Subatomic Particle

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New subatomic particle found

Mysterious quark blend hints at what holds atoms together.
3 May 2003


BaBar records the exotic particles that result from smashing electrons and positrons together.

Researchers have stumbled across a new subatomic particle. The mysterious body is causing theorists to rethink their ideas about the strong force, which binds subatomic particles together into atoms.

Dubbed 'Ds (2317)', the new-found particle is probably an unusual configuration of quarks - the entities that, in trios, form protons and neutrons. It could be one quark orbiting another, or perhaps a sort of molecule of four quarks.

Marcello Giorgi of the University of Pisa, Italy, and his group happened upon Ds (2317) while combing through three years' worth of data gathered by the BaBar detector at the Stanford Linear Accelerator Center (SLAC) in California1. When SLAC smashes together electrons and their antimatter counterparts, positrons, BaBar records the wealth of exotic particles that are created as a result. "We were surprised, but even more surprised by the mass," Giorgi says.

The particle's mass is lower than predicted, and is very clear. The mass of most high-energy particles is made fuzzy by uncertainties at the smallest of scales. Ds (2317) has a very definite weight of 2,317 mega-electron volts - the units that physicists use to describe both the mass and energy of particles.

This well-defined mass may help researchers to understand what holds atoms together, says particle-physics theorist Estia Eichten of the Fermi National Accelerator Laboratory in Batavia, Illinois. Because mass and energy are equivalent at small scales, knowing the mass of the new quark mix hints at the strong force's energy inside the particle.

Even more intriguing, Ds (2317) is thought to be made up of exotic heavy quarks. "Most of what we know about the strong force comes from studies of lighter quarks," explains David Cinabro, a high-energy physicist at Cornell University in Ithaca, New York. "Maybe there's something different about heavy ones."

So how unusual is it to find a new particle? Not very, apparently - there are catalogues full of similar quark combinations, says Cinabro.

What is it?

Quarks come in six different types: up, down, charm, strange, top and bottom. Triplets of ups and downs, the lightest and most common quarks, make up the protons and neutrons of everyday matter.

By contrast, Ds (2317) may be two quarks linked together into a rare particle called a meson. This meson may look a little like an atom, with a lighter anti-strange quark orbiting a heavier charm quark, Eichten speculates.

Others have more unusual interpretations. Jonathan Rosner, a theoretical physicist at the University of Chicago, suggests that the new-found particle could consist of couplets of paired quarks. The existence of such subatomic molecules has long been predicted.

"We don't have any evidence that these configurations exist," says Cinabro. "If it's the real thing, then that would be really weird."

Researchers at SLAC, at Cornell University's High Energy Synchrotron Source, and at the High Energy Accelerator Research Organization in Japan, are now rifling through more old data in search of more Ds (2317) particles to test their ideas.

Geoff Brumfiel is the Washington Physical Sciences Correspondent of the journal Nature

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  1. The BaBar collaborationObservation of a Narrow Meson Decaying to Ds+pi0 at a Mass of 2.32 GeV/c^2. Preprint, (2003). |Article|

© Nature News Service / Macmillan Magazines Ltd 2003



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