A team of physicists from the University of Zurich’s Physics Institute working with the Compact Muon Solenoid (CMS) detector at CERN has observed for the first time a new, excited beauty baryon known as the Ξ*b0 (or Xi_b^*0), a baryon with one light and two heavy quarks.
In particle physics, the baryon family refers to particles that are made up of three quarks. Quarks form a group of six particles that differ in their masses and charges.
The two lightest quarks, the so-called “up” and “down” quarks, form the two atomic components, protons and neutrons. All baryons that are composed of the three lightest quarks (“up”, “down” and “strange” quarks) are known. Only very few baryons with heavy quarks have been observed to date. They can only be generated artificially in particle accelerators as they are heavy and very unstable.
The particle Ξ*b0 comprises one “up”, one “strange” and one “bottom” quark (usb), is electrically neutral and has a spin of 3/2. Its mass is comparable to 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 discovery, which will be published in Physical Review Letters, is based on data gathered in the CMS detector at CERN.
The new particle cannot be detected directly as it is too unstable to be registered by the detector. However, Ξ*b0 breaks up in a known cascade of decay products. Ernest Aguiló, a postdoctoral student and a member of the team, identified traces of the respective decay products in the measurement data and was able to reconstruct the decay cascades starting from Ξ*b0 decays.
The calculations are based on data from proton-proton collisions at an energy of seven Tera electron volts (TeV) collected by the CMS detector between April and November 2011.
A total of 21 Ξ*b0 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.