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Bernd A. Kniehl

Publications and source records attributed to Bernd A. Kniehl.

At least 145 records · Page 8Linked to original sources
arXiv1999

Massive-Evolution Effects on Charmonium Hadroproduction

The fragmentation functions D_{a -> H}(x,mu^2) of a heavy hadron H, with mass m_H, satisfy the phase-space constraint D_{a - > H}(x,mu^2)=0 for x < m_H^2/mu^2, which is violated by the naive mu^2 evolution equations. Using appropriately generalized mu^2 evolution equations, we reconsider the inclusive hadroproduction of prompt J/psi mesons with high transverse momenta in the framework of the factorization formalism of nonrelativistic quantum chromodynamics, and determine the resulting shifts in the values of the leading colour-octet matrix elements, which are fitted to data from the Fermilab Tevatron.

hep-ph↗
arXiv1998

TEVATRON-HERA Colour-Octet Charmonium Anomaly Versus Higher-Order QCD Effects

Approximately taking into account the higher-order effects due to multiple-gluon initial-state radiation, we extract from the latest Tevatron data of prompt J/psi hadroproduction the leading colour-octet matrix elements within the nonrelativistic-QCD (NRQCD) factorization formalism proposed by Bodwin, Braaten, and Lepage. We find that the matrix elements which describe the formation of J/psi mesons from colour-octet cc-bar pairs in the angular-momentum states ^{2S+1}L_J = ^1S_0 and ^3P_J, with J = 0,1,2, which are responsible for the excess of the predicted cross section of inelastic J/psi photoproduction over the existing HERA data at high values of the inelasticity variable z, are significantly reduced. We conclude that it is premature to proclaim a discrepancy between the Tevatron and HERA measurements of inclusive J/psi production in the NRQCD framework. We also consider J/psi mesons originating from the radiative feed down of promptly produced chi_{cJ} mesons.

hep-ph↗
arXiv1997

Colour-Octet Contributions to J/psi Production via Fragmentation at HERA

We study J/psi photoproduction via fragmentation at next-to-leading order in the QCD-improved parton model, using the nonrelativistic factorization formalism proposed by Bodwin, Braaten, and Lepage. We predict that measurements of J/psi photoproduction at DESY HERA should show a distinctive excess over the expectation based on the colour-singlet model at small values of the inelasticity variable z.

hep-ph↗
arXiv1997

Theoretical aspects of hadron photoproduction

We review recent developments in the theoretical description of inclusive single-hadron production at next-to-leading order in the parton model of quantum chromodynamics. Fragmentation functions are extracted from fits to data of inclusive pion and kaon production in e^+e^- annihilation at different centre-of-mass energies. Exploiting sensitivity to the scaling violation, one can simultaneously fit the asymptotic scale parameter Lambda so as to obtain an independent determination of the strong coupling constant alpha_s. Owing to the factorization theorem, the fragmentation functions only depend on the species of the produced particles, but not on the process by which they are produced. This allows one to make absolute theoretical predictions for inclusive pion and kaon production in other types of experiments such as photon-photon, photon-hadron, or hadron-hadron scattering. Recent data of photoproduction taken by the H1 and ZEUS Collaborations at DESY HERA nicely agree with such next-to-leading-order predictions.

hep-ph↗
arXiv1997

Charmonium Production via Fragmentation at DESY HERA

The cross section for the photoproduction of large-p_T J/psi mesons at HERA is calculated at next-to-leading order, adopting a perturbative approach to describe the fragmentation of charm quarks and gluons into J/psi mesons. We treat the charm quark according to the massless factorization scheme, where it is assumed to be one of the active flavours inside the proton and the resolved photon. We present inclusive distributions in transverse momentum and rapidity, including the contributions due to direct and resolved photons. The importance of the colour-octet components of the J/psi wave function, which contribute to the fragmentation process, is emphasized. In addition to prompt J/psi production, we consider also the production of chi_{cJ} states followed by radiative decays to J/psi mesons, both in the colour-singlet and colour-octet channels.

hep-ph↗
arXiv1997

Color-Octet Contributions to J/psi Photoproduction via Fragmentation at HERA

We study J/psi photoproduction via fragmentation at next-to-leading order in the QCD-improved parton model, using the nonrelativistic factorization formalism proposed by Bodwin, Braaten, and Lepage. We consider direct and resolved photoproduction of prompt J/psi mesons and chi_{cJ} mesons radiatively decaying to J/psi+gamma, taking into account the formation of both color-singlet and color-octet c anti-c states. Adopting the values of the long-distance color-octet matrix elements extracted from fits to prompt-J/psi data recently taken at the Fermilab Tevatron, we predict that measurements of J/psi photoproduction at DESY HERA should show a distinctive excess over the expectation based on the color-singlet model at small values of the inelasticity variable z. This is complementary to the expected enhancement at z close to 1 due to the color-octet contribution to photon-gluon fusion.

