Researchers working at the FermiLab particle accelerator in the US have discovered two new kinds of particle, exotic relatives of the common-or-garden proton and neutron.
Dr Todd Huffman of the University of Oxford, one of the UK scientists in Collider Detector at Fermilab (CDF), said although the particles have been predicted by theory, they haven't been seen 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," he said.
Protons and neutrons are both baryons, a particle composed of three quarks. There are six different "flavours" of quark: up, down, top, bottom, strange, and charmed (u,d,t,b,s,c) (the nomenclature tells you something about particle physicists, not about quarks).
The combinations of flavours determine the characteristics of the particle. For example, protons contain two up quarks and one down quark (u-u-d), while neutrons have two down and one up (d-d-u).
These exotic new particles are also composed of three quarks, but the mixture is different: they have u-u-b and d-d-b quark combinations. They have been named Sigma-sub-b.
Quark theory predicts six types of three-quark particles with one bottom quark. Only one had been observed before, so catching these two is quite a feat.
A bottom quark has almostthe same mass as a lithium ion, so it takes very high energies in a particle accelerator to produce a baryon with a bottom quark. The Tevatron collider at Fermilab accelerates protons and antiprotons and makes them collide at the energy of two tera electron volts.
"These particles are like rare jewels that we mined out of our data," said Jacobo Konigsberg, University of Florida. "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." ®