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Fermilab discovers exotic relatives of protons & neutrons
(NC&T/PPARC) Dr Todd Huffman of the University of Oxford, one of the UK scientists in CDF explains the discovery "CDF has discovered two new particles which though predicted by our theories, have never been observed before. These are variants on the familiar proton and neutrons found in atoms, but in each case, one of the quarks inside them has been replaced with a much heavier bottom quark."
Like protons and neutrons, the new particles are made of three quarks, the building blocks of matter. There are six different types of quarks: up, down, strange, charm, bottom and top (u,d,s,c,b,t). Protons contain two up quarks and one down quark (u-u-d), while neutrons have two down and one up (d-d-u).
The CDF collaboration has discovered two new three-quark particles involving the bottom quark, featuring u-u-b and d-d-b quark combinations. Quark theory predicts six different types of baryons with one bottom quark. Only one had been observed in the past, and the CDF experiment now accounts for two additional ones.
"These particles, named Sigma-sub-b [Ób], are like rare jewels that we mined out of our data," said Jacobo Konigsberg, University of Florida, a spokesperson for the CDF collaboration. "Piece by piece, we are developing a better picture of how matter is built out of quarks. We learn more about the subatomic forces that hold quarks together and tear them apart. Our discovery helps complete the periodic table of baryons. "
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| A baryon. (Photo: Ian MacVicar, University of Glasgow) |
The CDF experiment identified 103 u-u-b particles, also known as positively charged Sigma-sub-b particles (Ó+b), and 134 d-d-b particles, or negatively charged Sigma-sub-b particles (Ó-b). In order to find this number of particles, scientists sifted through more than 100 trillion high-energy proton-antiproton collisions produced by the Tevatron.
Dr Mark Lancaster of University College London said "To find these rare particles, you have to produce huge numbers of collisions and then program dedicated micro-processors, called triggers, to pick out the very small number of events you are interested in. Without these triggers, which the UK helped to program, you'd have no chance of finding these particles in fact you're 100,000 times more likely to win the National Lottery than discover one of these particles by chance!"
Dr Christine Davies of the University of Glasgow is a particle physics theorist working on predicting these particles "The masses of these new particles can be calculated from the theory of how quarks interact with each other, but it is very complicated and difficult to solve. In the last couple of years we have managed to improve the accuracy of our theoretical calculations enormously and we have had huge success comparing our results to experiment for lots of different particles. The calculations take a long time, however, and these baryons are still on the to-do list. So the CDF results have jumped ahead of the accuracy of our predictions. That is great to see, and will serve to spur us on to greater efforts!"
The new particles are extremely short-lived and decay within a tiny fraction of a second. Because a bottom quark weighs nearly as much as a lithium atom, producing rare three-quark combinations with one or more bottom quarks requires accelerators that boost particles to high energies. The Tevatron collider at Fermilab accelerates protons and antiprotons and makes them collide at the energy of 2 tera electron volts. In the collisions, energy transforms into mass, according to Einstein's famous equation E=mc^2. To beat the low odds of producing bottom quarks, the Tevatron creates billions of collisions per second.
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 both types of Sigma-sub-b particles, u-u-b and d-d-b, were 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.
CDF is an international experiment of 700 physicists from 61 institutions and 13 countries. It is supported by the US Department of Energy, the National Science Foundation, the UK's Particle Physics and Astronomy Research Council and a number of international funding agencies. (The full list can be found at online. ) Using the Tevatron, the CDF and DZero collaborations at Fermilab discovered the top quark, the final and most massive quark, in 1995.
Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated under contract by Universities Research Association, Inc.
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