Semileptonic decays B c →(η c , J/Ψ)lν in the perturbativeQCD approach * WANG Wen-Fei( ) FAN Ying-Ying( ) XIAO Zhen-Jun( ) 1)

In this paper, we calculate the next-to-leading-order (NLO) twist-3 contribution to the form factors of B → π transitions by employing the k T factorization theorem. All the infrared divergences regulated by the logarithms ln(k 2 iT ) cancel between those from the quark diagrams and from the effective diagrams for the initial B meson wave function and the final pion meson wave function. An infrared finite NLO hard kernel is therefore obtained, which confirms the application of the k T factorization theorem to B meson semileptonic decays at twist-3 level. From our analytical and numerical evaluations, we find that the NLO twist-3 contributions to the form factors f +,0 (q 2 ) of B → π transition are similar in size, but have an opposite sign with the NLO twist-2 contribution, which leads to a large cancelation between these two NLO parts. For the case of f + (0), for example, the 24% NLO twist-2 enhancement to the full LO prediction is largely canceled by the negative ( about −17% ) NLO twist-3 contribution, leaving a small and stable 7% enhancement to the full LO prediction in the whole range of 0 ≤ q 2 ≤ 12 GeV 2 . At the full NLO level, the perturbative QCD prediction is F B→π (0) = 0.269 +0.054 −0.050 . We also studied the possible effects on the pQCD predictions when different sets of the B meson and pion distribution amplitudes are used in the numerical evaluation.

In this paper, we study the B → D ( * ) l −ν l semileptonic decays and calculate the branching ratios B(B → D ( * ) l −ν l ) and the ratios R(D ( * ) ) and R l,τ D by employing the perturbative QCD (pQCD) factorization approach. We find that (a) for R(D) and R(D * ) ratios, the pQCD predictions are R(D) = 0.430 +0.021 −0.026 , R(D * ) = 0.301 ± 0.013 and agree well with BaBar's measurements of R(D ( * ) ); (b) for the newly defined R l D and R τ D ratios, the pQCD predictions are R l D = 0.450 +0.064 −0.051 and R τ D = 0.642 +0.081 −0.070 , which may be more sensitive to the QCD dynamics of the considered semileptonic decays than R(D ( * ) ) and should be tested by experimental measurements. Key WordsB meson semileptonic decays; The pQCD factorization approach; Form factors; Branching ratios 1 INTRODUCTION The semileptonic decays B → Dτν τ and B → D * τν τ have been previously measured by both BaBar and Belle Collaborations with 3.8σ and 8.1σ significance [1-3]. Very recently, the BaBar collaboration with their full data greatly improved their previous analysis and reported their measurements for the relevant branching ratios and the ratios R(D ( * ) ) of the corresponding branching ratios [4]: R(D) = 0.440 ± 0.072, R(D * ) = 0.332 ± 0.030,where the isospin symmetry relations R(D 0 ) = R(D + ) = R(D) and R(D * 0 ) = R(D * + ) = R(D * ) have been imposed, and the statistical and systematic uncertainties have been combined in quadrature. These BaBar results are surprisingly larger than the standard model (SM) predictions as given in Ref.[5]:The combined BaBar results disagree with the SM predictions by 3.4σ [4,6].Since the report of BaBar measurements, this R(D ( * ) ) anomaly has been studied intensively by many authors, for example, in Refs. [7][8][9][10][11][12][13][14][15][16][17][18]. Some authors treat this 3.4σ deviation as the first evidence for new physics (NP) in semileptonic B meson decays to τ lepton [9-13], such as the NP contributions from the charged Higgs bosons in the Two-Higgs-Doublet models [10].Some other physicists, however, try to interpret the data in the framework of the SM but with their own methods. In Ref.[7] the authors presented their SM predictions R(D) SM = a xiaozhenjun@njnu.edu.cn