Nucleon Spin Structure Functions Research Articles

Resonance contributions to nucleon spin structure in holographic QCD

We study polarized inelastic electron-nucleon scattering at low momentum transfer in the Witten-Sakai-Sugimoto model of holographic QCD, focusing on resonance production contributions to the nucleon spin structure functions. Our analysis includes both spin 3/2 and spin 1/2 low-lying nucleon resonances with positive and negative parity. We determine, in turn, the helicity amplitudes for nucleon-resonance transitions and the resonance contributions to the neutron and proton generalized spin polarizabilities. Extrapolating the model parameters to realistic QCD data, our analysis, triggered by recent experimental results from Jefferson Lab, agrees with the observation that the ∆(1232) resonance gives the dominant contribution to the forward spin polarizabilities at low momentum transfer. The contribution is negative and tends to zero as the momentum transfer increases. As expected, the contribution of the ∆(1232) to the longitudinal-transverse polarizabilities is instead negligible. The latter, for both nucleons, turn out to be negative functions with zero asymptote. The holographic results, at least for the proton where enough data are available, are in qualitative agreement with the resonance contributions to the spin polarizabilities extracted from experimental data on the helicity amplitudes.

Nucleon spin structure functions, considering target mass correction, and higher twist effects at the NNLO accuracy and their transverse momentum dependence

Using recent and updated world data on polarized structure functions ${g}_{1}$ and ${g}_{2}$ we perform an analysis based on quantum chromodynamics (QCD) as a theory of strong interactions at next-next-to-leading-order accuracy. We include also target mass corrections and higher twist effects to get more precise results in our fitting procedure. To confirm the validity of our fitting results several sum rules are examined and we do a comparison for them with results from other models. In our analysis we employ the Jacobi polynomials approach to obtain analytical solutions of the Dokshitzer-Gribov-LipatovAltarelli-Parisi evolution equations for parton distribution functions (PDFs). Using the extracted PDFs from our data analysis as input, we also compute the $x$- and ${\mathbf{p}}_{T}$-dependence of some transverse momentum dependence PDFs in polarized case, based on covariant parton model. These functions are naively even time-reversal at twist-two approximation. The results for transverse momentum dependences indicate proper and acceptable behavior with respect to what are presented in other literatures.

Gluon transversity in polarized proton-deuteron Drell-Yan process

Nucleon spin structure functions have been investigated mainly by longitudinally-polarized ones for finding the origin of the nucleon spin. Other types of spin structure functions are transversely-polarized ones. In particular, quark transversity distributions in the nucleons have very different properties from the longitudinally-polarized quark distribution functions, especially in scaling violation, because they are decoupled from the gluon transversity, due to the fact that they are helicity-flip (chiral-odd) distributions. Such studies are valuable for finding not only the origin of the nucleon spin but also a signature on physics beyond the standard model, because the electric dipole moment of the neutron is proportional to the transversity distributions. Now, there is experimental progress on the quark transversity distributions; however, there is no experimental information on gluon transversity. In fact, the gluon transversity does not exist for the spin-1/2 nucleon due to the helicity-conservation constraint. One needs a hadron with spin more than or equal to one, so that the helicity flip of two units is allowed. A stable spin-1 target is, for example, the deuteron for studying the gluon transversity. In this work, we propose a possibility for finding the gluon transversity at hadron-accelerator facilities, especially in the proton-deuteron Drell-Yan process with the linearly-polarized deuteron, by showing theoretical formalism and numerical results. We show the dependencies of the Drell-Yan cross section on the dimuon-mass squared $M_{\mu\mu}^{\,2}$, the dimuon transverse-momentum $q_T$, the dimuon rapidity $y$ in the center-of-momentum frame, and the magnitude of the gluon transversity $\Delta_T g$.

Updating spin-dependent Regge intercepts

We use new high statistics data from CLAS and COMPASS on the nucleon's spin structure function at low Bjorken x and low virtuality, Q^2 < 0.5 GeV^2, together with earlier measurements from the SLAC E-143, HERMES and GDH experiments to estimate the effective intercept(s) for spin dependent Regge theory. We find alpha_{a_1} = 0.31 \pm 0.04 for the intercept describing the high-energy behaviour of spin dependent photoabsorption together with a new estimate for the high-energy part of the Gerasimov-Drell-Hearn sum-rule, -15 \pm 2 mu b from photon-proton centre-of-mass energy greater than 2.5 GeV. Our value of alpha_[a_1} suggests QCD physics beyond a simple straight-line a_1 trajectory.

