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J. G. Körner

Publications and source records attributed to J. G. Körner.

At least 73 records · Page 4Linked to original sources

Inclusive Decays of B Mesons into D_s and $D_s^{\ast}$ at $O(α_s)$ Including $D_s^{\ast}$ Polarization Effects

The dominant contribution to the inclusive decays of $B$ mesons into the charm-strangeness mesons $D_s$ and $D_s^{\ast}$ is expected to be given by the partonic process $b \to c+(D_s^-,D_s^{\ast -})$. We determine the nonperturbative $O(1/m_b^2)$ and the $O(α_s)$ radiative corrections to $b \to c+(D_s^-,D_s^{\ast -})$ and thereby to the inclusive decays $ \bar{B} \to X_c+(D_s^-,D_s^{\ast -})$. The new feature of our calculation is that we separately determine the nonperturbative and the $O(α_s)$ corrections to the longitudinal ($L$) and transverse ($T$) pieces of the spin 1 $D_s^{\ast -}$ meson. The longitudinal/transverse composition of the $D_s^{\ast -}$ can be probed through its two principal decay modes $D_s^{\ast -} \to D_s^- + γ$ and $D_s^{\ast -} \to D_s^- + π^0$ for which we write down the angular decay distributions.

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A new technique for computing the spectral density of sunset-type diagrams: integral transformation in configuration space

We present a new method to investigate a class of diagrams which generalizes the sunset topology to any number of massive internal lines. Our attention is focused on the computation of the spectral density of these diagrams which is related to many-body phase space in $D$ dimensional space-time. The spectral density is determined by the inverse $K$-transform of the product of propagators in configuration space. The inverse $K$-transform reduces to the inverse Laplace transform in any odd number of space-time dimensions for which we present an explicit analytical result.

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On the evaluation of sunset-type Feynman diagrams

We introduce an efficient configuration space technique which allows one to compute a class of Feynman diagrams which generalize the scalar sunset topology to any number of massive internal lines. General tensor vertex structures and modifications of the propagators due to particle emission with vanishing momenta can be included with only a little change of the basic technique described for the scalar case. We discuss applications to the computation of $n$-body phase space in $D$-dimensional space-time. Substantial simplifications occur for odd space-time dimensions where the final results can be expressed in closed form through rational functions. We present explicit analytical formulas for three-dimensional space-time.

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Heavy Baryons - Status and Overview (Theory)

We review recent progress in the understanding of the physics of heavy baryons. We begin our review by presenting some highlights of recent experimental findings on charm and bottom baryons and briefly comment on their theoretical implications. On the theoretical side we review new results on the renormalization of HQET, on the Isgur-Wise function for $Λ_b \to Λ_c$ transitions and on the flavour-conserving one-pion transitions between heavy baryons.

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Theory of Heavy Baryon Decay

We discuss various topics in the theory of heavy baryon decays. Among these are recent applications of the Relativistic Three Quark Model to semileptonic, nonleptonic, one-pion and one-photon transitions among heavy baryons, new higher order perturbative results on the correlator of two heavy baryon currents and on the semi-inclusive decay $Λ_b \to X_c + D_s^{(*)-}$.

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Transcendental numbers and the topology of three-loop bubbles

We present a proof that all transcendental numbers that are needed for the calculation of the master integrals for three-loop vacuum Feynman diagrams can be obtained by calculating diagrams with an even simpler topology, the topology of spectacles.

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Polarized top decay into polarized W: t(up)->W(up) + b at O(alpha_s)

We consider the decay of a polarized top quark into a polarized W-boson plus a bottom quark, followed by the decay of the W-boson into a pair of leptons or quarks. The polar angle distribution of the top spin relative to the W-momentum and the polar angle distribution of the lepton (or quark) in the W-rest frame is governed by three polarized and three unpolarized rate functions which are related to the double density matrix elements of the decay $t \to W^+ + b$. We obtain analytical expressions for the $O(α_s)$ radiative corrections to the three polarized and three unpolarized rate functions. We also provide a comprehensive discussion of the dependence of the longitudinal, transverse and normal polarization of the top quarks produced at $e^+e^-$-colliders on beam polarization parameters.

