New heavy-weight particle of the Standard Model seen

By Tushna Commissariat

This past year has seen a fair amount of excitement in the particle-physics community, with bumps and jumps and leaks and debates, but sadly without any spectacular discoveries. In fact, since the both the CDF and D0 experiments at the US Fermi National Accelerator Laboratory (Fermilab) reported the production of the top quarks in 2009, it’s been rather quiet on the particle front. So it was quite refreshing to hear that researchers at the CDF collaboration at Fermilab announced the observation of a new particle – the neutral “Xi-sub-b”. This particle is basically a baryon – a Standard Model particle that is formed of a combination of three quarks.

Common examples of baryonic particles are the proton – a combination of two up quarks and a down quark and the neutron – a combination of two down quarks and an up quark. This new addition consists of a strange quark, an up quark and a bottom quark (s-u-b). While its existence was predicted by the Standard Model, the observation of the neutral Xi-sub-b is significant because it strengthens our understanding of how quarks form matter. This new particle fits into the bottom baryons group, which are six times heavier than the proton and neutron because they all contain a heavy bottom quark. The particles are produced only in high-energy collisions, and are rare and very difficult to observe.

Once produced, the neutral Xi-sub-b travels a fraction of a millimetre before it decays into lighter particles. Combing through almost 500 trillion proton–antiproton collisions produced by researchers isolated 25 examples in which the particles emerging from a collision bore the signature of the neutral Xi-sub-b. The analysis established the discovery at a level of 7 sigma, clearing the 5 sigma threshold quite easily. (Image courtesy: Fermilab)

A paper detailing their results will be available on the arXiv preprint server soon.