BATAVIA, Ill., June 19, 2007 -- A new heavy particle has been discovered: the Ξb (pronounced "zigh sub b") baryon, with a mass of 5.774±0.019 GeV/c2, approximately six times the proton mass.
The electrically charged Ξb baryon, also known as the "cascade b," is made of a down, a strange and a bottom quark. It is the first observed baryon formed of quarks from all three families of matter. Its discovery and the measurement of its mass provide new understanding of how the strong nuclear force acts upon the quarks, the basic building blocks of matter.
Physicists of the DZero experiment at the Department of Energy's Fermi National Accelerator Laboratory (Fermilab) reported the discovery of the cascade b baryon in a paper recently submitted to Physical Review Letters.
Six quarks -- up, down, strange, charm, bottom and top -- are the building blocks of matter. Protons and neutrons are made of up and down quarks, held together by the strong nuclear force. The DZero experiment has discovered the Cascade-b particle, which contains a down quark (d), strange quark (s) and bottom quark (b). It is the first particle ever observed with one quark from each generation of particles. (Image courtesy Fermilab)
"Knowing the mass of the cascade b baryon gives scientists information they need in order to develop accurate models of how individual quarks are bound together into larger particles such as protons and neutrons," said physicist Robin Staffin, associate director for high-energy physics for the Department of Energy's Office of Science.
Dmitri Denisov, a Dzero experiment co-spokesperson at Fermilab, said, "Cascade b baryon is a heavy cousin of proton and neutron, just made of different quarks. As baryon, it contains three quarks. We call them 'down," 'strange' and 'bottom.' What is fundamentally interesting is that all matter around us in the universe is made of just six quarks (three out of six are named above) in different combinations. We are trying to put different quarks 'together' and see what particles we can make -- similar to the periodic table of chemical elements, where all elements are made of protons and neutrons."
It is not easy to predict all properties of "new quarks combinations" -- what will be mass, lifetime and other properties, he said, which is why it is so exciting to see the production of such particles and their subsequent decay and to measure their properties.
"While theory did guide us as to what properties this particle is supposed to have, it is similar to Columbus, who saw birds flying and guessed there should be a land somewhere ahead -- but until you actually stand on the land, it is just expectations," Denisov said. "Our feelings were similar to people first discovering new lands. When we started to see the first signs of a new particle nobody saw before, it was with great pleasure and excitement of discovering something new."
No practical application of the discovery exists, but Denisov said, "We are doing fundamental science. And who knew when electricity was discovered back in the 19th century how it would have transformed our lives some 200 years later?"
The cascade b is produced in high-energy proton-antiproton collisions at Fermilab's Tevatron. A baryon is a particle of matter made of three fundamental building blocks called quarks. The most familiar baryons are the proton and neutron of the atomic nucleus, consisting of up and down quarks. Although protons and neutrons make up the majority of known matter today, baryons composed of heavier quarks, including the cascade b, were abundant soon after the big bang at the beginning of the universe.
The Cascade-b particle (Ξb) is short-lived. Once produced, it travels only several millimeters before the action of the weak nuclear force causes it to disintegrate into two well-known particles called J/Ψ and Ξ-. The J/Ψ then promptly decays into a pair of muons, common particles that are cousins of electrons. The Ξ- baryon, on the other hand, travels several centimeters before decaying into yet another unstable particle called a Λ baryon, along with another long-lived particle called a pion. The Λ baryon too can travel several centimeters before ultimately decaying to a proton and a pion. (Image courtesy Fermilab)
The Standard Model elegantly summarizes the basic building blocks of matter, which come in three distinct families of quarks and their sister particles, the leptons. The first family contains the up and down quarks. Heavier charm and strange quarks form the second family, while the top and bottom, the heaviest quarks, make the third. The strong force binds the quarks together into larger particles, including the cascade b baryon. The cascade b fills a missing slot in the standard model.
Prior to this discovery, only indirect evidence for the cascade b had been reported by experiments at the Large Electron-Positron Collider at the CERN Laboratory near Geneva, Switzerland, Fermilab said.
"For the first time, the DZero experiment has positively identified the cascade b baryon from its decay daughter particles in a remarkably complex feat of detection," the lab said in a statement. "Most of the particles produced in high-energy collisions are short-lived and decay almost instantaneously into lighter stable particles. Particle detectors such as DZero measure these stable decay products to discover the new particles produced in the collision."
Once produced, the cascade b travels several millimeters at nearly the speed of light before the action of the weak nuclear force causes it to disintegrate into two well-known particles called J/Ψ ("jay-sigh") and Ξ- ("zigh minus"). The J/Ψ then promptly decays into a pair of muons, common particles that are cousins of electrons. The Ξ- baryon, on the other hand, travels several centimeters before decaying into yet another unstable particle called a lambda (Λ) baryon, along with another long-lived particle called a pion. The Λ baryon too can travel several centimeters before ultimately decaying to a proton and a pion. Sifting through data from trillions of collisions produced over the last five years to identify these final decay products, DZero physicists have detected 19 cascade b candidate events. The odds of the observed signal being due to something other than the cascade b are estimated to be one in 30 million.
DZero is an international experiment of about 610 physicists from 88 institutions in 19 countries. It is supported by the Department of Energy, the National Science Foundation and a number of international funding agencies. Fermilab is a national laboratory funded by the Office of Science of the US Department of Energy, operated under contract by Fermi Research Alliance LLC.
For more information, visit: www.fnal.gov
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