Updated: Thursday, March 03, 2011

BABAR Sees First Evidence of the h_b(1P) Bottomonium State

On February 21 BABAR presented first evidence for the h_b(1P) state of the bottomonium system at the 2011 Lake Louise Winter Institute. A paper describing this result was submitted to the journal Phys. Rev. D on February 23.

The large Υ(2S) and Υ(3S) data sets collected during the final months of data taking by the experiment in 2008 have provided a unique opportunity to study the properties of quarkonium states, in particular the physics of "bottomonium", bound states of a b-quark and a b-antiquark. The properties of the bottomonium system are predicted using quantum chromodynamics (QCD), the theory of the fundamental intractions of quarks via the so-called strong force. Theoretical calculations have produced testable predictions of particle decays and spectroscopy within the bottomonium system, in particular, predicting a spectrum of bottomonium particle states. These states are analogous to the energy level states of a multi-electon atom and can be characterized by the J^PC quantum numbers of the state.

Although the first of these b―b states, the Υ(1S) meson, was discovered more than thirty years ago, several predicted states have not been observed and many of the properties of the known states have not yet been measured. The study of the bottomonium system therefore provides an important test both of QCD and of relativistic corrections to basic precursory phenomenological models used to describe the quark-antiquark interaction.

A major success of the BABAR Υ(2,3S) run was the discovery of the ground state of the bottomonium system, the η_b(1S), the spin-singlet pseudoscalar partner (J^PC=0⁻⁺) of the Υ(1S) state (J^PC=1^--), (PRL 101, 071801 (2008)) in 2008 and subsequent evidence for η_b(1S) in decays of Υ(2S) radiative decays (PRL 103, 161801 (2009)). Building upon this success, BABAR has reported new measurements related to χ_bJ radiative and Υ(1D) hadronic decays (in preparation for PRD), and in particular results of searches for the h_b(1P), the spin-singlet axial vector partner of the χ_bJ(1P) states (J^PC = 0,1,2⁺⁺).

The h_b(1P) state (J^PC=1⁺⁻) is predicted to be produced in Υ(3S) decays accompanied by either a π⁰ or a pair of oppositely-charged pions and subsequently decaying predominantly via electromagnetic transitions to the η_b(1S). Competing theoretical models favor one or the other of these production modes, providing an opportunity for BABAR to discriminate between these theoretical predictions.  BABAR has performed searches for both signatures, finding no evidence for the h_b(1P) in the analysis of the dipion mode, but observing a 3.0σ excess of events in the search for the π⁰ decay mode. The mass of the h_b(1P) was measured in the BABAR analysis to be in good agreement with the theoretical expectations for this state, providing confirmation of the theories used to describe interquark forces and binding. The combination of the results of these dipion- and π⁰-mode searches clearly favors models predicting higher rates for the π⁰ mode, shedding further light on the nature of the fundamental interactions of quarks.

The preliminary results of both of h_b(1P) searches were initially reported at the ICHEP conference in summer 2010 (Proceedings of Science: ICHEP2010), but since then work has continued towards journal papers for both studies.

Content: Cowan
Page: Cowan