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R. Rückl

Publications and source records attributed to R. Rückl.

35 records · Page 2Linked to original sources
arXiv1998

Exclusive Decays of Charm and Beauty

Exclusive decays of B and D mesons are discussed with the focus on the theoretical problem how to calculate the relevant hadronic matrix elements of weak operators in the framework of QCD sum rules. The three lectures are devoted to 1. leptonic decays and decay constants 2. semileptonic decays and form factors 3. nonleptonic decays and nonfactorizable amplitudes. I shall introduce some of the basic concepts, describe various calculational techniques, and illustrate the numerical results. The latter are compared with lattice and quark model predictions and, where possible, with experimental data. Applications include the determination of V_{ub} from B -> π\bar{l} ν_l and B -> ρ\bar{l} ν_l, and an estimate of the phenomenological coefficient a_2 for B -> J/ψK.

hep-ph↗
arXiv1998

Exclusive Nonleptonic Decays of Heavy Mesons in QCD

We use operator product expansion and QCD light-cone sum rules to estimate the amplitude of the decay $B \to J/ψK $ taking into account leading nonfactorizable contributions. The result is very similar to the estimate obtained earlier from a four-point QCD sum rule. We discuss applications of this method to other nonleptonic $B$ decay modes.

hep-ph↗
arXiv1998

On the energy dependence of hadronic $B_c$ production

An estimate is presented of the production cross section of $B_c$ mesons between threshold and LHC energies using lowest-order perturbation theory and non-relativistic bound state approximation. It is shown that the ratio of the production cross sections for $B_c$ mesons and for $b$ quarks varies strongly with energy.

hep-ph↗
arXiv1998

The Scalar $B\to π$ and $D \to π$ Form Factors in QCD

QCD sum rules on the light-cone are derived for the sum $f^+ + f^-$ of the $B\to π$ and $D\to π$ form factors taking into account contributions up to twist four. Combining the results with the corresponding $f^+$ form factors calculated previously by the same method, we obtain the scalar form factors $f^0$. Our sum rule predictions are compared with lattice results, current-algebra constraints, and quark-model calculations. Furthermore, we calculate decay distributions and the integrated width for the semileptonic decay $B \to π\barτν_τ$ which is sensitive to $f^0$. Finally, the dependence of the sum rules on the heavy quark mass and the asymptotic scaling laws are discussed.

hep-ph↗
arXiv1998

QCD Sum Rules for Exclusive Decays of Heavy Mesons

Applications of QCD sum rules to exclusive decays of heavy mesons are reviewed. In detail, we discuss the calculations of the B and $D$ decay constants, the heavy-to-light form factors $f^+(p^2)$ and $f^0(p^2)$, and the strong couplings $g_{B^*Bπ}$ and $g_{D^*Dπ}$. Predictions are presented for the semileptonic weak decays $B \to π\bar{l} ν_l$ and $B \to ρ\bar{l} ν_l$, and used to extract $V_{ub}$ from the measured decay widths. Results are also shown on $D \to π\bar{l} ν_l$ and $D^* \to D π$. Furthermore, we reconsider the factorization hypothesis in nonleptonic two-body decays, and describe a sum rule estimate of the nonfactorizable contribution to the amplitude of $B \to J/ψK$.

hep-ph↗
arXiv1997

Perturbative QCD Correction to the $B\toπ$ Transition Form Factor

We report on the perturbative $O(α_s)$ correction to the light-cone QCD sum rule for the $B\to π$ transition form factor $f^+$. The correction to the product $f_Bf^+$ in leading twist approximation is found to be about 30%, that is similar in magnitude to the corresponding $O(α_s)$ correction in the two-point sum rule for $f_B$. The resulting cancellation of large QCD corrections in $f^+$ eliminates one important uncertainty in the sum-rule prediction for this form factor.

hep-ph↗
arXiv1997

QCD estimate of the long-distance effect in $B \to K^* γ$

We suggest to use operator product expansion and QCD sum rule techniques to estimate the long-distance contribution to the amplitude of the exclusive rare decay $B\to K^* γ$. In contrast to a phenomenological description in terms of $ψ$ resonances converting into a photon, the virtual charm quark loop interacting with soft gluons is represented by a new quark-gluon-photon operator. The matrix element of this operator is calculated in the same approximation as the matrix element of the leading magnetic penguin operator. The overall correction is found to be small, not more than about 5%.

hep-ph↗
arXiv1996

Sum-Rule Results on Exclusive Decays of Heavy Mesons

We review QCD sum rule applications to hadronic matrix elements of exclusive $B$ and $D$ decays. Results are presented for the form factors $f^+$ and $F_0$ of $B \rightarrow π$ and $D\rightarrow π$ transitions. The predictions are used to compute the width of $D^* \rightarrow D π$, and to extract the CKM parameter $V_{ub}$ from the measured $B \rightarrow πe ν$ width. Furthermore, we comment on weak annihilation in $B \rightarrow ργ$, as well as on the radiative decays $D \rightarrow ργ$, $D \rightarrow K^* γ$, and $B \rightarrow μνγ$.

