From: sichase@csa2.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: Quark-gluon plasmas
Date: 17 Aug 92 22:19:10 GMT
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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In article <1992Aug17.200335.19340@galois.mit.edu>, jbaez@euclid.mit.edu (John C. Baez) writes...
> 
>>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.  
> 
>I love to hear real physicists talk.  Mainly because I can't quite
>understand it.  I can sort of believe that something like Debye
>screening would occur between charm-anticharm pairs.  So then you're
>saying that this allows them to wander apart and, when they cool down,
>form a pair of charmed hadrons, e.g. J/Psi's?  Okay.  

Under normal conditions, the charm-anticharm pair will form a 
J/Psi when they separate to a distance roughly equal to the J/Psi 
radius.  However, in the QGP the "normal" antiscreening of the color
fields becomes screening instead.  The charm and anticharm drift apart.
Eventually, when the mess hadronizes, each particle will pick up a 
(probably lighter) partner and become an open charm meson of some kind.

>Now what are "dileptons"?... well, they've gotta be pairs of leptons. 

Sorry, I didn't mean to swamp you with lingo.  A very common experiment 
is to collide two things and measure the invariant mass distribution 
of opposite-sign lepton pairs.  Roughly speaking, this measures the 
virtual photon invariant-mass distribution in the early stage of a 
collision.  Leptons are a good probe of the early hot phase of a collision
because they are unperturbed by the many hadronic interactions that,
for example, a pion will suffer as it leaves the center of a fireball
on the way to your detector.   

The basic distribution you get is a "Drell-Yan" distribution, on 
top of which will be dileptons from resonance decay which give sharp
peaks at specific invariant mass on top of an otherwise more-or-less
featureless, exponentially-falling, distribution.

The down-side of such a measurement is statistics.  You don't get nearly
as many lepton pairs as pions from a typically collision.  But for 
measuring something like the J/Psi yeild, you can make it work quite
nicely.

>But when WHAT
>is near the J/Psi mass do you expect suppression of the formation of
>dileptons, and why?  The energy of the charm-anticharm pair?  Half that?

I don't think I meant what you think I meant.  What I was trying to say 
is this:  Imagine varying some experimental parameter such that you think
that in one case you will produce a hadron gas and in another extreme
of the parameter you will produce a QGP - at least for a fleeting instant
during the hottest densest part of the fireball evolution.  Typically
you might think of varying the beam energy, system mass, or centrality
of the collision (in the case of heavy ions, where this is fairly well
defined.)  

You can measure the integrated J/Psi yield for peripheral collision, for 
example, where the energy density is not expected to be high enough 
to produce a QGP, and compare it to the yield in central collisions (small 
impact parameter, high multiplicity, high transverse energy, etc.) in
which you hope conditions are right for QGP formation.  

If you find that the number of J/Psi's (modulo a carefully thought-out 
normalization between data sets) is smaller in the latter case, you 
declare that you have seen J/Psi suppression.  

The problem with such an experiment is that there is too much unknown 
about the classic nuclear physics of a high-energy heavy-ion collision 
to easily rule out conventional explanations of the effect.  For example,
in the "central" case, there is a considerable amount of "comoving" debris
which flys out of the collision, more or less along the beam axis, along 
with a hypothetical J/Psi which has been produced.  This stuff is absent
in the less violent collisions.  How does the J/Psi interact with this 
stuff?  What is the absorption cross section for J/Psi's on omegas, rhos,
etas, N*'s, etc?  You need to know these numbers in order to pin down 
the probability that our J/Psi is eliminated from the final state 
by conventional nuclear/particle physics processes.  

The only way to address this and other questions is to do systematic 
studies of J/Psi production over a wide variety of beam energies, system
mass, and trigger conditions.  Then, if the nuclear theorists can agree
on a prediction for the dependence of J/Psi reabsorption on some of these
variables, you might be able to clearly distinguish between conventional
physics and QGP formation.

Unfortunately, the problem of low statistics comes in here, and makes it
very difficult to produce the kind of comprehensive data needed.  In 
my judgement, the most interesting data would be proton-proton collisions
at high energy (20 GeV sqrt(s) or more) in a variety of event subselections.
But p-p collisions produce *very* few J/Psis.  The only reason why the
original CERN experiment was feasible was that they were using heavy 
ions - O16 and S32 on heavy targets - which give you many individual 
nucleon-nucleon interactions per heavy-ion collision.   Since that time,
a simlar experiment has been done at Fermilab using p-nucleus collisions.
But to the best of my knowledge, nobody has yet attempted the more fundamental 
p-p experiment to see if the effect can be isolated in a simpler system.

-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
Newsgroups: sci.image.processing