Target Mass Corrections to QCD Bjorken Sum Rule for Nucleon Spin Structure Functions

We discuss the possible target mass corrections in the QCD analysis of nucleon's spin-dependent structure functions measured in the polarized deep-inelastic leptoproduction. The target mass correction for the QCD Bjorken sum rule is obtained from the Nachtmann moment and its magnitude is estimated employing positivity bound as well as the experimental data for the asymmetry parameters. We also study the uncertainty due to target mass effects in determining the QCD effective coupling constant $\alpha_s(Q^2)$ from the Bjorken sum rule. The target mass effect for the Ellis-Jaffe sum rule is also briefly discussed.

The role of semi inclusive DIS data in determining polarized PDFs

A new combined NLO QCD analysis of the polarized inclusive and semi-inclusive DIS data is presented. In contrast to previous combined analyses, the 1/Q2 terms (kinematic –target mass corrections, and dynamic – higher twist corrections) in the expression for the nucleon spin structure function g1 are taken into account. The new COMPASS data are included in the analysis. The impact of the semi-inclusive (SIDIS) data on the polarized parton densities (PDFs) and on the higher twist corrections is demonstrated. The incompatibility of the polarized strange quark density obtained from DIS and from SIDIS data is discussed.

Determination of polarized parton densities from a QCD analysis of inclusive and semi-inclusive deep inelastic scattering data

A new combined next to leading order QCD analysis of the polarized inclusive and semi-inclusive deep inelastic lepton-hadron scattering (DIS) data is presented. In contrast to previous combined analyses, the $1/Q^2$ terms (kinematic - target mass corrections, and dynamic - higher twist corrections) in the expression for the nucleon spin structure function $g_1$ are taken into account. The new COMPASS data are included in the analysis. The impact of the semi-inclusive data on the polarized parton densities (PDFs) and on the higher twist corrections is discussed. The new results for the PDFs are compared to our (Leader, Sidorov, Stamenov) LSS'06 PDFs, obtained from the fit to the inclusive DIS data alone, and to those obtained from the de Florian, Sassot, Stratmann, and Vogelsang global analysis.

Target mass corrections to the matrix elements in nucleon spin structure functions

Target mass corrections to the twist-4 terms (f) over bar (p,n,d)(2) as well as to the leading-twist (a) over tilde (2) are discussed.

Twist-3 effects and target mass corrections

We estimate the higher-twist contributions using Nachtmann moments, which provide a method for handling target mass corrections. The corrections to the Cornwall-Norton moments of nucleon spin structure functions are also addressed. Their sizable role in the twist-3 ${\stackrel{~}{d}}_{2}^{p,n,d}$ extracted from the Cornwall-Norton moments is discussed. Moreover, their role in the isoscalar and isovector structure functions is also shown.

Quark-hadron duality of nucleon spin-structure functions and target mass corrections

Bloom-Gilman quark-hadron duality of nucleon spin-structure functions g 1,2 is discussed. The occurrence of the duality is also estimated. Moreover, target mass corrections to the nucleon spin-structure functions as well as to the quark-hadron duality are studied.

Target mass corrections and twist-3 in the nucleon spin structure functions

The Nachtmann moment is employed to study the contribution of twist-3 operator to the nucleon spin structure functions. Target mass corrections to the Cornwall–Norton moments of the spin structure functions g1,2 are discussed. It is found that the corrections play a sizeable role to the contribution of the twist-3 d˜2 extracted from the Cornwall–Norton moments.

Local quark–hadron duality of nucleon spin structure functions with target mass corrections

Target mass corrections to nucleon spin structure functions are analyzed. Our results show that the corrections are important to the structure functions in a large x region. Moreover, they play a remarkable role to the local quark–hadron duality of the nucleon spin structure functions in three individual inelastic resonance production regions.

Target-mass corrections to the truncated moments of nucleon spin structure functions and quark-hadron duality

The Cornell-Norton and Nachtmann moments of nucleon spin structure functions are discussed with a nonvanishing nucleon mass. Target-mass corrections to the truncated moments of the structure functions are calculated. Moreover, the corrections to the Bloom-Gilman quark-hadron dualities of ${g}_{1}^{p}$ and ${g}_{2}^{p}$ in both the inelastic resonance production region and the elastic one are analyzed.

