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- 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
- Lines: 107
- Distribution: na
- Message-ID: <25549@dog.ee.lbl.gov>
- References: <4537@news.duke.edu> <1992Aug14.195651.22652@galois.mit.edu> <25518@dog.ee.lbl.gov> <1992Aug17.200335.19340@galois.mit.edu>
- Reply-To: sichase@csa2.lbl.gov
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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
-