Scientists discover new particle

21 Jul 2011

Scientists at the US Department of Energy’s FermiNational Accelerator Lab – using a particle collider – have discovered the existence of a new particle: the neutral Xi-sub-b.

This particle contains three quarks: 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.

Fermilab physicist Pat Lukens, a member of the CDF collaboration, presented the discovery at Fermilab yesterday.

The neutral Xi-sub-b is the latest entry in the periodic table of baryons. Baryons are particles formed of three quarks, the most common examples being the proton (two up quarks and a down quark) and the neutron (two down quarks and an up quark). The neutral Xi-sub-b belongs to the family of bottom baryons, which are about 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 difficult to observe.

Although Fermilab’s Tevatron particle collider is not a dedicated bottom quark factory, sophisticated particle detectors and trillions of proton-antiproton collisions have made it a haven for discovering and studying almost all of the known bottom baryons.

Scientists at Fermilab and other US Department of Energy national laboratories use powerful computers to simulate quark interactions and understand the properties of particles comprised of quarks.

Passing the threshold for new discoveries

Once produced, the neutral Xi-sub-b travels a fraction of a millimetre before it decays into lighter particles. These particles then decay again into even lighter particles.

Combing through almost 500trn proton-antiproton collisions produced by Fermilab’s Tevatron particle collider, the CDF collaboration isolated 25 examples in which the particles emerging from a collision revealed the distinctive signature of the neutral Xi-sub-b.

The analysis established the discovery at a level of 7 sigma. Scientists consider 5 sigma the threshold for discoveries.

CDF also re-observed the already known charged version of the neutral Xi-sub-b in a never before observed decay, which served as an independent cross-check of the analysis. The newly analysed data samples offer possibilities for further discoveries, it says.

John Kennedy is a journalist who served as editor of Silicon Republic for 17 years