In about a weeks time the Large Hadron Collider will stop proton-proton physics for this year and the physicists working on ATLAS, CMS and LHCb will work hard on their 50/pb of data to try to figure out if supersymmetry exists in nature. Meanwhile the LHC will continue running for another month colliding lead ions instead of protons. The main experiment designed to take advantage of these heavy-ion collisions is ALICE, but ATLAS and CMS will also take a look.
One of the exciting features of the Heavy-Ion collisions will be the total amount of energy in each collision. I don’t know how high they will actually get in this first series of runs but the target seems to be about 2.7 TeV per nucleon. Lead nuclei have 207 nucleons so the total centre of mass-energy could be as high as 1100 TeV. That is enough energy to create a million protons. The LHC is about to become the worlds first Petatron collider. (Edit: actually it will be half this energy for this year with full energy in 2013, see comments)
Sadly this does not mean they will be exploring the particle spectrum at such high energies. When heavy ions collide it is more like lots of small collisions between the quarks and gluons in the nuclei so the energy does not get concentrated into the production of any single heavy particles. Instead you can get lots of lighter particles that can form a very hot plasma ball. The interactions involved are dominated by QCD so this experiment is mostly a study of QCD phenomena.
If you want an idea of what such a collision will look like have a look at what has been seen at the RHIC collider. Here is a typical example with thousands of particles produced. The RHIC uses energies of 500 GeV per nucleon so we can expect something like 5 times as many particles at the LHC.
The aim of these experiments is to find out something about the phase diagram of QCD. According to various theories it probably looks like this
All the stuff to the bottom right is what happens in neutron stars at very high densities of matter. There is not much possibility of recreating such conditions in any experiment because they only way to produce the densities required are by using the enormous pressures due to gravity that occur inside neutron stars.We will have to rely on astronomical observations to probe those regions of the phase diagram. RHIC and the LHC are better suited to looking at the top left where the enourmous collision energies produce a plasma at very high temperature.
The hadronic matter phase is what we are used to at low temperature and at most nuclear density. Here the quarks are confined inside hadrons and mesons. At higher temperatures and densities theory predicts that the quarks will enter a deconfined phase where hadrons do not form. Instead the quarks and gluons just form a liquid-like plasma where they can flow around freely. You can cross from the hadronic phase to the quark gluon plasma over a first order phase transition (the thick red line) where the two phases mix just like gas and liquid in boiling water. However, at lower densities you can pass from one phase to another without going through the phase transition. A similar thing happens with water turning to steam at high pressure. The first order phase transition stops at a critical point and one objective of RHIC has been to try to find this point experimentally. This requires running with lower energy, not higher energy, so the LHC is not looking for the same thing.
Instead the LHC will be able to explore the crossover region where there is a smooth change from confined to deconfined matter, but there is something else in this region. Another phase transition is though to be crossed over, but it is a second order phase transition, not first order. This is the phase transition for chiral symmetry breaking.
The QCD Lagrangian has an approximate symmetry known as chrial symmetry that relates different flavours of quarks and left and right chiral states. The symmetry is broken by any quark mass but the up and down quark masses are small enough for this symmetry to be a good approximation. the symmetry is also broken by the electric charges, but this is also a relatively small effect. The spontaneous symmetry breaking leaves a residual symmetry which is isospin and it generates Goldstone bosons such as the pion. The pion would be massless if the symmetry was exact. At high temperatures the chiral symmetry is restored, so there must be a transition. lattice calculations suggest that it is around temperatures corresponding to 170 MeV.
With symmetry breaking phase transitions it is not possible to have a cross-over region if the symmetry is exact. A symmetry is either broken or not. You cant go smoothly from one phase to another. However, chiral symmetry is not exact in QCD so the phase transition will not be sharp. It is though to coincide with the deconfining phase transition up to the critical point and then continue across to the zero density axis as shown on my diagram. Actually it could separate before the critical point, we don’t know for sure.
When the LHC starts Ion-collisions at ramped energies it will be a leap ahead of where the RHIC has been looking. It has been said that it is one of the largest jumps in energy for a specific type of accelerator ever taken. The regions explored are thought to be similar to the conditions in the big bang a little after inflation stopped. It will be very interesting to see what happens.
Posted by Philip Gibbs 
