Quark-parton Model Research Articles

Parton symmetries and the NuTeV anomaly

Recently the NuTeV Collaboration (Phys. Rev. Lett. 88 091802 (2002)) announced a new measurement of sin2 θw which was approximately three standard deviations above the currently accepted standard model value. The NuTeV analysis depends on the assumption that various quark-parton model symmetries are not broken. In particular the analysis takes \(\bar s(x) = s(x)and\;d_v^m(x) = u_v^p(x).\) However models which break these symmetries are known. We examine the predictions of these models and their effect on the NuTeV result. In most instances the effect is to decrease the discrepancy between the NuTeV result and the accepted value.

SPIN STRUCTURE FUNCTIONS AND INTRINSIC MOTION OF THE CONSTITUENTS

The spin structure functions of the system of quasifree fermions on mass shell are studied in a consistently covariant approach. Comparison with the basic formulas following from the quark-parton model reveals the importance of the fermion motion inside the target for the correct evaluation of the spin structure functions. In particular it is shown, that regarding the moment Γ1, both the approaches are equivalent for the static fermions, but differ by the factor 1/3 in the limit of massles fermions (m ≪ p0, in target rest frame). Some other sum rules are discussed as well.

Nuclear Effect Study Under K Factor's Nonconstancy**The project supported by the Chinese Engineering Physics Institute for Research Doctor Subsidizes (2001)

A consistent approach to estimating nuclear effect functions and based on numerical iteration technique is presented in the quark-parton model when taking into account the nonconstancy of quantum chromodynamics correction factor K. and correspond respectively to the valence quark distributions for one bound nucleon within the nucleus and to the sea quark ones. Related numerical analysis is given for nuclei , and . As the basis, it adopts both experimental data of the high energy proton-nucleus Drell–Yan process and of the high energy lepton-nucleus deep inelastic scattering.

Measurement of the hadronic photon structure function F2γ at LEP2

The hadronic structure function of the photon F2γ(x,Q2) is measured as a function of Bjorken x and of the photon virtuality Q2 using deep-inelastic scattering data taken by the OPAL detector at LEP at e+e− centre-of-mass energies from 183 to 209 GeV. Previous OPAL measurements of the x dependence of F2γ are extended to an average Q2 of 〈Q2〉=780 GeV2 using data in the kinematic range 0.15<x<0.98. The Q2 evolution of F2γ is studied for 12.1<〈Q2〉<780 GeV2 using three ranges of x. As predicted by QCD, the data show positive scaling violations in F2γ with F2γ(Q2)/α=(0.08±0.02+0.05−0.03)+(0.13±0.01+0.01−0.01)lnQ2, where Q2 is in GeV2, for the central x region 0.10–0.60. Several parameterisations of F2γ are in qualitative agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

Spin structure functions and intrinsic motion of the constituents

The spin structure functions of the system of quasifree fermions on mass shell are studied in a consistently covariant approach. A comparison with the basic formulas following from the quark-parton model reveals the importance of the fermion motion inside the target for the correct evaluation of the spin structure functions. In particular it is shown that, regarding the moment ${\ensuremath{\Gamma}}_{1},$ both the approaches are equivalent for the static fermions but differ by a factor $1/3$ in the limit of massles fermions $(m\ensuremath{\ll}{p}_{0},$ in the target rest frame). Some other sum rules are discussed as well.

Measurement of the hadronic cross-section for the scattering of two virtual photons at LEP

The interaction of virtual photons is investigated using the reaction ${\rm e^+e^-} \rightarrow{\rm e^+e^-}$ hadrons based on data taken by the OPAL experiment at ${\rm e^+e^-}$ centre-of-mass energies $\sqrt{s_{\rm ee}}=189-209$ GeV, for $W>5$ GeV and at an average $Q^{2}$ of 17.9 GeV $^2$ . The measured cross-sections are compared to predictions of the Quark Parton Model (QPM), to the Leading Order QCD Monte Carlo model PHOJET, to the NLO prediction for the reaction ${\rm e^+e^-}\rightarrow{\rm e^+e^-} q\bar{q}$ , and to BFKL calculations. PHOJET, NLO ${\rm e^+e^-}\rightarrow{\rm e^+e^-} q\bar{q}$ , and QPM describe the data reasonably well, whereas the cross-section predicted by a Leading Order BFKL calculation is too large.

