Quantum chromodynamics and hadron dynamics (original) (raw)
Dynamical determination of parton and gluon distributions in quantum chromodynamics
Nuclear Physics B, 1977
Using the dynamical assumption that at low resolution-energies hadrons consist of valence quarks only, we calculate uniquely nucleonic as well as pionic parton and gluon distributions within the framework of QCD, and give analytic expressions for theirx-and Q2_dependence. Applications to dilepton and W-boson Drell-Yan production in pN and nN reactions are illustrated and possible applications to high-pT processes are discussed.
Nuclear Physics B, 1996
The next-to-leading-order (NLO) cross section for the production of heavy quarks at large transverse momenta (p±) in 3,3/collisions is calculated with perturbative fragmentation functions (PFFs). This approach allows for a resummation of terms cx as ln(p~/m 2) which arise in NLO from colfinear emission of gluons by heavy quarks at large p± or from almost collinear branching of photons or gluons into heavy-quark pairs. We present single-inclusive distributions in p± and rapidity including direct and resolved photons for yy production of heavy quarks at e+e -colliders and at high-energy yy 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 as ln(p2/m e) terms in the PFF approach leads to a softer p± distribution and to a reduced sensitivity to the choice of the renormalization and factorization scales.
Exclusive processes in perturbative quantum chromodynamics
Physical Review D, 1980
We present a systematic analysis in perturbative quantum chromodynamics (@CD) of large-momentum-transfer exclusive processes. Predictions are given for the scaling behavior, angular dependence, helicity structure, and normalization of elastic and inelastic form factors and large-angle exclusive scattering amplitudes for hadrons and photons. We prove that these reactions are dominated by quark and gluon subprocesses at short distances, and thus that the dimensional-counting rules for the power-law falloff of these amplitudes with momentum transfer are rigorous predictions of @CD, modulo calculable logarithmic corrections from the behavior of the hadronic wave functions at short distances. These anomalous-dimension corrections are determined by evolution equations for process-independent meson and baryon "distribution amplitudes" Si(x, ,g) which control the valence-quark distributions in high-momentum-transfer exclusive reactions. The analysis can be carried out systematically in powers of a, (Q'), the QCD running coupling constant. Although the calculations are most conveniently carried out using light-cone perturbation theory and the light-cone gauge, we also present a gauge-independent analysis and relate the distribution amplitude to a gauge-invariant Bethe-Salpeter amplitude.
Perturbation theory and the parton model in QCD
Nuclear Physics B, 1979
We prove that for any process which admits a parton-model interpretation, the naive parton model can be modified to include the effects of QCD interactions to all orders in perturbation theory. This requires that the mass singularities in quark and gluon inclusive cross sections factor into universal functions which renormalize the naive parton model distribution and decay functions. We prove that this factorization takes place for all leading and non-leading logs and thus check the consistency of the parton model to all orders in perturbation theory.
Soft and hard hadronic processes in the nonperturbative approach of QCD
Physical Review D, 2000
A two-component model to analyze both soft and hard hadronic processes at high energies is suggested. The model is based on the topological 1/N expansion of the scattering amplitude and the theory of the supercritical Pomeron. The longitudinal component is given by the string model and determines the behavior of the cross section on longitudinal variables. The dependence on the transverse momentum is calculated on the basis of a two-gluon Pomeron model in which the Pomeron is modeled as an exchange of two nonperturbative gluons whose propagator is finite at q 2 ϭ0. Hard scattering of quarks on the ends of quark-gluon strings is calculated as a sequence of multi-Pomeron exchanges. It is shown that the propagator which vanishes as (q 2) Ϫ3 or faster allows one to reproduce hard distributions of secondary hadrons. The model is used to analyze the inclusive spectra of hadrons on the Feynman variable x F and transverse momentum p Ќ up to 10 GeV/c in a wide energy interval.
Physics of heavy-quark production in quantum chromodynamics
Physical Review D, 1987
For very heavy quark masses, QCD predicts that the inclusive hadronic production of heavy quarks is governed by quark and gluon hard scattering subprocesses. On general grounds, one expects corrections of order p/i&, where CL-300 MeV and MQ is the heavy quark mass. At the charm mass scale, such corrections could be important, possibly accounting for the anomalies observed in the nuclear number dependence, the longitudinal momentum distributions, and beam flavor dependence of charm hadroproduction. In this paper we present a general overview of such corrections. In particular, we discuss a "coalescence" correction, which substantially alters the cross section in situations where the heavy quark is known to have low velocity relative to one or more constituents of the spectator jet. In attractive channels the result is a large enhancement. In inclusive cross sections this final state interaction effect is suppressed by only a single power of the heavy quark mass.
Properties of hadrons from a non-perturbative approach
2016
The composition and structure of matter remains a fundamental problem in physics. The closest theory that tries to deal with this problem is the Standard Model of particle physics, which encodes the electromagnetic, weak and strong interactions. The strong interaction is the main ingredient in the description of hadronic properties such as their masses, distribution of charge and magnetisation, decay parameters, and interactions between them. These properties have the common feature that they are modelled at low four momentum transfer. At those conditions the theory of the strong interactions becomes impossible to use and the best approach is to calculate the hadronic properties using expensive numerical simulations called lattice QCD. Because lattice QCD requires a lot of computational resources, models close to the theory of strong interactions are still of great importance. The work presented in this thesis uses different quark models (in particular the NJL model), in the calcula...
Perturbative quantum chromodynamic prediction for the heavy quark fragmentation function
Physics Letters B, 1987
Within the framework of a parhcular model for meson production, a perturbatlve QCD analysis is presented for the reclusive production of pseudoscalar and vector mesons to derive the fragmentatmn function of heavy quarks produced in e+e-anmhllatmn. The results are compared with experimental data for bottom and charm quark fragmentatmn functmns. The new generation of high energy e+e-colliders such as TRISTAN, SLC, and LEP, is expected to provide a wealth of information about production and hadronization of heavy quarks (c, b, t). The production of heavy particles in these accelerators will be an important testing ground for the perturbative quantum chromodynamics (QCD) [ 1-3 ]. One of the interesting properties of heavy quarks which can be studied in the framework of perturbative QCD is the evolution of quarks into hadrons. The mechanism responsible for hadronization, in general, is specified by the fragmentation function D~ (z, s) which represents the fragmentation of the quark Q into the final state hadron H with the momentum fraction z= 2E/~s, where E is the energy of the hadron and s is the square of the total e+e-CM energy. Various phenomenological models, like the Lund model [ 4 ], the Cascade model [ 5 ], and the Peterson et al. model [ 6 ], motivated by QCD, have been developed to describe the fragmentation function D~. Since these models [ 4-6 ] involve parameters to fit the experimental data, it is not clear how to check the consistency of their results with a general theorem for the extreme case of heavy quark fragmentation given Work supported by the Department of Energy, contract DE-AC03-76SF00515.