arXiv subjects

B. A. Kniehl

Publications and source records attributed to B. A. Kniehl.

At least 163 records · Page 9Linked to original sources

arXiv1996

Electroweak Physics at HERA: Introduction and Summary

A high luminosity upgrade of HERA will allow the measurement of standard model parameters and the neutral current couplings of quarks. These results will have to be consistent with other precision measurements or indicate traces of new physics. The analysis of $W$ production will complement future results of LEP 2 and the Tevatron. We summarize the main results and conclusions obtained by the working group on Electroweak Physics concerning the potential of future experimentation at HERA.

hep-ph↗

arXiv1996

Dispersion Relations in Loop Calculations

These lecture notes give a pedagogical introduction to the use of dispersion relations in loop calculations. We first derive dispersion relations which allow us to recover the real part of a physical amplitude from the knowledge of its absorptive part along the branch cut. In perturbative calculations, the latter may be constructed by means of Cutkosky's rule, which is briefly discussed. For illustration, we apply this procedure at one loop to the photon vacuum-polarization function induced by leptons as well as to the $γf\bar f$ vertex form factor generated by the exchange of a massive vector boson between the two fermion legs. We also show how the hadronic contribution to the photon vacuum polarization may be extracted from the total cross section of hadron production in $e^+e^-$ annihilation measured as a function of energy. Finally, we outline the application of dispersive techniques at the two-loop level, considering as an example the bosonic decay width of a high-mass Higgs boson.

hep-ph↗

arXiv1996

Heavy-Higgs Lifetime at Two Loops

The Standard-Model Higgs boson with mass $ M_H >> 2M_Z $ decays almost exclusively to pairs of $W$ and $Z$ bosons. We calculate the dominant two-loop corrections of $ O( G_F^2 M_H^4 )$ to the partial widths of these decays. In the on-mass-shell renormalization scheme, the correction factor is found to be $ 1 + 14.6 % (M_H/TeV)^2 + 16.9 % (M_H/TeV)^4 $, where the second term is the one-loop correction. We give full analytic results for all divergent two-loop Feynman diagrams. A subset of finite two-loop vertex diagrams is computed to high precision using numerical techniques. We find agreement with a previous numerical analysis. The above correction factor is also in line with a recent lattice calculation.

hep-ph↗

arXiv1996

Higgs Physics at LEP2

In this report we review the prospects for Higgs physics at LEP2. The theoretical aspects and the phenomenology of Higgs particles are discussed within the Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM). The experimental search techniques are described and the discovery limits for Higgs bosons in the LEP2 energy range are summarized. In addition, opportunities of detecting Higgs particles in non-minimal extensions of the SM and the MSSM are investigated.

hep-ph↗

arXiv1996

Inclusive Hadron Production in Photon-Photon Collisions at Next-to-Leading Order

We study inclusive charged-hadron production in collisions of quasireal photons at NLO in perturbative QCD, using fragmentation functions recently extracted from PEP and LEP1 data. We superimpose the direct (DD), single-resolved (DR), and double-resolved (RR) gamma-gamma channels. First, we confront existing data taken by TASSO at PETRA and by MARK II at PEP with our NLO calculations. We also make comparisons with the neutral-kaon to charged-hadron ratio measured by MARK II. Then, we present NLO predictions for LEP2, a next-generation e+e- linear collider (NLC) in the TESLA design, and a Compton collider obtained by converting a NLC. We analyze transverse-momentum and rapidity spectra with regard to the scale dependence, the interplay of the DD, DR, and RR components, the sensitivity to the gluon density in the resolved photon, and the influence of gluon fragmentation. It turns out that the inclusive measurement of small-p_T hadrons at a Compton collider would greatly constrain the gluon density of the photon and the gluon fragmentation function.

hep-ph↗

arXiv1995

Large-$p_\perp$ Heavy-Quark Production in Two-Photon Collisions

The next-to-leading-order (NLO) cross section for the production of heavy quarks at large transverse momenta ($p_\perp$) in $γγ$ collisions is calculated with perturbative fragmentation functions (PFF's). This approach allows for a resummation of terms $\proptoα_s\ln(p_\perp^2/m^2)$ which arise in NLO from collinear emission of gluons by heavy quarks at large $p_\perp$ or from almost collinear branching of photons or gluons into heavy-quark pairs. We present single-inclusive distributions in $p_\perp$ and rapidity including direct and resolved photons for $γγ$ production of heavy quarks at $e^+e^-$ colliders and at high-energy $γγ$ colliders. The results are compared with the fixed-order calculation for $m$ finite including QCD radiative corrections. The two approaches differ in the definitions and relative contributions of the direct and resolved terms, but essentially agree in their sum. The resummation of the $α_s \ln(p_\perp^2/m^2)$ terms in the PFF approach leads to a softer $p_\perp$ distribution and to a reduced sensitivity to the choice of the renormalization and factorization scales.