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Gluon Polarization in e^+e^- -> t \bar t G: Polar Angle Dependence and Beam Polarization Effects

We calculate the linear and circular polarization of gluons produced in conjunction with massive quarks in the annihilation process $e^+e^-\to q \bar q G$. The linear polarization is calculated in the hadron event plane as well as in the gluon-beam plane. Beam polarization and polar orientation effects are included in our discussion. For typical top pair production energies at the Next-Linear-Collider (NLC) the degree of linear polarization in the hadron event plane remains close to its soft gluon value of 100% over most of the energy spectrum of the gluon. The linear polarization in the gluon-beam plane is generally smaller but peaks toward the hard end of the gluon spectrum. The dependence of the linear polarization on beam polarization and on the polar orientation of the gluon is small. The circular polarization is largest for maximal gluon energies and shows a strong dependence on the longitudinal beam polarization. The longitudinal polarization of the beam may therefore be used to tune the circular polarization of the gluon. The massive quark results are compared with the corresponding results for the massless quark case.

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On the rigidity of back-to-back top quark pairs in e^+e^- annihilation

We consider the effect of gluon radiation on the energy of top/antitop quarks and on the anticollinearity of top-antitop quark pairs produced in $e^+e^-$ annihilation. Our results are presented in terms of the $E_q$-dependence of the $t\bar tg$ cross section and the dependence on the cosine of the opening angle $θ_{12}$ between top and antitop for a center of mass energy of $\sqrt{q^2}=500 GeV$. We then go on to determine mean values for the top quark's energy as well as its longitudinal and transverse projections, and for the deviation of $\sinθ_{12}$ and $\cosθ_{12}$ from the anticollinearity limits $\sinθ_{12}=0$ and $\cosθ_{12}=-1$. For a center of mass energy of $500 GeV$ we obtain $ =248.22 GeV$, $ =247.24 GeV$ and $ =4.70 GeV$. Thus, at this energy gluon radiation causes a total average energy loss of 0.71% of the top quark's energy. The average energy loss in the longitudinal direction is 1.06% and the average energy gain in the transverse direction is 1.88%. These percentage figures go up to 3.77%, 5.19% and 6.06%, respectively, at $1000\GeV$. For the mean of the acollinearity angle $\barθ_{12}=180^0-θ_{12}$ we obtain $<\barθ_{12}>=1.25^0$ at $500 GeV$, the value of which goes up to $4.62^0$ at $1000 GeV$. From an analysis of the transverse momentum of the top we find that the mean transverse momentum of the top stays close to the mean total momentum of the gluon in the energy range from threshold to $1000 GeV$ showing that the gluon momentum has a large mean transverse component in this energy range.

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Massive Two-Loop Integrals and Higgs Physics

We describe in some detail the present features of an automatic loop calculation program as well as the integration techniques that go into it. The program, called XLOOPS 1.0, allows one to calculate massive one- and two-loop Feynman diagrams in the Standard Model including their tensor structure. UV divergences in UV divergent integrals are explicitly computed in dimensional regularization. One-loop integrals are calculated analytically in d \neq 4 dimensions whereas two-loop integrals are reduced to two-fold integral representations which the program evaluates numerically. We discuss Higgs decay at the two-loop level as a first application of the novel integration techniques that are incorporated into XLOOPS.

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Infinite Momentum Frame Calculation of Semileptonic Heavy Λ_b\toΛ_c Transitions Including HQET Improvements

We calculate the transition form factors that occur in heavy $Λ$-type baryon semileptonic decays as e.g. in $Λ_{b} \to Λ_{c}^{+} + l^{-} + \barν_{l} $. We use Bauer-Stech-Wirbel type infinite momentum frame wave functions for the heavy $Λ$-type baryons which we assume to consist of a heavy quark and a light spin-isospin zero diquark system. The form factors at $ q^{2} = 0 $ are calculated from the overlap integrals of the initial and final $Λ$-type baryon states. To leading order in the heavy mass scale the structure of the form factors agrees with the HQET predictions including the normalization at zero recoil. The leading order $ω$-dependence of the form factors is extracted by scaling arguments. By comparing the model form factors with the HQET predictions at ${\cal O}(1/m_{Q})$ we obtain a consistent set of model form factors up to ${\cal O}(1/m_{Q})$. With our preferred choice of parameter values we find that the contribution of the non-leading form factor is practically negligible. We use our form factor predictions to compute rates, spectra and various asymmetry parameters for the semi-leptonic decay $Λ_{b} \to Λ_{c}^{+} + l^{-} + \barν_{l} $.

