When you're a jet...

you may not be a jet all the way. At least not at the Relativistic Heavy-Ion Collider. "Jets" in this instance refers not to the local NFL team, but instead to streams of hadrons that are formed in the high-energy collisions that take place inside this machine. At high energies the formation of these jets is a prediction of perturbative Quantum Chromodynamics (QCD). Last Thursday's speaker was using the modifications in the patterns of formation of these jets at high temperatures and densities to try an learn about QCD in these extreme conditions. In particular "away-side jet suppression", where jets do not appear as regularly in collisions of gold nuclei as they do in collisions of protons, suggests that the smashing together of the roughly 400 protons and neutrons in two gold nuclei heats up the QCD vacuum a lot more than a proton-proton collision does. Yes, I know this is not a surprise, but the point is that the results are so different that there seems to be a phase transition to a "new state of matter": the so-called Quark-gluon plasma, that some have dubbed "the perfect liquid" because of its low viscosity.

I found it interesting to hear Dr Frantz say that predictions from pQCD do a good job of predicting much of the behaviour of the jets in the proton-proton collisions. But when it comes to the gold-gold collisions the hot "QCD plasma" (or, more conservatively "QCD fluid") that we think gets formed in the collision buffets the jet that has to traverse more of the plasma. Those jets lose a lot of energy to the plasma, and so we tend not to detect them as often. I liked the way that Dr Frantz could test this hypothesis by looking at how photons escape from this plasma. After all, if the jet is interacting, i.e. being accelerated by, the stuff in the plasma, then it should spit off photons too. But once a photon has been spat off, it will tend to get out of the plasma, since it does not carry "QCD charge". So photons, once emitted, can be photons all the way to the detector. And the data Dr Frantz and his colleagues collected bore out this hypothesis.

The future of this research would seem to be at the Large Hadron Collider, which has a heavy-ion program lined up for a period a few years from now when they hope to be done with discovering the Higgs boson, supersymmetry, and extra dimensions. (Yes, I am joking, although I am sure they hope to find all that.) Because the energies at the LHC will be much higher than those at RHIC they can test whether the jets continue to be suppressed in higher-energy collisions. Because QCD is an asymptotically free theory one would expect that the interactions of higher-energy jets with the plasma will be smaller than those of the 4-10 GeV jets they've looked at so far at RHIC. But this is asymptotic freedom, so it may take a while to get there. I guess once the LHC does this experiment we'll have more idea of what energy we have to go to before jets really are jets all the way.