D¯−ūasymmetry and semi-inclusive production of pions in deep inelastic scattering

We investigate unambiguities in the extraction of the $\bar d - \bar u$ asymmetry from semi-inclusive production of pions in DIS. The role of several effects beyond the quark-parton model (QPM) which lead to $N_p^{\pi^+} \ne N_n^{\pi^+}$ and $N_p^{\pi^-} \ne N_n^{\pi^-}$ and may therefore cloud such an extraction is studied. The results are discussed in the context of the recent HERMES data. We find that the interaction of the resolved photon with the nucleon significantly modifies the observed $\bar d - \bar u$ asymmetry. The exclusive elastic production of $\rho$ mesons plays a similar role for the large-$z$ data sample. Our estimate shows a rather small effect of the spectator mechanism. Nuclear effects in the deuteron also look potentially important but are difficult to estimate quantitatively. Throughout the paper we make in addition several general remarks concerning semi-inclusive and exclusive production of mesons.

Measurement of the photon structure function at high Q2 at LEP

The structure functions of real and virtual photons are derived from cross section measurements of the reaction e +e −→e +e −+ hadrons at LEP. The reaction is studied at s ≃91 GeV with the L3 detector. One of the final state electrons is detected at a large angle relative to the beam direction, leading to Q 2 values between 40 GeV 2 and 500 GeV 2. The other final state electron is either undetected or it is detected at a four-momentum transfer squared P 2 between 1 GeV 2 and 8 GeV 2. These measurements are compared with predictions of the Quark Parton Model and other QCD based models.

Double-tag events study with the L3 detector at √ s = 189 GeV

A preliminary study of double tag events using the L3 detector at center of mass energy √s ⋍ 189 GeV has been performed. The cross-section of γ ∗γ ∗ collisions is measured at average 〈 Q 2〉 = 14.5 GeV 2. The results are in agreement with predictions based on perturbative QCD, while the Quark Parton Model alone is insufficient to describe the data. The measurements lie below the LO and above the NLO BFKL calculations.

SPIN STRUCTURE FUNCTIONS

▪ Abstract We review the study of the internal spin structure of the proton and neutron. High-energy scattering of polarized leptons by polarized protons, neutrons, and deuterons provides a measurement of the nucleon spin structure functions. These structure functions give information on the polarized quark contributions to the spin of the proton and the neutron and allow tests of the quark-parton model and quantum chromodynamics. We discuss the formalism of deep inelastic scattering of polarized leptons on polarized nucleons, the past decade of experimental progress, and future programs to measure the polarized gluon contribution to the proton spin.

Electron and neutron electric dipole moments in the constrained MSSM

We analyze the effects of CP-violating phases on the electric dipole moment (EDM) of electron and neutron in the constrained minimal supersymmetric model. We find that the phases phi_{\mu} and phi_{A_0} have to be strongly correlated, in particular for small values of the SUSY mass parameters. We calculate the neutron EDM in two different models, the Quark-Parton Model and the Chiral Quark Model. It turns out that the predictions are quite sensitive to the model used. We show parameter regions in the M_0-M_1/2 plane which are excluded by considering simultaneously the experimental bounds of both electron and neutron EDM, assuming specific values for the phases phi_{\mu} and phi_{A_0}.

Measurement of the cross-section for the process [formula omitted] at LEP

Measurements of the two-photon interaction e +e − → e +e − + hadrons at s ≃ 91 GeV and s ≃ 183 GeV are presented. The double-tag events, collected with the L3 detector, correspond to interated luminosities of 140 pb −1 at 91 GeV and 52 pb −1 at 183 GeV. The cross-section of γ ∗γ ∗ collisions has been measured at 〈 Q 2〉 = 3.5 GeV 2 and 〈 Q 2〉 = 14 GeV 2. The data agree well with predictions based on perturbative QCD, while the Quark Parton Model alone is insufficient to describe the data.

Quark Distributions in the Nucleon by the Semi-Inclusive Reaction on the Proton and Helium-3 Target

We investigate the spin asymmetries which will be measured by semi-inclusive reactions on proton targets and helium-3 targets with tagged charged pions and kaons. By using the quark parton model, we relate the spin asymmetries to the quark spin distributions in the nucleon. We also use the parametrized fragmentation functions and the existing parametrizations of polarized and unpolarized quark distributions. Then we predict the spin asymmetries. We show that we can study the quark distributions in the neutron with the helium-3 target. We find useful properties for experimental analysis: The spin asymmetry changes little if kaons are taken into account, and it is almost independent of the z-cut.

