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Christine Nattrass | USLHC | USA

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The Quark Gluon Plasma

Much of the press coverage of the LHC has discussed the search for the Higgs, but ALICE was designed for something completely different.  We’re studying a hot, dense phase of nuclear matter called the Quark Gluon Plasma (QGP).

What we’re doing is mapping out the phase diagram of nuclear matter.  This is roughly what we know about the phase diagram of nuclear matter:

The x-axis, baryochemical potential, is the amount of energy needed to add a baryon (such as a proton or a neutron) to the system.  You can think of it as analogous to pressure in the phase diagram of water.   The y-axis is the temperature in MeV – one MeV is roughly 1160 megakelvin (MK) and 175 MeV is about 2,000,000MK.  That’s about 250,000 times hotter than the core of the sun.  At lower temperatures and lower baryochemical potentials (lower densities), we have a hadron gas.  (Neutrons and protons are hadrons.  The hadron gas is actually mostly made up of pions, a hadron with about 10% of the mass of the proton.)  At higher temperatures, we get the Quark Gluon Plasma.  Neutron stars are somewhere to the right, way off the chart.

We create a QGP in the lab by colliding two nuclei together at high energies.  The LHC will collide lead nuclei at a center of mass energy of 5.5 TeV per nucleon, which is over 27 times greater than the highest high energy nuclear collisions so far.  (Nucleons are the protons and neutrons in the nucleus.)  We expect our first data from lead-lead collisions this fall, if everything goes according to plan.  From studies of gold-gold collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island, we learned that a QGP is a liquid of quarks and gluons – and set record for the highest temperature reached in the laboratory in the process.  The fact that it acts like a fluid actually indicates the attraction between quarks and gluons isn’t negligible, as had been predicted – this is why it’s sometimes called a strongly-interacting QGP, or sQGP.  (At one point someone proposed a variation on this, the bound state QGP, but this name was thankfully abandoned.)  We’re actually not really sure what we’ll see at the LHC – will it still behave like a liquid?  Or will it behave like a gas?

What we’re doing is kind of like colliding two ice cubes together to study the phase diagram of water.  If you had two ice cubes in outer space (where it’s really cold) and you got them moving really fast and slammed them into each other, the ice would melt for a very short time and then as the system expanded, it’d freeze again and you’d end up with little flakes of ice scattering all over the place.  On the surface of the Earth, nuclei are essentially frozen nuclear matter.  We have to figure out the properties of melted nuclear matter by looking at the frozen shrapnel.  We have the luxury of having better ways to study water than by smashing ice cubes together, but we don’t have any other way of studying nuclear matter.

Everyone learns that water has three phases, solid, liquid and gas.  This is a pretty good approximation, but it is a simplification.  There are 15 different phases of ice (that we know about).  Analogously, the diagram above is definitely an approximation.  There’s probably a lot of structure in there – a lot of extra phases – we don’t know about.  In the era of the LHC, we’ll be able to move over about two orders of magnitude in temperature along this phase diagram because both the LHC and RHIC are actively taking data.  There have also been a lot of improvements in detector technology and data analysis techniques in the last couple decades, so we’ll be able to do better measurements than ever before.  We’re in the golden age of the field.

ps –  I haven’t forgotten about the questions from my first post on Facebook but I just haven’t gotten to them all.  I did post replies to the comments posted on the blog in the comments section.

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