Search

The decays ${B}_{c}^{+}\ensuremath{\rightarrow}{B}_{a}\overline{\ensuremath{\ell}}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ and ${B}_{c}^{+}\ensuremath{\rightarrow}{B}_{a}^{*}(\ensuremath{\rightarrow}{B}_{a}\ensuremath{\gamma})\overline{\ensuremath{\ell}}{\ensuremath{\nu}}_{\ensuremath{\ell}}$, with $a=s$, $d$ and $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, are studied in the Standard Model (SM) and in the extension based on the low-energy Hamiltonian comprising the full set of dimension-6 semileptonic $c\ensuremath{\rightarrow}s,d$ operators with left-handed neutrinos. Tests of $\ensuremath{\mu}/e$ universality are investigated using such modes. The heavy quark spin symmetry is applied to relate the relevant hadronic matrix elements and to exploit lattice QCD results on ${B}_{c}$ form factors. Optimized observables are selected, and the pattern of their correlations is studied to identify the effects of the various operators in the extended low-energy Hamiltonian.

Onset of chaos for the holographic dual of a QQ¯ system at finite temperature and baryon density is studied. We consider a string in the AdS Reissner–Nordstrom background near the black-hole horizon and investigate small time-dependent perturbations of the static configurations. The proximity to the horizon induces chaos, which is softened increasing the chemical potential. A background geometry including the effect of a dilaton is also examined. The Maldacena, Shenker, and Stanford bound on the Lyapunov exponents characterizing the perturbations is satisfied for finite baryon chemical potential and when the dilaton is included in the metric.

The $B$ meson semileptonic modes to $\ensuremath{\rho}(770)$ and ${a}_{1}(1260)$ are useful to pin down possible non--Standard Model effects. The four-dimensional differential $\overline{B}\ensuremath{\rightarrow}\ensuremath{\rho}(\ensuremath{\pi}\ensuremath{\pi}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ and $\overline{B}\ensuremath{\rightarrow}{a}_{1}(\ensuremath{\rho}\ensuremath{\pi}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ decay distributions are computed in the Standard Model and in extensions involving new lepton-flavor universality violating semileptonic $b\ensuremath{\rightarrow}u$ operators. The large energy limit for the light meson is also considered for both modes. The new effective couplings are constrained using the available data, and several observables in $\overline{B}\ensuremath{\rightarrow}\ensuremath{\rho}(\ensuremath{\pi}\ensuremath{\pi}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ in which new physics effects can be better identified are selected, using the angular coefficient functions. The complementary role of $\overline{B}\ensuremath{\rightarrow}\ensuremath{\rho}(\ensuremath{\pi}\ensuremath{\pi}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ and $\overline{B}\ensuremath{\rightarrow}{a}_{1}(\ensuremath{\rho}\ensuremath{\pi}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ is discussed.

We reconsider the possibility that the tension in the $|V_{cb}|$ determinations from inclusive and exclusive $B$ decay modes is due to a new physics effect. We modify the Standard Model effective Hamiltonian for semileptonic $b \to c$ transitions including a tensor operator with a lepton flavour dependent coupling $\epsilon_T^\ell$, and investigate separately the muon and electron modes. The interference term between SM and NP, proportional to the lepton mass, has different impact in the inclusive and exclusive $B$ modes to muon. Moreover, even when the lepton mass is small as for the electron, the NP effect is different in inclusive and exclusive $B$ channels. For both $\mu$ and $e$ we find a region of $\epsilon_T^{\mu,\,e}$ where the constraints from $B^- \to D^{(*)0} \ell^- {\bar \nu}_\ell$ and $B \to X_c \, \ell \, \bar \nu_\ell$ are satisfied for the same $|V_{cb}|$.

