Study of χ_{bJ}(2P)→ωϒ(1S) at Belle

Area of Science:

• Particle Physics
• Quantum Chromodynamics (QCD)
• Hadron Spectroscopy

Background:

• The study investigates hadronic transitions between excited bottomonium states (χ_{bJ}(2P)) and the Upsilon(1S) meson.
• Understanding these transitions is crucial for mapping the spectrum of heavy quarkonium and testing QCD predictions.
• Previous studies have focused on charmonium sector analogs, like the χ_{c1}(3872) decay.

Purpose of the Study:

• To search for and measure the branching fractions of χ_{bJ}(2P)→ωϒ(1S) transitions, where ω decays into three pions.
• To provide the first evidence for the near-threshold transition χ_{b0}(2P)→ωϒ(1S).
• To compare the measured ratios of transition rates with theoretical expectations, particularly from the QCD multipole expansion.

Main Methods:

• Analysis of 28.2×10^6 Υ(3S) mesons collected by the Belle detector.
• Identification of ω mesons via their decay into π^{+}π^{-}π^{0}.
• Statistical analysis to determine branching fractions and their uncertainties.

Main Results:

• First evidence for the near-threshold transition χ_{b0}(2P)→ωϒ(1S) with a branching fraction of (0.55±0.19±0.07)%.
• Measured branching fractions for χ_{b1}(2P)→ωϒ(1S) as (2.39_{-0.19}^{+0.20}±0.24)% and χ_{b2}(2P)→ωϒ(1S) as (0.47_{-0.12}^{+0.13}±0.06)%.
• The measured ratio of J=2 to J=1 transitions deviates by 3.3 standard deviations from QCD multipole expansion predictions.

Conclusions:

• The observation of the χ_{b0}(2P)→ωϒ(1S) transition, analogous to near-threshold charm decays, offers new insights into heavy quarkonium dynamics.
• The measured branching fractions for the P-wave states are confirmed.
• The discrepancy in the transition ratio suggests potential refinements needed in the theoretical framework of the QCD multipole expansion for heavy quarkonium.