Low energy electromagnetic processes based on the chiral effective field theory approach

Chiral effective field theory describes the interaction of nucleons and pions in the low-energy regime of QCD.This theory offers a consistent picture of nuclear forces and nuclear current operators.We study the electromagnetic processes based on ChEFT dynamical picture and compare our predictions to results obtained in the conventional framework.In particular,we consider low energy photo-disintegration of the deuteron at different orders of the chiral expansion.We investigate a role of different ingredients in the two-nucleon current operator.For the first time calculations involve consistent contributions from long-range two-pion exchange currents which appear at next-to-leading order of the chiral expansion.We present novel results for cross sections and various polarization observables.     

A next-to-leading order study of pion-pair production and comparison with E706 data

We discuss the production of pion pairs with a large invariant mass in hadronic collisions. We present a study based on a perturbative QCD calculation at full next-to-leading order accuracy, implemented in the computer programme DIPHOX. We provide a comparison of various predictions with the corresponding observables measured by the E706 fixed target experiment. Discrepancies between data and next-to-leading order calculation are carefully analysed. We classify the theoretical next-to-leading order distributions with respect to their infrared sensitivity, and explain which distributions need improvements. Further, we comment on the energy scale dependences of non pertubative effects. Received: 8 November 2001 / Published online: 5 April 2002     

Photoproduction of W bosons at HERA: QCD corrections

W bosons can be produced in the channels at HERA thus allowing to probe for anomalous trilinear couplings among the gauge bosons. We discuss the next-to-leading order (NLO) QCD corrections to the photoproduction of W bosons with finite transverse momentum at HERA. The higher-order QCD corrections reduce the factorization scale dependence significantly and modify the leading-order (LO) cross sections by . Received: 18 April 2002 / Published online: 30 August 2002 RID="*" ID="*" Present address: Deutsches Elektronen-Synchrotron DESY, 22603 Hamburg, Germany     

Measurements of the strong coupling constant and the QCD colour factors using four-jet observables from hadronic Z decays

Data from annihilation into hadrons, taken with the ALEPH detector at the Z resonance, are analyzed. The four-jet rate is studied as a function of the resolution parameter and compared to next-to-leading order calculations combined with resummation of large logarithms. Angular correlations in four-jet events are measured and compared to next-to-leading order QCD predictions. With these observables two different measurements are performed. In a first analysis the strong coupling constant is measured from the four-jet rate yielding . In a second measurement the strong coupling constant and the QCD colour factors are determined simultaneously from a fit to the four-jet rate and the four-jet angular correlations, giving in good agreement with the expectation from QCD. Received: 14 May 2002 / Published online: 15 January 2003     

ΔS = 0 effective weak chiral Lagrangianfrom the instanton vacuum

We investigate the ΔS = 0 effective chiral Lagrangian from the instanton vacuum. Based on the ΔS = 0 effective weak Hamiltonian from the operator product expansion and renormalization group equations, we derive the strangeness-conserving effective weak chiral Lagrangian from the instanton vacuum to order and the next-to-leading order in the 1/N_c expansion at the quark level. We find that the quark condensate and a dynamical term which arise from the QCD and electroweak penguin operators appear in the next-to-leading order in the 1/N_c expansion for the ΔS = 0 effective weak chiral Lagrangian, while they are in the leading order terms in the ΔS = 1 case. Three different types of form factors are employed and we find that the dependence on the different choices of the form factor is rather insensitive. The low-energy constants of the Gasser-Leutwyler type are determined and discussed in the chiral limit. Arrival of the final proofs: 2 December 2005 PACS: 12.40.-y, 14.20.Dh     

Chiral extrapolation of nucleon wave function normalization constants

Within the framework of two-flavor covariant baryon chiral perturbation theory we have expressed the Chernyak-Zhitnitsky, Ioffe and Dosch currents in terms of chiral fields to provide leading one-loop extrapolation formulae for the leading and next-to-leading twist normalization constants f _N , λ₁ and λ₂. Finite-volume effects due to pion loops have been taken into account. The occurring low-energy constants are fitted to data obtained from recent lattice QCD simulations in order to extract the values at the physical point.     