hep-ph↗
arXiv1996

Calculation of the $B \to π$ Transition Matrix Element in QCD

We describe the calculation of the $B\rightarrow π$ hadronic matrix element from operator product expansion near the light-cone in QCD. In the resulting sum rules, the form factors $f^+$ and $F_0$ determining this matrix element are expressed in terms of pion light-cone wave functions. We present predictions for $f^+$ and $F_0$ in the region of momentum transfer $0 \leq p^2 < m_b^2 - {\cal O}(\mbox{1 GeV}^2)$ taking into account the wave functions up to twist 4. We also discuss the extrapolation of the form factors to higher $p^2$ and their asymptotic dependence on the heavy quark mass.

hep-ph↗
arXiv1995

B Meson Form Factors and Exclusive Decays

We present results for the $B \rightarrow π$ and $D\rightarrow π$ form factors derived from QCD sum rules on the light-cone. Our predictions are compared with experiment and used to extract the quark mixing parameter $|V_{ub}|$ from a recent CLEO measurement of $B \ra πl^+ ν$. Furthermore, we discuss the factorization approximation for exclusive nonleptonic matrix elements, and describe a first QCD estimate of the nonfactorizable contribution to the amplitude of $B \rightarrow J/ψK $.

hep-ph↗
arXiv1995

Heavy Meson Exclusive Decays in the Framework of QCD Sum Rules

We discuss applications of QCD sum rules on the light-cone to the form factors of the exclusive transitions $B \rightarrow π$ and $D\rightarrow π$, and to the $B^*B π$ and $D^*D π$ coupling constants. In the light of our results we examine the pole dominance model for these form factors. A first estimate is given on the nonfactorizable amplitude of the decay $ B \ra J/ψK$.

hep-ph↗
arXiv1994

Production Mechanisms for $B_c$ Mesons in Photon--Photon Collisions

Using photon-photon collisions as a particularly transparent study case we investigate the production mechanisms for $B_c$ mesons. In nonrelativistic approximation and to $O(α^2α_s^2)$ it is shown that recombination of $\bar{b}$- and $c$-quarks dominates by far over $\bar{b}$ and $c$ fragmentation. This dominance persists up to the highest accessible transverse momenta and leads to distributions in energy which differ completely from the spectra expected on the basis of the fragmentation functions. For processes in which a $b \bar{b}$-pair is radiated from a primary $c$-quark, the fragmentation description is found to be inadequate. We anticipate important implications of these results for hadronic production of heavy quark resonances. Using realistic photon spectra we predict two-photon production rates for $B_c$ and $B_c^*$ at present and future $e^+e^-$-machines.

hep-ph↗
arXiv1994

$D^*Dπ$ and $B^*Bπ$ couplings in QCD

We calculate the $D^*Dπ$ and $B^*Bπ$ couplings using QCD sum rules on the light-cone. In this approach, the large-distance dynamics is incorporated in a set of pion wave functions. We take into account two-particle and three-particle wave functions of twist 2, 3 and 4. The resulting values of the coupling constants are $g_{D^*Dπ}= 12.5\pm 1$ and $g_{B^*Bπ}= 29\pm 3 $. From this we predict the partial width $ Γ(D^{*+} \ra D^0 π^+ )=32 \pm 5~ keV $. We also discuss the soft-pion limit of the sum rules which is equivalent to the external axial field approach employed in earlier calculations. Furthermore, using $g_{B^*Bπ}$ and $g_{D^*Dπ}$ the pole dominance model for the $ B \ra π$ and $D\ra π$ semileptonic form factors is compared with the direct calculation of these form factors in the same framework of light-cone sum rules.

hep-ph↗
arXiv1994

QCD Calculation of $B$ Meson Form Factors and Exclusive Nonleptonic Decays

The method of QCD sum rules has proven to be particularly useful in heavy quark physics, where a small distance scale is provided by the inverse heavy quark mass. We present two new examples for the application of this method to B-physics: a calculation of the heavy-to-light form factors $B \rightarrow π,K$ and an estimate of the weak decay amplitude for $B \rightarrow J/ψK $ beyond the factorization approximation.

hep-ph↗
arXiv1993

QCD Calculation of the $B \rightarrow π,K$ Form Factors

We calculate the form factors for the heavy-to-light transitions $B\rightarrow π,K $ by means of QCD sum rules using $π$ and $K$ light-cone wave functions. Higher twist contributions as well as gluonic corrections are taken into account. The sensitivity to the shape of the leading-twist wave functions and effects of SU(3)-breaking are discussed. The results are compared with quark model predictions and with the results from QCD sum rules for three-point correlators.

hep-ph↗
arXiv1992

QCD and QED Corrections to Higgs Boson Production in Charged Current $ep$ Scattering

First order QCD and leading QED corrections to Higgs boson production in the channel $e^-p \to νH^0 X; H^0 \to b\bar{b}$ are calculated for the kinematical conditions at LEP $\otimes$ LHC ($\sqrt{s} = 1360 \GeV$) and the interesting mass range $80 < M_H < 150 \GeV$. In the DIS scheme the QCD corrections (not including the corrections to the branching ratio, which are well-known) are found to be about 1\% for the total cross section and $-13\%$ to $-10\%$ for the observable cross section as defined by appropriate cuts. The latter results depend on the definition of these cuts. The QED corrections amount to about $-5\%$. Also obtainable with anonymous ftp from gluon.hep.physik.uni-muenchen.de.

hep-ph↗