On PM: New baryons at LHCb

The LHCb experiment at CERN has observed two new baryons — particles like the protons and neutrons that form the nuclei of normal matter — in data from the 2011-2012 run of the Large Hadron Collider. These particular baryons are called “cascade b”: they are dominated by the heavy b or “beauty” quark, and so are about 6 times heavier than a proton; they also contain a strange quark, and a down quark. The lightest cascade-b has been known for some time, but two related particles at slightly higher masses were expected based on very general quark-model arguments. It's these related particles that have just been seen.

I was interviewed briefly on ABC's PM program yeterday, to cover this discovery, and LHC-related news more generally. A technical account of the work can be found in the paper LHCb submitted to Phys. Rev. Lett., which is available on the arXiv.

(What I've written simplifies things considerably: for example, these cascade-b's are negatively charged; there's also a matching set of neutral cascade-b's, each with an up quark in place of the down. And why are there three related states close in mass? As well as being an empirical observation, such patterns can be understood by considering the possible ways to combine quarks with various flavours and spins, taking into account the symmetry of the resulting wavefunction in each case. Readers with a considerable amount of university physics should be able to follow the PDG's discussion of the charmed baryons: baryons carrying one unit of “beauty” are an analogous case.)