Behavior of the hadron potential at large distances and properties of the hadron spin-flip amplitude

The impact of the form of the hadron potential at large distances on the behavior of the hadron spin-flip amplitude at small angles is examined. The t-dependence of the spin-flip amplitude of high-energy hadron elastic scattering is analyzed under different assumptions on the hadron interaction. It is shown that the long tail of the nonGaussian form of the hadron potential of the hadron interaction in the impact parameter representation leads to a large value of the slope of the spin-flip amplitude (without the kinematical factor ) as compared with the slope of the spin-nonflip amplitude. This effect can explain the form of the differential cross-section and the analyzing power at small transfer momenta. The methods for the definition of the spin-dependent part of the hadron scattering amplitude are presented. A possibility to investigate the structure of the hadron spin-flip amplitude from the accurate measure of the differential cross-section and the spin correlation parameters is shown. Received: 14 December 2001 / Accepted: 17 January 2002     

Standard SANC modules for NLO QCD radiative corrections to single top-quark production

In this paper we present the results obtained with the newly created standard SANC modules for calculation of the NLO QCD corrections to single top-quark-production processes in s and t channels at the partonic level, as well as to top-decays. The main aim of these results is to prove the correct work of modules. A comprehensive comparison with the results of the CompHEP system is given, where possible. These modules are intended to be used in Monte Carlo generators for single top-quark-production processes at the LHC. As in our recent paper, devoted to the electroweak corrections to these processes, we study the regularization of the top-legs associated infrared divergences with the aid of the complex mass of the top quark. A comparison of QCD corrections with those computed by the conventional method is presented both for top production and for decays. For s-channel production we give an analytic proof of equivalence of the two methods in the limit of low top width.     

Higher-order QCD corrections to exclusive processes: The multiple scattering mechanism

The multiple-scattering diagram, calculated to lowest order in QCD, fits well topp elastic scattering data at larget. It is shown that higherorder QCD corrections seem to spoil this agreement: calculated as far as the insertion of two additional gluons, they give an exponential factor. This factor differs in detail from some previous conjectures. For colour-singlet bound states of quarks, the exponential factor is independent of whether or not the gluon mass is zero, unlike the case of simple quark-quark scatterin. Two régimes are considered:t ands of the same order, andt large but much less thans. Exponentiation seems to occur in both cases, but in a non-abelian theory more diagrams are important for the latter case.     

Wilson coefficients for parton decay at order αs

We calculate, at order α_s, certain Wilson coefficients that relate to the P_T spread of quark jets in QCD. Also included is a calculation of the quark decay function d(z) in a spacelike axial gauge.     

When hadrons become unstable: A novel type of non-analyticity in chiral extrapolations

Hadron masses show a specific dependence on the quark masses. Therefore, the variation of these masses can cause a resonance in a hadronic scattering amplitude to become a bound state. Consequently, the amplitude exhibits a non-analytic behavior at this transition. Crossed amplitudes, where the resonance can be exchanged in the t-channel, can be shown to exhibit the same phenomenon by s→t analytic continuation. This entails possible kinks in lattice quark-mass extrapolations needed to compute hadronic observables.     

Quark-Gluon Interactions at High Temperature and Large Distances

Continuum fourdimensional Quantum Chromodynamics (QCD) including quarks in the regime of high temperature and large distances (the HT regime) is studied to all perturbative orders. The imaginary time (τ) formalism is used. Then, as shown in previous works, QCD is described by a new generating functional Z_HT, in which quark fields retain their dependences on τ, while gluon and ghost fields are τ-independent. The invariance of Z_HT under BRST transformations in the HT regime is exhibited: it closed quark and electric gluon loops in the HT regime are obtained. The electric mass terms in Z_HT. Infinite sets of non-abelian Ward identities for the closed quark and electric gluon loops in the HT regime are obtained. The electric mass terms in Z_HT are shown to be infrared finite. We prove to all perturbative orders that one can regard as subdominant, and, hence, neglect consistently the contributions of :i) all closed electric gluon loops, ii) all closed quark loops with three or more vertices in diagrams having an even number of electric gluons (or none) in the external lines. In the HT regime, the axial anomalies are obtained: their expressions in terms of τ-independent gluon fields are similar to those for zero temperature. A non-trivial renormalization group (RG) equation in the HT regime, specifically due to the quark-gluon interaction, is presented. A positive beta function is obtained, and it is argued that interactions are not weak in that regime. The RG and the perturbative analysis to all orders appear to indicate that quarks and gluons may be confined in the HT regime (and, in particular in the Early Universe), due to the infrared divergent magnetic gluon sector. Other possibilities are also discussed.     

Medium temperature dependence of quark energy loss

By using the reaction operator approach and the single hard scattering approximation scenario, we study the medium temperature dependence of heavy quark radiation energy loss via introducing the medium temperature into QCD running coupling α _s and Debye mass μ _D . Our results indicate that there exist an equilibrum temperature T = 320 MeV in Au-Au collisions withe energy 130 GeV at RHIC, which is agreement with the prediction from hydrodynamic model. We also find that, in the low p _t region, the color coupling effect dominates the heavy quark radiation energy loss, and the energy loss will be dominated by the Debye screening effect when the quark p _t become large.     

QCD phase transition with strange quark in wilson formalism for fermions

The nature of QCD phase transition is studied with massless up and down quarks and a light strange quark, using the Wilson formalism for quarks on a lattice with the temporal direction extensionN _t=4. We find that the phase transition is of first order for the physical strange quark mass.     

