Physicists using the Large Hadron Collider (LHC) particle accelerator have discovered their first brand-new baryon, a particle consisting of three quarks.
There are six quarks altogether, which differ in their masses and charges. The two lightest – the so-called ‘up’ and ‘down’ quarks – form protons and neutrons. All baryons that are composed of the three lightest quarks – ‘up’, ‘down’ and ‘strange’ – are known.
But only a very few baryons with heavy quarks have ever been seen before: they can only be generated artificially in particle accelerators as they are heavy and very unstable.
Now, though, a group of scientists from the University of Zurich’s Physics Institute have managed to detect a baryon with one light and two heavy quarks.
The particle Xi_b^* comprises one ‘up’, one ‘strange’ and one ‘bottom’ quark, or ‘usb’, is electrically neutral and has a spin of 3/2 (1.5). Its mass is around the same as that of a lithium atom.
The new discovery means that two of the three baryons predicted in the usb composition by theory have now been observed.
The new particle can’t be detected directly as it is too unstable to be registered by the detector. However, it breaks up in a known cascade of decay products, which have now been identified.
A total of 21 Xi_b^* baryon decays were discovered – statistically sufficient to rule out a statistical fluctuation.
The discovery of the new particle confirms the theory of how quarks bind – and therefore helps to understand the strong interaction, one of the four basic forces of physics which determines the structure of matter, say the scientists.