Quark-parton Model Research Articles

Measurement of D ∗± production in two-photon processes at TRISTAN

The inclusive cross section for the production of charmed D ∗± mesons in two-photon processes is measured with the AMY detector at the TRISTAN e +e − collider. D ∗± mesons are identified from the distribution of charged-particle transverse momenta relative to the jet axis. A data sample corresponding to an integrated luminosity of 176 pb −1 at a center-of-mass energy of 58 GeV is used to determine a cross section σ(e +e − → e +e −D ∗±X) = 270 ± 49(stat) ± 38(syst) pb. The results are compared with theoretical expectations based on the Vector Meson Dominance, direct quark-parton model, and resolved photon processes.

Parton sum rules and improved scaling variable

The effect from quark masses and transversal motion on the Gottfried, Bjorken, and Ellis-Jaffe sum rules is examined by using a quark-parton model of nucleon structure functions based on an improved scaling variable. Its use results in corrections to the Gottfried, Bjorken, and Ellis-Jaffe sum rules. We use the Brodsky-Huang-Lepage prescription of light-cone wavefunctions to estimate the size of the corrections. We constrain our choice of parameters by the roughly known higher twist corrections to the Bjorken sum rule and find that the resulting corrections to the Gottfried and Ellis-Jaffe sum rules are relevant, though not large enough to explain the observed sum rule violations.

Quark model description of polarised deep inelastic scattering and the prediction of g2

We show how the operator product expansion evaluated in the approximation of ignoring gluons leads to the covariant formulation of the quark parton model. We discuss the connection with other formulations and show how the free quark model prediction, g 2 = 0, changes smoothly into the Wandzura-Wilczek (WW) relation for quark masses small relative to the nucleon mass. Previous contradictory parton model predictions are shown to follow from an inconsistent treatment of the mass shell conditions. The description is extended to include quark mass corrections.

Observation of highly virtual photon-photon collisions to hadrons at TRISTAN

We have observed highly virtual ( Q 2 > 1.05 GeV 2) photon-photon collisions to hadronic final states at S e+e−= 58 GeV . The integrated luminosity of the data sample was 241 pb −1. Both scattered beam-electrons and scattered beam-positrons were detected using low-angle calorimeters (i.e., both photons were highly virtual, “ouble-tag”); we obtained 115 hadronic events with an estimated background of 10.2 ± 1.1. The cross section obtained was 4.11 ± 0.66 pb in the 2 < W γγ < 25 GeV and Q γ,min 2 >2 GeV region, while the lowest order quark-parton model predicted 3.00 pb.

Spin-dependent structure functions g^1 and g^2 for inclusive spin-1/2 baryon production in electron-positron annihilation.

Two spin-dependent structure functions $\hat g_1$ and $\hat g_2$ for the inclusive spin-half baryon production in electron-positron annihilation are studied in the context of QCD factorization as well as in the naive quark parton model. As a result, it is found that the sum of $\hat g_1$ and $\hat g_2$ is related to $\hat h_1$ and $\hat g_T$, two quark fragmentation functions defined by Jaffe and Ji. In connection with the measurement of quark fragmentation functions, the possible phenomenological consequences are discussed.

Measurements of the Q2-dependence of the proton and deuteron spin structure functions g1p and g1d

The ratio g 1 F 1 has been measured over the range 0.03 < x < 0.6 and 0.3 < Q 2 < 10 (GeV/ c) 2 using deep-inelastic scattering of polarized electrons from polarized protons and deuterons. We find g 1 F 1 to be consistent with no Q 2-dependence at fixed x in the deep-inelastic region Q 2 > 1 ( GeV/ c) 2. A trend is observed for g 1 F 1 to decrease at lower Q 2. Fits to world data with and without a possible Q 2-dependence in g 1 F 1 are in agreement with the Bjorken sum rule, but Δ q is substantially less than the quark-parton model expectation.

Inclusive charged particle distributions in deep inelastic scattering events at HERA

A measurement of inclusive charged particle distributions in deep inelasticep scattering forγ*p centre-of-mass energies 75<W<175 GeV and momentum transfer squared 10<Q 2<160 GeV2 from the ZEUS detector at HERA is presented. The differential charged particle rates in theγ*p centre-of-mass system as a function of the scaled longitudinal momentum,x F, and of the transverse momentum,p* t and 〈p* t 2〉, as a function ofx F, W andQ 2 are given. Separate distributions are shown for events with (LRG) and without (NRG) a rapidity gap with respect to the proton direction. The data are compared with results from experiments at lower beam energies, with the naive quark parton model and with parton models including perturbative QCD corrections. The comparison shows the importance of the higher order QCD processes. Significant differences of the inclusive charged particle rates between NRG and LRG events at the sameW are observed. The value of 〈p* t 2〉 for LRG events with a hadronic massM x, which excludes the forward produced baryonic system, is similar to the 〈p* t 2〉 value observed in fixed target experiments atW≈M x.

