Lawrence Berkeley National Laboratory
An ordinary proton or neutron (foreground) is formed of three quarks bound together by gluons, carriers of the color force. Above a critical temperature, protons and neutrons and other forms of hadronic matter "melt" into a hot, dense soup of free quarks and gluons (background), the quark-gluon plasma.
updated 6/23/2011 5:23:31 PM ET 2011-06-23T21:23:31

By creating a soup of subatomic particles similar to what the Big Bang produced, scientists have discovered the temperature boundary where ordinary matter dissolves.

Normal atoms will be converted into another state of matter — a plasma of quarks and gluons — at a temperature about 125,000 times hotter than the center of the sun, physicists said after smashing the nuclei of gold atoms together and measuring the results.

While this extreme state of matter is far from anything that occurs naturally on Earth, scientists think the whole universe consisted of a similar soup for a few microseconds after the Big Bang about 13.7 billion years ago.

Physicists could re-create it only inside powerful atom smashers like the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory on Long Island, which has a 2.4-mile-long ring. Researchers there accelerated the nuclei of gold atoms to incredible speeds, then crashed them into each other. The inferno created in this explosion was enough to give rise, briefly, to particle soup.

Quark-gluon plasma
"Normal matter like we are, nuclear matter, is called hadronic matter. If you excite the system to a very high temperature, normal matter will transform into a different type of matter called quark-gluon plasma," said physicist Nu Xu of the U.S. Department of Energy's Lawrence Berkeley National Laboratory in Berkeley, Calif.

Xu and his colleagues created quark-gluon plasma by crashing together gold nuclei inside the STAR experiment (Solenoidal Tracker at RHIC), which is inside the ring of the RHIC accelerator.

The nuclei of gold atoms consist of 79 protons and 118 neutrons. Both protons and neutrons are made of quarks, held together by massless, chargeless particles called gluons. (Protons contain two "up" quarks and one "down," while neutrons have two "down" quarks and an "up.")

When two of these gold nuclei slammed into each other head-on, they melted down into their constituent parts, an incoherent swarm of quarks and gluons. The researchers found that this occurred when the particles reached an energy of 175 million electron volts.

This corresponds to about 3.7 trillion degrees Fahrenheit (2 trillion degrees Celsius), which is about 125,000 times hotter than the center of the sun.

"If you can heat the system to that temperature, any hadron will be melted into quarks and gluons," Xu told LiveScience.

A new breakthrough
This wasn't the first time physicists had created quark-gluon plasma. The first hints that RHIC had produced the extreme state of matter came in 2005, and firm evidence that it had been achieved was announced in 2010.

But until now, scientists had never been able to precisely measure the temperature at which the nuclei transitioned into the quark-gluon plasma state.

The discovery allows researches to compare hard measurements with predictions from a theory called quantum chromodynamics (QCD), which describes how matter is fundamentally put together, including how quarks assemble to form protons and neutrons. The interactions involved in quark-gluon plasma are governed by a framework called lattice gauge theory.

"This is the first time we compare the experimentally measured quantities with that of QCD lattice gauge calculations," said Xu, who is the spokesman for the STAR experiment. "It is the start of the era of precision measurements in high-energy nuclear collisions. It is very exciting."

Xu and his colleagues, led by Sourendu Gupta of India's Tata Institute of Fundamental Research, published their findings in Friday's issue of the journal Science.

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Soupy caldron
By creating the soupy caldron of quarks and gluons, researchers hope to learn not just about how matter is put together, but how our whole universe began.

According to the Big Bang theory, the universe began extremely hot and dense, then cooled and expanded. A few microseconds after the Big Bang, scientists think, matter was still hot enough that it existed in a quark-gluon plasma state; it was only after the quarks cooled enough that they could bind together with gluons and form the protons and neutrons that make up the matter we see today.

Through studies like the one at RHIC, as well as at the world's largest particle accelerator, CERN's Large Hadron Collider near Geneva, Switzerland, researchers hope to create more of this extreme matter to probe just how this happened.

"With many more results expected from the RHIC experiments in the near future, additional insights into the details of the transition from ordinary matter to quark matter are within reach," wrote physicist Berndt Müller of Duke University in an essay published in the same issue of Science. Müller was not involved in the new study.

An earlier version of this report included an incorrect figure for the comparison between the temperature boundary and the temperature of the sun.

You can follow senior writer Clara Moskowitz on Twitter @ClaraMoskowitz. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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Explainer: What’s a hadron? Tour the particle zoo

  • Image: Elementary particles

    The Standard Model of particle physics is one of science's most successful theories, enabling the development of devices ranging from light bulbs, to microwave ovens and television, to quantum computing devices. The Standard Model is also one of the oddest theories, because it lays out a dizzying menagerie of hundreds of subatomic particles. At its heart are 16 types of elementary particles ... plus at least one more mysterious particle that scientists are spending billions of dollars to detect.

