Parton Model Of Quantum Chromodynamics Research Articles

Intrinsic asymmetries in semi-inclusive deeply inelastic scattering at the Electron-Ion Collider

We calculate the neutral current jet production semi-inclusive deeply inelastic scattering process in this paper. Neutral current implies that interactions can be mediated by the photon, $Z^0$-boson and their interference. The initial electron is assumed to be polarized and then scattered off by a target particle with spin-1/2. Calculations are carried out up to twist-3 level in the quantum chromodynamics parton model by applying the collinear expansion formalism where multiple gluon scattering is taken into account and gauge links are obtained automatically. After obtaining the differential cross section, we introduce the definition of the intrinsic asymmetry. This quantity reveals the asymmetry in the distribution of the quark intrinsic transverse momentum. We find that these asymmetries can be expressed in terms of the transverse momentum dependent parton distribution functions and the electroweak couplings. As a result, our calculations provide a set of new quantities for analyzing the parton distribution functions and the electroweak couplings. It is helpful to understand the hadronic weak interactions and strong interactions in the deeply inelastic scattering process simultaneously.

Computational modeling of the Balitsky–Kovchegov equation and its numerical solution using hybrid B-spline collocation technique

The dynamics of high-energy scattering of hadrons are governed by Quantum Chromodynamics (QCD). In the parton model of QCD, scattering can be represented as a kind of reaction–diffusion process and consequently can be included in the category of the Fisher–Kolmogorov–Petrovsky–Piscounov (FKPP) equation. We perform a numerical study in the mean-field approximation of QCD evolution given by the Balitsky–Kovchegov (BK) equation, using the hybrid B-spline (HB-spline) collocation technique. The Rubin–Graves type linearization process is used to linearize the nonlinear terms. The accuracy and efficiency of the method are checked by taking numerical examples. It is noticed that the present method gives more accurate results than existing methods. The von Neumann stability analysis is also discussed for the discretized system of the BK equation.

Parity-violating semi-inclusive deeply inelastic scattering at the Electron-Ion Collider

We present a systematic calculation of the current jet production semi-inclusive deeply inelastic scattering process at the Electron-Ion Collider energy. Contributions from weak interactions are considered which give rise to parity violating effects. We consider the general form of the polarized electron beam scattering off the polarized target which has spin 1. The calculations are carried out up to twist-3 level in the quantum chromodynamics parton model by applying the collinear expansion where multiple gluon scattering is taken into account and gauge links are obtained automatically. We present complete results for structure functions and spin/azimuthal asymmetries in terms of the gauge invariant transverse momentum dependent parton distribution functions. Both the unpolarized and the polarized electron beam cases correspond to 24 azimuthal asymmetries, in which 6 of them appear at the leading twist, while 18 of them contribute at twist-3 level. In addition we also calculate the parity-violating asymmetries which arise from the interference of the electromagnetic and weak interactions.

What is the structure and spin decomposition of the proton?

The structure of the is the forefront of studies on the deep structure of matter. With developments of high energy experiments in particular deeply inelastic lepton nucleon scattering (DIS), a dynamical picture of the structure of the has been developed, i.e., the quantum chromodynamics (QCD) based parton model. According to this picture, a moving close to the speed of light can be viewed as a bunch of free partons, and the collision between two high energy protons can be viewed as the collision between partons. Partons in the are described by parton distribution functions (PDFs). This picture is very successful in describing data from different experiments. It has been widely accepted in high energy physics community and is now taken as the standard picture for the dynamical structure of hadrons. The picture has changed the concepts of structures of matter in the sense that the number of partons in a depends on the space and time resolution of the probe. The studies of the structure provide not only the necessary initial condition of high energy collision but also the most active and fruitful frontier for studying properties of QCD where new QCD application techniques have been developed such as the collinear expansion and factorization theorem. With developments of experimental techniques, the studies have been extended to include spin degree of freedom and to use different probes such as electrons and muons. These studies have lead to many new discoveries, initiated the well known proton spin crisis and the proton radius puzzle, and attracted much more attention in the field of particle and nuclear physics. The experimental data from polarized DIS has revealed that quark spin contributes to only a small fraction of the spin and initiated the question where the spin comes from. The electromagnetic radius of the measured from the electron-proton interaction does not coincide with that measured from the muon-proton interaction and initiated the radius puzzle. The experimental data on single-spin asymmetries in inclusive hadron production in proton-proton collisions have lead the studies of the QCD parton model from the simplest one dimensional longitudinal distribution case to the three dimensional case including spin and transverse momentum dependences. The physics in the three-dimensional case is definitely much more abundant and interesting. The transverse momentum dependent (TMD) PDFs include the simple number densities, the helicity distributions, the transversity, and many spin-momentum correlation functions as well, such as the Sivers function, the Boer-Mulders function, the Collins function, worm-gear, pretzelosity and also many so-called higher twist distributions describing quantum interference effects. Both experimental and theoretical efforts have been made in last years and many progresses have been made. The planned new instrumentations such as the Electron-Ion Collider (EIC) in the United States will definitely bring the field to a new era of rapid developments. Several new equipment discussed in China, including Super-Z Factory, Circular Electron Positron Collider (CEPC) and EIC of China (EicC) at High Intensity heavy ion Accelerator Facility (HIAF) and so on can also bring very significant contributions even play dominant role in some of the aspects in this exciting field.