Complete O( α) QED corrections to the process ep → eX in mixed variables

The complete set of O( α) QED corrections with soft photon exponentiation to the process ep → eX in mixed variables ( y = y h , Q 2 = Q l 2) is calculated in the quark-parton model, including the lepton-quark interference and the quarkonic corrections which were unknown so far. The interference corrections amount to a few percent or less and become negligible at small x. The leading logarithmic terms proportional to ln( Q 2 m q 2 ) from radiation off quarks are discussed and the non-logarithmic quarkonic corrections are found to be negligible for almost all experimentally accessible x and y.

New quark distributions and semi-inclusive electroproduction on polarized nucleons

The quark-parton model calculation including the effects of intrinsic transverse momentum and of all six twist-two distribution functions of quarks in polarized nucleons is performed. It is demonstrated that new twist-two quark distribution functions and polarized quark fragmentation functions can be investigated in semi-inclusive DIS at leading order in Q 2. The general expression for the cross-section of semi-inclusive DIS of polarized leptons on polarized nucleons in terms of structure functions is also discussed.

Gluon and charm distributions in the photon.

We study systematically the sensitivity of the photon structure function data on the gluon contents of the photon, by using the leading order Altarelli--Parisi (AP) equations. Charm quark contribution is studied in the quark parton model and by using the massive quark AP equations of Gl\"uck, Hoffmann and Reya. The present photon structure function data are found to allow wide range of gluon distributions in the photon. We give a set of six scale-dependent parton distributions in the photon (WHIT1 to WHIT6), which have systematically different gluon contents. Sensitivity of the structure function at small $x$ and that of the total charm quark production cross section to the effective gluon distribution are discussed.

Precision Measurement of the Proton Spin Structure Functiong1p

We have measured the ratio [ital g][sup [ital p]][sub 1]/[ital F][sup [ital p]][sub 1] over the range 0.029[lt][ital x][lt]0.8 and 1.3[lt][ital Q][sup 2][lt]10 (GeV/[ital c])[sup 2] using deep-inelastic scattering of polarized electrons from polarized ammonia. An evaluation of the integral [integral][ital g][sup [ital p]][sub 1]([ital x],[ital Q][sup 2])[ital dx] at fixed [ital Q][sup 2]=3 (GeV/[ital c])[sup 2] yields 0.127[plus minus]0.004(stat)[plus minus]0.010(syst), in agreement with previous experiments, but well below the Ellis-Jaffe sum rule prediction of 0.160[plus minus]0.006. In the quark-parton model, this implies [Delta][ital q]=0.27[plus minus]0.10.

First evidence of hard scattering processes in single tagged γγ collisions

For the first time, multihadronic production from single tagged γγ collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared (〈 Q 2〉 = 0.06 (GeV/ c 2) 2). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (ϱ, ω or π), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these structure functions were used and the predictions compared with the data. This study confirms recent results from no-tag experiments in requiring a QCD-based component to successfully describe the data, indicating that the photon has a significant partonic content.

SLAC measurements of the neutron spin-structure function

Results from a measurement of the neutron spin-dependent structure functiong1n(x) over a range inx from 0.03 to 0.6 and withQ2>1 (GeV/c)2 are presented. The experiment consisted of scattering a longitudinally polarized electron beam from the Stanford Linear Accelerator off a polarized3He target and detecting scattered electrons in two magnetic spectrometers. The results are interpreted in the quark-parton model and used to test the Bjorken sum rule.

Study of hard scattering processes in multihadron production from ?? collisions at LEP

The production of multihadronic states in γγ collisions at LEP has been studied with the DELPHI detector. The analyzed data correspond to an integrated luminosity of about 32pb−1, collected in the LEP runs of 1990–1992. Minimum bias data and a sample of events with jets at highpT have been selected under the requirement that no scattered electron or positron is observed. The two data sets have been compared to Monte Carlo predictions. The non-perturbative contribution described by the vector meson dominance Model and direct\(q\bar q\) production from pointlike photons described by the quark parton model were found to be insufficient to reproduce the data. It has been necessary to include a third interaction component, which is due to perburbative hard scattering of the partonic constituents of the photon. Several parametrisations of the quark and gluon densities of the photon have been tested. The interplay with the cut in jet transverse momentum, which is necessary for the separation of the perturbative and non-perturbative regions, is discussed. The data favour parametrisations with rather soft partonic content of the photon.

