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July 22, 2003

Pentaquarks: a new form of matter

ANN ARBOR, Mich.—Evidence for the pentaquark has very recently been reported by experiments in Japan, Russia and the United States. The new particle's properties like mass, decay time and decay mode fit to theoretical predictions of a five-quark system, a pentaquark. Such a particle would constitute a new form of matter.

"It is important that the signature for this new particle has been found at various laboratories around the world, but in many different reactions. This enhances the credibility of the existence of this new particle," said Wolfgang Lorenzon, who participated in a University of Michigan team that confirmed the existence of the new particle in the debris of high-energy collisions, together with a team led by Moskov Amarian from the German Zeuthen research group.

Gamma rays hitting the nuclei of heavy hydrogen atoms may create pentaquarks and other subatomic particles. A pentaquark is composed of 5 quarks. This new particle constitutes a new form of matter.

The U-M physics team, consisting of research fellow Avetik Airapetian and Lorenzon, a physics professor, is part of HERMES, a multinational experimental collaboration working at the HERA particle accelerator at the DESY laboratory in Hamburg, Germany.

By sifting through many millions of interactions of high-energy positrons with deuterium nuclei, the scientists isolated 30 events that cannot be explained in terms of the standard known particles. The combination of particles detected in these events indicates the decay of a new particle with a mass of 1.6 times that of a proton.

The observed properties of this particle are remarkably consistent with those predicted in 1997 by a group of Russian scientists who said it would have five quarks bound together.

Up to now only combinations of two or three quarks have been observed in nature. On the other hand, physicists have been searching for many years for more exotic combinations, which are also permitted by the basic theory of quark interactions known as Quantum Chromo Dynamics or QCD. With the new data from HERMES, which have been measured at much higher beam energy, the existence of this new form of matter now seems well established.

For more information on Lorenzon and HERMES, visit http://www.physics.lsa.umich.edu/department/directory/bio.asp?ID=207
or
http://ipumich.temppublish.com/public/experts/ExpDisplay2.cfm?expertloc=910

Contact: Carol Rabuck
Phone: (734) 763-2588
E-mail: crabuck@umich.edu