New CLEO measurement of B->rho l nu and |Vub|
Exclusive semileptonic b->u l nu decays are an active area of experimental and theoretical study. These rare processes can be used to extract the magnitude of Vub, one of the smallest and least well known elements of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix Because |Vub/Vcb| is about 0.08, the branching fractions for exclusive b->u l nu processes are small, of order 10^-4, and they have only recently become experimentally accessible. The present analysis confirms the initial CLEO observation of B->rho l nu, improves the precision on the branching fraction and |Vub|, and provides the first information on the q2 dependence for this decay.
Using a sample of 3.3 million Y(4S)->BBbar events collected with the CLEO II detector at the Cornell Electron Storage Ring (CESR), we measure the branching fraction for B->rho l nu, |Vub|, and the partial rate (Delta Gamma) in three bins of q2 = (pB-prho)^2. We find B(B^0->rho^- l^+ nu) = (2.69 +- 0.41 +0.35-0.40 +- 0.50) x 10^-4, |Vub| = (3.23 +- 0.24 +0.23-0.26 +- 0.58) x 10^-3, Delta Gamma (0 lt q2 lt 7 GeV^2/c^4) = (7.6 +- 3.0 +0.9-1.2 +- 3.0) x 10^-2 ns^-1, Delta Gamma (7 lt q2 lt 14 GeV^2/c^4) = (4.8 +- 2.9 +0.7-0.8 +- 0.7) x 10^-2 ns^-1, and Delta Gamma (14 lt q2 lt 21 GeV^2/c^4) = (7.1 +- 2.1 +0.9-1.1 +- 0.6) x 10^-2 ns^-1. Here, l=e or mu, but not both, and the quoted errors are statistical, systematic, and theoretical. The method is sensitive primarily to B->rho l nu decays with leptons in the energy range above 2.3 GeV. Averaging with the previously published CLEO results for B->rho l nu, we obtain B(B^0->rho^- l^+ nu) = (2.57 +- 0.29 +0.33-0.46 +- 0.41) x 10^-4 and |Vub| = (3.25 +- 0.14 +0.21-0.29 +- 0.55) x 10^-3.
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