We report improved measurements of branching fractions for charmless hadronic two-body B meson decays containing an ω meson in the final state. The results are based on a data sample of 78 fb −1 collected on the Υ(4S) resonance by the Belle detector. We measure the branching fractions B(B + → ωK + ) = (6.5 +1.3 −1.2 ± 0.6) × 10 −6 and B(B + → ωπ + ) = (5.7 +1.4 −1.3 ± 0.6) × 10 −6 . We give 90% confidence upper limits for B(B 0 → ωK 0 ) < 7.6 × 10 −6 and B(B 0 → ωπ 0 ) < 1.9 × 10 −6 . We also obtain the partial rate asymmetries ACP = 0.06 +0.21 −0.18 ± 0.01 for B ± → ωK ± and ACP = 0.50 +0.23 −0.20 ± 0.02 for B ± → ωπ ± .PACS numbers: 13.25.Hw, 14.40.Nd
INTRODUCTIONCharmless hadronic B decays play an important role in the understanding of CP violation in the B system. These decays proceed primarily through interfering b → s loop penguin diagrams and b → u tree spectator diagrams. Studies of B → ωh, where h denotes K + , π + , K 0 , and π 0 , are important examples of such decays. Charge conjugates are implied unless otherwise stated. We also assume equal production of B + B − and B 0B0 pairs from the Υ(4S). ωK + 3.2 +2.4 −1.9 ± 0.8 5.5 ± 0.9 ± 0.5 9.2 +2.6 −2.3 ± 1.0 ωπ + 11.3 +3.3 −2.9 ± 1.4 4.8 ± 0.8 ± 0.4 4.2 +2.0 −1.8 ± 0.5 Table I lists the branching fractions from previous measurements [1,2,3,4,5], which indicate some discrepancies for B + → ωK + . Naive factorization and QCD factorization approaches [6,7] yield values of B(B + → ωπ + ) consistent with the experimental results. However, these approaches predict B(B + → ωπ + ) to be a factor of two larger than B(B + → ωK + ), which is not supported by Belle's previous experimental results that were based on a 29.4 fb −1 data sample [5]. In this paper, we update our previous measurements on ωK + and ωπ + with a 78.1 fb −1 data sample. We also report measurements of ωK 0 and ωπ 0 decay modes.
APPARATUS AND DATA SETThe data sample used was collected with the Belle detector at the KEKB asymmetric energy e + e − collider [8], which collides 8.0 GeV e − and 3.5 GeV e + beams at a small crossing angle (±11 mrad). The data sample contained 85.0 × 10 6 BB pairs produced at the Υ(4S) resonance. A 8.8 fb −1 data sample taken at a center-ofmass energy 60 MeV below the Υ(4S) is used to characterize continuum background. In order to establish the event selection criteria, we use a Monte Carlo (MC) generator [9] to generate signal, generic b → c, and other charmless rare B decays. The GEANT3 package [10] is used for detector simulation.The Belle detector measures charged particles and photons with high efficiency and precision [11]. Charged particle tracking is provided by a silicon vertex detector (SVD) and a central drift chamber (CDC) that surround the interaction region. The charged particle acceptance covers the laboratory polar angle region between θ = 17 o and 150 o . Charged hadrons are distinguished by combining the responses from an array of silica aerogelČerenkov counters (ACC), a barrel-like array of 128 time-of-flight scintillation counters (TOF), and dE/dx...