CDF collaboration finds new baryons that contain b quarks

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 Σ_b particles, made up of quark combinations uub and ddb. Until now the Λ_b⁰ (udb) was the only baryon (three quark) state containing a b quark to have been observed.

The quark model predicts the existence of Σ_b particles, in which the spins of the light quarks (u or d) combine with spin-parity, J^P = 1⁺ to give a ground-state baryon with J^P = 1/2⁺, and excited state, Σ_b^*, with J^P = 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 Σ_b are expected to decay strongly to Λ_b⁰ states by emitting pions, the CDF team searched first for the Λ states via the decay chain, Λ_b⁰ → Λ_c⁺π, Λ_c⁺ → pK^–π⁺. They then looked for narrow resonances in the mass difference m(Λ_b⁰pi;) – m(Λ_b⁰) – m_π, 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 Σ_b⁺ or Σ_b^*-, while the negative states correspond to ddb, that is Σ_b⁺ or Σ_b^*-, where the Σ_b^* states have slightly higher masses. Working from CDF’s measurement of the Λ_b⁰, the four new states have masses in the region of 5800 MeV/c².