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O(α_s) Corrections to Longitudinal Spin-Spin Correlations in e+e- -> q qbar

We calculate the $O(α_s)$ corrections to longitudinal spin-spin correlations in $e^+e^-\to q\bar q$. For top quark pair production the $O(α_s)$ corrections to the longitudinal spin-spin asymmetry amount to less than 1% in the $q^2$-range from above $t\bar t$-treshold up to $\sqrt{q^2}= 1000 GeV$. In the $e^+e^-\to b\bar b$ case the $O(α_s)$ corrections reduce the asymmetry value from its $m=0$ value of -1 to approximately -0.96 for $q^2$-values around the Z-peak. This reduction can be traced to finite anomalous contributions from residual mass effects which survive the $m\to 0$ limit. We discuss the role of the anomalous contributions and the pattern of how they contribute to spin-flip and no-flip terms.

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Gluon Polarization in $e^+e^-\to t\bar tG$

We calculate the linear polarization of gluons radiated off top quarks produced in e^+e^- annihilations. For typical top pair production energies at the Next-Linear-Collider (NLC) the degree of linear polarization remains close to its soft gluon value of 100% over almost the whole energy spectrum of the gluon. The massive quark results are compared with the corresponding results for the massless quark case.

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Resummation analysis of the $τ$ decay width using the four-loop $β$-function

We extract the strong coupling constant $α_s(m_τ^2)$ from the semileptonic $τ$ decay width taking into account resummation effects from the running of the strong coupling constant. In the $\overline{\rm MS}$ scheme the result reads $α_s=0.375\pm 0.007$ to third order and $α_s=0.378\pm 0.007$ to fourth order in the $β$-function, respectively, where we use the recently computed four-loop coefficient $β_3$. These values for the coupling constant have to be compared with the value $α_s=0.354\pm 0.005$ derived from a third order analysis of $τ$ decays. We determine the exact value of the convergence radius of the perturbation series by analyzing the singularity structure of the complex coupling constant plane.

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An Analysis of Diagonal and Non-diagonal QCD Sum Rules for Heavy Baryons at Next-to-Leading Order in α_S

We consider diagonal and non-diagonal QCD sum rules for the ground state heavy baryons to leading order in $1/m_Q$ and at next-to-leading order in $α_S$. In the non-diagonal case we evaluate the eight different two-loop diagrams which determine the perturbative $α_S$-corrections to the Wilson coefficient of the quark condensate in the Operator Product Expansion. The QCD corrections to the non-diagonal sum rules are moderate compared to the QCD corrections in the diagonal case. We also consider constituent type sum rules using constituent type interpolating currents. The obtained results are in reasonable agreement with the corresponding results obtained in the diagonal case. As central values for the bound state energies we find $m(Λ_Q)-m_Q = 760 MeV$ and $m(Σ_Q)-m_Q = 940 MeV$. The central values for the residues are given by $F(Λ_Q) = 0.030 GeV^3$ and $F(Σ_Q) = 0.038 GeV^3$.

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Perturbative relations between $e^+e^-$ annihilation and $τ$ decay observables including resummation effects

By exploiting the analyticity properties of the two-point current-current correlator we obtain numerical predictions for the $e^+e^-$ moments in terms of the $τ$ decay rate. We perform a partial resummation of the pertinent perturbative series expansion by solving the renormalization group equation for Adler's function. Our predictions are renormalization scheme independent but depend on the order of the perturbative $β$-function expansion. The analysis involves the unknown five-loop coefficient $k_3$ for which we give some new estimates.

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Inclusive Semileptonic Decays of Polarized $Λ_b$ Baryons into Polarized $τ$-Leptons

We employ OPE techniques within HQET to calculate the inclusive semileptonic decays of polarized $Λ_b$ baryons. Lepton mass effects are included which enables us to also discuss rates into polarized $τ$-leptons. We present explicit results for the longitudinal polarization of the $τ$ in the $Λ_b$ rest frame as well as in the $(τ^-, \bar ν_τ)$ c.m. frame. In both cases we make use of novel calculational techniques which considerably simplify the calculations. The transverse polarization components of the $τ$ are calculated in the $(τ^-,\barν_τ)$ c.m. frame. We delineate how to measure the full set of 14 polarized and unpolarized structure functions of the decay process by angular correlation measurements. A set of observables are identified that allow one to isolate the contributions of the two $O(1/m_b^2)$ nonperturbative matrix elements $K_b$ and $ε_b$.

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New high order relations between physical observables in perturbative QCD

We exploit the fact that within massless perturbative QCD the same Green's function determines the hadronic contribution to the $τ$ decay width and the moments of the $e^+e^-$ cross section. This allows one to obtain relations between physical observables in the two processes up to an unprecedented high order of perturbative QCD. A precision measurement of the $τ$ decay width allows one then to predict the first few moments of the spectral density in $e^+e^-$ annihilations integrated up to $s\sim m_τ^2$ with high accuracy. The proposed tests are in reach of present experimental capabilities.

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