hep-ph↗

arXiv1995

Three-Loop O(alpha_s^2 G_F M_t^2) Corrections to Higgs Production and Decay at e^+e^- Colliders

We evaluate the next-to-leading-order QCD corrections of O(alpha_s^2 G_F M_t^2) to the Standard-Model l^+l^-H, ZZH, and W^+W^-H couplings in the heavy-top- quark limit. Exploiting these results together with knowledge of Delta rho to the same order, we analyze a variety of production and decay processes of low- mass Higgs bosons at e^+e^- colliders. Specifically, we consider H->l^+l^-, H->l^+l^-l'^+l'^-, e^+e^-->ZH, Z->ff-barH, and e^+e^-->ff-barH, with f=l,nu. We work in the electroweak on-shell scheme formulated with G_F and employ both the on-shell and MS-bar definitions of the top-quark mass in QCD. As expected, the scheme and scale dependences are greatly reduced when the next-to-leading- order corrections are taken into account. In the on-shell scheme of top-quark mass renormalization, the O(alpha_s^2 G_F M_t^2) corrections act in the same direction as the O(alpha_s G_F M_t^2) ones and further increase the screening of the O(G_F M_t^2) terms. The coefficients of (alpha_s/pi)^2 range from -6.847 for the ZZH coupling to -16.201 for the l^+l^-H coupling. This is in line with the value -14.594 recently found for Delta rho.

hep-ph↗

arXiv1995

Dependence of Electroweak Parameters on the Definition of the Top-Quark Mass

The QCD corrections to electroweak parameters depend on the renormalization scheme and scales used to define the top-quark mass. We analyze these dependences for the W-boson mass predicted via Delta r to O(alpha alpha_s) and O(alpha alpha_s^2) in the on-shell and MS-bar schemes. These variations provide us with a hint on the magnitude of the unknown higher-order QCD effects and contribute to the theoretical error of the prediction.

hep-ph↗

arXiv1995

Virtual Top Effects on Low-Mass Higgs Interactions at Next-to-Leading Order in QCD

We present the next-to-leading-order QCD corrections of O(alpha_s^2 G_F M_t^2) to the low-M_H effective l^+l^-H, ZZH, and W^+W^-H interaction Lagrangians in the high-M_t limit. In the on-shell scheme formulated with G_F, the O(alpha_s^2 G_F M_t^2) corrections support the O(alpha_s G_F M_t^2) ones and further increase the screening of the O(G_F M_t^2) terms. The coefficients of (alpha_s/pi)^2 range from -6.847 to -16.201, being in line with the value -14.594 recently found for Delta rho. All four QCD expansions converge considerably more rapidly, if they are written with mu_t=m_t(mu_t), where m_t(mu) is the MS-bar mass, rather than the pole mass, M_t.

hep-ph↗

arXiv1995

Neutral-Kaon Production in e^+e^-, ep, and p\bar p Collisions at Next-to-Leading Order

We present new sets of fragmentation functions for neutral kaons, both at leading and next-to-leading order. They are fitted to data on inclusive K^0 production in e^+e^- annihilation taken by MARK II at PEP (\sqrt s = 29 GeV) and by ALEPH at LEP. Our fragmentation functions lead to a good description of other e^+e^- data on inclusive K^0 production at various energies. They also nicely agree with the K_S^0 transverse-momentum spectra measured by H1 at the DESY ep collider HERA, by UA5 at the CERN Sp\bar pS Collider, and by CDF at the Fermilab Tevatron.

hep-ph↗

arXiv1995

Cross Sections for Charm Production in $ep$ Collisions: Massive versus Massless Scheme

The next--to--leading order inclusive cross section for large-$p_\perp$ photoproduction of charm quarks at HERA is calculated in two different approaches. In the first approach the charm quarks are treated as massive objects which are strictly external to the proton and the photon while in the second approach the charm mass is neglected and the $c$ quark is assumed to be one of the active flavours in the proton and photon structure functions. We present single-inclusive distributions in transverse momentum and rapidity including direct and resolved photons. The cross section in the massless approach is found to be significantly larger than in the massive scheme. The deviation originates from several contributions which are disentangled. We argue that large-$p_\perp$ photoproduction of charm quarks at HERA will be sensitive to the charm content of the photon structure function.

hep-ph↗

arXiv1995

Pion and Kaon Production in $e^+e^-$ and $ep$ Collisions at Next-to-Leading Order

