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Hard Probes (and Soft Ones) to Test the Quark-Gluon Soup

“We call short-wavelength probes ‘hard’ — the shorter the wavelength, the smaller the features it can resolve,” says theorist Xin-Nian Wang of Berkeley Lab’s Nuclear Science Division (NSD). “For example, you can study smaller objects with x‑rays than with visible light. We need the hardest probes of all to study the hot, dense state of matter that exists when two heavy nuclei like gold collide with enough energy to temporarily free the quarks and gluons in their constituent protons and neutrons.”

Elemental mercury
In the laboratory frame of reference, two nuclei approaching at relativistic speed appear contracted. Nucleons are shown in red and white, pions, the most abundant collision products, in green. Each pair of colliding nucleons produces a pair of jets, not apparent at this scale. (Images by Ultrarelativistic Quantum Molecular Dynamics group)

Slamming gold and other kinds of heavy nuclei together at high energy is the stock in trade of RHIC, the Relativistic Heavy Ion Collider located at Brookhaven National Laboratory on Long Island. The state of matter thus created is called a quark-gluon plasma — although, unlike what we usually think of as a plasma, what’s created in RHIC resembles a perfect liquid, not a gas.

As NSD experimentalist Peter Jacobs cautions, “There’s no way we can look into such a short-lived, dense event from the outside. What we call ‘probes’ are coming from inside, jets of quarks and gluons, or the hadrons into which they decay, plowing through the medium and strongly interacting with other matter. From many such probes coming from different parts of the event we build up a tomographic picture, like a CAT scan, of what’s going on inside.”

Playing Hard Ball

Symmetrical, back-to-back jets of particles flying out of head-on collisions are a well-known phenomenon in high-energy physics. But a few years ago Xin-Nian Wang and his colleagues, including Miklos Gyulassy, then at Berkeley Lab, came up with a new wrinkle in the study of jets: the idea of jet quenching.

Because a collision of gold nuclei where RHIC’s twin beams meet is actually a collision of many individual nucleons — protons, of which gold has 79, and neutrons, of which gold normally has 118 — most of those nucleon collisions will be off center. One of the back-to-back jets will have an easier time escaping to the edge of the event, whereas the other, the recoil jet, will be “quenched” — hidden from detection — by the event’s superdense interior.

Jet quenching has indeed been observed in RHIC experiments, including the giant STAR detector whose core Time Projection Chamber was built at Berkeley Lab. Says Jacobs, a member of the STAR collaboration, “at first we studied the jets that were easier to see, which are mostly from the edge of the event. What’s new is that we’re now starting to plot what happens to [the other jet,] the recoil guy who needs to plow through all this stuff.”

“One effect is that the recoil jet heats matter, and we’re beginning to see how the medium responds,” says Wang. “These are the color-charge interactions of the strong nuclear force.”

The strong nuclear force, carried by appropriately named gluons, is what “glues” a proton or neutron’s quarks together. Quarks are fermions, of which no two in the same system can have the same quantum number, so theorists came up with color charge to label each quark in a nucleon (which has three quarks) with a different quantum number, whimsically called red, green, and blue.

A typical U.
Spacer image A nucleon contains three quarks, two ups and a down for the proton, two downs and an up for the neutron, no two with the same color charge. The nucleon’s quarks are bound by the strong force, which is carried by gluons, themselves consisting of a quark and an antiquark.

A gluon is a boson and consists of a colored quark and antiquark (a kind of particle also called a meson). When a nucleon blows apart in an energetic collision of nuclei and instantly recondenses into new particles, the escaping jets bear signs of those color-charge interactions, the proper field of study for Quantum Chromodynamics, or QCD.

Advances in jet studies were one of the main topics of discussion at the Second International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions, held this summer in Pacific Grove, California. A topic of lively interest was the kind of particles that could be identified in jets. Initially, the only quarks that could be easily identified were lighter quarks, like the up and down quarks that combine to make ordinary protons and neutrons. More recently, very massive charm and bottom quarks have been identified. But they don’t behave as expected.

“Theoretical expectations were that heavy quarks would lose less energy than light quarks do as the jet pushes through the medium,” says Wang. “The data don’t correspond to those expectations — the heavy quarks are suppressed just like the light ones — and the theorists are facing a dilemma.”

Peter Jacobs cautions that experiments aren’t definitive yet, because the signals that have been employed to identify charm and bottom quarks can’t distinguish between them. “To do that, we’ll need vertex detectors closer to the interaction point that can better measure the decay products.”

Vertex detectors, usually cylindrically mounted dense arrays of pixels on semiconductors, surround the collision point and track the path and energy of emerging charged particles. The closer to the beam point, the more precise the measurement. Berkeley Lab physicists and engineers are at work on an upgrade to STAR’s existing vertex detector.

Probing the Softer Side

While the 2006 Hard Probe conference was indeed mostly devoted to “hard” probes — those that can be calculated using QCD — attention was also devoted to “soft” probes that interact electromagnetically, namely, photons and dileptons such as positron-electron pairs.

“Electromagnetic interactions are a lot weaker than strong interactions,” says Wang, “exactly 1/137th weaker, in fact, than the color interaction. These experiments have also thrown the theorists into turmoil.”

The problem arises because of how quarks acquire their mass. The total mass of three up and down quarks is a lot less than the mass of the proton or neutron they constitute, but the effective mass of the quarks is greater because of something called “chiral symmetry breaking.” Chiral symmetry is broken at low temperatures, not at the high temperature attained in quark-gluon plasmas. In quark-gluon plasmas, quark masses are expected to drop.

As a probe, dilepton pairs are sensitive to chiral symmetry breaking, but the results of an experiment at CERN are not what theorists expected to see, and seem to rule out the dropping of the mass of rho mesons, which are pairs of up or down quarks and antiquarks.

Elemental mercury
The Second International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions was held June 9 to 16, 2006, at the Asilomar Conference Grounds in Pacific Grove, California.

On a different tack, conference participants were also stimulated by the successes of at least one brand of string theory in interpreting some experimental puzzles much more easily than QCD can deal with the same phenomena. “We don’t understand how these theories are related,” says Wang, referring to the well-established QCD and the popular if somewhat controversial string theory, but in this case “there’s a startling resemblance in their results.”

As might be expected, a final area of intense discussion at the summer conference was progress toward future heavy-ion experiments, especially at CERN’s Large Hadron Collider.

“The Large Hadron Collider at CERN won’t just accelerate protons,” says Jacobs. “It will also be able to accelerate heavy ions like lead to energies never before achieved.”

Jacobs and NSD Director James Symons are heading an effort to equip a CERN experiment named ALICE with a detector called an electromagnetic calorimeter, which will push heavy-ion experiments and the physics of jets into new realms of exploration.

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