    Click on "Next" to get the full rundown.

  • Quarks

    Image: Quarks and gluons
    Berkeley Lab

    Six "flavors" of quarks have been detected: up and down, charm and strange, top and bottom. Quarks are almost always found in different combinations, bound together by gluons (more on those later). Particles built up from quarks and gluons are called hadrons. The Large Hadron Collider is so named because it's a large collider that smashes hadrons together.

    Three-quark combinations fit in the category of baryons. The best-known baryons are the proton (with two up quarks and one down quark) and the neutron (with two down quarks and one up quark).

    Particles that have one quark and one antiquark fit in the category of mesons. For example, the pion, or pi meson, contains an up quark and an anti-down quark.

  • Leptons

    Image: Single electrons in helium
    Brown University

    Six "flavors" of leptons have been detected: The negatively charged electron is the best-known lepton — along with its antimatter counterpart, the positron. This photo shows the path of single electrons passing through liquid helium, in an experiment devised by Brown University researchers.

    The muon is also negatively charged, but it's about 207 times as massive as the electron. ("Who ordered that?" physicist Isidor Rabi reportedly asked.) The negatively charged tau particle is even bigger — 3,477 times as massive as the electron — but it decays into other particles in less than a trillionth of a second.

    Each of those leptons has a neutrino associated with it: the electron neutrino, the muon neutrino and the tau neutrino. Neutrinos interact only weakly with other particles, and they zip through our planet virtually without a trace. Physicists only recently determined that they have mass, but there's still a great deal of mystery surrounding the ghostly particles.

  • Force carriers

    Image: Graviton

    The Standard Model sets aside a category for particles that are associated with force fields. The effect of a field can be viewed as involving an exchange of such force-carrying particles.

    Four elementary force-carrying particles have been detected. The best-known force carrier is the photon — which plays a part in the electromagnetic spectrum, including visible light. The gluon binds quarks together through the strong nuclear force. The weak nuclear force involves the exchange of W and Z bosons. The W boson can carry a positive or a negative charge, while the Z boson is neutral.

    If gravity could be incorporated into the Standard Model, the force-carrying particle would be called the graviton (shown here in an artist's depiction). However, gravitons have not yet been detected, and at least for now, such particles are not accounted for in the Standard Model.

  • Bosons vs. fermions

    Image: Bosons vs. fermions
    Rice Univ. via AIP

    All force-carrying particles are bosons, but not all bosons are force carriers. The difference has to do with a property known as particle spin. Particles with a fractional spin value (for example, electrons, protons and neutrons) are fermions. Two identical fermions cannot occupy the same quantum state. This is a property that keeps electrons from collapsing into a jumble, and thus makes chemical reactions possible.

    All particles with a whole-integer spin value are classified as bosons, and such particles can occupy the same quantum state even if they're identical. The photon is the best-known type of boson.

    Even atoms can be classified as fermions and bosons. This photo shows how atom clouds of lithium-7 (bosons) and lithium-6 (fermions) behave at low temperatures. The bosons collapse into a compact cloud, while the fermions can't squeeze that closely together.

  • The mysterious Higgs

    Image: Higgs as seen by CMS
    Ianna Osborne / CERN / CMS Collection

    The Higgs boson is the only particle predicted by the Standard Model that has not yet been detected. The Higgs is the main quarry for physicists at the Large Hadron Collider. This image is a simulation of the Higgs' signature as it might appear in one of the LHC's detectors.

    The Higgs boson, named after Scottish theorist Peter Higgs, is thought to be associated with a field that endows some particles (such as the weak nuclear force's W and Z bosons) with mass, while leaving the electromagnetic force's photons without mass.

    This Higgs field may have played a role at the very beginnings of the universe: Physicists believe that at the highest energies, the electromagnetic and weak nuclear forces were unified, but something led to "electroweak symmetry breaking" as the infant cosmos cooled. That would be why the electromagnetic force and the weak nuclear force are distinct in the current universe. The Large Hadron Collider could shed new light on this mysterious Higgs mechanism.

  • Why so complicated?

    Image: Particle zoo
    Tim Jones / McDonald Observatory / HETDEX

    Hadrons and leptons? Baryons and mesons? Fermions and bosons? Sometimes it seems as if particle physicists set up these classifications just to keep outsiders totally confused. But for researchers, these occasionally overlapping categories are useful for figuring out how different types of particles interact with each other.

    In a sense, it's as if we've been talking about the game of chess but have gotten only to the point of naming the different pieces on the board: black pieces and white ones, pawns and knights, bishops and rooks, kings and queens. The real meaning of the game comes out when you start studying how the pieces perform and interact.

    To delve into the deeper meaning of the Standard Model, you can visit The Particle Adventure at Lawrence Berkeley National Laboratory, "A Subatomic Venture" at CERN, or Particle Physics UK.


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