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Bernd A. Kniehl

Publications and source records attributed to Bernd A. Kniehl.

At least 127 records · Page 7Linked to original sources
arXiv2002

Hadron Production in Hadron-Hadron and Lepton-Hadron Collisions

We review a recent global analysis of inclusive single-charged-hadron production in high-energy colliding-beam experiments, which is performed at next-to-leading order (NLO) in the parton model of quantum chromodynamics endowed with nonperturbative fragmentation functions (FFs). Comparisons of p p-bar data from CERN S p p-bar S and the Fermilab Tevatron and gamma p data from DESY HERA with the corresponding NLO predictions allow for quantitative tests of the scaling violations in the FFs and their universality. We emphasize the potential of new measurements at the Tevatron to place tight constraints in the large-x region and on the gluon FF, complementary to those from e^+ e^- data, which are indispensible in order to reliably predict the pi^0 background for the H -> gamma gamma signal of the intermediate-mass Higgs boson at the Tevatron and the CERN LHC. Adopting a similar theoretical framework for b-hadron production, we show that the notorious excess of the Tevatron data over existing theoretical calculations can be ascribed, at sufficiently large values of p_T, to nonperturbative fragmentation effects inadequately included previously.

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arXiv2002

Quarkonium production in deep-inelastic scattering

We discuss the inclusive production of J/psi mesons in deep-inelastic scattering (DIS) via the electromagnetic, weak neutral, and charged currents within the factorization formalism of nonrelativistic quantum chromodynamics. Theoretical predictions are confronted with experimental data of e p and nu N DIS taken by the H1 Collaboration at DESY HERA and the CHORUS Collaboration at CERN, respectively.

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arXiv2002

Associated Production of Heavy Quarkonia and Electroweak Bosons at Present and Future Colliders

We investigate the associated production of heavy quarkonia, with angular-momentum quantum numbers ^{2S+1}L_J = ^1S_0, ^3S_1, ^1P_1, ^3P_J (J = 0, 1, 2), and photons, Z bosons, and W bosons in photon-photon, photon-hadron, and hadron-hadron collisions within the factorization formalism of nonrelativistic quantum chromodynamics providing all contributing partonic cross sections in analytic form. In the case of photoproduction, we also include the resolved-photon contributions. We present numerical results for the processes involving J/psi and chi_{cJ} mesons appropriate for the Fermilab Tevatron, CERN LHC, DESY TESLA, operated in the e^+ e^- and gamma gamma modes, and DESY THERA.

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arXiv2002

QCD Corrections to t anti-b H^- Associated Production in e^+ e^- Annihilation

We calculate the QCD corrections to the cross section of e^+ e^- -> t anti-b H^- and its charge-conjugate counterpart within the minimal supersymmetric extension of the Standard Model. This process is particularly important if m_t < m_H + m_b and root(s) < 2 m_H, so that t -> b H^+ and e^+ e^- -> H^+ H^- are not allowed kinematically. Large logarithmic corrections that arise in the on-mass-shell scheme of quark mass renormalization, especially from the t anti-b H^- Yukawa coupling for large values of tan(beta), are resummed by adopting the modified minimal-subtraction scheme, so that the convergence behavior of the perturbative expansion is improved. The inclusion of the QCD corrections leads to a significant reduction of the theoretical uncertainties due to scheme and scale dependences.

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arXiv2002

Elimination of Threshold Singularities in the Relation Between On-Shell and Pole Widths

In a previous communication by two of us, Phys. Rev. Lett. 81, 1373 (1998), the gauge-dependent deviations of the on-shell mass and total decay width from their gauge-independent pole counterparts were investigated at leading order for the Higgs boson of the Standard Model. In the case of the widths, the deviation was found to diverge at unphysical thresholds, m_H = 2 root{xi_V} m_V (V = W,Z), in the R_xi gauge. In this Brief Report, we demonstrate that these unphysical threshold singularities are properly eliminated if a recently proposed definition of wave-function renormalization for unstable particles is invoked.