We use a holographic method to investigate thermalization of a boost-invariant strongly interacting non-Abelian plasma. Boundary sourcing, a distorsion of the boundary metric, is employed to drive the system far from equilibrium. Thermalization is analyzed through nonlocal probes: the equal-time two-point correlation function of large conformal dimension operators in the boundary theory, and Wilson loops of different shapes. We study the dependence of the thermalization time on the size of the probes, and compare the results to the ones obtained using local observables: the onset of thermalization is first observed at short distances., Comment: 18 pages, 11 figures, 1 table, accepted for publication on Physical Review D

The lowest dimensional gluon condensate $G_2$ is analyzed at finite temperature and chemical potential using a holographic model of QCD with conformal invariance broken by a background dilaton. Starting from the free energy of the model, the thermodynamical quantities needed to determine the $T$ and $\mu$ dependence of the gluon condensate are evaluated. At high temperature the gluon condensate is independent of chemical potential. Moreover, at $\mu=0$ and in the string frame, the temporal and spatial Wilson loops at low temperature are computed; they are related to the (chromo) electric and magnetic components of $G_2$, respectively. The $T$-dependence of the two components is separately determined., Comment: 19 pages, 11 figures. Added references; dilaton formulation adopted; matches the published version

The BABAR measurements of the ratios $\mathcal{R}({D}^{(*)})=\frac{\mathcal{B}(B\ensuremath{\rightarrow}{D}^{(*)}\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}})}{\mathcal{B}(B\ensuremath{\rightarrow}{D}^{(*)}\ensuremath{\mu}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}})}$ deviate from the standard model expectation, while new results on the purely leptonic $B\ensuremath{\rightarrow}\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ mode show a better consistency with the standard model, within the uncertainties. In a new physics scenario, one possibility to accommodate these two experimental facts consists in considering an additional tensor operator in the effective weak Hamiltonian. We study the effects of such an operator in a set of observables, in semileptonic $B\ensuremath{\rightarrow}{D}^{(*)}$ modes as well as in semileptonic $B$ and ${B}_{s}$ decays to excited positive-parity charmed mesons.

We consider the problem of determining the matrix element of the heavy quark kinetic energy operator {ital K} between {Lambda}{sub {ital b}} baryonic states by three-point function QCD sum rules in the heavy quark effective theory. We discuss the main features and the uncertainties associated with such an approach. Numerically, we find the result {l_angle}{ital K}{r_angle}{sub {Lambda}{sub {ital b}}}{approx_equal}{minus}0.6 GeV{sup 2}, qualitatively similar to the analogous quantity {l_angle}{ital K}{r_angle}{sub {ital B}{sub {ital d}}}. {copyright} {ital 1996 The American Physical Society.}

We use QCD sum rules to calculate the hadronic matrix elements governing the rare decays {ital B}{r_arrow}{ital K}l{sup +}l{sup {minus}} and {ital B}{r_arrow}{ital K}{sup {asterisk}}l{sup +}l{sup {minus}} induced by the flavor-changing neutral current {ital b}{r_arrow}{ital s} transition. We also study relations among semileptonic and rare {ital B}{r_arrow}{ital K}{sup ({asterisk})} decay form factors. The analysis of the invariant mass distribution of the lepton pair in {ital B}{r_arrow}{ital K}{sup ({asterisk})}l{sup +}l{sup {minus}} and of the angular asymmetry in {ital B}{r_arrow}{ital K}{sup {asterisk}}l{sup +}l{sup {minus}} provides us with interesting tests of the standard model and its extensions. {copyright} {ital 1996 The American Physical Society.}

We use three-point function QCD sum rules to calculate the form factor [ital A][sub 0]([ital q][sup 2]) appearing in the matrix element of the flavor-changing axial vector current between the [ital D]([ital B]) state and a vector meson state. We describe the role of this form factor in nonleptonic [ital D]([ital B])[r arrow][ital PV] decays and analyze the light SU(3)[sub [ital F]]-symmetry-breaking effects. We also discuss a proposal to relate the branching ratio of [ital B][r arrow][ital K][sup *][gamma] to the spectrum of the semileptonic [ital B][r arrow][rho][ital l][nu] decay.

