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February 10, 2000

Particle Physicists Getting Closer to the Bang That Started It All

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    Scientists in Geneva have re-created a primordial form of matter that physicists believe last existed in abundance when the universe was an exploding fireball only a fraction of a second old.

    The new material is a highly compressed gas of the particles called quarks and gluons, the building blocks of ordinary particles like the protons and neutrons within all the atoms in the universe today. In scientific importance, the long-sought achievement might be compared to the first splitting of the atom to reveal its individual parts.

    The achievement will be announced today at CERN, the European particle physics laboratory where the work was carried out. The finding moves experimental physics closer than it has ever been to the presumed moment at which the universe came into being and could help cosmologists better understand the driving forces behind the primordial explosion itself. The matter's existence confirms one of the most abstruse of all predictions by theoretical particle physicists.

    Quarks, and the gluons that powerfully bind them, are normally joined to form protons and neutrons and cannot be shaken loose individually no matter how hard pairs of the ordinary particles are smashed together. To create the new material, the scientists have, in effect, compressed and heated a ball of protons and neutrons so that they melted into their constituent quarks and gluons, which then floated freely in a laboratory for the first time.

    The compression was achieved by smashing together entire lead nuclei containing hundreds of protons and neutrons each, rather than mere pairs of them.

    "It does indicate that a new state of matter is created," said Dr. Johanna Stachel, a physicist at the University of Heidelberg in Germany who is the spokeswoman for one of the multinational collaborations that operates a large particle detector, called NA45, at the Geneva laboratory. "This new state we think the universe was in until about 10 microseconds after the Big Bang, and then crystallized into the particles as we know them now."

    The Big Bang is the colossal explosion in which most cosmologists believe the universe was born, some 15 billion years ago. A microsecond is a millionth of a second.

    The laboratory's experiment, said Dr. Michael Turner, a cosmologist at the University of Chicago, "helps take us back to when the universe was a soup of the most fundamental particles we know."

    Because of the tight connection to cosmology, said Dr. Edward Shuryak, a physicist at the State University of New York in Stony Brook, the Geneva laboratory's achievement is being called the "Little Bang."

    Known more technically as "quark-gluon matter," the material is also a boon for theoretical physicists, since their theory of strong particle interactions, called quantum chromodynamics, had predicted that the bizarre state should exist.

    Physicists know of six different types of quarks, which go by the somewhat whimsical designations up, down, charm, strange, top and bottom. Pairs and threesomes of quarks bind together to make up ordinary particles of matter. Protons, for example, consist primarily of two up quarks and one down quark, while the less common particles called kaons consist of a strange quark and either an up or a down.

    Oddly, the strength with which gluons bind quarks turns out to be weak when the quarks are close together and grows powerful when they are distant from each other, as if they were connected by elastic.

    But Dr. Shuryak and others came to the conclusion that quarks could roam free if enough protons and neutrons could be heated to a temperature about 100,000 times higher than the center of the sun and compressed to a density roughly 10 times that of an ordinary atomic nucleus. Small clouds of quarks effectively screen one another from the gluon force of more distant quarks, cutting the elastic connections.

    The equations of quantum chromodynamics are so complex that the theoretical properties of quark-gluon matter can be explored only on the world's largest computers.

    To test whether this state of matter can exist in reality, the scientists in Geneva used their Super Proton Synchrotron to accelerate lead nuclei to an energy of 33 trillion electron volts. Traveling at nearly the speed of light, those nuclei were smashed into a lead foil, producing hot, dense matter in the collisions.

    After a fleeting existence, the quark-gluon matter should then cool and condense into ordinary matter and explode in a hail of thousands of ordinary particles. Seven different particle detectors examined the residue of millions of lead collisions for evidence that the quark-gluon matter had been created.

    "It is sort of a criminal court procedure, where you have proof by circumstantial evidence," said Dr. Reinhard Stock, a physicist at the University of Frankfurt who is the spokesman for a collaboration centered on a detector called NA49.

    The fingerprints of the deed are clear, said Dr. Stock. They include detection of many more particles that contain strange quarks than an ordinary smash-up would produce, and fewer particles containing charm quarks -- the indications seen by the Geneva lab's detectors.

    "We have opened the door," said Dr. Claude Détraz, the laboratory's director of research, calling the new results "compelling evidence that we have created a new state of matter in which quarks are 'deconfined.' "

    Cosmologists believe that much of the character of the universe, and perhaps the fury of the Big Bang explosion itself, was determined by a series of so-called phase transitions like the coalescence of ordinary matter from the quark-gluon plasma.

    "This is really a concrete illustration of how cosmologists can benefit from accelerators, which can recreate the conditions that existed during the earliest moments of the universe," said Dr. Turner of the University of Chicago.

    Though quark-gluon matter is rare, physicists theorize that small amounts of it may be generated when energetic particles from space called cosmic rays crash into planetary bodies like Earth.

    The announcement sets the stage for much more powerful experiments, expected to begin this spring, using the Relativistic Heavy Ion Collider at the federal Brookhaven National Laboratory in Upton, N.Y.

    Those experiments will initially collide gold nuclei with 10 times higher energy than CERN has mustered, said Dr. Thomas Ludlam, a physicist who is an administrator of the program at Brookhaven. The experiments should produce an even more exotic entity called a quark-gluon plasma and allow for much more intensive study of the substances.

    "It's as if you're trying to discover steam and you can make a few little puffs of steam," he added. "But with a better pressure cooker you have enough time to stick a thermometer in and discover the thermodynamic properties of steam."

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