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Friday, June 15, 2007

Fermilab Physicists Discover "triple-scoop" Baryon

Physicists of the DZero experiment at the Department of Energy's Fermi National Accelerator Laboratory have discovered a new heavy particle, the Ξb (pronounced "zigh sub b") baryon, with a mass of 5.774±0.019 GeV/c2, approximately six times the proton mass. The newly discovered 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.

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.


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, in which six Louisiana Tech researchers participated, 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. (Graphic and information supplied by 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.

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.

Direct observation of the strange b baryon Xi_b^{-}. 2007. Authors: D0 Collaboration, V. Abazov, et al., Physical Review Letters, preprint.