SCIENCE

More signals from the QGP

Jet quenching is another signal of the quark-gluon plasma, first described in 1990 by Xin-Nian Wang, a senior scientist in NSD’s Nuclear Theory Group, and Miklos Gyulassy, then of Berkeley Lab and now at Columbia University.

When a proton or neutron collision scatters quarks and gluons back-to-back, some of their energy is absorbed before they quickly condense into jets of particles traveling in opposite directions. If these jets occur inside a dense fireball, in the wake of a collision of heavy nuclei, one jet will slow down more than the other or even be absorbed. Jets that escape carry valuable information about the dense, fiery medium where they were created and which they have just traveled through.

Jet quenching was first discovered at RHIC, with members of the Nuclear Science Division leading the effort at the STAR experiment. At the LHC the ATLAS experiment, soon followed by another general-purpose experiment, CMS, was first to report observing evidence for jet quenching. An ALICE paper posted less than a week later detailed the “suppression of charged particles with large transverse momentum” – another way of saying jet quenching.

Because the first ATLAS results depended on data from a limited region of the general-purpose experiment, Seth Zenz of Berkeley Lab’s Physics Division, a member of the Lab’s ATLAS team at CERN, was drafted to cross-check the results among different parts of the detector. What ATLAS reported, as well as CMS, was a marked imbalance in energies between the two jets of a pair.

“For anyone who wanted not to believe RHIC’s evidence of jet quenching, they might have found refuge in the limited coverage of the RHIC experiments,” says Ian Hinchliffe of the Physics Division, who heads the Lab’s participation in ATLAS. “They no longer have that excuse.”

Particle tracks from some of the first collisions of lead nuclei in ALICE.

ALICE’s observation of the suppression of high-momentum particle jets, like its observation of elliptic flow and its particle counts for head-on collisions, confirms studies initially made at RHIC and extends those results to much higher energies.

“We’re measuring jet quenching by looking at the primary charged particles produced when quarks and gluons break up into hadrons, like energetic pions,” says Jacobs. “This is the same way we did it at RHIC.”

ALICE has found that the production rate of high-momentum particles is more strongly suppressed than at RHIC, suggesting that their parent quarks or gluons suffered larger energy losses – and therefore that the quark-gluon plasma at the LHC is much denser than at RHIC.

“It’s an open question whether the ALICE and ATLAS measurements of jet quenching can be reconciled quantitatively,” says Jacobs. “The results are very new and we need to understand exactly what was done in each analysis. But our theory friends are already hard at work on the issue as well.”

Although ALICE is the LHC experiment that focuses on heavy-ion collisions, ALICE was already under construction before jet quenching was discovered at RHIC in 2002. An ALICE component specifically designed to target rare jet-quenching events is the electromagnetic calorimeter, dubbed EMCal.

“ATLAS and CMS provide very broad coverage, but when EMCal is completed, ALICE will be able to measure complete jets with extraordinary precision,” Jacobs says.

The modular EMCal was only partly installed before the present lead-lead run was begun and won’t be finished until January, during the LHC maintenance period. Meanwhile, anticipation of the physics which can be explored by studying back-to-back jet quenching has led to a plan to expand EMCal by adding more modules to cover more of the area around the collision point, an extension dubbed DCal, for di-jet calorimeter.

Already a new picture of nuclear physics, at extremes rarely experienced since the big bang except in the hearts of exploding stars, is gradually emerging from high-energy, heavy-ion collisions at RHIC and the LHC. Berkeley Lab continues to make essential contributions to a field it has done much to define.

“From RHIC to the LHC we see the evolution of the phenomena,” Jacobs says. “Results from RHIC or the LHC alone would be much less interesting than both together.”

Additional information
Initial ALICE results from lead-lead collisions announced in symmetry breaking

CERN’s press release on the first lead-lead results from the LHC

The ATLAS collaboration’s release on their lead-lead results

“Charged-particle multiplicity density at mid-rapidity in central Pb-Pb collisions at √sNN = 2.76 TeV,” by the ALICE Collaboration

“Elliptic flow of charged particles in Pb-Pb collisions at √sNN = 2.76 TeV,” by the ALICE Collaboration

“Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √sNN = 2.76 TeV with the ATLAS Detector at the LHC,” by the ATLAS Collaboration

“Suppression of Charged Particle Production at Large Transverse Momentum in Central Pb–Pb Collisions at √sNN = 2.76TeV,” by the ALICE Collaboration

More about studying jet quenching at ALICE

More about installing ALICE’s EMCal