Quark-parton Model Research Articles

Non-perturbative interactions among partons

We propose a model of interactions among «wee» quarks and antiquarks. Our fundamental assumption is that such partons may give rise —through nonperturbative interactions— to pseudoscalar\(q - \bar q\) bosons and to scalar diquarks; in particular we suppose that sea is organized in pseudoscalar\(q - \bar q\) bosons. Some of the interactions that a valence quark has, either with another valence quark or with sea, modify the predictions of the naive quark-parton model about the spin and spin-averaged quark densities. In order to take into account these modifications we have suitably parametrized the probability densities relative to the interactions in question; moreover we have fixed all the parameters involved, in such a way that our model matches some data of nucleon structure functions and of low-energy hadron physics. Our model has some predictive power with respect to other data of deep inelastic scattering, in particular with respect to the recent EMC and NMC data; furthermore some predictions do not depend on the specific parametrizations chosen for the probability densities, but only on the kinds of interactions assumed. The predictions are in good agreement with available data.

Polarization of partons in the proton.

The spin-dependent distribution functions of quarks and gluons in a proton are studied so as to explain the European Muon Collaboration {ital g}{sub 1}{sup {ital p}}({ital x}) data by introducing a new model, in which characteristics of both the static quark and the quark-parton model are taken into account. The {ital x} dependence of {ital g}{sub 1}{sup {ital p}}({ital x}) is reproduced well. It is shown that polarized gluons through the anomaly play a significant role and the resultant sum of quark spin is 0.375. Furthermore, {ital g}{sub 1}{sup {ital n}}({ital x}) as well as {ital g}{sub 1}{sup {ital p}}({ital x}) is predicted for future experiments.

Neutral strange particle production inK + andπ + collisions with Al and Au nuclei at 250 GeV/c

Data are presented on inclusiveK 0 s and Λ production inK + and π+ collisions with Al and Au nuclei at 250 GeV/c. Results are given on total inclusive cross sections and theA dependence, as well as on distributions in Feynman-x F , rapidityy and transverse momentum. Ratios ofK 0 S and of Λ to π+ production are presented. The data are compared with predictions of the quark-parton model FRITIOF.

Neutron structure function from the convolution formalism and the quark-parton model

A hybrid model is proposed to extract the electromagnetic neutron structure function by combining the convolution formalism in the small- x region and the quark-parton model in the medium- to large- x region. (μp)- and (μd)-data in the small- x region, and (μp)-, (νp)- and ( ν p)- data in the large- x region are combined. Possible inconsistencies in mixing the ν, ν- data with μ-data are carefully sorted out. The new method can circumvent both the experimental anomaly between the (μp)- and (μd)-data and the theoretical ambiguity of using the nonrelativistic spectroscopic function in the large- x region.

Nucleon structure functions from high energy neutrino interactions

Structure functions obtained from high energy neutrino and antineutrino scattering from an iron target are presented. These were extracted from the combined data of Fermilab experiments E616 and E701; these utilized narrow band beam runs between 1979–1982. The structure functions are used to test the validity of quarkparton model (QPM) predictions and to extract the QCD scale parameter Λ from fits to the Altarelli-Parisi equations.

Gottfried and Bjorken sum rules: The role of vector diquarks

A quark-diquark model of the nucleon is considered and the value of the Gottfried sum ruleSG, recently measured by the NMC group and found to differ considerably from the quark parton model expectation,SG=1/3, is computed allowing for both elastic and inelastic contributions from the diquarks. This approach fails to reproduce the data, unless vector diquark contributions persist at very largeQ2 values, contrary to the usual assumptions. The Bjorken sum rule andSU(6) violations due to the quark-diquark structure of the nucleon are also briefly discussed.

THE SPIN-DEPENDENT STRUCTURE FUNCTIONS OF THE NUCLEON

This is an introduction to the study of the nucleon spin structure from deep inelastic lepton–nucleon scattering experiments on polarized targets. A review of the existing data is presented. The derivation of the spin–dependent structure functions from the measured spin asymmetries is analyzed and their physical interpretation is discussed in the quark–parton model. The prospects opened by several new experiments currently in preparation are also described.

HERACLES: an event generator for ep interactions at HERA energies including radiative processes

A Monte Carlo event generator simulating neutral and charged current ep interactions at HERA energies and beyond is described. The present version 4.0 of the generator optionally treats the ep scattering either by means of structure-function parametrizations or on the basis of parton distribution functions in the framework of the quark-parton model. Single-photon emission from the lepton line as self-energy corrections and the complete set of one loop weak corrections can be included. These corrections are sufficient to describe the cross-section with an accuracy of a new percent or, if Q 2 ≤2 x 10 3 GeV 2, of less than 1%.

Is the qq-bar sea in the nucleon isospin symmetric?

The New Muon Collaboration (NMC) has recently performed a detailed analysis of the difference between the proton and neutron structure functions. They found that the Gottfried sum {integral}{ital dx}({ital F}{sub 2}{sup {ital p}}({ital x}){minus}{ital F}{sub 2}{sup {ital n}}({ital x}))/{ital x} is significantly below the simple quark-parton model prediction of 1/3, which indicates a sizable isospin asymmetry of the {ital q{bar q}} sea in the nucleon. We have carried out a theoretical analysis of the Gottfried sum on the basis of a chiral-quark-soliton model, and obtained a satisfactory agreement with the NMC measurement.