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arXiv2002

J/psi Inclusive Production in nu N Neutral-Current Deep-Inelastic Scattering

We calculate the cross section of J/psi inclusive production in neutrino-nucleon deep-inelastic scattering via the weak neutral current within the factorization formalism of nonrelativistic quantum chromodynamics. Besides J/psi single production via the Z-gluon fusion mechanism, we also consider J/psi plus hadron-jet associated production. We take into account both direct production and feed-down from directly-produced heavier charmonia. We present theoretical predictions for the J/psi transverse-momentum and rapidity distributions, which can be measured in the CHORUS and NOMAD experiments at CERN, including conservative error estimates. In order to interpret a recent CHORUS measurement of the total cross section, we also estimate the contribution due to J/psi prompt production via diffractive processes using the vector-meson dominance model.

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arXiv2002

Width and Partial Widths of Unstable Particles in the Light of the Nielsen Identities

Fundamental properties of unstable particles, including mass, width, and partial widths, are examined on the basis of the Nielsen identities (NI) that describe the gauge dependence of Green functions. In particular, we prove that the pole residues and associated definitions of branching ratios and partial widths are gauge independent to all orders. A simpler, previously discussed definition of branching ratios and partial widths is found to be gauge independent through next-to-next-to-leading order. It is then explained how it may be modified in order to extend the gauge independence to all orders. We also show that the physical scattering amplitude is the most general combination of self-energy, vertex, and box contributions that is gauge independent for arbitrary s, discuss the analytical properties of the NI functions, and exhibit explicitly their one-loop expressions in the Z-gamma sector of the Standard Model.

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arXiv2001

J/psi Inclusive Production in ep Deep-Inelastic Scattering at DESY HERA

We calculate the cross section of J/psi plus jet associated production in ep deep-inelastic scattering within the factorization formalism of nonrelativistic quantum chromodynamics. Our analytic results disagree with previous analyses, both for the colour-singlet and colour-octet channels. Our theoretical predictions agree reasonably well with recent data taken by the H1 Collaboration at DESY HERA, significantly better than those obtained within the colour-singlet model.

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arXiv2001

Theoretical Aspects of Standard-Model Higgs-Boson Physics at a Future e^+ e^- Linear Collider

The Higgs boson is the missing link of the Standard Model of elementary particle physics. We review its decay properties and production mechanisms at a future e^+ e^- linear collider and its e^- e^-, e^+- gamma, and gamma gamma modes, with special emphasis on the influence of quantum corrections. We also discuss how its quantum numbers and couplings can be extracted from the study of appropriate final states.

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arXiv2001

On the Field Renormalization Constant for Unstable Particles

A recently proposed normalization condition for the imaginary part of the self-energy of an unstable particle is shown to lead to a closed expression for the field renormalization constant Z. In turn, the exact expression for Z is necessary, in some important cases, in order to avoid power-like infrared divergences in high orders of perturbation theory. In the same examples, the width plays the rôle of an infrared cutoff and, consequently, Z is not an analytic function of the coupling constant.

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arXiv2001

Nonabelian alpha_s^3/(m_q r^2) heavy-quark-antiquark potential

We calculate the leading mass-suppressed two-loop contribution, proportional to alpha_s^3/(m_q r^2), to the potential of a heavy quark-antiquark system. This contribution originates in the nonabelian nature of quantum chromodynamics (QCD) and has no analogue in quantum electrodynamics (QED). For this purpose, we elaborate a technique based on the implementation of the threshold expansion within the effective theory of nonrelativistic QCD (NRQCD). We discuss phenomenological implications of our result for heavy-quarkonium physics. We also confirm a previous result for the two-loop O(alpha_s^2) correction to the static potential.

