Polarized Quark Distributions Research Articles

Inclusive and semi-inclusive analysis from polarised deep-inelastic muon scattering

We present new results for the spin-dependent structure function on the proton and for the polarised quark distributions in the nucleon from semi-inclusive spin asymmetries. With the inclusive asymmetry from deep inelastic scattering of polarised muons on polarised protons, it is found that the Ellis-Jaffe sume rule is violated. Using our results for γ d 1 the Bjorken sum rule is confirmed with an accuracy of about 15%. From semi-inclusive spin asymmetries and SMC inclusive spin asymmetries, we determine the polarised quark distributions of valence u and d quarks to be positive and negative, respectively, while the non-strange sea distribution to be consistent with zero.

Nucleon spin structure measurements at DESY

Measurements are reported for spin asymmetries in inclusive and semi-inclusive deep inelastic scattering on on polarised 3 He and hydrogen targets. The data were collected by the HERMES experiment at HERA/DESY and cover a kinematic range of 0.023 – 0.6 in ξ. The extraction of polarised quark distributions is described and preliminary results are presented. Several options for future measurements addressing the spin structure of the nucleon at DESY are mentioned.

Measurement of polarized quark distributions at HERMES

Preliminary results for inclusive and semi-inclusive hadron asymmetries are presented for 3He and 1H targets, measured in 1995 and 1996 at the HERMES experiment. Under the assumption of spin-independent fragmentation, these asymmetries may be used to extract the polarizations of individual quark flavors. The extraction requires a knowledge of unpolarized parton distributions, an understanding of the fragmentation process, a precise description of the HERMES spectrometer, and an assumption about the sea polarization. Preliminary results for the uv, dv, and averaged sea quark polizations are presented.

Quark spin distributions and axial anomaly in polarized deep inelastic scattering

We present a measurement of inclusive and semi-inclusive spin asymmetries from deep inelastic scette ring of polarized muons on polarized protons and deuterons in the range 0.003<x<0.7 and Q 2>1 GeV2. From these asymmetries we determine the polarized quark distributions of valence quarks and non-strange sea quarks at Q 2=10 GeV2. The polarized u valence quark distribution is positive and d valence quark distribution is negative. The non-strange sea spin distribution is consistent with zero over the whole measured range of x. We calculate moments of those distributions and, for valence quarks, we find the first moments consi stent with values obtained from weak decay constants and the second moments consistent with lattice QCD predictions. We also examine a hypothesis of anomalous contribution to the nucleon spin from colour background field at high x by fitting this hypothesis to the world data on inclusive and semi-inclusive asymmet ries.

QCD corrections to double and single spin asymmetries in vector boson production at polarized hadron colliders

The production of W ± and Z 0 bosons in collisions of longitudinally polarized protons allows one to access various combinations of the polarized quark distributions from the study of double and single spin asymmetries. We compute the O (α s) corrections to these asymmetries as function of the vector boson rapidity y in the hadron-hadron centre-of-mass frame. Detailed studies of the numerical impact of the next-to-leading order corrections are presented. The theoretical uncertainties of an extraction of polarized quark distributions from future vector boson production data are investigated in detail.

Drell-Yan non-singlet spin cross sections and spin asymmetry to O( αs2) (II)

We present predictions for the non-singlet Drell-Yan longitudinal spin cross sections and spin asymmetry, A LL , in proton-proton collisions at large p T at the RHIC energy of 200 GeV at next-to-leading order QCD. The higher order corrections to the non-singlet polarized cross section, σ NS LL , are sizeable and similar to those found for the unpolarized cross section. The non-singlet asymmetry parameter, A LL NS , is very stable against higher order corrections and is a direct measurement of the non-singlet (i.e., valence) polarized quark distributions within the proton.

Measuring polarized gluon and quark distributions with meson photoproduction

We calculate polarization asymmetries in the photoproduction of high transverse momentum mesons, focusing on charged pions, considering the direct, fragmentation, and resolved photon processes. The results at very high meson momentum measure the polarized quark distributions and are sensitive to differences among the existing models. The results at moderate meson momentum are sensitive to the polarized gluon distribution and can provide a good way to measure it. Suitable data may come as a by-product of deep inelastic experiments to measure ${g}_{1}$ or from dedicated experiments.