We study the radiative $B \to K \eta^{(\prime)} \gamma$ decays, which are important to investigate CP violation, and are also relevant to assess the role of the exclusive modes induced by the $b \to s \gamma$ transition to saturate the inclusive $B \to X_s \gamma$ decay rate. Moreover, these channels do not display the same hierarchy as $B \to K \eta^{(\prime)}$ modes, for which the decay into $\eta^\prime$ is enhanced with respect to one into $\eta$. The three-body radiative decays reverse the role: we find that this experimentally observed behavior (although affected by a large uncertainty in the case of the $\eta^\prime$) is reproduced in the theoretical analysis. We compute a $B^* \to K$ form factor, needed for this study, using light cone QCD sum rules, and discuss a relation expected to hold in the large energy limit for the light meson. Finally, we examine $B\to K \eta \gamma$ in two extensions of the standard model with universal extra dimensions, to investigate the sensitivity of this rare mode to such a kind of new physics effects.

We consider the vertex function of two vector and one axial-vector currents using the soft-wall holographic model of QCD with the Chern-Simons term. Two structure functions $w_L$ and $w_T$ describe such a vertex in the special case in which one of the two vector currents corresponds to an on-shell soft photon. We briefly review the QCD results for these functions, obtained from triangular loop diagrams with quarks having mass $m_q=0$ or $m_q \neq 0$, we compute $w_L$ and $w_T$ in the soft-wall model and compare the outcome to the QCD findings. We also calculate and discuss the two-point $\Pi_{VV}-\Pi_{AA}$ correlation function, together with a few low-energy constants, which turn out to be close to the QCD results., Comment: LaTeX, 29 pages. Matches the published version

We consider the $O({p}^{4})$ predictions of chiral perturbation theory ($\ensuremath{\chi}\mathrm{PT}$) in the process $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$, together with the study of the existing experimental measurements and the planned new ones. At the Frascati $\ensuremath{\Phi}$ factory $\mathrm{DA}\ensuremath{\Phi}\mathrm{NE}$ the rate of the events ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ is large enough to allow for a measurement of the cross section with a good statistical sensitivity. However, a quantitative analysis of the commonly used approximation of considering only real photons for the detection of the $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ signal is still lacking. This motivates us to compute the cross section in $\ensuremath{\chi}\mathrm{PT}$ for the production of ${\ensuremath{\pi}}^{0}$ pairs due to off-shell photons. A measurement of the azimuthal correlations would test the higher order $\ensuremath{\chi}\mathrm{PT}$ corrections independently from the measurement of the cross section. We approximate these corrections by including the contribution of the low-lying vector resonances in the scattering amplitude.

We analyze nonleptonic ${B}_{s}$ decays to a charmonium state and a light meson, induced by the $b\ensuremath{\rightarrow}c\overline{c}s$ transition, which are useful to access the ${B}_{s}\mathrm{\text{\ensuremath{-}}}{\overline{B}}_{s}$ mixing phase ${\ensuremath{\beta}}_{s}$. We use generalized factorization and $SU(3{)}_{F}$ symmetry to relate such modes to correspondent $B$ decay channels. We discuss the feasibility of the measurements in the various channels, stressing the importance of comparing different determinations of ${\ensuremath{\beta}}_{s}$ in view of the hints of new physics effects recently emerged in the ${B}_{s}$ sector. Finally, adopting a general parametrization of new physics contributions to the decay amplitudes, we discuss how to experimentally constrain new physics parameters.

We use gauge/string duality to investigate the free energy of two static color sources (a heavy quark-antiquark pair) in a Yang-Mills theory in strongly interacting matter, varying temperature and chemical potential. The dual space geometry is Anti-de Sitter with a charged black-hole to describe finite temperature and density in the boundary theory, and we also include a background dilaton field to generate confinement. The resulting phase diagram in the chemical potential-temperature $\mu-T$ plane is in a quite good agreement with lattice results and effective models of QCD., Comment: 4 pages, 5 figures, version published on PRD

