Spin-dependent Quark Distributions Research Articles

Unified Description of Polarized and Unpolarized Quark Distributions in the Proton.

We propose a unified new approach to describe polarized and unpolarized quark distributions in the proton based on the gauge-gravity correspondence, light-front holography, and the generalized Veneziano model. We find that the spin-dependent quark distributions are uniquely determined in terms of the unpolarized distributions by chirality separation without the introduction of additional free parameters. The predictions are consistent with existing experimental data and agree with perturbative QCD constraints at large longitudinal momentum x. In particular, we predict the sign reversal of the polarized down-quark distribution in the proton at x=0.8±0.03, a key property of nucleon substructure which will be tested very soon in upcoming experiments.

Re-examining valence quark spin distributions

The observed deep inelastic nucleon spin asymmetries ${A}_{1}^{p,n}$ and the spin dependent quark distributions break SU(6) spin-flavor symmetry. In quark models two mechanisms for breaking SU(6) symmetry are well known: the hyperfine interaction and the pion cloud of the nucleon. I re-evaluate these mechanisms and show how the breaking of SU(6) in each case affects spin dependent valence quark distributions of the proton. In particular I investigate the properties of these mechanisms in the kinematic region $0.3<x<0.7$ and attempt to test these against known data. Both mechanisms are able to explain the quantitative features of the spin asymmetries.

Spin-Dependent Coupled Altarelli-Parisi Equations in the NLO and the Method of Successive Approximations

The coupled Altarelli-Parisi (AP) equations for polarized singlet quark distribution and polarized gluon distribution, when considered in the small x limit of the next to leading order (NLO) splitting functions, reduce to a system of two first order linear nonhomogeneous integrodifferential equations. We have applied the method of successive approximations to obtain the solutions of these equations. We have applied the same method to obtain the approximate analytic expressions for spin-dependent quark distribution functions with individual flavour and polarized structure functions for nucleon.

Transverse momentum dependent quark distributions and polarized Drell-Yan processes

We study the spin-dependent quark distributions at large transverse momentum. We derive their transverse momentum behaviors in the collinear factorization approach in this region. We further calculate the angular distribution of the Drell-Yan lepton pair production with polarized beams and present the results in terms of the collinear twist-three quark-gluon correlation functions. In the intermediate transverse momentum region, we find that the two pproaches: the collinear factorization and the transverse momentum dependent factorization approaches are consistent in the description of the lepton pair angular distributions.

Single transverse-spin asymmetry in semi-inclusive deep inelastic scattering

Abstract We study the single transverse-spin asymmetry in semi-inclusive hadron production in deep inelastic scattering. We derive the leading contribution to the asymmetry at moderate transverse momentum P h ⊥ of the produced hadron in terms of twist-three quark–gluon correlation functions, and compare with the result obtained from the approach based on factorization at fixed transverse momentum which involves asymmetric transverse-momentum and spin-dependent quark distributions. We verify that the two approaches yield identical results in this regime. Comparing with our earlier calculations for the single-spin asymmetry in the Drell–Yan process, we confirm the sign difference between the time-reversal-odd transverse-momentum-dependent quark distributions in the two processes.

Spin-Dependent Structure Functions in Nuclear Matter and the Polarized EMC Effect

An excellent description of both spin-independent and spin-dependent quark distributions and structure functions has been obtained with a modified Nambu--Jona-Lasinio model, which is free of unphysical thresholds for nucleon decay into quarks--hence incorporating an important aspect of confinement. We utilize this model to investigate nuclear medium modifications to structure functions and find that we are readily able to reproduce both nuclear matter saturation and the experimental F2N(A)/F2N ratio, that is, the European Muon Collaboration (EMC) effect. Applying this framework to determine g1p(A), we find that the ratio g1p(A)/g1p differs significantly from unity, with the quenching caused by the nuclear medium being about twice that of the spin-independent case. This represents an exciting result, which, if confirmed experimentally, will reveal much about the quark structure of nuclear matter.

Single transverse spin asymmetries in inclusive hadron production

A consistent phenomenological approach to the computation of transverse single spin asymmetries in inclusive hadron production is presented, based on the assumed generalization of the QCD factorization theorem to the case in which quark intrinsic motion is taken into account. New k ⊥ and spin dependent quark distribution and fragmentation functions are considered: some of them are fixed by fitting data on p ↑ p → πX and predictions are given for single spin asymmetries in ℓp ↑ → πX and γ ∗p ↑ → πX processes.