hep-ph↗
arXiv1996

Theoretical Aspects of Higgs Physics

We review the present status of Higgs physics within the standard model and its extensions. First, we briefly summarize the current experimental exclusion limits from the direct searches with LEP1 and the Tevatron, and assess the discovery potential of LEP2. Then, we report the mass bounds resulting from global fits to the latest electroweak precision data, and compile various theoretical principles which lead to restrictions - or even determinations - of the Higgs-boson masses. Perturbative upper bounds are discussed in some detail. Finally, we survey the recent progress in the computation of higher-order radiative corrections to the bb-bar decay width of the standard-model Higgs boson.

hep-ph↗
arXiv1996

Two-Loop O(alpha_s G_F M_Q^2) Heavy-Quark Corrections to the Interactions between Higgs and Intermediate Bosons

By means of a low-energy theorem, we analyze at O(alpha_s G_F M_Q^2) the shifts in the Standard-Model W^+W^-H and ZZH couplings induced by virtual high-mass quarks, Q, with M_Q >> M_Z, M_H, which includes the top quark. Invoking the improved Born approximation, we then find the corresponding corrections to various four- and five-point Higgs-boson production and decay processes which involve the W^+W^-H and ZZH vertices with one or both of the gauge bosons being connected to light-fermion currents, respectively. This includes e^+e^- -> f anti-f H via Higgs-strahlung, via W^+W^- fusion (with f = nu_e), and via ZZ fusion (with f = e), as well as H -> 2V -> 4f (with V = W, Z).

hep-ph↗
arXiv1996

Mixing Renormalization in Majorana Neutrino Theories

The renormalization of general theories with inter-family mixing of Dirac and/or Majorana fermions is studied at the one-loop electroweak order. The phenomenological significance of the mixing-matrix renormalization is discussed, within the context of models based on the SU(2)_L x U(1)_Y gauge group. The effect of radiative neutrino masses present in these models is naturally taken into account in this formulation. As an example, charged-lepton universality in pion decays is investigated in the heavy- neutrino limit. Non-decoupling heavy-neutrino effects induced by mixing renormalization are found to considerably affect the predictions in these new-physics scenarios.

hep-ph↗
arXiv1995

Low-Energy Theorems in Higgs Physics

We present low-energy theorems for the calculation of loop amplitudes with external scalar or pseudoscalar Higgs bosons which are light compared to the loop particles. Starting from existing lowest-order versions of these theorems, we show how their applicability may be extended to the two--loop level. To illustrate the usefulness of these theorems, we discuss a number of applications to Higgs production and decay at and beyond the one--loop order.

hep-ph↗
arXiv1995

SEMILEPTONIC LEPTON-NUMBER/FLAVOUR-VIOLATING tau DECAYS IN MAJORANA NEUTRINO MODELS

Motivated by the recent investigation of neutrinoless $τ$-lepton decays by the CLEO collaboration, we perform a systematic analysis of such decays in a possible new-physics scenario with heavy Dirac/Majorana neutrinos, including heavy-neutrino nondecoupling effects, finite quark masses, and quark as well as meson mixings. We find that $τ$-lepton decays into an electron or muon and a pseudoscalar or vector meson can have branching ratios close to the experimental sensitivity. Numerical estimates show that the predominant decay modes of this kind are $τ^-\to e^-ϕ$, $τ^-\to e^-ρ^0$, and $τ^-\to e^-π^0$, with branching ratios of order $10^{-6}$.

hep-ph↗
arXiv1995

Two-Loop ${\cal O}(α_sG_Fm_t^2)$ Corrections to Higgs Production at LEP

We evaluate the two-loop ${\cal O}(α_sG_Fm_t^2)$ correction to the $ZZH$ coupling in the Standard Model by means of a low-energy theorem, assuming that the top quark is much heavier than the Higgs boson. We then construct a heavy-top-quark effective Lagrangian for the $ZZH$ interaction that accommodates the ${\cal O}(G_Fm_t^2)$ and ${\cal O}(α_sG_Fm_t^2)$ corrections and derive from it the corresponding corrections to the $H\to ZZ$ decay as well as those to Higgs-boson production at LEP1, via $Z\to f\bar fH$, and at LEP2, via $e^+e^-\to ZH$. In all cases, the leading ${\cal O}(G_Fm_t^2)$ terms are considerably screened by their QCD corrections, if the on-shell renormalization scheme with $G_F$ as a basic parameter is employed.

hep-ph↗
arXiv1994

Two-loop ${\rm O}\left(G_F^2M_H^4\right)$ corrections to the fermionic decay rates of the Higgs boson

