Spin-dependent parton distributions from polarized structure function data

In the past year, polarized deep inelastic scattering experiments at CERN and SLAC have obtained structure function measurements off proton, neutron and deuteron targets at a level of precision never before achieved. The measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and also to obtain information on the parton distributions in polarized nucleons. We perform a global leading-order QCD fit to the proton deep inelastic data in order to extract the spin-dependent parton distributions. By using parametric forms which are consistent with theoretical expectations at large and smallx, we find that the quark distributions are now rather well constrained. We assume that there is no significant intrinsic polarization of the strange quark sea. The data are then consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained, and several qualitatively different shapes are suggested. The spin-dependent distributions we obtain can be used as input to phenomenological studies for future polarized hadron-hadron and lepton-hadron colliders.     

Implications of new deep inelastic scattering data for parton distributions

We perform a next-to-leading order structure function analysis of μN and νN deep inelastic data in an attempt to resolve the disagreement between recent EMC and BCDMS measurements of F₂ for μp scattering. Equally acceptable QCD fits are obtained including either set of μN data, but a comparison with Drell-Yan data appears to favour the parton distributions derived from the BCDMS data.     

Quark-parton model relations in deep inelastic lepton scattering

A complete listing of the quark-parton model relations among deep inelastic lepton-nucleon scattering processes is given, to be compared with forthcoming neutrino and anti-neutrino data. We also note the stronger relations that come from specializing in various ways, including antiquark and strange quark suppression, and the dual valence plus sea model. Finally we construct illustrative parametrizations, fitted to the electron data and minimizing the antiquark momentum content, in accord with early results for the neutrino/antineutrino cross-section ratio.     

Semi-inclusive deep inelastic lepton scattering off complex nuclei

It is shown that in semi-inclusive deep inelastic scattering (DIS) of electrons off complex nuclei, the detection, in coincidence with the scattered electron, of a nucleus (A−1) in the ground state, as well as of a nucleon and a nucleus (A−2), also in the ground state, may provide unique information on several long standing problems, such as: i) the nature and the relevance of the final state interaction in DIS; ii) the validity of the spectator mechanism in DIS; iii) the medium induced modifications of the nucleon structure function; iv) the origin of the EMC effect. Received: 4 March 1999     

Tagged-photon events in polarized deep inelastic scattering

Deep inelastic scattering events of a longitudinally polarized electron by a polarized proton with a tagged collinear photon radiated from the initial-state electron are considered. The corresponding cross section is derived in the Born approximation. The model-independent radiative corrections to the Born cross section are also calculated. The obtained result is applied to the elastic scattering.     

Spin-dependent parton distributions and structure functions

Nuclear parton distributions and structure functions are determined in an effective chiral quark theory. We also discuss an extension of our model to fragmentation functions.     

Spin-dependent effects in inelastic scattering

The differential cross sections for the inelastic scattering of 10 MeV, 19.6 MeV, 30.4 MeV, 40 MeV and 49.35 MeV protons to the 2⁺ state (1.409 MeV) in ⁵⁴Fe and of 19.6 MeV protons to the 2⁺ state (0.846 MeV) in ⁵⁶Fe are analyzed in conjunction with the available data on the asymmetries and spin-flip probability amplitudes. The scattering amplitudes for both one step (valence plus core polarization) and two step (intermediate resonance) processes are evaluated using an antisymmetrized distorted wave approximation. Collective model representations for both the one step (core polarization) and two step (intermediate resonance) processes are used, and included are the effects of deforming the full Thomas spin-orbit potentials. The one step processes are fixed by the analyses of the scattering of 30.4, 40 and 49.35 MeV protons, with the core polarization contributions being constrained by the B(E2) values for the γ-ray deexcitation of the 2⁺ states. The analyses of the 19.6 MeV data demonstrates the need for an extra (two step) contribution to the reaction process and are consistent with the virtual formation of an L = 3 giant resonance. The 10 MeV data most certainly demonstrate compound nucleus effects but could also have some strength due to the virtual formation of an intermediate L = 2 giant resonance. The resonance parameters are consistent with recent information concerning the mass variation of giant resonances.     

