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Bernd A. Kniehl

Publications and source records attributed to Bernd A. Kniehl.

At least 109 records · Page 6Linked to original sources
arXiv2006

Two-loop virtual top-quark effect on Higgs-boson decay to bottom quarks

In most of the mass range encompassed by the limits from the direct search and the electroweak precision tests, the Higgs boson of the standard model preferably decays to bottom quarks. We present, in analytic form, the dominant two-loop electroweak correction, of O(G_F^2 m_t^4), to the partial width of this decay. It amplifies the familiar enhancement due to the O(G_F m_t^2) one-loop correction by about +16% and thus more than compensates the screening by about -8% through strong-interaction effects of order O(alpha_s G_F m_t^2).

hep-ph↗
arXiv2006

Simple Approach to Renormalize the Cabibbo-Kobayashi-Maskawa Matrix

We present an on-shell scheme to renormalize the Cabibbo-Kobayashi-Maskawa (CKM) matrix. It is based on a novel procedure to separate the external-leg mixing corrections into gauge-independent self-mass and gauge-dependent wave-function renormalization contributions, and to implement the on-shell renormalization of the former with non-diagonal mass counterterm matrices. Diagonalization of the complete mass matrix leads to an explicit CKM counterterm matrix, which automatically satisfies all the following important properties: it is gauge independent, preserves unitarity, and leads to renormalized amplitudes that are non-singular in the limit in which any two fermions become mass degenerate.

hep-ph↗
arXiv2006

Prompt J/psi plus photon associated electroproduction at DESY HERA

We study the production of a prompt J/psi meson in association with a prompt photon in ep deep-inelastic scattering within the factorisation formalism of non-relativistic quantum chromodynamics (NRQCD) and demonstrate that this process provides a clean probe of the colour-octet mechanism at DESY HERA. Our analysis is based on an updated set of non-perturbative NRQCD matrix elements obtained through a joint fit to data on charmonium inclusive hadroproduction from runs I and II at the Fermilab Tevatron.

hep-ph↗
arXiv2006

Charmed-Hadron Fragmentation Functions from CERN LEP1 Revisited

In Phys. Rev. D 58, 014014 (1998) and 71, 094013 (2005), we determined non-perturbative D^0, D^+, D^{*+}, D_s^+, and Lambda_c^+ fragmentation functions, both at leading and next-to-leading order in the MS-bar factorization scheme, by fitting e^+e^- data taken by the OPAL Collaboration at CERN LEP1. The starting points for the evolution in the factorization scale mu were taken to be mu_0 = 2m_Q, where Q = c,b. For the reader's convenience, in this Addendum, we repeat this analysis for mu_0 = m_Q, where the flavor thresholds of modern sets of parton density functions are located.

hep-ph↗
arXiv2006

Calculating four-loop tadpoles with one non-zero mass

An efficient method to calculate tadpole diagrams is proposed. Its capability is demonstrated by analytically evaluating two four-loop tadpole diagrams of current interest in the literature, including their $O(ε)$ terms in $D=4-2ε$ space-time dimensions.

hep-ph↗
arXiv2006

Light-hadron electroproduction at next-to-leading order and implications

We review recent results on the inclusive electroproduction of light hadrons at next-to-leading order in the parton model of quantum chromodynamics implemented with fragmentation functions and present updated predictions for HERA experiments based on the new AKK set. We also discuss phenomenological implications of these results.

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arXiv2005

Heavy-quarkonium production at next-to-leading order

We review recent progress in the description of heavy-quarkonium production in 2->2 processes at next-to-leading order in the factorization framework of nonrelativistic quantum chromodynamics. Specifically, we consider the production of prompt charmonium in association with a hadron jet or a prompt photon in two-photon collisions and exclusive double-charmonium production in e^+e^- annihilation.

hep-ph↗
arXiv2005

Dominant Two-Loop Electroweak Correction to H -> gamma gamma

We discuss a recent analysis of a dominant two-loop electroweak correction, of O(G_F M_t^2), to the partial width of the decay of an intermediate-mass Higgs boson into a pair of photons. The asymptotic-expansion technique was used in order to extract the leading dependence on the top-quark mass plus four expansion terms that describe the dependence on the W- and Higgs-boson masses. This correction reduces the Born result by approximately 2.5%. As a by-product of this analysis, also the O(G_F M_t^2) correction to the partial width of the Higgs-boson decay to two gluon jets was recovered.

