THE search for quark-gluon plasma, the ultra-hot soup of matter that filled
the Universe for its first few microseconds, is turning into a war of words
between rival teams of physicists in the US and Europe.
At last week鈥檚 Quark Matter 2001 conference at the State University of New
York, Stony Brook, researchers from Brookhaven National Laboratory on Long
Island reported the first data from their year-old accelerator, the Relativistic
Heavy Ion Collider (RHIC), which slams gold ions together at near light speed.
The results showed intriguing hints that the machine could be making the
long-sought plasma.
Meanwhile, researchers from CERN, the particle physics lab near Geneva,
restated a year-old claim that their ageing Super Proton Synchrotron (SPS)
already makes the stuff. Physicists can鈥檛 even agree on what it will take to
settle the issue.
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The protons and neutrons of normal matter are composed of tightly bound
bundles of particles called quarks. An individual quark can鈥檛 strike out on its
own, but stays bound to either two other quarks or an antiquark by a swarm of
particles called gluons. But under intense heat and pressure this 鈥渃onfinement鈥�
breaks down, allowing quarks, antiquarks and gluons to mingle freely in a
chaotic plasma.
Data from the RHIC鈥檚 first run suggests that these conditions were reproduced
inside colliding gold atoms, says John Harris, spokesperson for STAR, one of
four detectors arrayed around the RHIC鈥檚 3.8-kilometre ring. Each collision
spewed out thousands of particles, but surprisingly few high-energy ones, he
says. Some RHIC researchers believe that unbound quarks and gluons are slowing
these fast particles.
But Bill Zajc of the PHENIX detector team at the RHIC, says it is too early
to claim the collider is producing the plasma. 鈥淧eople are reluctant on the
basis of data that is so new and so fresh to make bold claims.鈥�
Meanwhile, CERN researchers are standing by last year鈥檚 controversial claim
that they have already seen the plasma. 鈥淲e have not been able to falsify the
hypothesis that we have seen the quark-gluon plasma at CERN,鈥� says Carlos
尝辞耻谤别苍莽辞.
The CERN claim rests largely on the observation that collisions of lead ions
produce few J/y particles, Louren莽o says. The J/y consists of a 鈥渃harm鈥�
quark and its antimatter partner. It won鈥檛 form in the plasma, physicists
believe, because the other particles swarming around the quark and antiquark
hide them from one another. CERN physicists could clinch their case by showing
that collisions of lighter indium ions also produce few J/y particles,
Louren莽o says. This experiment is planned for 2002.
But others are sceptical. Jean-Paul Blaizot of France鈥檚 Atomic Energy
Commission in Saclay says that data from several experiments will be needed to
prove that the plasma is there, and that most of these will be done over several
years at the RHIC. 鈥淭he most likely scenario,鈥� he says, 鈥渋s that we come to
understand what鈥檚 going on at the RHIC and then look back and say we understand
what was going on at the SPS.鈥