Flavor decomposition of the sea-quark helicity distributions in the nucleon from semiinclusive deep inelastic scattering

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.     

Semi-inclusive deep inelastic lepton scattering in a pion cloud model

In this note we explore the consequences of the pion cloud model for semi-inclusive deep inelastic lepton scattering. We argue that by scattering on a few-nucleon target, the detection of the recoiling target would provide a valuable test of the meson cloud model. We estimate the semi-inclusive cross section for deep inelastic lepton scattering on a³He target, as function of Bjorkenx and target recoil momentum.     

Spin-dependent deep inelastic electron scattering in an asymptotically free gauge theory

We compute the logarithmic deviations from Bjorken scaling for the spin dependent structure functions of deep inelastic electron scattering in an asymptotically free gauge theory. Continuations to the elastic limit and to the Regge region are also considered.     

Quasielastic versus inelastic and deep inelastic lepton scattering in nuclei at x > 1

We have made a thorough investigation of the nuclear structure function W_2A in the region of 0.8 < x < 1.5 and Q² < 20 GeV², separating the quasielastic and inelastic plus deep inelastic contributions. The agreement with present experimental data is good giving support to the results for both channels. Predictions are made in yet unexplored regions of x and Q² to assert the weight of the quasielastic or inelastic channels. We find that at Q² < 4 GeV² the structure function is dominated by the quasielastic contributions for x < 1.5, while for values of Q² > 15 GeV² and the range of x studied the inelastic channels are over one order of magnitude bigger than the quasielastic one. The potential of the structure function at x > 1 as a source of information on nuclear correlations is stressed once more.     

New sum rules for polarized deep inelastic scattering

Relations between the structure functions g₁ and g₂ of polarized DIS are derived in the leading perturbative QCD approximation, starting with the sum rules for twist-3 parton correlation functions which follow from gauge invariance and the equation of motion of the quark fields. For the first moments they coincide with the approximate relations proposed earlier by Wandzura and Wilczek as well as Bukhvostov, Kuraev and Lipatov.     

Approach to scaling in deep inelastic scattering and lepton pair production

The covariant parton model is used to investigate the approach to scalling in deep inelastic lepton scattering and in lepton pair production by hadron beams. The subasymptotic effects in these two reactions are controlled by rather different features. Of particular importance for lepton pair production is how far the partons are off shell before they annihilate, while for deep inelastic scattering what matters is rather the mass of the parton after it has absorbed the virtual photon. There are uncertainties because of problems with gauge invariance, but it seems that subasymptotic effects in lepton pair production may be large even at SPS/Fermilab energies.Transverse momentum and x distributions of partons in hadrons are discussed in a very simple model. In particular it is found that, while the model is constructed such that F₂(x) ～ (1 ? x)³ as x → 1, for values of x up to 0.85 F₂(x) is better approximated by (1 ? x)⁴.     

Radiative effects in deep inelastic scattering of polarized leptons by polarized nucleons

Low-order electromagnetic corrections to deep inelastic scattering of polarized leptons by polarized nucleons are derived and investigated using a covariant method. Thorough numerical calculations of the corrections to cross sections and to the measured asymmetry are carried out for μp (ep) scattering in the range of lepton energy E = 200–500 GeV (10–16 GeV). The sensitivity of the corrections to the choice of the structure functions is investigated. The electromagnetic corrections to the asymmetry are found to be less than 10% within the greatest part of the kinematical region, but if x ～ 0, y ～ 1 they can reach a few tens of percent. The lowest order neutral current contribution to the asymmetry is obtained within the framework of standard electroweak interaction theory and the quark parton model. It is found this effect may be significant, especially in the kinematical range where electromagnetic corrections are negligible.