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Misbehavin' Meson: Perplexing particle flouts the rules

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3:11pm, June 23, 2004
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The discovery of what appears to be a new subatomic particle with bizarre properties is challenging theorists' understanding of how matter behaves on the scale of protons and neutrons.

What seems especially weird about the particle, dubbed DsJ(2632), is its unexpected breakdown pattern. It produces a particle called an eta meson six times as often as the K meson that theory says should be more prevalent, explains James S. Russ of Carnegie Mellon University in Pittsburgh, leader of the experimental team.

"It's like watching a water bucket with a large hole and a small hole in the bottom. For some reason, the water is pouring out the small hole six times faster than it's coming out of the large one," Russ says.

Moreover, DsJ(2632) survives three times as long as similar but lighter mesons do. Typically, heavier mesons have shorter lives than less-massive ones. Theorist Eric S. Swanson of the University of Pittsburgh says that if the finding holds up, "it's a conundrum."

Like other mesons, DsJ(2632) is thought to contain one quark—a strange quark—and one antiquark—a charm antiquark.

The particle appears to be a newfound member of the family known as Ds mesons that last year was already causing theorists to wonder whether something was amiss with their models. Physicists had found signs of two new Ds mesons that were not as massive as theory said they should be (SN: 5/24/03, p. 333: Available to subscribers at New particles pose puzzle).

Those findings inspired Russ and his team to dig back into data that they had collected at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., during 1996 and 1997. At that time, the experimenters had been bombarding thin films of copper or diamond with a beam of particles known as sigmas to create particles containing charm quarks (SN: 7/6/02, p. 14: Available to subscribers at Twice-charmed particles spotted?). While reviewing their data, Russ' group came upon the baffling DsJ(2632).

The team announced its latest findings at a seminar at Fermilab on June 18.

Theorists propose a number of explanations for the new particle's characteristics. For one, the results may be wrong. "It's moderately strong evidence, but it needs to be confirmed," comments William A. Bardeen, a Fermilab theorist who is not part of Russ' collaboration.

If other laboratories verify the particle's properties, then theorists will need to stretch their thinking. They're confident of the basic correctness of the current theory known as quantum chromodynamics, or QCD. What may be in question, however, is the enterprise of extrapolating from the infinitesimal, quark-scale realm of QCD to the experimentally measurable traits and behaviors of much larger particles, such as protons, neutrons, and mesons.

One possibility, Bardeen notes, is that DsJ(2632) contains four constituents—quarks or antiquarks—instead of two. Until a year ago, there were no particles reported to have more than three such constituents, but then researchers reported several particles with up to five quarks (SN: 4/24/04, p. 270: Available to subscribers at Signs of new five-quark particle).

Such quark-rich entities might be tight combinations of particles with fewer quarks rather than a single, quark-laden particle, Swanson says. He adds that he has yet to identify any specific combinations that could account for DsJ(2632).

"This guy is really unusual," Swanson says. "I mean, I can't find even an unusual explanation for it."

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