image: The decay pattern of the Ξb particle.view more
Credit: Fermilab
Batavia, Ill. - Physicists of the DZero experiment at the Department of Energy's Fermi National Accelerator Laboratory have discovered a new heavy particle, the Îb (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 Îb 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 DZero experiment has reported the discovery of the cascade b baryon in a paper submitted to Physical Review Letters on June 12.
"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.
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 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. 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. 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/Ø ("jay-sigh") and Î- ("zigh minus"). The J/Ø then promptly decays into a pair of muons, common particles that are cousins of electrons. The Î-baryon, on the other hand, travels several centimeters before decaying into yet another unstable particle called a &Lambda ("lambda") baryon, along with another long-lived particle called a pion. The &Lambda 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.
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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 U.S. Department of Energy, operated under contract by Fermi Research Alliance, LLC.
Notes for editors:
The DZero paper on the cascade b discovery is available in the hep/ex preprint location at http://arxiv.org/abs/0706.1690.
Fachbereich Physik, University of Wuppertal, Wuppertal, Germany
Panjab University, Chandigarh, India
Delhi University, Delhi, India
Tata Institute of Fundamental Research, Mumbai, India
University College Dublin, Dublin, Ireland
Korea Detector Laboratory, Korea University, Seoul, Korea
SungKyunKwan University, Suwon, Korea
CINVESTAV, Mexico City, Mexico
FOM-Institute NIKHEF and University of Amsterdam/NIKHEF, Amsterdam, The Netherlands
Radboud University Nijmegen/NIKHEF, Nijmegen, The Netherlands
Joint Institute for Nuclear Research, Dubna, Russia
Institute for Theoretical and Experimental Physics, Moscow, Russia
Moscow State University, Moscow, Russia
Institute for High Energy Physics, Protvino, Russia
Petersburg Nuclear Physics Institute, St. Petersburg, Russia
Lund University, Royal Institute of Technology, Stockholm University, and Uppsala University, Sweden
Lancaster University, Lancaster, United Kingdom
Imperial College, London, United Kingdom
University of Manchester, Manchester, United Kingdom
University of Arizona, Tucson, Arizona, USA
Lawrence Berkeley National Laboratory and University of California, Berkeley, California, USA
California State University, Fresno, California, USA
University of California, Riverside, California, USA
Florida State University, Tallahassee, Florida, USA
Fermi National Accelerator Laboratory, Batavia, Illinois, USA
University of Illinois at Chicago, Chicago, Illinois, USA
Northern Illinois University, DeKalb, Illinois, USA
Northwestern University, Evanston, Illinois, USA
Indiana University, Bloomington, Indiana, USA
University of Notre Dame, Notre Dame, Indiana, USA
Purdue University Calumet, Hammond, Indiana, USA
Iowa State University, Ames, Iowa, USA
University of Kansas, Lawrence, Kansas, USA
Kansas State University, Manhattan, Kansas, USA
Louisiana Tech University, Ruston, Louisiana, USA
University of Maryland, College Park, Maryland, USA
Boston University, Boston, Massachusetts, USA
Northeastern University, Boston, Massachusetts, USA
University of Michigan, Ann Arbor, Michigan, USA
Michigan State University, East Lansing, Michigan, USA
University of Mississippi, University, Mississippi, USA
University of Nebraska, Lincoln, Nebraska, USA
Princeton University, Princeton, New Jersey, USA
State University of New York, Buffalo, New York, USA
Columbia University, New York, New York, USA
University of Rochester, Rochester, New York, USA
State University of New York, Stony Brook, New York, USA
Brookhaven National Laboratory, Upton, New York, USA
Langston University, Langston, Oklahoma, USA
University of Oklahoma, Norman, Oklahoma, USA
Oklahoma State University, Stillwater, Oklahoma, USA
Brown University, Providence, Rhode Island, USA
University of Texas, Arlington, Texas, USA
Southern Methodist University, Dallas, Texas, USA
Rice University, Houston, Texas, USA
University of Virginia, Charlottesville, Virginia, USA
University of Washington, Seattle, Washington, USA
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