From: sichase@csa2.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: Symmetries, groups, and categories
Date: 16 Aug 92 18:55:30 GMT
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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In article <1992Aug14.195651.22652@galois.mit.edu>, jbaez@zermelo.mit.edu (John C. Baez) writes...
> 
>Another cool thing is that superdense superheated plasmas of baryons and
>mesons can get so hot that the quarks become effectively free... I think
>Scott is working on torturing particles in this manner.

Correct.  But I would describe it a little bit differently.  If you heat
up a chunk of nuclear matter (most closely approximated in the laboratory
by colliding heavy nuclei at high energy) you will convert the nuclear 
liquid into a hadronic gas.  Further heat or pressure will convert the
hadron gas into a plasma of free quarks and qluons, called (for some
strange reason) a quark-gluon plasma, or QGP.  At least that is what
many people believe on the basis of zillions of CPU hours of lattice
gauge calculation.

The experimental problem is a nightmare.  In the latest generation
experiment (on which I have worked a little when I should have been
working on my thesis expt.) Au nuclei will collide at 200 GeV/nucleon
in the C.M. at a new accelerator at BNL called RHIC (Relativistic Heavy
Ion Collider.)  Typical particle multiplicities are 6000 per collision,
of which 1000 will land in the acceptance of our central TPC.  By
comparison, CDF sees multiplities of order 100.  

The idea is to have so many particles that the system approximates a 
thermodynamic state resembling infinite nuclear matter, with properties
which can be calculated using the statistical mechanics of strong
interactions.

The big problem is identifying an unambiguous "signature" for QGP formation
during the hot dense phase of the interaction.  After all, after a few 
Fermi/c, the system expands and, if it ever *was* a QGP, rehadronizes into
mostly pions and other junk which fly into your detector and look an awful
lot like the mess you would see if the collision had produced a hadron gas
instead.  

One signature that has gotten some broad attention is "J/Psi suppression."
The idea is that in a collision which passes through a QGP phase, 
charm-anticharm pairs which are formed in individual parton-parton interactions 
are Debye screened from one another due to the "melted vacuum" of the QGP,
which is no longer confining.  There will be a larger tendency toward
producing open charm in such a collision than one in which a QGP is not formed,
leading to a suppression of dileptons at the J/Psi mass, a fairly 
straightforward experimental signature.  So you vary the beam energy, 
system mass, and trigger, and you watch what happens to the J/Psi yield in
a dilepton experiement.  Having seen suppression in events with high
transverse energy (a typical trigger condition) relative to minimum bias,
the crux of the issue is whether there can be a "standard" nuclear physics
explanation of the same effect.

There are many other signatures being studied.  Our experiment, STAR, at
RHIC, will study, among other things, "jet quenching" which is a process
by which hard quarks passing through a bubble of QGP loose energy, so that
in the end there are fewer hard jets than in non-QGP-forming collisions.

The nuclear physics of many-particle collisions is complex and not well
understood, so we are really on the hairy edge of experimental possibility.
Deciphering what any experiment ultimately means is a difficult business.
It's not as clear as many particle physics experiments, where the result
is the measurement of a single well-defined quantity.  Rather, evidence
for quark deconfinement in heavy-ion collisions is likely to be a cumulative
body of many indirect pieces of evidence.

There's so much more to say about the subject, that if I don't stop now
I'll end up writing a book - which really should wait until I graduate.

-Scott
--------------------
Scott I. Chase			"The question seems to be of such a character
SICHASE@CSA2.LBL.GOV		that if I should come to life after my death
				and some mathematician were to tell me that it
				had been definitely settled, I think I would
				immediately drop dead again."      - Vandiver