The measurement of the ${\ensuremath{\eta}}_{b}$ mass, together with a QCD result for the hyperfine splitting ${E}_{\mathrm{HFS}}={M}_{\ensuremath{\Upsilon}(1S)}\ensuremath{-}{M}_{{\ensuremath{\eta}}_{b}}$, allows us to determine the strong coupling constant ${\ensuremath{\alpha}}_{s}$ at a low energy scale. The result ${\ensuremath{\alpha}}_{s}({M}_{\ensuremath{\Upsilon}(1S)})=0.197\ifmmode\pm\else\textpm\fi{}0.002{|}_{\ensuremath{\Delta}{E}_{\mathrm{HFS}}^{\mathrm{exp} }}\ifmmode\pm\else\textpm\fi{}0.002{|}_{\mathrm{scheme}}\ifmmode\pm\else\textpm\fi{}0.002{|}_{\ensuremath{\delta}⟨{G}^{2}⟩}\ifmmode\pm\else\textpm\fi{}0.006{|}_{\ensuremath{\delta}{m}_{b}}\ifmmode\pm\else\textpm\fi{}0.005{|}_{\mathrm{ho}}$, ${\ensuremath{\alpha}}_{s}({M}_{{Z}^{0}})=0.124\ifmmode\pm\else\textpm\fi{}0.001{|}_{\ensuremath{\Delta}{E}_{\mathrm{HFS}}^{\mathrm{exp} }}\ifmmode\pm\else\textpm\fi{}0.001{|}_{\mathrm{scheme}}\ifmmode\pm\else\textpm\fi{}0.001{|}_{\ensuremath{\delta}⟨{G}^{2}⟩}\ifmmode\pm\else\textpm\fi{}0.003{|}_{\ensuremath{\delta}{m}_{b}}\ifmmode\pm\else\textpm\fi{}0.002{|}_{\mathrm{ho}}$ is compatible with the current world average of ${\ensuremath{\alpha}}_{s}$ reported by the Particle Data Group, and shows that the experimental lowest-lying $\overline{b}b$ hyperfine splitting can be reproduced in terms of a perturbative and nonperturbative QCD contribution.

We compute the B{sub s{yields}}f{sub 0}(980) transition form factors using light-cone QCD sum rules at leading order in the strong coupling constant, and also including an estimate of next-to-leading order corrections. We use the results to predict the branching fractions of the rare decay modes B{sub s{yields}}f{sub 0}l{sup +}l{sup -} and B{sub s{yields}}f{sub 0{nu}{nu}}, which turn out to be O(10{sup -7}) (B{sub s{yields}}f{sub 0}(980)l{sup +}l{sup -}, with l=e, {mu}), O(10{sup -8}) (B{sub s{yields}}f{sub 0}(980){tau}{sup +{tau}-}) and O(10{sup -6}) (B{sub s{yields}}f{sub 0}(980){nu}{nu}). We also predict the branching ratio of B{sub s{yields}}J/{psi}f{sub 0}(980) decay under the factorization assumption, and discuss the role of this channel for the determination of the B{sub s} mixing phase compared to the golden mode B{sub s{yields}}J/{psi}{phi}. As a last application, we consider D{sub s{yields}}f{sub 0} form factors, providing a determination of the branching ratio of D{sub s{yields}}f{sub 0}e{sup +{nu}}{sub e}.

We study exclusive ${B}_{s}$ decays to final states with $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$, induced by the rare $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $b\ensuremath{\rightarrow}s\ensuremath{\nu}\overline{\ensuremath{\nu}}$ transitions. Differential decay rates and total branching fractions are predicted in the standard model, adopting the flavor scheme for the description of the $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\eta}}^{\ensuremath{'}}$ mixing. We discuss the theoretical uncertainty related to the hadronic matrix elements. We also consider these decay modes in a new physics scenario with a single universal extra dimension, studying the dependence of branching ratios and decay distributions on the compactification scale ${R}^{\ensuremath{-}1}$ of the extra dimension.

By using a QCD relativistic potential model we compute several physical quantities for heavy-light-quark {ital Q{bar q}} mesons: current-particle matrix elements, leptonic decay constants in the limit {ital m}{sub {ital Q}}{r arrow}{infinity}, and the Kobayashi-Maskawa matrix elements {ital V}{sub {ital b}{ital u}} and {ital V}{sub {ital b}{ital c}} from recent CLEO and ARGUS data on semileptonic inclusive {ital B} decays. A comparison with other theoretical approaches is also presented.

