Jetting into the Quark-Gluon Plasma

January 15, 2010 by Paul Preuss
Gold nuclei collide in the STAR experiment at RHIC, creating a fireball in which the quark-gluon plasma briefly appears. Its properties are reconstructed from particle tracks captured in STAR's Time Projection Chamber.

After the quark-gluon plasma filled the universe for a few millionths of a second after the big bang, it was over 13 billion years until experimenters managed to recreate the extraordinarily hot, dense medium on Earth. The JET Collaboration, a team from six universities and three national laboratories led by Berkeley Lab’s Nuclear Science Division, is now developing a new and highly detailed theoretical picture of this unique state of the early universe.

The Department of Energy’s Office of recently named Berkeley Lab’s Nuclear Science Division to lead a nine-institution collaboration investigating the “Quantitative Jet and Electromagnetic Tomography of Extreme Phases of Matter in Heavy-Ion Collisions” - JET, for short.

The JET Collaboration is a five-year theoretical effort to understand the properties of the extraordinarily hot and dense state of matter known as the quark-gluon . The quark-gluon plasma filled the Universe a few millionths of a second after the big bang but instantly vanished, condensing into the protons and and other particles from which the present Universe descended.

Some 13.7 billion years later, experimenters recreated the quark-gluon plasma on Earth, using the Relativistic Heavy (RHIC) at Brookhaven National Laboratory. The first heavy-ion collisions occurred at RHIC in 2000, but confirming the occurence of the quark-gluon plasma in these events took several more years of data collection and analysis.

Freeing the quarks

come in three different “colors,” and it takes three quarks to build a proton or a neutron; as carriers of the color charge, an aspect of the strong nuclear interaction, gluons literally glue the quarks together.

Under ordinary conditions neither quarks nor gluons are ever free. The farther apart they get, the stronger the force between them. Because mass and energy are interchangeable, as described by Einstein’s E=Mc2, eventually the energy that would be needed to separate them goes into creating new bound quarks instead.

RHIC was designed to collide heavy nuclei (as heavy as gold, whose nucleus consists of 79 protons and 118 neutrons) at energies so high that during the near-light-speed collisions, conditions cease to be anything like ordinary. Dense, hot fireballs blossom in the collisions, forming a plasma in which neither quarks nor gluons are bound together; instead they move independently with almost complete freedom.

The RHIC results held some surprises. Unlike more familiar plasmas in which electrically charged particles are separated from one another, the quark-gluon plasma consists of color charges. The quark-gluon plasma produced at RHIC turned out to be more like a liquid than a gas.

“One of the main discoveries at RHIC is that the quark-gluon plasma produced in heavy-ion collisions behaves as a perfect fluid with very small viscosity,” says Xin-Nian Wang, a senior scientist in the Nuclear Theory Group in Berkeley Lab’s Nuclear Science Division (NSD). Wang is the co-spokesperson and project director of the JET Collaboration.

Perfect fluidity arises because the plasma’s constituents are strongly coupled, causing their collective flow. And the quark-gluon plasma flows freely, like low-viscosity motor oil in a hot engine - much more freely, in fact, Wang says, because its specific shear viscosity is “an order of magnitude less than that of water.”

Another RHIC discovery was the predicted but never-before-seen “jet quenching.” When individual particles collide in a vacuum - as when protons collide in CERN’s Large Hadron Collider, for example - the debris often flies out in a pair of jets; particles like pions or kaons detected on one side of the detector are correlated, in terms of total momentum and energy, with particles detected on the opposite side.

“But when heavy ions collide, they produce an incredibly dense medium, 30 to 50 times as dense as an ordinary nucleus,” Wang says. “The farther a jet of particles has to push through this strongly interacting nuclear matter, the more energy it loses. One jet from the back-to-back pair may not escape the fireball at all.”

The energy of the trapped jet has to go somewhere. The energetic particles that are initially produced decay to softer ones which further interact with the medium, producing shock waves in the fluid. As with the sonic boom from a jet plane “breaking the sound barrier” - flying faster than the speed of sound in air - the shock wave from a jet swallowed by the quark-gluon plasma could be used to measure the velocity of sound in the plasma.

The debris from heavy-ion collisions indicates that free quarks and gluons recombine into hadrons (which include pions and kaons made of two quarks and protons and neutrons made of three quarks) while the plasma is cooling; this also affects how the jets propagate.

Protons and neutrons (upper left) are made of up and down quarks bound by color charges carried by gluons. But in a hot, dense quark-gluon plasma (right), quarks and gluons are unbound and free to move independently.

