Event display recorded by ATLAS of a lead-ion collision where one jet was emitted with large transverse energy and no evident recoiling jet.

CERN announced today that the ATLAS experiment has published its first measurements from lead-ion collisions. The measurement, of a phenomenon called jet quenching, opens up a new era in the ability of scientists to probe the behavior of the hot, dense matter--the quark gluon plasma--that existed microseconds after the Big Bang.

The publication of the ATLAS paper comes three weeks after the first lead-ion collisions in the LHC, and one week after the ALICE experiment published its first two measurements of different properties of lead-ion collisions. Scientists from the CMS experiment have confirmed that they also see the jet quenching effect in their data, and will publish their result in the coming weeks.

Jets are sprays of particles that fly out from high-energy collisions of particles like protons or lead ions. When protons or lead ions collide at high energies, what really collides are their component particles: quarks and gluons. Since nature has made it impossible for quarks or gluons to exist in isolation, as they move away from the point of collision they immediately turn into a narrow cascade, or jet, of particles.

Jets are a common sight in collisions of protons at the LHC, usually appearing in pairs as narrow cones of particles heading away in opposite directions from the collision point. They are equally common in collisions of lead ions, but with a twist. The ATLAS measurement showed that the more "head-on" the collisions of lead ions, the more unbalanced the energies of the jets streaming out in opposite directions from the collision point. While one jet may still appear as a narrow cone of particles, the second jet has a much lower energy, and the narrow cone of particles has become much more diffused.

This is the phenomenon of jet quenching, first observed indirectly in 2003 by experiments at Brookhaven National Laboratory's Relativistic Heavy Ion Collider. In the years since, scientists have theorized that the quenching effect occurs when quarks scatter and lose energy as they travel through the quark gluon plasma. Depending on where the pairs of quarks are created and their paths out of the plasma, they may lose different amounts of energy. In the extreme case, where the jets are produced near the surface, one jet may escape unscathed while the other jet travels through the bulk of the plasma losing a large fraction of its energy.

Precisely measuring jets as they emerge from heavy-ion collisions thus provides an excellent tool to probe the quark gluon plasma itself, about which relatively little is known. But the indirect measurements of jet quenching possible at RHIC severely limited scientists' ability to explain exactly what was happening to the jets as they passed through the plasma. ATLAS has shown in its publication today that direct measurements of jet quenching are possible at the LHC, and that the phenomenon is even stronger than expected from the RHIC results.

"This result, the fact that the interactions are so strong, was completely unexpected," says Brian Cole from Columbia University, who led the ATLAS analysis. The effect is so strong at the LHC that scientists in both the ATLAS and CMS experiments were able to see it with their own eyes as they looked at event displays on computer monitors. "We also see that the jets remain exactly back-to-back, even when they're quenched. This is startling, and was the kind of insight we didn't get at RHIC."

ATLAS and CMS are able to directly measure jet quenching for two reasons. First, the higher collision energy at the LHC means that jets have enough energy to make it out of the collision zone and into the particle detectors. Second, unlike the RHIC detectors, the ATLAS and CMS detectors capture almost all of the particles that stream outward from the collision point, and are sophisticated enough to record all of the particles that make up a jet.

"In ATLAS the jets have nowhere to hide," says Peter Steinberg from Brookhaven National Laboratory, co-leader of the heavy ion physics group within ATLAS. "We see one jet that's a sharp cone, and debris in the other direction that seems to originate from the recoiling jet. It's analogous to comparing a drop of ink falling in air, and in water. In one case the drop goes straight ahead and in the other it diffuses and spreads as it interacts with the water, but in either case nothing is lost."

But scientists, including Cole, caution that this is just another step in a process that will span years and perhaps decades, as scientists seek to understand exactly how quarks and gluons interact within the plasma. But with the knowledge in hand that the jet quenching effect is strong at the LHC, and the ability to directly measure the phenomenon, scientists from ATLAS, CMS and ALICE will forge ahead with more analysis using more lead-ion collision data. Theoretical physicists will turn to the task of interpreting the new measurements, all with the goal of learning more about the behavior of the universe's fundamental particles and forces.

The ATLAS result has been accepted for publication by the journal Physical Review Letters, and a draft is available on the CERN document server.