Quark Gluon Plasma

In absence of quality thinking time, I will provide you with one of my favorite shortcuts to thinking: pictures. In this case, I found some particularly nice pictures in one of Stefan's talks. They depict very nicely the difference between common hadronic matter and the quark gluon plasma.

The possibility to produce a state of matter as hot and dense as it was in the first moments of the universe is one of the primary goals for Heavy Ion collisions. For this purpose, heavy nuclei like those of lead and gold, are collided with highest energies and form an intermediate hot and dense state, the so-called fireball. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) performs such experiments, with which we hope re-create conditions similar to those in the very early universe.

The top energy for the collision of gold nuclei at RHIC is about 200 GeV per nucleon. Such a gold nucleus consists of 197 nucleons (79 protons and 118 neutrons), which adds up to 40 TeV: that is roughly the energy of a mass of 10 milligram (a grain of coarse salt) falling from a height of 2.5 cm (or 1 inch), its macroscopic!

In a head-on, central collision with such an energy, some 1000 particles are produced. The tracks of the charged particles are measured in the large detectors, and they typically look like this:

(picture credits go to Jens Berger, from the STAR team)

Naive as I am, I always thought there are ten-thousands of nuclei in such a collision, but actually the number is roughly 70.

Matter in the particle beam before the collision is present in its usual form of nuclei, which consists of neutrons and protons. In this state, the quarks are confined to these color-neutral bound states, which are called hadrons. This usual matter, which consists of protons and neutrons, is called hadronic matter. The fireball created in the collisions is so hot and dense, that it is expected to allow the quarks to move almost freely, thus removing the confinement into hadrons. This state, then is called the quark gluon plasma. The pictures below show both possibilities.

This is hadronic matter:

where the grey bubbles indicate the color-neutral hadrons, inside which you see the valence quarks (large) and further virtual colored particles - so called sea quarks, and gluons. The arrows indicate the isospin of the quarks. The protons and neutrons consist of up and down quarks, with isospin +1/2 (up quark), and isospin -1/2 (down quark), respectively.

This, in contrast is the quark gluon plasma:

Here the colored constituents are able to move freely within the blob of heated, deconfined matter. The initial temperature of the fireball which creates such extreme conditions is some hundred MeV. My unit converter says that is about 10¹³ Kelvin, a pretty hot soup.

The challenge for the theoretical physicists is now to come up with observables, which allow to distinguish in which form the matter was present in the fireball. After all, in the dectors, nobody has ever found an isolated quark, its always hadrons.

At RHIC, it is now quite sure that matter is not hadronic any more in the collision. But what it is instead, that is still not completely clear. At the moment, the state of matter created is dubbed a quark gluon liquid, which is a a liquid of strongly interacting quarks and gluons rather than a gas of weakly interacting quarks and gluons. If someone has a picture for that, I'd really like to see it.

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TAGS: SCIENCE, PHYSICS, NUCLEAR PHYSICS