Probing the plasma

Jets are called “hard probes.” Although by nature strongly interacting, they are moving so fast and with so much energy that their interaction with the surrounding free quarks and gluons in the plasma is actually relatively weak. A jet’s ability to transfer energy and momentum to the medium as it moves through the fireball is known as the jet transport coefficient (JTC), which is related to the plasma’s viscosity: the smaller the viscosity - and the viscosity of the quark-gluon plasma is very small indeed - the larger the JTC.

It’s not just the degree of jet quenching, a figure that emerges in the data from millions of collision events, but the orientation, directionality, and composition of the jets that have much to tell about what’s inside the fireball, and thus about the properties of the quark-gluon plasma.

Another kind of probe, an electromagnetic probe, is so weak there is virtually no interaction with the medium at all. Electromagnetic probes appear when a jet of particles in one direction is balanced not by another jet but by a single, very energetic photon.

The task of the JET Collaboration is to use the existing evidence from the RHIC results to calculate in detail what’s really going on inside the strongly interacting quark-gluon plasma - the kind of three-dimensional picture of an otherwise invisible interior that’s called tomography, as in computed axial tomography, the familiar CAT scan.

Three kinds of phenomena are critical to the completion of the task: collectivity, to determine the viscosity of the medium; jets, to determine the jet transport coefficient; and the excitation of the medium, to determine the velocity of sound within it.

More than one kind of calculation will be required. Different assumptions and different codes must be used to model different kinds of interactions and different properties, and the results don’t always agree. The JET Collaboration includes representatives from major institutions that have made significant contributions to the study of the hot, dense matter in heavy-ion collisions, often approaching the question from different points of view. Working together, a consistent picture of the quark-gluon plasma will emerge.

Once the calculations are complete, having taken into account the entire energy spectrum of particles emerging from millions of evanescent fireballs, the new theoretical picture of this unique state of the will be tested against observations at the newly upgraded RHIC and at the ALICE experiment at the Large Hadron Collider (LHC) at CERN. (The LHC collides for most of the year, but for a month each year it will collide heavy ions in the form of lead nuclei.)

Jets are "hard probes" of the quark-gluon plasma. Especially revealing information can be derived when one of the pair of jets is unable to escape the hot, dense medium.

The JET Collaboration

In the JET Collaboration, Berkeley Lab will be represented by theorists Wang, Volker Koch, and Feng Yuan. The Lab’s leadership in both the theory of the quark-gluon plasma and in its experimental exploration through the Relativistic Nuclear Collision (RNC) group uniquely positions the Lab to head the Collaboration.

The idea of jet quenching was first proposed for proton-proton collisions in the early 1980s, by James Daniel Bjorken of the Stanford Linear Accelerator Center. The theory linking jet quenching to the quark-gluon plasma in heavy-ion collisions was later developed by Xin-Nian Wang and Miklos Gyulassy; Gyulassy was with Berkeley Lab at the time and is now at Columbia University, where he is a member of the JET Collaboration.

On the experimental side, the heart of the STAR experiment at RHIC is a time projection chamber built at Berkeley Lab and invented here by David Nygren of the Physics Division; STAR is one of many time projection chambers around the world, including the heart of the ALICE experiment at the LHC. The electromagnetic calorimeter, EMCal, which will trigger the recording of interesting jet events in ALICE, is being constructed by an international team led by U.S. members of ALICE, with project management by Berkeley Lab’s Peter Jacobs of NSD and Joseph Rasson of Engineering.

Other DOE labs participating in the JET Collaboration are Lawrence Livermore, represented by Ramona Vogt, and Los Alamos, represented by Ivan Vitev. In addition to Columbia University, represented by Gyulassy, other universities include Duke, represented by Steffen Bass and Berndt Mueller, the JET Collaboration’s co-spokesperson, plus Charles Gale and Sangyong Jeon of McGill, Ulrich Heinz and Abhijit Majumder of Ohio State, Denes Molnar of Purdue, and Rainer Fries and Che-Ming Ko of Texas A&M.

JET is one of three topical collaborations established by DOE’s Office of Nuclear Physics. Over a period of five years, with a budget of $2.5 million, the JET Collaboration will not only develop theory but work closely with experimentalists, train students and postdoctoral fellows, and form associations with a wide range of researchers in the nuclear science community at institutions in the U.S. and abroad.