Fermi-Dirac distributions for quark partons

We propose to use Fermi-Dirac distributions for quark and antiquark partons. It allows a fair description of the $x$-dependence of the very recent NMC data on the proton and neutron structure functions $F_2^p(x)$ and $F_2^n(x)$ at $Q^2=4$ GeV$^2$, as well as the CCFR antiquark distribution $x\overline q(x)$. We show that one can also use a corresponding Bose-Einstein expression to describe consistently the gluon distribution. The Pauli exclusion principle, which has been identified to explain the flavor asymmetry of the light-quark sea of the proton, is advocated to guide us for making a simple construction of the polarized parton distributions. We predict the spin dependent structure functions $g_1^p(x)$ and $g_1^n(x)$ in good agreement with EMC and SLAC data. The quark distributions involve some parameters whose values support well the hypothesis that the violation of the quark parton model sum rules is a consequence of the Pauli principle.

New results on the Gottfried sum and [formula omitted] in the nucleon sea

The NMC has re-evaluated the Gottfried sum using new results on F 2 d and F 2 n / F 2 p . The new value, S G = 0.235 ± 0.026, remains below the quark-parton model value of 1 3 , which can be interpreted as due to an excess of down- over up- quarks in the sea of the proton. The NA51 experiment at CERN has measured the asymmetry ( σ pp − σ pn )/( σ pp + σ pn ) in cross sections for the Drell-Yan muon pair production at 450 GeV. The result is consistent with u/ d( x = 0.18) = 0.56 ± 0.06 .

Λ hyperon production at backward angles in the reaction [formula omitted

The first measurement of the invariant cross sections and polarizations of Λ hyperons produced by the reaction π − 6 Li → ΛX at 4 GeV/c in the backward angular region ( 70° < ≁ lab < 145° ) has been done. In the measured region the production with free nucleons was kinematically forbidden. All the experimental results are reproduced by a quark-parton model calculation.

Cosmic prompt-muon spectrum at super high energies estimated from the JACEE primary spectrum using recent charmed-particle production cross-sections

The prompt-muon flux generated directly from the decay of short-lived particles like D-mesons and Λc-hyperons produced during primary cosmic proton-air collisions has been estimated. The inclusive production cross-sections of charmed particles based on the recombination of quark parton model developed by Bugaevet al. and the primary-nucleon spectrum based on the latest JACEE balloon flight data have been used in this investigation. The derived direct muon spectrum has been compared with the earlier results of Elbertet al., Inazawaet al. Bugaevet al. and Allkofer and Bhattacharyya.

The hard QCD contribution to nucleon structure functions at x→1

The hard QCD contribution to the nucleon structure functions at x→1 expressed in terms of the V-A projection of the nucleon wave function has been obtained in the framework of the quark-parton model in the infinite momentum frame. Comparison with experiment shows that for any reasonable nucleon wave function the data cannot be described by hard QCD contributions.

Tests of perturbative QCD using CCFR data for measurements of nucleon structure functions

We present measurements of nucleon structure functions, F 2( ϰ, Q 2) and xFs( ϰ, Q 2), from the high-statistics, high-energy neutrino-iron scattering experiment at the Fermilab Tevatron. The existing high-statistics xF 3 determination by the CDHSW collaboration is compared to our data. The data presented here constitute the first corroboration of the QCD prediction of xF 3 evolution at low-ϰ, and yields Λ MS =210±28±41 MeV, and a determination of the GLS sum rule at Q 2 = 3 GeV 2, S GLS = ∫ ϰ 1 xF s ϰ =2.50±0.018 (stat.)±0.078 (syst.) . Our value of Λ MS yields α 3( M z ) = .111 ± .002 ± .003 ± .003 (scale). Comparison of the neutrino determination of F 2( ϰ, Q 2) with that obtained from the charged-lepton ( e or μ) scattering leads to a precise test of the mean-square charge prediction by the Quark Parton Model. The SLAC-CCFR and BCDMS structure function provide a consistent and precise set of structure functions over a large range of Q 2.