New Quarktet: Subatomic oddity hints at pentaparticle family

Physicists at a European particle accelerator say they’ve spotted a never-before-seen elementary particle composed of five of the fundamental constituents known as quarks and antiquarks. In contrast, protons and neutrons contain three quarks, and no particle is known to have four quarks. The new report marks only the second sighting ever of a five-quark particle, the first one having been found last summer by three independent groups working in the United States, Japan, and Russia (SN: 7/5/03, p. 3: Wild Bunch: First five-quark particle turns up).

The detection of this second so-called pentaquark bolsters the theoretical hunch that a family of five-quark particles exists, says Gunther M. Roland of the Massachusetts Institute of Technology (MIT), a member of the team that spotted the newest particle. Physicists expect others in the new particle family, like these initial two members, to consist of four quarks and one antiquark.

“This is really the beginning of a new era,” Roland says. “I think this will lead to a big program to find further [pentaquarks] and to understand their properties in detail.”

“It’s all very exciting,” says theorist Harry J. Lipkin of the Weizmann Institute in Rehovot, Israel, who is not associated with the team making the claim.

Evidence for about 40 of the new particles turned up in an analysis of millions of proton-proton collisions from 2000 and 2001 at the Super Proton Synchrotron accelerator at the European Organization for Nuclear Research (CERN) near Geneva. The experiment has yet to be replicated.

On Oct. 8, the CERN team posted a report about the new pentaquark on ArXiv, an Internet site where many physics results appear initially (http://www.arXiv.org/abs/hep-ex/0310014).

The discovery of a new family of quark-containing particles may help physicists fill in blanks in their understanding of quark interactions, says theorist Frank Wilczek of MIT. For one thing, it could end what had been a puzzling absence of evidence for particles with groupings containing more than three quarks or antiquarks, which theorists for decades have been expecting to show up in accelerators.

Larger groupings of quarks and antiquarks may have existed in the early universe and may persist today in extremely dense stars, says theorist Marek Karliner of Tel Aviv University. Like recently discovered extrasolar planets that reveal new planetary configurations to astronomers (SN: 3/15/03, p. 164: Planet’s Slim-Fast Plan: Extrasolar orb is too close for comfort), pentaquarks are alerting physicists to possible new subatomic arrangements, he adds.

Some theorists, including Karliner, Lipkin, and Wilczek, propose that pentaquarks may involve two-quark subgroups known as diquarks. These are quark-quark or antiquark-antiquark pairs that have seemed to play only bit roles in quark interactions, Wilczek says. Others theorize that the new findings rule out such arrangements.

“Do not believe any theoretical model at this stage,” says Lipkin.

What’s most important about the CERN evidence for now, says Karliner, is that it puts to rest gnawing doubts about the initial pentaquark sighting last summer.

“There were slight discrepancies between the experiments [that identified the first pentaquark], and people were worried that something was wrong,” Karliner recalls. “Having discovered another member of this exotic family is very reassuring.”

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