ALICE’s first two papers on lead-lead collisions were submitted yesterday, about a week and a half after the first lead-lead collisions.
One paper is a measurement of the charged particle multiplicity. This analogous to the multiplicity measurements in proton-proton collisions, except a more particles are produced in a lead-lead collision. In p+p the models were off by around 10-15%. This is a plot from the lead-lead paper:
The red point shows the ALICE measurement. The x-axis is a measure of the number of particles produced in the collision. (Specifically it is the number of charged particles produced in the collision per unit pseudorapidity for pseudorapidities from -0.5 to 0.5.) The black points are different predictions. Notice is that the predictions vary from 1000-2000 particles. This is a rather large theoretical uncertainty, especially compared to proton-proton collisions. So less than two weeks after our first collisions, we have already gained a much deeper understanding of lead-lead collisions.
The other new paper is a bit more abstract than the number of particles created in the collision. Think of a heavy ion collision as like slamming two ice cubes at each other. If you slammed two ice cubes together fast enough, they’d melt when they hit each other. If you did this in space – where it’s about 3K (about -270 Celcius and -454 Fahrenheit) – the water would immediately freeze again. This is roughly what happens in a lead-lead collision. Nuclei are basically frozen quarks and gluons, and when we collide them fast enough, they melt. But by the time we see the remnants of the collision in our detector, the quarks and gluons have frozen again. However, they went through a phase where they were a liquid. A liquid can flow. We can see evidence that the liquid of quarks and gluons was flowing because we can see in our detector that the particles are all moving in a preferred direction.
This plot compares ALICE’s measurement to earlier measurements:
The x-axis is the collision energy per nucleon (proton or neutron) in the center of mass. The y-axis is a measure of how much the liquid is flowing. [Technical audience: The y-axis is the coefficient of the second term of the Fourier decomposition of the distribution of particles in azimuth with respect to the reaction plane.] Measuring how much the Quark Gluon Plasma is flowing gives us some insight into its viscosity. There are lots of technical details and subtleties in interpreting these data that I’m skipping over. But already – less than two weeks after the first lead-lead collisions – we have two measurements that give us deep insight into the properties of the Quark Gluon Plasma.