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LHC Experiments Shed Light on Primordial Universe
Aug 2012
GENEVA, Aug. 14, 2012 — The primordial particles of matter that existed just after the big bang are now being re-created in the Large Hadron Collider (LHC) to advance understanding of the kind of matter that existed in the first seconds of the universe.

Seconds after the big bang, quarks and gluons — basic building blocks of matter — were not confined inside composite particles such as protons and neutrons as they are today. Instead, they moved freely in a state of matter known as quark-gluon plasma.

Now, experiments using heavy ions at CERN’s LHC have re-created, for a fleeting moment, conditions similar to those of the early universe. By examining more than 1 billion lead ion collisions, the ALICE (A Large Ion Collider Experiment), ATLAS (A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid) collaboration experiments have enabled precise measurements of the properties of matter under these extreme conditions. The findings are based on the four-week LHC run with lead ions in 2011.

“The field of heavy-ion physics is crucial for probing the properties of matter in the primordial universe, one of the key questions of fundamental physics that the LHC and its experiments are designed to address,” said Rolf Heuer, CERN’s director general. “It illustrates how, in addition to the investigation of the recently discovered Higgs-like boson, physicists at the LHC are studying many other important phenomena in both proton-proton and lead-lead collisions.”

Heavy-ion collision recorded by ALICE in 2011 (Image: CERN)

The ALICE, ATLAS and CMS collaborations will present more refined characterizations of the densest and hottest matter ever studied in the laboratory — 100,000 times hotter than the interior of the sun and denser than a neutron star — this week at the Quark Matter 2012 conference in Washington.

Physicists from ALICE will present new results on all aspects of the evolution of high-density, strongly interacting matter in both space and time; ALICE is one of the largest experiments in the world devoted to research in the physics of matter at an infinitely small scale.

One of its studies deals with “charmed particles,” which contain a charm or anticharm quark. Charm quarks, 100 times heavier than the up and down quarks that form normal matter, are significantly decelerated by their passage through quark-gluon plasma, offering scientists a unique tool to probe its properties. ALICE physicists will report indications that the flow in the plasma is so strong that the heavy charmed particles are dragged along by it. The experiment also has observed indications of a thermalization phenomenon, which involves the recombination of charm and anticharm quarks to form “charmonium.”

“This is only one leading example of the scientific opportunities in reach of the ALICE experiment,” said Paolo Giubellino, an ALICE spokesman. “With more data still being analyzed and further data taking scheduled for next February, we are closer than ever to unraveling the properties of the primordial state of the universe: the quark-gluon plasma.”

The initial dissociation of charmonium was proposed in the 1980s as a direct signature for the formation of quark-gluon plasma, and the first experimental indications of this dissociation were reported from fixed-target experiments at CERN’s Super Proton Synchrotron in 2000. LHC’s much higher energy makes it possible for the first time to study similar tightly bound states of the heavier beauty quarks. The hypothesis was that, depending on their binding energy, some of these states would “melt” in the plasma produced, while others would survive the extreme temperature. The CMS particle detector, designed to see particles and phenomena produced in high-energy collisions in the LHC, has observed clear signs of the expected sequential suppression of the “quarkonium” (quark-antiquark) states.

“CMS will present important new heavy-ion results not only on quarkonium suppression, but also on bulk properties of the medium and on a variety of studies of jet quenching,” said CMS spokesman Joseph Incandela. “We are entering an exciting new era of high-precision research on strongly interacting matter at the highest energies produced in the laboratory.”

The quenching of jets is the phenomenon in which highly energetic sprays of particles break up in the dense quark-gluon plasma, giving scientists detailed information about the density and properties of the produced matter. ATLAS, a particle physics experiment at the LHC, will report new findings on jet quenching, including a high-precision study of how the jets fragment in matter, and on the correlations between jets and electroweak bosons. The results are complementary to others, including groundbreaking findings on the flow of the plasma.

“We have entered a new phase in which we not only observe the phenomenon of quark-gluon plasma, but where we can also make high-precision measurements using a variety of probes,” said ATLAS spokesman Fabiola Gianotti. “The studies will contribute significantly to our understanding of the early universe.”

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