that case, theory predicts that the particles in the smashup would
no longer bounce cleanly off one another; the melted mess would be sloppier,
the particles splashing off one another like droplets of water instead of rebounding
like chunks of ice. By analyzing the sprays of particles created by colliding
various atoms, the RHIC physicists hoped to determine whether collisions become
softer as the nuclei get bigger and carry more energy-- a sign of a quark-gluon
plasma, a state of matter that hasn't existed since the big bang.
Last year, RHIC seemed to be seeing just that. For example, trackers found proportionately fewer high-momentum particles spraying away from powerful gold collisions, a phenomenon known as jet quenching (Science, 26 January 2001, p. 573). Although jet quenching could be due to some new, subtle effect caused by the particles' travels through dense nuclear matter, it is consistent with the creation of a quark-gluon plasma: The particles slow down as they fly through the sticky, soft goop in a plasma, rather than merely ricocheting off the components of the nucleus.
This tidy picture has just become considerably messier. With the higher energies
and better statistics of RHIC's second year of
running, physicists could classify the particles zooming away from the collisions.
What they saw was a shock.
Measurements at PHENIX indicate that some of the particles flying away from the smashup are moving more slowly than normal, as one would expect in a soft collision, but others are caroming out of the wreck as if from a hard collision (see figure). Scientists know of no plausible mechanism for this discrepancy. "It's a true puzzle," says Zajc.
Part of the problem is that most of the particles PHENIX detects are born after the collision--spawned from more or less identical quarks and gluons (collectively dubbed "partons") that scatter off one another at the moment the two atoms crash together. The flying partons only then recombine into two-quark or three-quark ensembles ("hadrons," such as protons and neutrons). Because identical partons are doing the scattering, the hadrons they produce should all look as if they were born in the same sort of collision, soft or hard.
But that isn't what PHENIX sees, says Julia Velkovska, a Brookhaven physicist who is also associated with the PHENIX experiment. Pions, two-quark ensembles made of up and down quarks and antiquarks (and a handful of gluons) bound in an uneasy package, "behave more or less exactly like predicted" for a particle traveling through a sticky medium like a quark-gluon plasma, she says, whereas protons and antiprotons, three-quark ensembles also made of up and down quarks and antiquarks (and a handful of gluons), behave as if they were formed by a hard collision.
"Gee whiz," said Sean Gavin, a theorist at Wayne State University
in Detroit, Michigan, when told of the results for the first
time. "That's really interesting." But so far neither he nor anybody
else can account for the difference. Zajc suggests that exotic gluon
configurations might make two-quark ensembles (mesons such as pions) behave
differently from three-quark ensembles (baryons such as protons). Velkovska
says that perhaps a parton flying away from a collision somehow "knows
from the beginning that it's going to be a baryon." But both admit that
these are wild guesses at the moment.
James Thomas of Lawrence Berkeley National Laboratory in California, who works with the RHIC detector called STAR, says that data due to be collected in 2004 will reveal whether a similar pattern holds with heavier baryons and mesons, such as the lambda baryon and the K meson. The next RHIC run, however, will collide deuterium with gold and protons with protons--a lower energy regime than gold-on-gold collisions. If the anomaly disappears under these lower energy conditions, physicists will be much more confident that this effect and others stem from the formation of some sort of dense plasma, rather than from partons traversing the nucleus.
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* 2002 fall meeting of the American Physical Society's Division of Nuclear Physics,
9-12 October.
Volume 298, Number 5594, Issue of 25 Oct 2002, pp. 718-719.
Copyright © 2002 by The American Association for the Advancement of Science.
All rights reserved.