Researchers at the Tevatron at Fermilab have found two new heavy particles and two of their excited states. The CDF collaboration has observed the first Σb particles, made up of quark combinations uub and ddb. Until now the Λb0 (udb) was the only baryon (three quark) state containing a b quark to have been observed.

The quark model predicts the existence of Σb particles, in which the spins of the light quarks (u or d) combine with spin-parity, JP = 1+ to give a ground-state baryon with JP = 1/2+, and excited state, Σb*, with JP = 3/2+. CDF is well placed to search for new particles like these, as the collaboration has the world's largest data set of baryons containing the b quark, thanks to the displaced track trigger that the experiment uses and a total of proton–antiproton luminosity around 1 fb-1 from the Tevatron, collected between February 2002 and February 2006. As the ground states of Σb are expected to decay strongly to Λb0 states by emitting pions, the CDF team searched first for the Λ states via the decay chain, Λb0 → Λc+π, Λc+ → pK-π+. They then looked for narrow resonances in the mass difference m(Λb0pi;) - m(Λb0) - mπ, where they found signals corresponding to a hundred or so examples of positively charged states, and rather more with negative charge.

The states with positive charge correspond to uub and are consistent with being either Σb+ or Σb*-, while the negative states correspond to ddb, that is Σb+ or Σb*-, where the Σb* states have slightly higher masses. Working from CDF's measurement of the Λb0, the four new states have masses in the region of 5800 MeV/c2.