INELASTIC PHOTOPRODUCTION OF W-BOSON

We study the WWγ vertex through the process γq→Wq′ where q denotes the quark at a future high energy collider. The differential cross-section for the photoproduction process γq→Wq′ is computed with both anomalous magnetic moment κ and electric quadrupole moment λ for the W-boson. The deep inelastic photoproduction of W± is also calculated in a quark-parton model.

Atmospheric muon flux at sea level, underground, and underwater

The vertical sea-level muon spectrum at energies above 1 GeV and the underground/underwater muon intensities at depths up to 18 km w.e. are calculated. The results are particularly collated with a great body of the ground-level, underground, and underwater muon data. In the hadron-cascade calculations, the growth with energy of inelastic cross sections and pion, kaon, and nucleon generation in pion-nucleus collisions are taken into account. For evaluating the prompt muon contribution to the muon flux, we apply two phenomenological approaches to the charm production problem: the recombination quark-parton model and the quark-gluon string model. To solve the muon transport equation at large depths of homogeneous medium, a semi-analytical method is used. The simple fitting formulas describing our numerical results are given. Our analysis shows that, at depths up to 6-7 km w. e., essentially all underground data on the muon intensity correlate with each other and with predicted depth-intensity relation for conventional muons to within 10%. However, the high-energy sea-level data as well as the data at large depths are contradictory and cannot be quantitatively decribed by a single nuclear-cascade model.

Spin structure of the proton from polarized inclusive deep-inelastic muon-proton scattering

We have measured the spin-dependent structure function $g_1^p$ in inclusive deep-inelastic scattering of polarized muons off polarized protons, in the kinematic range $0.003 < x < 0.7$ and $1 GeV^2 < Q^2 < 60 GeV^2$. A next-to-leading order QCD analysis is used to evolve the measured $g_1^p(x,Q^2)$ to a fixed $Q^2_0$. The first moment of $g_1^p$ at $Q^2_0 = 10 GeV^2$ is $\Gamma^p = 0.136\pm 0.013(stat.) \pm 0.009(syst.)\pm 0.005(evol.)$. This result is below the prediction of the Ellis-Jaffe sum rule by more than two standard deviations. The singlet axial charge $a_0$ is found to be $0.28 \pm 0.16$. In the Adler-Bardeen factorization scheme, $\Delta g \simeq 2$ is required to bring $\Delta \Sigma$ in agreement with the Quark-Parton Model. A combined analysis of all available proton and deuteron data confirms the Bjorken sum rule.

Proton spin structure in the rest frame

It is shown that the quark-parton model in the standard infinite momentum approach overestimate the proton spin structure function $g_1(x)$ in comparison with the approach taking consistently into account the internal motion of quarks described by a spherical phase space in the proton rest frame. Particularly, it is shown the first moment of the spin structure function in the latter approach, assuming only the valence quarks contribution to the proton spin, does not contradict to the experimental data.

Nontrivial Q2-Dependence in the Gottfried Sum Induced by Nonperturbative Effect

In consideration of the lowest dimensional condensate contributions to the free quark propagator, we obtain the nonperturbative quark propagator with the nonvanishing vacuum average value for quark composite operator , Using the -corrected quark propagator and basing on the simple quark-parton model, we discuss the nonperturbative effect in the nucleon structure function. It is shown that the nonperturbative effect modifies the conventional quark-parton model formula of the nucleon structure function at finite Q2 and suggests a nontrivial Q2-dependence in the Gottfried sum.

Precision Determination of the Neutron Spin Structure Functiong1n

We report on a precision measurement of the neutron spin structure function $g^n_1$ using deep inelastic scattering of polarized electrons by polarized ^3He. For the kinematic range 0.014<x<0.7 and 1 (GeV/c)^2< Q^2< 17 (GeV/c)^2, we obtain $\int^{0.7}_{0.014} g^n_1(x)dx = -0.036 \pm 0.004 (stat) \pm 0.005 (syst)$ at an average $Q^2=5 (GeV/c)^2$. We find relatively large negative values for $g^n_1$ at low $x$. The results call into question the usual Regge theory method for extrapolating to x=0 to find the full neutron integral $\int^1_0 g^n_1(x)dx$, needed for testing quark-parton model and QCD sum rules.

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.