RHIC finds hints to why we exist

APS Meeting: Physics Today : Physicists at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) have discovered some additional experimental hints of why there is matter in the universe by replicating the conditions of the first microseconds after the Big Bang .The results from RHIC’s Pioneering High Energy Nuclear Interaction eXperiment (PHENIX) and STAR detector were discussed at the April meeting of the American Physical Society yesterday and published in Physical Review Letters.When the Big Bang occurred, according to the symmetry rules that govern the universe, equal parts of matter and antimatter should have been created leading to all matter being annihilated. But this was not the case: Why? A hot startTo find out, you have to replicate conditions of the early universe. RHIC can duplicate the conditions of the first microseconds by colliding gold atoms together near the speed of light. The resulting collision is hot enough to melt protons and neutrons into a quark gluon plasma, according to the PHENIX’s researchers.Predictions made prior to RHIC’s initial operations in 2000 expected that the quark-gluon plasma would exist as a gas. But RHIC’s first three years of operation showed that the matter produced at RHIC behaves as a liquid, whose constituent particles interact very strongly among themselves. This liquid matter has been described as nearly “perfect” in the sense that it flows with almost no frictional resistance, or viscosity. Such a “perfect” liquid doesn’t fit with the picture of “free” quarks and gluons physicists had previously used to describe the quark-gluon plasma.In papers published in 2005, RHIC physicists laid out a plan of crucial measurements to clarify the nature and constituents of this “perfect” liquid. Measuring the temperature early in the collisions was one of those goals. Models of the evolution of the matter produced in RHIC collisions had suggested that the initial temperature might be high enough to melt protons, but a more direct measurement of the temperature required detecting photons—particles of light—emitted near the beginning of the collision, which travel outward undisturbed by their surroundings."This was an extraordinarily challenging measurement,” explained PHENIX spokesperson Barbara Jacak . “There are many ways that photons can be produced in these violent collisions. We were able to ‘eliminate’ the contribution from these other sources by exploiting RHIC’s flexibility to measure them directly and to make the same measurement in collisions of protons, rather than of gold nuclei. Thus we could pin down excess production in the gold-gold collisions, and determine the temperature of the matter that radiated the excess photons. By matching theoretical models of the expanding plasma to the data, we can determine that the initial temperature of the ‘perfect’ liquid has reached about four trillion degrees Celsius.” At its hottest stage, the infusion may reach 7 trillion °C."This is the hottest matter ever created in the laboratory” and qualifies as the “highest temperature known in our present universe,” said Steven Vigdor, Brookhaven’s associate laboratory director for nuclear and particle physics , who oversees research at RHIC. Breaking the universe’s rulesCosmologists have predicted that the solution to this dichotomy of why equal amounts of matter and antimatter were not created would be bubbles or local regions in which there would be a breakdown in the existing rules governing the behavior of particles in the universe.The symmetry “rule” suggests that events should occur in exactly the same way whether seen directly or in a mirror, with no directional dependence. But STAR has observed regions in the quark-gluon plasma at the heart of the RHIC collisions in which asymmetric charge separations occurs.STAR observed that positively charged quarks may prefer to emerge parallel to the magnetic field in a given collision event, while negatively charged quarks prefer to emerge in the opposite direction. Because this preference would appear reversed if the situation were reflected through a mirror, it appears to violate mirror symmetry."In all previous studies of systems governed by the strong force among quarks and gluons, it has been found to very high precision that events and their mirror reflections occur at exactly the same rate, with no directional dependence,” said Vigdor. “So this observation at STAR is truly intriguing."STAR data also suggest the local breaking of another form of symmetry, known as charge conjugation–parity or CP invariance. According to this fundamental physics principle, when energy is converted to mass or vice versa according to Einstein’s famous E= mc ² equation, equal numbers of particles and oppositely charged antiparticles must be created or annihilated. If CP symmetry had not been broken at some very early time in the evolution of our universe, the particles and antiparticles created in equal numbers in the Big Bang would subsequently have annihilated one another in pairs, leaving no matter to form the galaxies, stars, planets.While some small violations of CP symmetry have been found in previous laboratory experiments, those violations are far too weak to account for the amount of matter remaining in the universe today."These new results thus suggest that RHIC may have a unique opportunity to test in the laboratory some crucial features of symmetry-altering bubbles,” said Vigdor.The signs of possible local CP violation at STAR cannot completely explain the global predominance of matter in today’s universe, but they may offer some insight into how such symmetry violations occur. CERN’s Large Hadron Collider, which restarts this week, will eventually produce collisions 3 times more powerful than those at RHIC to see if this quark-gluon plasma actually does transition into a gas.RHIC will be upgraded over the next few years to investigate these broken symmetry effects more closely but there could also be less sexy explanations for the observed charge separation, said Berndt Mueller, a theorist at Duke University in Durham, North Carolina, to ScienceNOW‘s Lauren Schenkman . If more refined analyses do turn up conclusive evidence of parity violation, it would be like mining for silver and finding gold, said Mueller.Paul Guinnessy Related coverage Atom smasher shows vacuum of space in a twist New Scientist The hottest science experiment on the planet Discover magazine Scientists re-create high temperatures from Big Bang USA Today Hottest temperature ever heads science to Big Bang Reuters In Brookhaven collider, scientists briefly break a law of nature New York Times Particle collision puts twist in early universe ScienceNOW