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Pentaquark

Physics

Quark Matters

Rensselaer researchers are part of an international team of physicists that has provided the best evidence to date of the existence of a new form of atomic matter, dubbed the “pentaquark.”

The research team confirmed the existence of pentaquarks by using a different approach that greatly increased the rate of detection compared to previous experiments. The results were published in the Jan. 23 issue of the journal Physical Review Letters.

“The latest, and most conclusive evidence of this five-quark particle — the ‘pentaquark’ — could bring immense insight in understanding the laws and structure of universal matter in its most fundamental form,” said lead author and Rensselaer research scientist Valery Kubarovsky. The research was carried out at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Lab).

An atomic nucleus is composed of protons and neutrons. In the last four decades, physicists have discovered that these subatomic particles are composed of even smaller particles, called quarks. Each proton and neutron is composed of three quarks, for example.

For years, scientists have predicted that five-quark particles also could exist under unusual conditions. Yet, no proof surfaced until late 2002 when a Japanese team announced its discovery of the pentaquark in particle-smashing experiments. When the researchers zapped carbon atoms with high-energy gamma rays, they observed that, after gamma ray photons “crashed” into the neutrons, a few neutrons “grew” into five-quark particles.

Still, the results of subsequent experiments by researchers globally have been mixed until now.

“Detection is difficult because we are unable to ‘see’ the pentaquark itself, which lives less than one hundredth of a billionth of a billionth of a second, before decaying into two separate particles,” said Paul Stoler, Rensselaer physics professor and chair of the Jefferson Lab Users Board of Directors. “But even the two-particle, tell-tale sign is difficult to detect because of the many irrelevant reactions, or ‘debris,’ that also occur in the same experiments.”

To limit the debris, the Jefferson Lab team searched for a simpler mode of production. Since they could not isolate a single neutron — stable neutrons cannot exist freely — they turned to the single proton as a target.

The Jefferson Lab team liquefied hydrogen, an element composed of a single proton, at a temperature that reached a few degrees above absolute zero before zapping the element with gamma rays.

“Shifting our focus from neutrons to protons dramatically altered our results,” Kubarovsky says. “We strongly increased the previous success rates for detecting pentaquarks.”

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Rensselaer Magazine: Spring 2004
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