Higher twists in polarized DIS and the size of the constituent quark

The spontaneous breaking of chiral symmetry implies the presence of a short-distance scale in the QCD vacuum, which phenomenologically may be associated with the size of the constituent quark, $\ensuremath{\rho}\ensuremath{\approx}0.3\text{ }\text{ }\mathrm{fm}$. We discuss the role of this scale in the matrix elements of the twist-4 and 3 quark-gluon operators determining the leading power ($1/{Q}^{2}$)-corrections to the moments of the nucleon spin structure functions. We argue that the flavor-nonsinglet twist-4 matrix element, ${f}_{2}^{u\ensuremath{-}d}$, has a sizable negative value of the order ${\ensuremath{\rho}}^{\ensuremath{-}2}$, due to the presence of sea quarks with virtualities $\ensuremath{\sim}{\ensuremath{\rho}}^{\ensuremath{-}2}$ in the proton wave function. The twist-3 matrix element, ${d}_{2}$, is not related to the scale ${\ensuremath{\rho}}^{\ensuremath{-}2}$. Our arguments support the results of previous calculations of the matrix elements in the instanton vacuum model. We show that this qualitative picture is in agreement with the phenomenological higher-twist correction extracted from an NLO QCD fit to the world data on ${g}_{1}^{p}$ and ${g}_{1}^{n}$, which include recent data from the Jefferson Lab Hall A and COMPASS experiments. We comment on the implications of the short-distance scale $\ensuremath{\rho}$ for quark-hadron duality and the $x$-dependence of higher-twist contributions.

Bloom–Gilman Duality of Nucleon Spin Structure Function andElastic Peak Contribution

By employing the parametrization form of the nucleon spin structure functionin the resonance region, which includes the contributions of the resonancepeaks and of nonresonance background, we study Bloom–Gilman quark-hadronduality of g1 both in the inelastic resonance region and elastic one.

Study of target mass effects on the moments of [formula omitted] and Bloom–Gilman quark–hadron duality

We study the Nachtmann moments and Bloom–Gilman quark–hadron duality of nucleon spin structure function. The target mass corrections to the moments of g 1 are discussed. Moreover, the onset of Bloom–Gilman quark–hadron duality of g 1 is estimated to be ∼ 2 GeV 2 .

Higher moments of nucleon spin structure functions in heavy baryon chiral perturbation theory and in a resonance model

The third moment $d_2$ of the twist-3 part of the nucleon spin structure function $g_2$ is generalized to arbitrary momentum transfer $Q^2$ and is evaluated in heavy baryon chiral perturbation theory (HBChPT) up to order ${\mathcal{O}}(p^4)$ and in a unitary isobar model (MAID). We show how to link $d_2$ as well as higher moments of the nucleon spin structure functions $g_1$ and $g_2$ to nucleon spin polarizabilities. We compare our results with the most recent experimental data, and find a good description of these available data within the unitary isobar model. We proceed to extract the twist-4 matrix element $f_2$ which appears in the $1/Q^2$ suppressed term in the twist expansion of the spin structure function $g_1$ for proton and neutron.

Nucleon spin structure function and quark-hadron duality

The nucleon spin structure function ${g}_{1}$ is studied based on a simple constituent quark model. Effects of low-lying nucleon resonances are discussed and the onset of Bloom-Gilman quark-hadron duality for the nucleon spin structure function is estimated. Our result qualitatively shows that in the range of ${Q}^{2}&lt;0.75{\mathrm{GeV}}^{2},$ quark-hadron duality for ${g}_{1}^{p}$ is violated.

Nucleon Spin Structure Functions in the Resonance Region and the Duality**The project supported by National Natural Science Foundation of China under Grant No. 10075056, and by Foundations of the Chinese Academy of Sciences (X-37), of the Chinese Ministry of Education (B-22), and of the Institute of Theoretical Physics. Thanks also go to CAS Knowledge Innovation Project No. KC2-SW-N02

We discuss the nucleon spin structure function g1 and the difference between the proton and neutron targets , based on quark model calculation. Quark-hadron duality for the nucleon spin structure function is also analyzed. Effects of the Δ(1232) and Roper P11(1440) resonances on the spin structure function and on the difference are mentioned. The results of different models for the Roper resonance are also addressed.

Results from the HERMES experiment

A summary of results from the HERMES experiment is presented. Inclusive and semi-inclusive asymmetries have yielded measurements of the nucleon spin structure function g 1( x) and of the flavor separated polarized parton distributions Δ q( x). First indications of the gluon polarisation and of the transversity distribution are reported, along with the first measurement of a single spin azimuthal asymmetry in semi-inclusive pion production. A short overview of other results is also presented.