Hunting quark-gluon plasma

4 October 2010

A simulated high-multiplicity lead ion collision. Energy thresholds are applied in the calorimeter, allowing a clear view of the tracks reconstructed by the tracking software.

From the outset, the LHC experiments' Heavy Ion Programmes have had the mysterious quark gluon plasma state of matter in their sights. The ATLAS programme kicks off in just over a month, but many people still don't know what to expect from it.

“At every iteration [of experimental heavy ion physics], what has come out has always been different than the expectations,” smiles ATLAS heavy ion expert Peter Steinberg. In his last appointment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, Peter witnessed the research focus shift very quickly from trying to establish whether or not QGP – essentially unbound quarks and gluons at unthinkably high temperatures or densities – exists, to instead characterising the unidentified medium that was being produced. “We didn't know what it was, but we could measure its properties,” he explains.

The LHC machine set-up for the Heavy Ion Programme should get underway on November 6th and, beginning on November 11th, the LHC will spend four weeks smashing together lead nuclei, at a nucleon-nucleon centre-of-mass energy of 2.76 TeV, within the ATLAS, CMS and ALICE detectors.

Unlike at the start of the proton-proton physics run, months of data collection and analysis mean that the ATLAS detector is now well understood and the heavy ion physics programme can begin in earnest. One thing that's still unknown though is how many particles will be produced, relative to simply scaling up from single proton-proton collisions.

“It will be like for low luminosity proton-proton [running], in that the event rate will be relatively low,” explains Peter, “but imagine 100 or so collisions piled up on top of each other, for every single event: it looks quite complicated!”

The number of particles produced in the collision of lead ions is still a hotly debated detail. “It turns out if you ask our community of theorists how many particles should come out, the answers vary by a factor of two to three,” says Peter. “For the proton-proton program, people were arguing over 20 per cent differences. For heavy ions though – all bets are off.”

The problem is complicated by having to consider the fact that nucleons undergo more than one collision amongst the crowd of nucleons as they pass through the oncoming nucleus. And when they do, do they have the same chance of producing jets or mini-jets each time? “That's the kind of hypothesis that will be tested,” Peter assures. “Within a few hours, we'll have enough data to actually know.”

How this 'multiplicity' depends on energy will likely be the first big result coming out of the programme. Following that, probing the bulk features – viscosity, temperature, density – of the now-infamous QGP medium is on the agenda. Studies from RHIC show that it reaches thermal equilibrium quickly. “So it very quickly turns from individual particles into a collective state of matter,” explains Peter.

In this state, in which the constituent particles are strongly coupled, the system behaves like a tiny drop of liquid, in the sense that it expands geometrically in a way related to its original shape. “How the system responds to changing the shape is actually described by hydrodynamics, which seems surprising because hydrodynamics is not a theory of particles per se,” Peter considers. “So we care about things like the systematic changes induced as we change the geometry of the collision … and we're very interested in characterising this ultra-dense matter with a variety of tools.”

Measurements of photons, muons, and electrons – particles able to pass right through the QGP matter – will help to build a picture of what is going on inside it as it evolves. Jet properties and rates will be another early focus, after RHIC noted dramatic jet suppression related to the curious matter:

“When we counted [the jets], there were far too few,” Peter reports. “We still don't know exactly why just yet, because RHIC doesn't have large-acceptance detectors like ATLAS. We don't know if jets are essentially not created as often or if somehow, as they go through [the QGP matter], they're broadening or being modified so much that the energy which would go straight ahead is being spread out and dispersed, leading you to reconstruct fewer of them.”

Information from the ATLAS calorimeters and inner detector will be used to investigate how the shape of the overlapping region between a pair of colliding ions affects the overall shape of the energy flow through ATLAS. The calorimeters will be run in much the same way as for proton-proton collisions, but the tracking required tuning to bring down the level of fake tracks caused by the abundance of hits.

“It's definitely a challenge,” Peter smiles. After the ATLAS tracking group helped them put a tracking package together, the Heavy Ion group “had several people for several months changing nearly every parameter to try to understand whether or not there was any way to eke out a better and better performance” out of it. “What's pretty amazing is that after months of work, we arrived back to just about where we started,” Peter says.

Although there is an order of magnitude between the heavy ion collision energies seen at RHIC and those which will soon be seen at the LHC, the two programmes are likely to run side by side for years, exploring different properties of the same matter.

“The really interesting thing is that we don't know yet what the difference between RHIC and the LHC is going to be,” says Peter. “We know that [at the LHC] the energy is higher, we know that the multiplicity will be higher. But we don't know yet if essentially we'll be in the same regime of quantum chromodynamics, or a in another regime altogether – one which may well be weakly coupled after all.”


Ceri Perkins

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