QCD phase transition with strange quark in wilson formalism for fermions

The nature of QCD phase transition is studied with massless up and down quarks and a light strange quark, using the Wilson formalism for quarks on a lattice with the temporal direction extensionN _t=4. We find that the phase transition is of first order for the physical strange quark mass.     

Uncertainties of Drell-Yan production cross section in pp collisions at the LHC

The results of calculations of uncertainties of the Drell-Yan production cross section at the energy √s = 13 TeV are presented. Systematic errors related to uncertainties in the quark and gluon distribution functions, the choice of the QCD factorization scale, and the QCD running coupling constant are considered.     

Ratio of a strange quark mass ms to up or down quark mass mu,d predicted by a quark propagator in the frameworkof the chiral perturbation theory

Based on the fully dressed quark propagator and chiral perturbation theory, we study the ratio of the strange quark mass m_s to up or down quark mass m_u,d. The ratio is related to the determination of quark masses which are fundamental input parameters of QCD Lagrangian in the Standard Model of particle physics and can not be directly measured since the quark is confined within a hadron. An accurate determination of these QCD free parameters is extremely important for both phenomenological and theoretical applications. We begin with a brief introduction to the non-perturbation QCD theory, and then study the mass ratio in the framework of the chiral perturbation theory (χ PT) with a parameterized fully dressed quark propagator which describes confining fully dressed quark propagation and is analytic everywhere in the finite complex p²-plane and has no Lehmann representation so there are no quark production thresholds in any theoretical calculations of observable data. Our prediction for the ratio m_s/m_u,d is consistent with other model predictions such as Lattice QCD, instanton model, QCD sum rules and the empirical values used widely in the literature. As a by-product of this study, our theoretical results, together with other predictions of physical quantities that used this quark propagator in our previous publications, clearly show that the parameterized form of the fully dressed quark propagator is an applicable and reliable approximation to the solution of the Dyson-Schwinger Equation of quark propagator in the QCD.     

Heavy quark production at a photon linear collider

Heavy quark production at future Compton colliders is investigated. The impact of polarization has been studied. Perturbative QCD corrections to the effective cross section are calculated and found to be at the level of 40% for ae ⁺ e ^? collider of 500 GeV and a top quark mass of 150 GeV. The η _t resonance structure is incorporated.     

Forward-backward asymmetry in top quark semileptonic decay

The semileptonic decay of the top quark t → bW⁺ → bl⁺ν_l is analyzed in the rest system of the W. The forward-backward asymmetry of the lepton l⁺ with respect to the heavy quark direction is defined and computed. It is argued that this observable will be an ideal tool to study top quark properties at Tevatron and LHC. Higher order QCD corrections are calculated and their structure is elucidated in some detail.     

Instantons and meson correlation functions in QCD

Various QCD correlators are calculated in the instanton liquid model in zeromode approximation and 1/N _c expansion. Previous works are extended by including dynamical quark loops. In contrast to the original “perturbative” 1/N _c approximation, not all quark loops are suppressed. Renormalization of the instanton density allows the identification of the density with the gluon condensate even in presence of dynamical quark loops. In the flavor singlet meson correlators a chain of quark bubbles survives the N _c → ∞ limit causing a massive η′ in the pseudoscalar correlator while keeping massless pions in the triplet correlator. The correlators are plotted and meson masses and couplings are obtained from a spectral fit. They are compared to the values obtained from numerical studies of the instanton liquid and to experimental results.     

Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Tests of QCD at LEP

The four experiments, ALEPH, DELPHI, L3 and OPAL at LEP, have performed a large number of precise measurements to test Quantum Chromodynamics. The strong coupling constant has been measured with high precision:α _s (M_z) = 0.123±0.004. The coupling constant has been found to be independent of quark flavour. The running ofα _s has been demonstrated by the data. Second order QCD matrix element calculations have been tested from several measured distributions in 3-jet and 4-jet events. These distributions give evidence of the vector nature of the gluon and provide measurements of the QCD colour factors. The hadronic distributions are well reproduced by QCD Monte Carlo programs as well as by analytical calculations with soft gluon coherence effects.     

Small QCD corrections in inclusive 0−+ quarkonium decay for the bound-state renormalization scheme

A consistent field theoretic computation of the first-order correction to the total decay of a non-relativistic heavy quarkonium state must consider contributions from the annihilation amplitude, with a typical scale of the order of the quark mass m, and from the bound-state wave function, where the typical scale is of the order of the Bohr momentum α_sm. Therefore, not only the latter smaller scale is the one to be taken for the total decay rate, but also a quite specific renormalization, the “bound-state renormalization scheme”, must be used for a computation of the total inclusive decay rate of 0^?+→hadrons. Due to important cancellations between large individual contributions, the net result turns out to be small, encouraging the use of pure perturbative QCD in such systems.     

QCD Evolution of Nuclear Structure Functions at Large X: EMC Effect and Cumulative Processes

QCD evolution of nuclear structure functions at large x is reviewed within the an approach based on QCD factorization for hard processes and multiquark flucton model. In this approach, x > 1 region of the nuclear structure functions is intimately related with x < 1 region due to manifestation of quark and gluon degrees of freedom in nuclei. Properties of QCD evolution and observed EMC-ratio for nuclear structure functions at x < 1 result in an admixture of hard extra sea quark distribution. This extra nuclear quark sea provides a bump above unity for EMC-ratio at small x and becomes dominant in the nuclear quark sea for cumulative region x > 1. It leads to a striking prediction, confirmed by data, for the same spectrum slopes of all cumulative hadrons in nuclear fragmentation region.