We study light scalar mesons in the AdS/QCD soft-wall model with a background dilaton field. The masses and decay constants are compatible with experiment and QCD determinations if $a_0(980)$ and $f_0(980)$ are identified as the lightest scalar mesons; moreover, the states are organized in linear Regge trajectories with the same slope of vector mesons. Comparing the two-point correlation function of scalar operators in AdS and QCD, information about the condensates can be derived. Strong couplings of scalar states to pairs of light pseudoscalar mesons turn out to be small, at odds with experiment and QCD estimates: this discrepancy is related to the description of chiral symmetry breaking in this model., LaTeX, 17 pages, 1 figure

We study the flavor changing neutral current ${B}_{s}\ensuremath{\rightarrow}\ensuremath{\phi}\ensuremath{\gamma}$, $\ensuremath{\phi}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ and ${\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}\ensuremath{\Lambda}\ensuremath{\gamma}$, $\ensuremath{\Lambda}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ transitions in the standard model and in a scenario with a single universal extra dimension. In particular, we focus on the present knowledge of the hadronic uncertainties and on possible improvements. We discuss how the measurements of these modes can be used to constrain the new parameter involved in the extra-dimensional scenario, the radius $R$ of the extra dimension, completing the information available from $B$-factories. The rates of these $b\ensuremath{\rightarrow}s$ induced decays are within the reach of new experiments, such as LHCb.

We study how to assign the recently observed ${D}_{sJ}(2700)$ meson to an appropriate level of the $c\overline{s}$ spectrum by the analysis of its decay modes in final states comprising a light pseudoscalar meson. We use an effective Lagrangian approach with heavy quark and chiral symmetries, obtaining that the measurement of the ${D}^{*}K$ decay width would allow one to distinguish between two possible assignments.

We study the branching fractions and the lepton polarization asymmetries of rare B{yields}X{sub s}{tau}{sup +}{tau}{sup -} and B{yields}K{sup (*)}{tau}{sup +}{tau}{sup -} modes within the standard model and in the Appelquist-Cheng-Dobrescu model, which is a New Physics scenario with a single universal extra dimension. In particular, we investigate the sensitivity of the observables to the radius R of the compactified extra dimension, which is the only new parameter in this New Physics model. For the exclusive transitions we study the uncertainties due to the hadronic matrix elements using two set of form factors, finding that in many cases such uncertainties are small. In particular, we show that the {tau}{sup -} polarization asymmetries become free of hadronic uncertainties if Large Energy relations for the form factors, valid for low momentum transfer to the lepton pair, are used. We also consider K* helicity fractions and their sensitivity to the compactification parameter.

We analyze the exclusive rare B{yields}K{sup (*)}l{sup +}l{sup -}, B{yields}K{sup (*)}{nu}{nu}, and B{yields}K*{gamma} decays in the Applequist-Cheng-Dobrescu model, which is an extension of the standard model in presence of universal extra dimensions. In particular, we consider the case of a single universal extra dimension. We study how spectra, branching fractions and asymmetries depend on the compactification parameter 1/R, and whether the hadronic uncertainty due to the form factors obscures or not such a dependence. We find that, using present data, it is possible in many cases to put a sensible lower bound to 1/R, the most stringent one coming from B{yields}K*{gamma}. We also study how improved experimental data can be used to establish stronger constraints to this model.

We study radiative decays of D{sub sJ}*(2317) and D{sub sJ}(2460) using light-cone QCD sum rules. In particular, we consider the decay modes D{sub sJ}*(2317){yields}D{sub s}*{gamma} and D{sub sJ}(2460){yields}D{sub s}{sup (}*{sup )}{gamma},D{sub sJ}*(2317){gamma} and evaluate the hadronic parameters in the transition amplitudes analyzing correlation functions of scalar, pseudoscalar, vector, and axial-vector cs currents. In the case of D{sub sJ}*(2317){yields}D{sub s}*{gamma} we also consider determinations based on two different correlation functions in heavy quark effective theory (HQET), in order to estimate the finite charm-quark mass effects. The mode D{sub sJ}(2460){yields}D{sub s}{gamma} turns out to have the largest decay rate among the radiative D{sub sJ}(2460) channels, as experimentally observed, a result thus favoring the interpretation of D{sub sJ} states as ordinary cs mesons.