QCD corrections to J/psi and Upsilon production at hadron colliders

We calculate the cross section for hadroproduction of a pair of heavy quarks in a (3)S(1) color-singlet state at next-to-leading order in QCD. This corresponds to the leading contribution in the nonrelativistic QCD expansion for J/psi and Upsilon production. The higher-order corrections have a large impact on the p(T) distributions, enhancing the production at high p(T) at both the Fermilab Tevatron and the CERN Large Hadron Collider. The total decay rate of a (3)S(1) into hadrons at next-to-leading order is also computed, confirming for the first time the result obtained by Mackenzie and Lepage in 1981.     

A detailed next-to-leading order QCD analysis of deeply virtual Compton scattering observables

We present a detailed next-to-leading order (NLO) leading twist QCD analysis of deeply virtual Compton scattering (DVCS) observables, for several different input scenarios, in the scheme. We discuss the size of the NLO effects and the behavior of the observables in skewedness , momentum transfer,t, and photon virtuality, . We present results on the amplitude level for unpolarized and longitudinally polarized lepton probes, and unpolarized and longitudinally polarized proton targets. We make predictions for various asymmetries and for the DVCS cross section and compare with the available data. Received: 30 November 2001 / Revised version: 12 February 2002 / Published online: 15 March 2002     

K– mixing in the 1/Nc expansion

We present the result for the invariant factor of K⁰– mixing in the chiral limit and to next-to-leading order in the 1/N_c expansion. We explicitly demonstrate the cancellation of the renormalization scale and scheme dependences between short- and long-distance contributions in the final expression. Numerical estimates are then given, by taking into account increasingly refined short- and long-distance constraints of the underlying QCD Green's function which governs the factor.     

Heavy quark diffusion in perturbative QCD at next-to-leading order

We compute the momentum diffusion coefficient of a heavy quark in a hot QCD plasma, to next-to-leading order in the weak-coupling expansion. Corrections arise at [see formula]; physically they represent interference between overlapping scatterings, as well as soft, electric scale (p approximately gT) gauge field physics, which we treat using the hard thermal loop effective theory. In 3-color, 3-flavor QCD, the momentum diffusion constant of a fundamental representation heavy quark at next-to-leading order is kappa = 16pi/3alpha(s)(2)T(3)(ln1/g(s)+0.07428+1.9026 g(s)). The convergence of the perturbative expansion is poor.     

Nonperturbative condensate contributions to the effective quark-gluon coupling

We study the effective quark-gluon coupling at low-energy scale, which is defined as the amplitude of a quark emitting or absorbing a gluon with some momentum at low-energy scale. This amplitude is determined from the fermionic three-point Green’s functions of QCD including the leading order contributions of nonperturbative condensates through use of the operator-product expansion. By this approach, we discuss the relationship between the constituent quark and the quark of QCD Lagrangian, and estimate the scale of chiral symmetry breaking and the size of a constituent quark in participating the strong interaction process, such as form factors and radii.     

On methods of analyzing scaling violation in deep inelastic scattering

We discuss relative merits and shortcomings of various methods used in analyses of deep inelastic scattering data within the framework of leading twist QCD perturbation expansion. We advocate the use of Jacobi polynomials method as by far the simples, fastest and simultaneously very accurate way of analysing theQ ²-evolution of nucleon structure functions. Detailed comparison with the numerical solution of the corresponding evolution equation to the next-to-leading order is presented.     

