Rights: The University of Waikato Published 17 September 2009 Download

What happens when lead ions, each containing large numbers of protons and neutrons collide at extremely high energies in the LHC? This is the question that forms the basis of David Krofcheck’s research at the LHC.

David is interested in the ‘herd behaviour’ or collective flow shown by the quarks and gluons that make up protons and neutrons. It is this collective flow that may give further insight into the strong nuclear force found in the nucleus.

Point of interest
Find out more about quarks. What are they? How many different types are there? What do gluons have to do with quarks?


The goal of the physics is to try to unify the forces of nature, and as we go higher in beam energies, there are theoretical pictures that have supporting evidence that show all the forces, except maybe gravity, come together at some very ultra high energy, and we are looking at hints of that behaviour to unify the laws of physics. The LHC will start up with proton-proton collisions and that will provide us an order of magnitude for how many particles get produced, for example, when 2 protons collide and they get hundreds of particles.

For the lead collisions, what I’m more interested in is what happens when you have not just one proton hitting one proton, which might mean one quark in a proton hitting another quark in a proton, but what happens when you have hundreds of quarks colliding head on with hundreds of other quarks. Lead has 208 protons and neutrons – compared to just one proton and one proton colliding, lead will collide 208 protons and neutrons with 208 other protons and neutrons. 416 protons and neutrons give you a lot of quarks colliding at the same time and that sets off the herd behaviour of the quarks.

We know from lower beam energies that they start showing collective behaviour, like a crowd mentality behaviour, herd mentality – something shows up that isn’t just the sum of individual quark-quark collisions – but there’s a collective motion that I’m extremely interested in. That collective motion tells us some characteristics of the very strong nuclear force. And we want to see, for the heavy nucleus collisions, what we call the quark-gluon plasma, where we melt the protons and neutrons into individual quarks and gluons, which then flow collectively, and that is the name of the phenomena – collective flow. So by cracking open the prisms, the bags of quarks and gluons, we see there’s hints of collective flow behaviour at lower beam energies. We need to push those studies harder now with the Large Hadron Collider.