Semileptonic decays B → $$D^{(*)}$$ D ( ∗ ) lν in the perturbative QCD factorization approach

Abstract

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

Perturbative corrections to B → D form factors in QCD

“…However, both calculations for the vacuum-to-D-meson correlation function with an interpolating current for the B-meson presented in [21,22] were carried out at tree level without specifying the power counting scheme for the charm-quark field and without implementing the perturbative QCD constraints for the D-meson DA. Yet another QCD-based approach to evaluate B → D form factors in the framework of transverse-momentum-dependent (TMD) factorization was proposed in [23,24] with the power counting m b m c Λ, which was further updated in [25] recently. Albeit with the technical progresses on the computations of perturbative matching coefficients [26][27][28][29], TMD factorization for hard exclusive processes is still not well established conceptually due to the lack of a definite power counting scheme for the intrinsic transverse momentum [30] and the Wilson-line structure of TMD wave functions needs to be constructed appropriately to avoid both the rapidity and pinch singularities in the infrared subtraction [31].…”

Semileptonic decays of $$B_c$$ B c meson to S-wave charmonium states in the perturbative QCD approach

“…The perturbative QCD (pQCD) [20,21] is one of the recently developed theoretical tools based on QCD to deal with the nonleptonic and semileptonic B decays. So far the semileptonic B u,d,s,c decays have been studied systematically in the pQCD approach [22][23][24][25]. One may refer to the review paper [26] and the references therein.…”

The B → D(*)lv decays in the pQCD approach with the Lattice QCD input

“…which has invoked intensive studies in the framework of the SM and/or various NP models, for example, in Refs. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. As is well-known, the theoretical predictions for the ratios R(D) and R(D * ) have a direct strong dependence on the values and the shapes of the B → D ( * ) transition form factors F (+,0) (q 2 ), V (q 2 ) and A 0,1,2 (q 2 ).…”

“…In Ref. [26], we calculated the ratios R(D ( * ) ) by employing the pQCD factorization approach, where the relevant hadronic form factors are evaluated in the low q 2 region by employing the pQCD factorization approach and then extrapolated to the higher q 2 region by using the pole model parametrization [30]. The resulted pQCD predictions are R(D) = 0.430 [26].…”