The proton’s strange new cousin

Its existence further validates the standard model of particle physics

September 4, 2008 at 1:50 pm

A new heavy cousin of the proton was found hiding in a pile of data at the Fermi National Accelerator Laboratory in Batavia, Ill.

BIG FOR SMALL While a mighty new particle accelerator is starting up in Europe, Fermilab’s Tevatron, outside Chicago, still has a few cards up its sleeve. Physicists working at the DZero detector (hosted in the facility on the top right, along the accelerator’s 6.3-kilometer ring in the background) announced the discovery of a new particle called the omega-b-minus. Fermilab

By detecting sets of five particles — two muons, a kaon, a pion and a proton — physicists have deduced the existence of a new particle called omega-b-minus. This illustration shows how the particle decays into the telltale debris. DZero collaboration

The new particle, long predicted to exist, is made of a bottom quark — the second-heaviest of all quarks — and two, much lighter strange quarks essentially orbiting around it, says Fermilab physicist Dmitri Denisov. The laboratory announced the discovery on September 3 and submitted a paper for publication to Physical Review Letters.

The particle, known as omega-b-minus (Ωb^–), is one of many possible combinations of quarks predicted by the standard model of particle physics, the accepted foundation of the subject. The 1964 discovery of a particle made of three strange quarks was the landmark that established the mathematical basis for what would later be the theory of quarks, says Michael Peskin, a theoretical physicist at the Stanford Linear Accelerator Center in Menlo Park, Calif. “This much later discovery is just another feather in the cap of this excellent theory,” he says.

Researchers working at Fermilab’s DZero detector, which smashes together protons and antiprotons circling almost at light speed inside the Tevatron accelerator, took about one year to sift through data gathered between 2002 and 2006.

The data described the debris coming from the proton-antiproton collisions, in which the particles’ huge energy — equivalent by E=mc² to roughly 1,000 times their normal mass — turns in part into matter, creating hundreds of new quarks and other particles.

The physicists were looking for telltale signs of the rare collision events that led some of the quarks to combine into the new particle. “We needed 100 trillion events to select events which can be interpreted as an omega-b-minus,” and only a handful fit the bill, says Denisov, DZero’s co-spokesperson. In each case, the researchers did not observe the new particle itself, because it decayed almost immediately. Instead, the DZero detector picked up a signature combination of five particles leftover by omega-b-minus decay. The experiment also produced a roughly equal number of antimatter versions of the new particle, as predicted by theory, Denisov says, for a total of 18 particles.

The physicists were able to calculate the new particle’s mass, which at 6.2 billion electron volts is about six times that of a proton — very close to theorists’ predictions.

A similar effort last year led physicists at DZero and also at CDF, the Tevatron’s other detector, to discover a particle called the cascade baryon (SN: 7/7/07, p. 13). The omega-b-minus is the 13th particle to be discovered out of 20 predicted proton “cousins” — particles made of three quarks, as is the proton, and having magnetic properties similar to a proton’s.

Fermilab physicists are now searching for signs of one more such particle. Some of the remaining ones probably lie outside the Tevatron’s reach, Denisov says, but could be found at the Large Hadron Collider, the new, more powerful particle accelerator that’s starting up this fall deep below the Swiss-French border (SN 7/19/08, p. 16). The LHC is also expected to find new elementary particles (rather than combinations of other particles), some of which, if found to exist, may require extending or rethinking the standard model.