Isospin breaking in low-energy charged pion–kaon elastic scattering

We use chiral perturbation theory to evaluate the scattering amplitude for the process at leading and next-to-leading orders in the chiral counting and in the presence of isospin breaking effects. We also discuss the influence of the latter on the combination of the -wave scattering lengths which is relevant for the 2S–2P energy level shift of atoms. Received: 19 November 2001 / Revised version: 21 December 2001 / Published online: 5 April 2002     

Borel convergence of the variationally improved mass expansion and dynamical symmetry breaking

A modification of perturbation theory, known as the delta expansion (variationally improved perturbation), gave rigorously convergent series in some D=1 models (oscillator energy levels) with factorially divergent ordinary perturbative expansions. In a generalization of the variationally improved perturbation technique appropriate to renormalizable asymptotically free theories, we show that the large expansion orders of certain physical quantities are similarly improved, and prove the Borel convergence of the corresponding series for , with the new (arbitrary) mass perturbation parameter. We argue that non-ambiguous estimates of quantities relevant to dynamical (chiral) symmetry breaking in QCD are possible in this resummation framework. Received: 25 February 2002 / Published online: 8 May 2002     

Analytic approach to the approximate solution of the independent DGLAP evolution equations with respect to the hard-Pomeron behavior

We show that it is possible to use hard-Pomeron behavior to the gluon distribution and singlet structure function at low x. We derive a second-order independent differential equation for the gluon distribution and the singlet structure function. In this approach, both singlet quarks and gluons have the same high-energy behavior at small x. These equations are derived from the next-to-leading order DGLAP evolution equations. All results can be consistently described in the framework of perturbative QCD, which shows an increase of gluon distribution and singlet structure functions as x decreases.     

Hadronic top-quark pair production in association with a hard jet at next-to-leading order QCD: phenomenological studies for the Tevatron and the LHC

We report on the calculation of the next-to-leading order QCD corrections to the production of top–antitop-quark pairs in association with a hard jet at the Tevatron and at the LHC. Results for integrated and differential cross sections are presented. We find a significant reduction of the scale dependence. In most cases the corrections are below 20% indicating that the perturbative expansion is well under control. Moreover, the forward–backward charge asymmetry of the top quark, which is analyzed at the Tevatron, is studied at next-to-leading order. We find large corrections, suggesting that the definition of the observable has to be refined.     

Heavy-baryon properties with NLO accuracy in perturbative QCD

We present an analysis of the static properties of heavy baryons at next-to-leading order in the perturbative expansion of QCD. We obtain analytical next-to-leading order three-loop results for the two-point correlators of baryonic currents with one finite-mass quark field for a variety of quantum numbers of the baryonic currents. We consider both the massless limit and the HQET limit of the correlator as special cases of the general finite-mass formula and find agreement with previous results. We present closed-form expressions for the moments of the spectral density. We determine the residues of physical baryon states using sum-rule techniques.     

Renormalization group estimate of the hadronic decay width of the Higgs boson

The total hadronic decay width of the Weinberg-Salam type Higgs boson is estimated in QCD for the Higgs boson mass much larger than the ordinary hadronic mass scale, by use of the operator product expansion and renormalization group equation. We give an explicit formula for the decay width in terms of quark masses including strong interaction corrections up to the next-to-leading order. A numerical analysis of the hadronic decay width of the Higgs boson is made in the six-quark model. The next-to-leading order correction is found to be significant, e.g., 30-20% of the leading term for m_H of oue interest, m_H ? 1 TeV. Application of our scheme to the decay rates of heavy Higgs bosons of other types is also discussed.     

Next-to-leading order QCD corrections to the production of two bottom-antibottom pairs at the LHC

We report the results of a computation of the full next-to-leading order QCD corrections to the production of two bb pairs at the LHC. This calculation at the parton level provides predictions for well separated b jets. The results show that the next-to-leading order corrections lead to an enhancement of the cross section for the central scale choice by roughly 50% with respect to the leading order result. The theoretical uncertainty estimated by variation of the renormalization and factorization scales is strongly reduced by the inclusion of next-to-leading order corrections.