We give an estimate of the {ital C}-even {ital B}{sup 0}{bar {ital B}}{sup 0} background coming from the radiative decay {Upsilon}(4{ital S}){r_arrow}{ital B}{sup 0}{bar {ital B}}{sup 0}{gamma} at {ital B} factories. Our result {ital B}{bold (}{Upsilon}(4{ital S}){r_arrow}{ital B}{sup 0}{bar {ital B}}{sup 0}{gamma}{bold )}{approx_equal}3{times}10{sup {minus}9} shows that such a background could be safely neglected in the analyses of {ital CP}-violating effects. {copyright} {ital 1996 The American Physical Society.}

Nonleptonic B to charmonium decays generally show deviations from the factorization predictions. For example, the mode ${B}^{\ensuremath{-}}\ensuremath{\rightarrow}{K}^{\ensuremath{-}}{\ensuremath{\chi}}_{c0}$ has been experimentally observed with a sizable branching fraction while its factorized amplitude vanishes. We investigate the role of rescattering effects mediated by intermediate charmed meson production in this class of decay modes, and consider ${B}^{\ensuremath{-}}\ensuremath{\rightarrow}{K}^{\ensuremath{-}}{h}_{c}$ with ${h}_{c}$ the ${J}^{\mathrm{PC}}{=1}^{+\ensuremath{-}}\overline{c}c$ meson. Using an effective Lagrangian describing interactions of pairs of heavy-light $Q\overline{q}$ mesons with a quarkonium state, we relate this mode to the analogous mode with ${\ensuremath{\chi}}_{c0}$ in the final state. We find $\mathcal{B}{(B}^{\ensuremath{-}}\ensuremath{\rightarrow}{K}^{\ensuremath{-}}{h}_{c})$ large enough to be measured at B factories, so that this decay mode could be used to study the poorly known ${h}_{c}.$

The form factors parameterizing the B_c semileptonic matrix elements can be related to a few invariant functions if the decoupling of the spin of the heavy quarks in B_c and in the mesons produced in the semileptonic decays is exploited. We compute the form factors as overlap integral of the meson wave-functions obtained using a QCD relativistic potential model, and give predictions for semileptonic and non-leptonic B_c decay modes. We also discuss possible experimental tests of the heavy quark spin symmetry in B_c decays.

The decay process $B\to D^+ D^- \pi^0$ is an interesting channel for the investigation of CP violating effects in the $b-$ sector. We write down a decay amplitude constrained by a low-energy theorem, which also includes the contribution of resonant $S-$ and $P-$wave beauty and charmed mesons, and we determine the relevant matrix elements in the infinite heavy quark mass limit, assuming the factorization ansatz. We estimate the rate of the decay: ${\cal B}(B \to D^+ D^- \pi^0)\simeq 1 \times 10^{-3}$. We also analyze the time-independent and time-dependent differential decay distributions, concluding that a signal for this process should be observed at the B-factories. Finally, we give an estimate of the decay rate of the Cabibbo-favoured process $B\to D^+ D^- K_S$.

We use QCD sum rules, in the framework of the heavy quark effective theory, to calculate the universal form factor ${\ensuremath{\tau}}_{1/2}(y)$ parametrizing the semileptonic transitions $\stackrel{\ensuremath{\rightarrow}}{B}{D}_{0}l\overline{\ensuremath{\nu}},$ $\stackrel{\ensuremath{\rightarrow}}{B}{D}_{1}^{*}l\overline{\ensuremath{\nu}},$ where ${D}_{0}$ and ${D}_{1}^{*}$ are the members of the excited charmed doublet with ${J}^{P}{=(0}_{1/2}^{+}{,1}_{1/2}^{+}).$ We include two-loop corrections in the perturbative contribution to the sum rule, and present a complete next-to-leading order result. As a preliminary part of our analysis we also compute, up to order ${\ensuremath{\alpha}}_{s},$ the leptonic constant ${F}^{+}$ of the doublet ${D}_{0},$ ${D}_{1}^{*}.$ Finally, we discuss the phenomenological implications of this calculation.