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.
Credit: Lawrence Berkeley National Laboratory
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 (3.8 km) 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.
"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. [Behind the Scenes at Humongous U.S. Atom Smasher]
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 (MeV).
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. [The Coolest Little Particles in Nature]
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 the June 24 issue of the journal Science.
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.
Editor's Note: This story was updated to correct the fact that the transition temperature is 125,000, not 250,000, times hotter than the center of the sun.