UCL News


Exotic relatives of protons and neutrons discovered

25 October 2006

Physicists participating in the CDF collaboration at the Fermi National Accelerator Laboratory, Fermilab, yesterday announced the discovery of two rare types of particles, exotic relatives of the much more common proton and neutron.

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.

Dr Mark Lancaster, (UCL Physics & Astronomy), one of the UK scientists involved in the project, said: "To find these rare particles, you have to produce huge numbers of collisions and then program dedicated microprocessors, 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!"

Jacobo Konigsberg, University of Florida, spokesperson for the CDF collaboration, said: "These particles, named Sigma-sub-b, are like rare jewels that we mined out of our data. 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."

'Baryon' is the term for the class of particles that contain three quarks, and scientists hope to catalogue all of them. While the matter around us is comprised of baryons (protons, neutrons) that contain up and down quarks, exotic matter abundant in the early universe contains other quarks as well. Employing Fermilab's Tevatron collider, the world's most powerful particle accelerator, physicists can recreate the conditions present in the early formation of the universe, reproducing exotic matter.

The new particles are extremely shortlived 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.

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. Using the Tevatron, the CDF and DZero collaborations at Fermilab discovered the top quark, the final and most massive quark, in 1995.