Tuesday, February 16, 2010

Particle Collision in Universe

The newborn universe was a fast learner: At barely a millionth of a second old, it already had an inkling of the difference between right and left. The conclusion comes from physicists working with a colossal atom smasher that, for the briefest moment, reproduces the soup of free-flying elementary particles thought to have filled the early universe. Within each infinitesimal dollop of the soup, interactions between particles are not mirror symmetric so that the soup develops a kind of left-right imbalance, the nuclear physicists report.

For physicists, symmetry helps frame their understanding of fundamental particles and the forces between them. A particle interaction or decay can be symmetric in several ways. For example, a process such as an electron bumping into a positron might look the same running backward instead of forward in time. Likewise, things might be symmetric if right were swapped for left in a kind of mirror reflection, a symmetry known as parity.

Nature sometimes bucks the aesthetic, however. For example, certain radioactive decay processes violate parity. The effect shows up in the so-called "weak" decay of, say, a nucleus of cobalt-60 into a nucleus of nickel-60, which happens when a neutron in the nucleus turns into a proton. In the process, an electron comes out always spinning to the left, whereas parity symmetry would predict that it should emerge spinning to the left or right with equal probabilities. That insight, which garnered a Nobel Prize in 1957, revolutionized particle physics.

Parity might also be violated, after a fashion, in so-called strong interactions, the forces that hold protons and neutrons together in the nucleus in the first place. Or so suggest data from the 4-kilometer-long Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. RHIC smashes gold ions together with such violence that the protons and neutrons in them melt into their constituent parts--particles called quarks and gluons--to make a mind-bogglingly hot mess known as a quark-gluon plasma. Within the plasma, parity symmetry goes askew, researchers reported here yesterday at the April meeting of the American Physical Society.

The "symmetry breaking" is subtle. Imagine, as is most likely the case, that two gold ions collide not precisely head on, but slightly off center. The resulting drop of quark-gluon plasma spins a bit like Earth on its axis. That internal churning creates a strong magnetic field. Within the plasma, positively charged quarks and antiquarks appear to flock to one pole and negatively charged quarks and antiquarks flee to the other, Brookhaven's Dmitri Kharzeev told the meeting. That separation of charges violates parity, which would predict equal numbers of positive and negative particles rushing in both directions, he said.

The team cautions that the parity violation is qualitatively different than in the weak interaction. In a weak decay, an electron always emerges spinning the same way. In the quark-gluon plasma, the positive particles sometimes go to the north pole, sometimes to the south. So the parity violation only shows up "locally," when events are analyzed one at a time. Tally up all the events, though, and positive particles don't have a preference for one direction or the other, so overall the strong interactions still preserves parity, as researchers observed.

While the observations are intriguing, the team can't rule out other, less sexy explanations for the observed charge separation, said Berndt Mueller, a theorist at DukeUniversity in Durham, North Carolina. If more refined analyses do turn up conclusive evidence of parity violation, it would be like mining for silver and finding gold, Mueller said. The differences between the quark-gluon plasma and the world we live in would be "even more exciting and even more dramatic than what has been generally expected."

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