Studies of multihadronic final states in photon-photon interactions

We have measured the cross section andQ2-dependence of the reaction γγ→ multihadrons, using the CELLO detector at DESY, for visible final-state masses between 4 and 9 GeV. The data are well fitted by an incoherent sum of Generalised Vector Meson Dominance and Quark Parton Model terms, but at lowQ2 require the addition of a third component which resembles the QPM but is less two-jet-like. With and without this term, the GVDM cross section atQ2=0 is found to be 200±20 and 250±25 nb, respectively.

The lowest order total electromagnetic correction to the deep inelastic scattering of polarized leptons on polarized nucleons

Compact formulae for a total 1-loop electromagnetic correction, including the contribution of electromagnetic hadron effects to the deep inelastic scattering of polarized leptons on polarized nucleons in the quark-parton model have been obtained. The cases of longitudinal and transverse nucleon polarization are considered in detail. A thorough numerical calculation of corrections to cross sections and polarization asymmetries at muon (electron) energies over the range of 200-2000 GeV (10-16 GeV) has been made. It has been established that the contribution of corrections to the hadron current considerably affects the behaviour of longitudinal asymmetry, particularly at y to 1 (y is the standard scaling variable). A satisfactory agreement is found between the model calculations of corrections to the lepton current and the phenomenological calculation results, which makes it possible to find the total 1-loop correction within the framework of a common approach.

On neutron charge structure

The aim of the present work is to investigate the charge structure of neutron by calculating its root mean square charge radius 〈rn2〉1/2 and the corresponding charge form factor,Gn(q), within the general framework of quark-parton model. The neutron (the nucleon in general) is considered to be disc-like in conformity with the idea of Lorentz invariance. Using the proton charge radius, 〈rp2〉=0·707 fm2 as input, our calculations not only reproduce the observed negative sign correctly but also give reasonable value for the magnitude of 〈rn2〉 when compared with the experimental limits. −0·16≦〈rn2〉≦−0·11 (fm2). Predictiona are also made for the proton and the neutron charge form factors and the results compared with the data.

D-Meson production by neutrinos

We have arrived at the form of the fragmentation function of the charmed quark with the help of quark-parton model and Regge phenomenology. We took into consideration the theoretical prejudice that the pion multiplicity increases logarithmically with energy. We would like to reproduce this relation in the case of charmed hadrons. The fragmentation function of the charm quarks derived here seems to be compatible with neutrino-Ne scattering.

New results in quark parton model and recent data

The CHARM Collaboration has recently reported high statistics d σ/d y measurements. We derive several new results using the QPM and compare them with CHARM data. Other aspects of the data are analyzed and compared with information from v( v) Nx distributions and/or EMC data.

Isospin symmetry violation of the nucleon sea or diquarks?

The preliminary results of the NMC on deep inelastic muon scattering experiments show a violation of the Gottfried sum rule, which, in the quark parton model of the nucleon, seems to imply a violation of the u and d flavour symmetry of the sea partons. We argue that this is not necessarily the case; a small admixture of vector diquarks in a quark—diquark model of the nucleon leads naturally to the experimental result. From the data we areled to very mild Q 2 dependence.

$$\bar K^{0*} (892)$$ andK 0*(892) production at lowp t and the quark parton model

Data on the production of the neutralK*(892) resonances at lowpt by 200 GeVK− and π− is compared with the predictions of various models based on the quark parton model of hadrons.

Quark momentum distribution in nucleons

The momentum distributions of constituent quarks in nucleons are studied within the framework of non-relativistic quark models for baryons. The corrections due to one-gluon-exchange effects are discussed comparing different quark-quark potential approximations. Applications to deep inelastic lepton-nucleon scattering are discussed within a quark-parton model in the laboratory frame.

Inclusive D∗± production in photon-photon collisions

Abstract The TPC/Two-Gamma Collaboration has measured the inclusive cross section for production of charmed D∗± mesons in photon-photon collisions. The reaction utilized was e+e-→e+e-D∗±X, with D∗±→DOπ+-, DO→K-+π ±, and either zero or one outgoing e± detected. The result, σ(e + e - → e + e - D ∗± X) = 74±26±19 pb , is in agreement with the quark parton mo del prediction for e+e- → e+e-c c , combined with a Lund model for the hadronization of the charmed quarks.

Inclusive production of vector mesons in π+ p interactions at 250 GeV/c

We report on a study of ρ0, ρ+, ω,\(\bar K^{*0} (892)\) andK*0 (892) inclusive production in π+p interactions at 250 GeV/c, for ρ+,\(\bar K^{*0} (892)\) for the first time in a π+p experiment. The data are compared withK+p data in the same experiment, with results of other experiments and with quark-parton models. Interesting differences are found between ρ+,0 and ω production.

Where is the anomaly of polarized deep-inelastic scattering?

We demonstrate that the anomalous gluon contribution to the polarized proton structure function ${\mathit{G}}_{1}$, while in principle present, cannot be of sufficient magnitude to resolve the present dichotomy between the predictions of perturbative QCD and the results of the European Muon Collaboration experiment. In particular, we clarify the argument that data on unpolarized sea distributions can be used to rule out any large singlet component of ${\mathit{G}}_{1}$. Although, a priori, such a contribution could indeed be significant, the general requirements of positivity and a consistent application of the light-cone operator-product expansion to the quark-parton model imply rather stringent bounds on its magnitude. We interpret this failure of perturbative QCD as clear evidence that it is not applicable to physical processes involving real hadronic states.