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June 2006 (Volume 15, Number 6)
The proton is not as strange as some people might have thought, according to results reported at the APS April meeting. Members of the HAPPEx collaboration at Jefferson Lab described their most recent findings on the contribution of the strange quark to some of the proton’s properties.
Protons are composed of two up quarks and a down quark, as well as a sea of virtual quark-antiquark pairs that flit into and out of existence. It has been an open question how much these sea quarks contribute to properties of the proton, such as its charge distribution and magnetic moment, said HAPPEx collaborator Krishna Kumar of the University of Massachusetts.
A number of experiments have put limits on the strange quark contribution to the nucleon’s properties. At the April meeting the HAPPEx collaboration reported that the strange quark contributes at most 4% of the proton’s magnetic moment, and at most 1% of its charge distribution. Both of these measurements are consistent with zero. The researchers also found that the strange quark-antiquark pairs in the nucleon are on average separated by less than about 2x10-17 meters.
The HAPPEx experiment studies scattering of a polarized beam of 3 GeV electrons from liquid hydrogen and from helium, and measures the elastically scattered electrons. The beam’s polarization is alternated throughout the experiment. Because the electromagnetic force is mirror symmetric while the weak force is not, the scientist can separate the effects of these two forces by noting differences in the number of scattered electrons when the beam’s polarization changes. They then deduce the contribution of the sea quarks to the nucleon’s properties.
“The proton is much less strange today than we thought it was two weeks ago,” said HAPPEx member Paul Souder of Syracuse University. Tony Thomas, JLab’s chief scientist, called the new measurement “the best test of what the sea of the nucleon looks like.”
Some previous theories and experiments had hinted that the strange quark could contribute as much as ten percent to the proton’s magnetic moment.
Meanwhile, other April Meeting speakers reported on some recently discovered surprising properties of the quark- gluon matter produced at the Relativistic Heavy Ion Collider. Barbara Jacak of SUNY Stony Brook, a member of the PHENIX collaboration at RHIC, described some of these properties during a plenary talk and press conference.
Previous investigation had focused on whether quarks and gluons, normally bound into hadrons, can become free and melt into a so-called quark-gluon plasma, Jacak said. She focused instead on whether the quark- gluon matter produced at RHIC really is a plasma.
The RHIC experiments collide gold ions together at very high energies to recreate a state of matter thought to have existed microseconds after the Big Bang. Four detectors analyze the complicated mess of particles that spew out of the collisions.
At last year’s April Meeting, all four RHIC detector groups announced that the soup of quarks and gluons they had produced in these collisions behaved like a nearly perfect fluid of strongly interacting quarks, rather than a gas of weakly interacting quarks.
Now, Jacak and colleagues have investigated how the quark- gluon soup affects heavy quarks. Some charm quarks are produced in the gold-gold collisions, but the researchers observed that jets of charm quarks coming out of the soup were suppressed.
It seems that the charm quarks get caught up in interactions in the mixture of mostly lighter quarks and gluons. In the RHIC matter, heavy quarks are flowing along with the lighter ones, Jacak said. This is surprising because the charm quarks should be too heavy to be affected by the quark-gluon matter. She likened the phenomenon to a river picking up rocks and carrying them along. Jacak believes this is evidence that the matter being produced at RHIC really is a plasma of unbound quarks and gluons, because quarks bound into hadrons would not be likely to affect charm quarks in this way.
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