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arXiv2005

D^0, D^+, D_s^+, and Lambda_c^+ Fragmentation Functions from CERN LEP1

We present new sets of nonperturbative fragmentation functions for D^0, D^+, and D_s^+ mesons as well as for Lambda_c^+ baryons, both at leading and next-to-leading order in the MSbar factorization scheme with five massless quark flavors. They are determined by fitting data of e^+e^- annihilation taken by the OPAL Collaboration at CERN LEP1. We take the charm-quark fragmentation function to be of the form proposed by Peterson et al. and thus obtain new values of the epsilon_c parameter, which are specific for our choice of factorization scheme.

hep-ph↗
arXiv2004

Next-to-leading-order predictions for D^{*+-} plus jet photoproduction at DESY HERA

We study the photoproduction of a D^{*+-} meson in association with a hadron jet at next-to-leading order in the parton model of QCD with non-perturbative fragmentation functions extracted from LEP1 data of e^+e^- annihilation. The transverse-momentum and rapidity distributions recently measured at DESY HERA in various kinematic ranges nicely agree with our theoretical predictions. This provides a useful test of the universality and the scaling violations of the fragmentation functions predicted by the factorization theorem. These comparisons also illustrate the significance of the charm component in the resolved photon. This is elaborated by investigating the cross-section distributions in x_obs^gamma and cos(theta^*).

hep-ph↗
arXiv2004

Relation between bottom-quark MS-bar Yukawa coupling and pole mass

We calculate the O(alpha alpha_s) corrections to the relationships between the MS-bar Yukawa couplings and the pole masses of the first five quark flavours in the standard model. We also present the corresponding relationships between the MS-bar and pole masses, which emerge as by-products of our main analysis. The occurring self-energies are evaluated using the method of asymptotic expansion.

hep-ph↗
arXiv2004

Two-loop electroweak correction of O(G_F M_t^2) to the Higgs-boson decay into photons

We compute the dominant two-loop electroweak correction, of O(G_F M_t^2), to the partial width of the decay of an intermediate-mass Higgs boson into a pair of photons. We use the asymptotic-expansion technique in order to extract the leading dependence on the top-quark mass plus four expansion terms that describe the dependence on the W- and Higgs-boson masses. This correction reduces the Born result by approximately 2.5%. As a by-product of our analysis, we also recover the O(G_F M_t^2) correction to the partial width of the Higgs-boson decay to two gluon jets.

hep-ph↗
arXiv2004

Associated Production of Bottomonia and Higgs Bosons at Hadron Colliders

We study the associated production of bottomonia and Higgs bosons at hadron colliders within the factorization formalism of nonrelativistic quantum chromodynamics providing all contributing partonic cross sections in analytic form. While such processes tend to be suppressed in the standard model, they may have interesting cross sections in its minimal supersymmetric extension, especially at large values of tan(beta), where the bottom Yukawa couplings are enhanced. We present numerical results for the processes involving the lighter CP-even h^0 boson and the CP-odd A^0 boson appropriate for the Fermilab Tevatron and the CERN LHC.

hep-ph↗
arXiv2003

Prompt J/psi production in charged-current deep-inelastic scattering

We calculate the cross section of J/psi inclusive production in lepton-nucleon deep-inelastic scattering via the weak charged current within the factorization formalism of nonrelativistic quantum chromodynamics. The hadronic system produced in association with the J/psi meson can be of light-quark origin or contain charm. We take into account both direct production and feed-down from directly-produced heavier charmonia. We provide the cross sections of all contributing partonic subprocesses in analytic form. We present theoretical predictions for the J/psi transverse-momentum and rapidity distributions, which can be measured in current neutrino-nucleon scattering experiments at CERN and Fermilab, and possibly at the future electron-proton collider THERA at DESY. We find the cross section to be at the edge of observability at DESY HERA.

hep-ph↗
arXiv2003

Heavy-quarkonium creation and annihilation with O(alpha_s^3 ln(alpha_s)) accuracy

We calculate the O(alpha_s^3 ln(alpha_s)) contributions to the heavy-quarkonium production and annihilation rates. Our result sheds new light on the structure of the high-order perturbative corrections and opens a new perspective for a high-precision theoretical analysis. We also determine the three-loop anomalous dimensions of the nonrelativistic vector and pseudoscalar currents.

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