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arXiv2001

Theoretical Aspects of Inclusive Light-Hadron Production

We summarize the key results of a recent global analysis of inclusive single-charged-hadron production in high-energy colliding-beam experiments. In the framework of the parton model of quantum chromodynamics at next-to-leading order (NLO), fragmentation functions (FFs) for charged pions, charged kaons, and (anti) protons are extracted from experimental data of e^+e^- annihilation at the Z-boson resonance and at centre-of-mass energy root(s) = 29 GeV. This fit also yields a new NLO value for the strong-coupling constant, namely alpha_s^(5)(M_Z) = 0.1170 +- 0.0073. The scaling violations encoded in the FFs through the Altarelli-Parisi evolution equations are tested by confronting (e^+e^-)-annihilation data from DESY DORIS, DESY PETRA, and CERN LEP2 with NLO predictions based on these FFs. Comparisons of p p-bar data from CERN S p p-bar S and the Fermilab Tevatron, gamma p data from DESY HERA, and gamma gamma data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem.

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arXiv2000

Order alpha^7 ln(1/alpha) Contribution to Positronium Hyperfine Splitting

The logarithmically enhanced alpha^7 ln(1/alpha) contribution to the hyperfine splitting of the positronium ground-state energy levels is calculated in the framework of dimensionally regularized nonrelativistic quantum electrodynamics. The correction is negative and amounts to about 1/3 of the leading logarithmic alpha^7 ln^2(1/alpha) one. The discrepancy between the experimental measurements and the theoretical prediction is reduced.

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arXiv2000

Higgs-Boson Production and Decay Close to Thresholds

At one loop in the conventional on-mass-shell renormalization scheme, the production and decay rates of the Higgs boson H exhibit singularities proportional to (2 M_V - M)^{-1/2} as the Higgs-boson mass M approaches from below the pair-production threshold of a vector boson V with mass M_V. This problem is of phenomenological interest because the values 2 M_W and 2 M_Z, corresponding to the W- and Z-boson thresholds, lie within the M range presently favoured by electroweak precision data. We demonstrate how these threshold singularities are eliminated when the definitions of mass and total decay width of the Higgs boson are based on the complex-valued pole of its propagator. We illustrate the phenomenological implications of this modification for the partial width of the H -> W^+ W^- decay.

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arXiv2000

Order alpha^3 ln(1/alpha) Corrections to Positronium Decays

The logarithmically enhanced alpha^3 ln(1/alpha) corrections to the para- and orthopositronium decay widths are calculated in the framework of dimensionally regularized nonrelativistic quantum electrodynamics.In the case of parapositronium, the correction is negative, approximately doubles the effect of the leading logarithmic alpha^3 ln^2(1/alpha) one, and is comparable to the nonlogarithmic O(alpha^2) one. As for orthopositronium, the correction is positive and almost cancels the alpha^3 ln^2(1/alpha) one. The uncertainties in the theoretical predictions for the decay widths are reduced.

hep-ph↗
arXiv2000

Polarization of Prompt J/psi at the Tevatron

The polarization of prompt J/psi at the Fermilab Tevatron is calculated within the nonrelativistic QCD factorization framework. The contribution from radiative decays of P-wave charmonium states decreases, but does not eliminate, the transverse polarization at large transverse momentum. The angular distribution parameter alpha for leptonic decays of the J/ψis predicted to increase from near 0 at p_T = 5 GeV to about 0.5 at p_T = 20 GeV. The prediction is consistent with measurements by the CDF Collaboration at intermediate values of p_T, but disagrees by about 3 standard deviations at the largest values of p_T measured.

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arXiv2000

Polarized J/psi from chi_{cJ} and psi' Decays at the Tevatron

We calculate the partonic cross sections for the hadroproduction of polarized chi_{cJ} mesons using the factorization formalism of nonrelativistic QCD. We also describe how the polarization is passed on to the J/psi mesons in the radiative feed-down chi_{cJ} -> J/psi + gamma and in the double-cascade decays of psi' mesons via chi_{cJ} intermediate states. These represent the missing ingredients needed to predict the polarization of prompt J/psi mesons at the Fermilab Tevatron.

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