Polarised quark distributions in the nucleon from semi-inclusive spin asymmetries

We present a measurement of semi-inclusive spin asymmetries for positively and negatively charged hadrons from deep inelastic scattering of polarised muons on polarised protons and deuterons in the range $0.003<x<0.7$ and $Q^2>$1 GeV$^2$. Compared to our previous publication on this subject, with the new data the statistical errors have been reduced by nearly a factor of two. From these asymmetries and our inclusive spin asymmetries we determine the polarised quark distributions of valence quarks and non-strange sea quarks at $Q^2$=10 GeV$^2$. The polarised $u$ valence quark distribution, $\Delta u_v(x)$, is positive and the polarisation increases with $x$. The polarised $d$ valence quark distribution, $\Delta d_v(x)$, is negative and the non-strange sea distribution, $\Delta \bar q(x)$, is consistent with zero over the measured range of $x$. We find for the first moments $\int_0^1 \Delta u_v(x) dx = 0.77 \pm 0.10 \pm 0.08$, $\int_0^1 \Delta d_v(x) dx = -0.52 \pm 0.14 \pm 0.09$ and $\int_0^1 \Delta \bar q(x) dx= 0.01 \pm 0.04 \pm 0.03$, where we assumed $\Delta \bar u(x) = \Delta \bar d(x)$. We also determine for the first time the second moments of the valence distributions $\int_0^1 x \Delta q_v(x) dx$.

New measurements of proton polarized structure functions in charged current processes at DESY HERA

Estimates for longitudinal spin asymmetries which single out new polarized nucleon structure functions in deeply inelastic charged current interactions at HERA energies are given, exploiting their interpretation in terms of polarized quark distributions. These asymmetries turn out to be large and allow a measurement of the new polarized structure functions $g_1^{W}$ and $g_5^{W}$, which would add valuable tests and information on the spin content of quarks inside a polarized proton. We also show that single spin asymmetries in neutral current interactions are very small.

Probing polarized parton distributions with meson photoproduction

Polarization asymmetries in photoproduction of high transverse momentum mesons are a flavor sensitive way to measure the polarized quark distributions. We calculate the expected asymmetries in several models, and find that the asymmetries are significant and also significantly different from model to model. Suitable data may come as a by-product of deep inelastic experiments to measure g 1 or from dedicated experiments.

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the newest Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperon $\beta$-decay data. We use the combination of proton and neutron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking gluonic degrees of freedom into account.

Phenomenological determination of polarized quark distributions in the nucleon

Phenomenological determination of polarized quark distributions in the nucleon

Hard part of the gamma -gluon cross section in deep-inelastic scattering off polarized target.

We analyze in detail the problem of separation of the hard $\ensuremath{\gamma}$-gluon cross section in polarized deep-inelastic scattering, closely related to the current dispute over the existence of the so-called anomalous gluon contribution to the flavor-singlet part of the nucleon polarized structure function ${g}_{1}(x)$. We find that maintaining the factorization property of the full $\ensuremath{\gamma}$-nucleon cross section beyond the leading order in ${\ensuremath{\alpha}}_{s}$ gives an unambiguous definition of the gluon contribution to the first moment of ${g}_{1}(x)$, independent of the value of soft momentum scales, such as quark and gluon masses, involved in the problem. Consideration of the soft contribution, which cannot be calculated reliably in perturbation theory, hints at the possible violation of a naive, constituent-quark-model pattern in polarized quark distributions for light flavors, as advocated earlier by Jaffe and Manohar. We argue that the latter conclusion should have an important impact on the current phenomenology of polarized quark distributions in the nucleon.

Parity and charge symmetry violations in high-energy lepton pair production with polarized hadrons

With polarized beams or targets the nature of the neutral weak current can be explored in the production of lepton pairs in high-energy hadronic collisions. The production of lepton pair masses of order 10 GeV, if dominated by quark-antiquark annihilation process and described by the Weinberg-Salam theory, will exhibit parity violations of the order of a few tenths of a percent and charge symmetry violations of the order of a few percent due to electro-weak interference terms. Both differential and integrated asymmetries are presented here. The dependence on the polarization of the constituent quarks is discussed with emphasis on how measurements of these asymmetries yield information on the polarized quark distributions.