Chiral-odd transversity spin structure function h1( x) of the nucleon in a constituent quark model

We study the chiral-odd transversity spin-dependent quark distribution function h 1( x) of the nucleon in a constituent quark model. The twist-2 structure functions, f 1( x), g 1( x) and h 1( x) are calculated within the diquark spectator approximation. Whereas an inequality f 1( x) > h 1( x) > g 1( x) holds with the interaction between quark and diquark being scalar, the axial-vector effective quark-diquark interaction, which contributes to the d-quark distribution, does not lead to such a simple relation. We find that h 1( x) for the d-quark becomes somewhat smaller than g 1 d ( x), when we fix the model parameter to reproduce known other structure functions. We also include corrections due to the non-trivial structure of the constituent quark, which is modeled by the Goldstone boson dressing. This improves agreements of f 1( x) and g 1( x) with experiments, and brings further reduction of h 1 d ( x) distribution. Consequences for semi-inclusive experiments are also discussed.

SPIN STRUCTURE OF THE PROTON

By assuming that there is no significant intrinsic polarization of the gluon, we have computed the polarized quark contributions to the proton’s spin under SU(3) flavor symmetry breaking for the polarized sea and have performed a global leading-order QCD fit to obtain the spin-dependent quark distributions, which could be used as input for analyzing lepton-hadron and hadron-hadron collisions.

O( alpha s) spin-dependent weak structure functions.

The complete next to leading logarithmic (${\cal O}(\alpha_{s})$) corrections to the spin dependent weak deep inelastic structure functions $g_{1},g_{3}$ and $g_{4}$, are calculated using dimensional regularization within the HVBM method. Analysing the quark and gluon initiated contributions to these corrections for different values for the quark masses, a consistent factorization prescription for spin dependent quark distributions, which safely removes soft contributions, is defined. It is shown that within this scheme, quark initiated corrections are comparable in magnitude to those of gluonic origin, even though their contributions to the moments are small.

Hadron structure and left-right asymmetry in inclusive production in single-spin hadron-hadron collisions.

Theoretical arguments and experimental facts are presented which show the following. Left-right asymmetries are expected to exist in a number of single-spin inclusive production processes. Measuring such asymmetries by using different types of projectile-target combinations, one can obtain useful information on the spin structure of hadrons in general, and determine the spin-dependent quark distribution functions in particular. Quantitative predictions for these asymmetries in various reactions are presented.

Gluonic contribution to g1 and its relationship to the spin-dependent parton distributions.

We examine the suggestion of Altarelli and Ross and of Carlitz, Collins, and Mueller that there is a hard gluonic contribution to the first moment of the proton's spin-dependent structure function ${g}_{1}$. We find that if the soft (collinear) divergence in the gluonic contribution is regulated dimensionally or with a quark mass, then the first moment vanishes. More generally, we suggest that the hard gluonic contribution to ${g}_{1}$ be identified by subtracting certain contributions that are attributable to the spin-dependent quark distributions. We show that the first moment of the resulting hard gluonic contribution vanishes, provided that the UV regulator for the spin-dependent quark distributions respects gauge invariance, Lorentz invariance, and the analyticity structure of the unregulated distributions. However, by relaxing the Lorentz-invariance requirement, we are able to construct quark distributions such that the hard gluonic contribution to the first moment of ${g}_{1}$ is nonzero. The corresponding quark distributions are related to matrix elements of Lorentz-variant operators. Hence, they have no analogue in the standard operator-product expansion and do not satisfy the usual forms of the quark sum rules. We conclude that the size of the gluonic contribution to the first moment of ${g}_{1}$ is entirely a matter of the convention used in defining the quark distributions.

Polarized lepton deep-inelastic scattering from few-nucleon targets

The structure functions for deep-inelastic scattering of polarized leptons from polarized few-nucleon targets (nucleon, 2H, 3He) are calculated in a parton model. Spin-dependent quark distributions constructed along the lines of Carlitz-Kaur model are used. The asymmetry for scattering from polarized 3He is small in magnitude and dominated by the neutron contribution. For 2H, cancellation between proton and neutron contributions leads to a very small asymmetry below x≈0.1. Otherwise the asymmetry is large but dominated by the proton.