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Subatomic soup is hot stuff

Twisting vortices seen in fireballs could unravel matter-antimatter conundrum.

By Eric Hand

Physicists have measured the temperature inside the hottest fireball on Earth, a four-trillion-degree jumble of melted protons and neutrons.

This soup of subatomic particles, created in collisions of gold nuclei at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York, is yielding other intriguing discoveries. Chief among them is evidence of tiny bubbles in the soup that could help explain a fundamental asymmetry: Why, if the Big Bang created matter and antimatter in equal parts, did matter win out moments later?

"This asymmetry is crucial to our existence," says Dmitri Kharzeev, a Brookhaven theorist whose 1998 proposal for the existence of the bubbles is now being validated. He and other scientists unveiled the results on 15 February at a meeting of the American Physical Society in Washington, D.C.

Five years ago, RHIC scientists announced that they had begun to understand the mash of melted protons and neutrons created by smashing gold nuclei in their machine, a collider ring nearly 4 kilometers around. The nuclei are composed of hadrons--particles such as protons and neutrons--which are in turn made up of tightly-bound quarks and gluons. Smashed together at 200 gigaelectronvolts, the gold nuclei unleash their constituent particles in a "quark-gluon plasma." In 2005, RHIC scientists found this behaved like a perfect liquid, with particles slipping past each other frictionlessly (see Early Universe was a liquid).

Hot soup

But they are only now succeeding in measuring its temperature, which can be up to 40 times hotter than the core of a supernova and will be reported in an upcoming issue of Physical Review Letters. "We can't just stick a thermometer in there," says physicist Barbara Jacak, spokesperson for an instrument at RHIC that measured the energy of emitted gamma rays as a proxy for the incredible peak temperatures.

The conditions in the quark-gluon plasma are a model for a moment just a microsecond after the Big Bang, when forces and particles were emerging in a fast-paced sequence. And the bubbles within the plasma offer clues about what might have happened moments earlier in the Universe's history, when several radical things had to happen in order for antimatter to disappear.

One of those things is that a particular law of conservation--that in particle interactions or decays, the number of constituent sub-particles is conserved--was somehow violated. RHIC scientists say the lack of symmetry in the twisting, fleeting vortices of gluons could point towards a long-sought mechanism for violating this conservation.

Kharzeev has another use for the bubbles in the quark-gluon plasma. He says they could explain the magnetic fields of the Universe's galaxies, which are far too large for current theories to explain. As the tiny bubbles dissipate, they give rise to remnant magnetic fields; this mechanism, applied to the early Universe, could generate the large magnetic fields, he says. Even more provocatively, he suggests that these magnetic fields might be imprinted on the Universe in the same places where seeds of matter began to clump and eventually form galaxies. Charles Gale, a theorist at McGill University in Montreal, Canada, calls that an "enticing" idea.

Questions about the quark-gluon plasma will also soon be probed at the Large Hadron Collider (LHC) at CERN, Europe's particle-physics laboratory near Geneva, Switzerland. In addition to its experiments that smash protons in a search for new particles and new physics, the LHC will also collide lead nuclei. Running at 7 teraelectronvolts next year, the LHC will produce quark-gluon plasmas that reach temperatures two or three times higher that that achieved by RHIC.

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