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Experimenters at Fermilab discover exotic relatives of protons and neutrons: photos and graphics

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The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. The Tevatron, four miles in circumference, is the world's most powerful accelerator, producing collisions at the energy of 2 tera electron volts (TeV). In a tiny volume, these collisions recreate the conditions of the early universe. Two experiments, CDF and DZero, record the particles emerging from billions of collisions per second.


The CDF detector, about the size of a 3-story house, weighs about 6,000 tons. Its subsystems record the "debris" emerging from high-energy proton-antiproton collisions, unveiling the secrets of the early universe. The detector surrounds the collision point and records the path, energy and charge of exotic, short-lived particles emerging from the collisions.


The center of the upgraded CDF detector features a silicon vertex detector, installed in January 2001. The improved detector has taken data since March 2001. The vertex detector allows experimenters to record tracks of charged particles with utmost precision. Tracing the particle tracks back to their origin, scientists discover what processes take place at the core of proton-antiproton collisions.


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 CDF experiment has discovered exotic relatives of the proton and neutron, particles that include a bottom quark.


Baryons are particles made of three quarks. The particles can exist in a ground state (J=1/2) and an excited state (J=3/2). The CDF experiment discovered the positively charged Sigma-sub-b and the negatively charged Sigma-sub-b in both spin configurations. The graphic shows the various three-quark combinations with J=3/2 that are possible using the three lightest quarks--up, down and strange--and the bottom quark. Past experiments discovered all of the baryons made of light quarks. The CDF discovery is the first observation of baryons with one bottom quark and spin J=3/2. Theory predicts four more such particles to exist. There are additional baryons involving the charm quark, which are not shown. The top quark, discovered at Fermilab in 1995, is too short-lived to become part of a baryon.

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    In a scientific presentation on Friday, October 20, CDF physicist Petar Maksimovic, professor at Johns Hopkins University, presented the discovery to the particle physics community at Fermilab. He explained that the two types of Sigma-sub-b particles are produced in two different spin combinations, J=1/2 and J=3/2, representing a ground state and an excited state, as predicted by theory.


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  • Last modified: 04/28/2009 |