Explore further: Ultracold gas mimics ultrahot plasma

More information: How heavy ions collide at RHIC to create the quark-gluon plasma

Wikepedia’s article on the quark-gluon plasma

The STAR experiment at RHIC

The ALICE experiment at the LHC

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3 / 5 (2) Jan 15, 2010
The absorbtion of jet demonstrates, how easily the QG plasma can be feeded by matter under formation of black hole or strangelets.

In this connection it may be significant, CMB cold spot is unpaired too (if the observable Universe is formed by black hole, we could see through polar jet into hyperuniverse). The parity violation of gallaxies should be corellated to CMB spot direction, too. Because jets of black holes are exaggerated example of gravitational brightening, we could observe CP symmetry violation by difference of polar temperatures at the case of giant stars, too.
5 / 5 (1) Jan 15, 2010
I forgot to explain, the violation of jet symmetry can be understood as an example of CP symmetry violation, which was observed first at the case of spin polarized and cooled cobalt-60 nuclei (1956). These nuclei are emanating electrons in asymmetric way. The same stuff we can observed at the case of jets of black holes, which are asymmetric often too, i.e. in similar way, like QG plasma during collider experiments.


5 / 5 (1) Jan 15, 2010
..the plasma’s constituents are strongly coupled, causing their collective flow..
This coupling is basically well known Yukawa coupling, which can be explained by presence of Higgs field. I explained already here, formation of top-quark pairs can be interpreted like formation of Higgs bosons, which are of the same rest mass and dilepton decay mechanism.

Quantum foam gets more dense with increasing energy density in similar way like soap foam under shaking and it changes itself into foam with small spherical bubbles similar to fluid, which results into collective motion of particles involved.
5 / 5 (1) Jan 15, 2010
This would be way more interesting if they actually looked at one event instead of averaging them all out.
But I dont think their equipment works on that timescale?

How interesting is an average cow? Not very..
One old long horn, now that is interesting.
5 / 5 (1) Jan 17, 2010
I forgot to explain, the violation of jet symmetry can be understood as an example of CP symmetry violation, which was observed first at the case of spin polarized and cooled cobalt-60 nuclei (1956). These nuclei are emanating electrons in asymmetric way. The same stuff we can observed at the case of jets of black holes, which are asymmetric often too, i.e. in similar way, like QG plasma during collider experiments.

It is questionable the 1956 experiment referred to above really is proof of symmetry violation. There is a completely different explanation. In short, the weak force (radioactive decay) may be caused by a background of neutral neutrino radiation. Ever so slightly more of the neutral radiation "rains" down from above as opposed to rising up from below because the earth absorbs about 15 parts per million. Neutrino oscillation is not needed to explain the missing neutrinos that travel through the earth.
Jan 17, 2010
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5 / 5 (1) Jan 17, 2010
BTW if violation of weak force is caused by neutrino (or whatever else particle) drag, it would mean, the direction of all black hole jets and jets in collider experiments or radioactivity during Chien-Shiung Wu experiments should be oriented toward Virgo cluster - or not?

I don't see such explanation quite realistic. If the symmetry violation is caused by background particle drag, then these particles must be scale invariant - i.e. we are rather dealing with fundamental geometric principles here. For example, we can asume, every observer INSIDE of our Universe must be of POSITIVE surface curvature - if he wouldn't, he would become LARGER then Universe, in which he resides.
not rated yet Jan 19, 2010
There is certain possibility, the pear shape, which deforms Earth ellipsoid by elevation of about two hundred meters at north pole is related to CP asymmetry, too - being balanced by ring of dark matter at equatorial plane. The testing of such hypothesis is simple, if we find pear shape for other rotating stars or planets related to chirality of rotation.


It's interesting, Christopher Columbus considered it in 17th century already, while promoting westward voyage to Cathay (China) or Zipangu (Japan).
1 / 5 (1) Jan 19, 2010
Christopher Columbus considered it in 17th century already
Christopher Columbus died at the beginning of the 16th century.
while promoting westward voyage to Cathay (China) or Zipangu (Japan)
One century earlier, admiral Zheng He did travel westwards - probably as far as Iran.
not rated yet Jan 19, 2010
Christopher Columbus died at the beginning of the 16th century.
Thanx for correction, but my thinking is time invariant here. The symmetry violation during jet formation could be explained by omnidirectional space-time expansion during torus rotation - the inner part of ring always rotates faster, so it's dragged in axis direction. Personally I consider funny the idea, Columbus could be a promoter of SUSY and CP symmetry violation in this connection.

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