Inclusive light-hadron production in photon-photon collisions at NLO

Abstract I review 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 √s = 29 GeV. Comparisons of γγ data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem. Possible modifications of the theory in view of the found discrepancy of the NLO predictions with recent data from the L3 Collaboration are shortly discussed.

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 the experimental data of e+e− annihilation at the Z-boson resonance and at centre-of-mass energy √s = 29 GeV. This fit also yields a new NLO value for the strong-coupling constant, namely α(5)s(MZ) = 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 pp̄ data from CERN Spp̄S and the Fermilab Tevatron, γp data from DESY HERA and γγ data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem.

Testing the universality of fragmentation functions

Using fragmentation functions for charged pions, charged kaons, and (anti)protons recently extracted from experimental data of e^+e^- annihilation at the Z-boson resonance and at centre-of-mass energy root(s) = 29 GeV, we perform a global study of inclusive charged-hadron production in p anti-p, gamma p, and gamma gamma collisions at next-to-leading order in the parton model of quantum chromodynamics. Comparisons of our results with p anti-p data from CERN S p anti-p S and the Fermilab Tevatron, gamma p data from DESY HERA, and gamma gamma data from CERN LEP2 allow us to test the universality of the fragmentation functions predicted by the factorization theorem. Furthermore, we perform comparisons with (e^+e^-)-annihilation data from LEP2 so as to test the scaling violations predicted by the Altarelli-Parisi evolution equations.

SINGLE SPIN ASYMMETRIES IN INCLUSIVE HIGH ENERGY HADRON–HADRON COLLISION PROCESSES

It has been realized for quite a long time that single-spin experiments, in which one of the colliding objects is transversely polarized, can be helpful in studying the properties of strong interaction in general and in testing quantum chromodynamics (QCD) in particular. Striking effects have been observed in the past few years which deviate drastically from the expectation of the perturbative QCD parton model. These effects have received much attention. New experiments of the similar type are underway and/or planned. Different theoretical attempts have been made to understand these effects. In this review, the special role played by singly polarized high-energy hadron–hadron collisions in high energy spin physics is emphasized. Characteristics of the available data for inclusive hadron productions are briefly summarized. Different theoretical approaches for such processes are reviewed with special attention to a nonperturbative model which explicitly takes the orbital motion of the valence quarks and hadronic surface effects into account. The connection between such asymmetries and hyperon polarization in unpolarized reactions is discussed. An example of the possible application of such experimental results in other processes is given.

Nonpointlike-parton model with asymptotic scaling and with scaling violation at moderateQ2values

A nonpointlike-parton model is formulated on the basis of the assumption of energy-independent total cross sections of partons and the current-algebra sum rules. No specific strong-interaction Lagrangian density is introduced in this approach. This model predicts asymptotic scaling for the inelastic structure functions of nucleons on the one hand and scaling violation at moderate ${Q}^{2}$ values on the other hand. The predicted scaling-violation patterns at moderate ${Q}^{2}$ values are consistent with the observed scaling-violation patterns. A numerical fit of ${F}_{2}$ functions is performed in order to demonstrate that the predicted scaling-violation patterns of this model at moderate ${Q}^{2}$ values fit the data, and to see how the predicted asymptotic scaling behavior sets in at various $x$ values. Explicit analytic forms of ${F}_{2}$ functions are obtained from this numerical fit, and are compared in detail with the analytic forms of ${F}_{2}$ functions obtained from the numerical fit of the quantum-chromodynamics (QCD) parton model. This comparison shows that this nonpointlike-parton model fits the data better than the QCD parton model, especially at large and small $x$ values. Nachtman moments are computed from the ${F}_{2}$ functions of this model and are shown to agree with data well. It is also shown that the two-dimensional plot of the logarithm of a nonsinglet moment versus the logarithm of another such moment is not a good way to distinguish this nonpointlike-parton model from the QCD parton model.

Hadron production at transverse momenta from 0.5 up to 2.2 GeV/ c in proton-proton collisions at 70 GeV

The cross sections for π ±, K ±, p, and p production in pp collisions have been measured at transverse momenta from 0.48 up to 2.21 GeV/ c at 70 GeV. The data are compared with results obtained at lower and higher proton energies and also with the quantum chromodynamics parton model (QPM) calculations. Common behaviour of the cross sections of the form g(p ⊥)⨍(x ⊥) in the energy range above 200 GeV does not take place at lower energies. Qualitatively QPM fits the data and the best agreement is for π +/ π − and K +/ π + ratios.