We calculate the dominant ${\rm O}\left(G_F^2M_H^4\right)$ two-loop electroweak corrections to the fermi\-onic decay widths of a heavy Higgs boson in the Standard Model. Use of the Goldstone-boson equivalence theorem reduces the problem to one involving only the physical Higgs boson $H$ and the Goldstone bosons $w^\pm$ and $z$ of the unbroken theory. The two-loop corrections are opposite in sign to the one-loop electroweak corrections, exceed the one-loop corrections in magnitude for $M_H>1114\ {\rm GeV}$, and increase in relative magnitude as $M_H^2$ for larger values of $M_H$. We conclude that the perturbation expansion in powers of $G_FM_H^2$ breaks down for $M_H\approx 1100\ {\rm GeV}$. We discuss briefly the QCD and the complete one-loop electroweak corrections to $H\rightarrow b\bar{b}, \,t\bar{t}$, and comment on the validity of the equivalence theorem. Finally we note how a very heavy Higgs boson could be described in a phenomenological manner.

hep-ph↗
arXiv1994

Standard Model of the Electroweak Interaction: Theoretical Developments

We review recent theoretical progress in the computation of radiative corrections beyond one loop within the standard model of electroweak interactions, both in the gauge and Higgs sectors. In the gauge sector, we discuss universal corrections of $Ø(G_F^2M_H^2M_W^2)$, $Ø(G_F^2m_t^4)$, $Ø(α_sG_FM_W^2)$, and those due to virtual $t\bar t$ threshold effects, as well as specific corrections to $Γ\left(Z\to b\bar b\right)$ of $Ø(G_F^2m_t^4)$, $Ø(α_sG_Fm_t^2)$, and $Ø(α_s^3)$ including finite-$m_b$ effects. We also present an update of the hadronic contributions to $Δα$. Theoretical uncertainties, other than those due to the lack of knowledge of $M_H$ and $m_t$, are estimated. In the Higgs sector, we concentrate on $Γ\left(H\to f\bar f\,\right)$ and consider in $Ø(α_sG_Fm_t^2)$ the universal corrections and those which are specific for the $\scriptstyle b\bar b$ mode, as well as $Ø(α_s^2)$ corrections in the $q\bar q$ channels including the finite-$m_q$ terms.

hep-ph↗
arXiv1994

Status of Higher-Order Corrections in the Standard Electroweak Theory

We review recent theoretical progress in the computation of radiative corrections beyond one loop within the standard model of electroweak interactions, both in the gauge and Higgs sectors. In the gauge sector, we discuss universal corrections of $Ø(G_F^2M_H^2M_W^2)$, $Ø(G_F^2m_t^4)$, $Ø(α_sG_FM_W^2)$, $Ø(α_s^2G_Fm_t^2)$, and those due to virtual $t\bar t$ threshold effects, as well as specific corrections to $Γ\left(Z\to b\bar b\right)$ of $Ø(G_F^2m_t^4)$, $Ø(α_sG_Fm_t^2)$, and $Ø(α_s^3)$ including finite-$m_b$ effects. We also present an update of the hadronic contributions to $Δα$. Theoretical uncertainties, other than those due to the lack of knowledge of $M_H$ and $m_t$, are estimated. In the Higgs sector, we report on the $Ø(α_sG_Fm_t^2)$ corrections to $Γ\left(H\to f\bar f\,\right)$ including those which are specific for the $b\bar b$ mode, the $Ø(α_s^2)$ corrections to $Γ\left(H\to q\bar q\right)$ including the finite-$m_q$ terms, and the $Ø(α_s)$ corrections to $Γ(H\to gg)$.

hep-ph↗
arXiv1994

Two-Loop $ O(α_sG_Fm_t^2)$ Correction to the $H\to b\bar b$ Decay Rate

We present the two-loop ${\cal O}(α_sG_Fm_t^2)$ correction to the $b\bar b$ decay rate of the Stan\-dard-Model Higgs boson, assuming that the $t$ quark is much heavier than the Higgs boson. Apart from the universal correction connected with the renormalizations of the wave function and the vacuum expectation value of the Higgs field, this involves vertex corrections specific to the presence of beauty in the final state. We calculate the latter by means of a low-energy theorem. All would-be mass singularities related to the $b$ quark can be absorbed into the running Higgs-bottom Yukawa coupling. It turns out that the total ${\cal O}(α_sG_Fm_t^2)$ correction screens the leading high-$m_t$ behaviour of the one-loop result by 71\% to 75\% for $M_H$ between 60~GeV and $2M_W$.

hep-ph↗
arXiv1994

On the relation between the fermion pole mass and MSbar Yukawa coupling in the standard model

We present the full one-loop correction to the relation between the running MSbar Yukawa coupling, $\overline h_f(μ)$, at some renormalization scale, $μ$, and the pole mass, $m_f$, of a fermion, $f$, in the standard model. Our result complements previous analyses that included just the QCD correction to this relation in the case of quarks. This allows us to convert, without loss of information, the threshold value, $\overline h_f(m_f)$, of $\overline h_f(μ)$, which satisfies a renormalization-group equation and may be driven up to some high-energy scale of new physics, to physical observables at low energy. We investigate the limit of the Higgs boson and/or the top quark being much heavier than all the other particles. We also list simple and yet rather precise approximation formulae, which are convenient for applications.

hep-ph↗