We present new sets of fragmentation functions for charged pions and kaons, both at leading and next-to-leading order. They are fitted to data on inclusive charged-hadron production in $e^+e^-$ annihilation taken by TPC at PEP ($\sqrt s=29$~GeV) and to similar data by ALEPH at LEP, who discriminated between events with charm, bottom, and light- flavour fragmentation in their charged-hadron sample. We treat all partons independently and to properly incorporate the charm and bottom thresholds. Due to the sizeable energy gap between PEP and LEP, we are sensitive to the scaling violation in the fragmentation process, which allows us to extract a value for the asymptotic scale parameter of QCD, $Λ$. Recent data on inclusive charged-hadron production in tagged three-jet events by OPAL and similar data for longitudinal electron polarization by ALEPH allow us to pin down the gluon fragmentation functions. Our new fragmentation functions lead to an excellent description of a multitude of other $e^+e^-$ data on inclusive charged-hadron production, ranging from $\sqrt s=5.2$~GeV to LEP energy. In addition, they agree nicely with the transverse-momentum spectra of single charged hadrons measured by H1 and ZEUS in photoproduction at the $ep$ collider HERA, which represents a nontrivial check of the factorization theorem of the QCD-improved parton model.

hep-ph↗

arXiv1994

Diffractive Photoproduction of Jets with a Direct Pomeron Coupling at HERA

We investigate in detail the effect of a direct pomeron coupling to quarks on the production of jets in $ep$ scattering with almost real photons. Jet production via a direct pomeron coupling is compared with the resolved--pomeron mechanism. We consider both direct and resolved photoproduction. Rapidity and transverse momentum distributions are calculated and compared with preliminary H1 and ZEUS data.

hep-ph↗

arXiv1994

Next-to-Leading Order Fragmentation Functions for Pions and Kaons

We present new sets of fragmentation functions for charged pions and kaons, both at leading and next-to-leading order. They are fitted to TPC data taken at energy $\sqrt s=29$~GeV and describe excellently a wealth of other $e^+e^-$ data on charged-hadron production, ranging from $\sqrt s=5.2$~GeV way up to LEP~1 energy. They also agree with data on the production of neutral pions and kaons, if one makes the natural assumption that the respective fragmentation functions are related to the charged counterparts by SU(2) symmetry. We also list simple parameterizations of the $x$ and $Q^2$ dependence of our results, which may be implemented conveniently in applications.

hep-ph↗

arXiv1994

Quantum Effects on Higgs-Boson Production and Decay due to Majorana Neutrinos

We analyze the phenomenological implications for new electroweak physics in the Higgs sector in the framework of SU(2)_L x U(1)_Y theories that naturally predict heavy Majorana neutrinos. We calculate the one-loop Majorana-neutrino contributions to the decay rates of the Higgs boson into pairs of quarks and intermediate bosons and to its production cross section via bremsstrahlung in e^+e^- collisions. It turns out that these are extremely small in three- generation models. On the other hand, the sizeable quantum corrections generated by a conventional fourth generation with a Dirac neutrino may be screened considerably in the presence of a Majorana degree of freedom.

hep-ph↗

arXiv1993

CP Violation in Higgs Decays due to Majorana Neutrinos

The possibility of observing CP violation in the decays of Higgs particles into top-quark, W- and Z-boson pairs induced by heavy Majorana neutrinos is discussed. In the context of minimal "see-saw" models with interfamily mixings, we find that Majorana neutrinos may give rise to sizable CP-odd effects at the one-loop electroweak order. We present numerical estimates of typical CP-odd observables that might be triggered at high-energy e+e- colliders.

hep-ph↗

arXiv1993

CP Violation Induced by Heavy Majorana Neutrinos in the Decays of Higgs Scalars into Top-Quark, W- and Z-Boson Pairs

We analyze the possibility of $CP$ violation induced by heavy Majorana neutrinos in the decays of the Higgs particle into top-quark, $W$- and $Z$-boson pairs. In the framework of various ``see-saw" models with interfamily mixings, we find that Majorana neutrinos may give rise to sizable $CP$-odd observables at the one-loop electroweak order. Numerical estimates of these $CP$-violating effects that may be detected in high-energy colliders are presented.

hep-ph↗

arXiv1993

Higgs Bosons: Intermediate Mass Range at e+e- Colliders

We elaborate on the production of the Standard Model Higgs particle at high-energy $e^+e^-$ colliders through the reaction $e^+e^- \rightarrow ZH$. Particular emphasis is put on the intermediate mass range. In addition to the signal we discuss in detail the background processes. Angular distributions which are sensitive to the spin and parity of the Higgs particle are analyzed.

hep-ph↗