Particle discovery fills a gap

Neutral xi-sub-b was predicted by theory

Scientists have found a previously unseen particle using the Tevatron, the most powerful particle accelerator in the United States.

GONE WITH A TRACE Xi-sub-b, a particle spotted for the first time at the Tevatron, decayed too quickly to be observed directly, but physicists were able to detect the shower of particles the baryon left behind — a xi particle (Ξ-), a lambda particle (Λ), pions (π) and a proton (p).   CDF collaboration

Predicted by the standard model of particle physics, the new particle fills a blank space in the family tree of baryons, making it a distant cousin of the neutron and the proton. Baryons are made of three quarks, small, fundamental building blocks of matter that come in different flavors. The newly discovered xi-sub-b (Ξb0) contains a strange quark, an up quark and a bottom quark.

“There are no big surprises, but we have an obligation to look everywhere we can to confirm whether the particles we predict are there or not,” says Pat Lukens, a physicist with the Tevatron’s Collider Detector, or CDF, team. Lukens presented the result, submitted to Physical Review Letters, July 20 at the home of the Tevatron, the Fermi National Accelerator Laboratory in Batavia, Ill.

To find the rare particle, the CDF team sifted through data they collected from 2001 to 2008. The circular collider creates new particles by smashing together protons and antiprotons travelling at close to the speed of light — and colliding at energies of almost 2 trillion electron volts. That’s about twice the energy of a flying mosquito concentrated into a spot the size of a subatomic particle.

The CDF detector did not directly observe the neutral xi-sub-b, which breaks down almost immediately. Instead, the detector spotted debris left behind by this decay — a shower of six lighter, longer-lived particles created during a complicated four-stage decay process.

“We weren’t sure at all that we could see this particle’s decay,” says CDF spokesperson Giovanni Punzi of the University of Pisa and the National Institute of Nuclear Physics in Italy. “All the particles that this decays into are very common particles, particles that we have millions of, that are very hard to recognize from the background.”

Only an estimated 25 out of the almost 500 trillion collisions in the data matched this telltale decay signature. CDF’s sister detector, DZero, hasn’t collected any data that could confirm this result, Punzi says. But the chance of the CDF observations being a statistical fluke is less than four in a trillion.

 The Fermilab physicists calculate the mass of the particle to be about six times greater than that of a neutron. That measurement agrees with theoretical calculations reported in the Annals of Physics in 2009.

 Their estimate is “right on, bang on,” says Jonathan Rosner, a high energy physicist at University of Chicago who coauthored the 2009 paper.

“I wouldn’t have expected anything other than this, but it’s still reassuring,” says Rosner. “We’re always on the lookout for surprises.”

Seven other baryons containing a single bottom quark have been confirmed so far, including a sibling of the new particle with a down quark instead of an up quark, found by Fermilab scientists in 2007. A ninth, also predicted by theory, remains at large. Finding that last family member, Lukens says, is a task well suited to the Large Hadron Collider, the world’s most powerful particle accelerator.

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