Spin Physics abstracts

Measurement of the Double Spin Asymmetriesin the quasi-real photo production of High-p_T Hadrons at HERMES

The {\sc Hermes} experiment uses the polarized positron beam of the {\sc Hera} accelerator together with a polarized internal gas target to study the spin structure of the nucleon. One of the open questions in the spin structure of the nucleon is the contribution of gluons to the nucleon spin. Studying double spin asymmetries in inclusive quasi-real photo production of single$^1$ or pairs$^2$ of hadrons gives the opportunity to study the polarized gluon and quark distributions. New data on double spin asymmetries for hadron pairs taken on a polarized deuteron target will be presented. First results on single hadron asymmetries on a proton target will be shown. Both datasets show a significant asymmetry at high transverse hadron momenta. An interpretation with respect to their sensitivity of both datasets to the gluon polarization was done using the theoretical model included in PYTHIA$^3$.

Measuring transversity with T-odd one-particle production

The transversity distribution can be measured in leading orderasymmetries in one-particle inclusive DIS, where it appears in connection with chiral-odd fragmentation functions. Among these functions, we focus on the time-reversal odd ones. They occur in fragmentation to unpolarized hadrons or polarized spin-one hadrons. Using a simple model, we show how this functions can be non-vanishing.

Marc Beckmann

Measurements of Polarized Quark Distributions at HERMES

This talk will present HERMES results on the polarized quarkdistributions for the up-, down-, and sea-flavors. The results are based on a LO analysis of polarized semi-inclusive deep inelastic scattering events, taken at the HERA electron storage ring with internal gas targets of polarized 3-helium, hydrogen and deuterium. The data set on the deuterium target employs a RICH detector, which provides the identification of pions, kaons and proton over almost the full kinematic range. In particular, semi-inclusive asymmetries for leading kaons provide an enhanced sensitivity on the polarized strange quark distribution in the sea. These new data will ultimately allow to test whether SU(3) flavor symmetry is broken by extracting the individual quark polarizations for the up, anti-up, down, anti-down, and strange flavors. Latest results on the semi-inclusive asymmetries and the polarized quark distributions will be shown.

Edgar Berger / Markus Diehl

Timelike Compton scattering

 We discuss the inverse process to DVCS, namely gamma p -> \mu^+ \mu^- p with a large invariant mass of the lepton pair. In this       case  this process can be described in a factorised form similar to DVCS and thus provides access to generalised parton distributions. We discuss the basic physics and phenomenology of this process and show numerical estimates for its cross section.
Les Bland

RHIC spin program

Johannes Bluemlein

QCD analysis of polarized structure functions

In this paper we determine the polarized parton densities with errors,lamba-QCD, alpha_s(M_z) (including the effects due to the theoretical uncertainty by the choice of the factorization and renormalization scale.) Moreover, we also perform a factorization-scheme invariant evolution of related physical observables (which was not yet done in the foregoing literature). Our analysis finishes with polarized parton densities being parameterized, including the parameterization of errors, which might be of great help in estimating the error due to these quantities for more involved measurements in the field.

Transversity Single Spin Asymmetries

I will review theoretical aspects of the two leading twist transversity single spin asymmetries, one arising due to the Collins effect and one from the interference fragmentation functions. For these two options I intend to discuss the issues of factorization theorems, evolution and Sudakov factors for the relevant observables. I will discuss some existing and future data and try to map out the road towards a global transversity analysis.

Ingo Bojak

Determining Dg : Polarized Hadro- and Photoproduction of Heavy Quarks in NLO QCD

We present the first calculation of the complete NLO QCD corrections to the hadroproduction of heavy flavors with longitudinally polarized hadrons. Predictions for the extraction of the gluon helicity distribution at RHIC using this reaction are shown, their theoretical uncertainties are briefly discussed, and previous unpolarized calculations are checked. The possibility of a "win-win" situation at RHIC concerning the heavy quark "enigma" on one hand and Dg on the other hand is mentioned. We also show the potential of the EIC project concerning the corresponding photoproduction process, which we had calculated previously in NLO QCD, in comparison to the possibilities at COMPASS. Here it will become feasible in future to take the "resolved" photon contributions into account in NLO QCD thanks to the now completed hadroproduction calculation.

Umberto D'Alesio

Longitudinal Lambda polarization in polarized semi-inclusive DIS

We calculate, within the parton model at leading orders, the expression of the longitudinal polarization of spin 1/2 hadrons (typically Lambda baryons), produced in polarized semi-inclusive DIS in all possible cases in which weak interactions are involved. We present some numerical estimates in few cases for which data are or will soon be available and discuss how to gather new information on polarized fragmentation functions.

Abhay Desphande

The Electron Ion Collider (EIC)

Most of what we know today about {\em the polarized and the unpolarized structure of the nucleon} is based on the experimental investigations usingDeep Inelastic Scattering techniques and the theoretical progress made along with it in the past 40 years. Every instance in this history when the experimental horizon in $x$ and $Q^{2}$ was furthered, new and exciting aspects of QCD were revealed. We stand at this juncture once again in polarized DIS, and will be there in unpolarized DIS after the scheduled operation of HERA with no higher energy e-p scattering facility immediately available.

The Electron Ion Collider (EIC) with it's a) variable energy electron beam ($\sim$5-10 GeV), b) variable variable energy proton beam ($\sim$50-250 GeV), c) possible nuclear beams with energies (up to $\sim$100 GeV/A) for ion species as heavy as Pb, and d) a very high luminosity, will provide an ideal facility where exciting future DIS investigations could be carried out. The electron, the proton and some light ion beams would be polarizable, enabling spin structure measurements at the EIC. The minimum luminosity planned for the EIC is $10^{33}~{\rm cm}^{-2} {\rm sec}^{-1}$ for e-p, where as for e-Au it is $10^{31}~{\rm cm}^{-2} {\rm sec}^{-1}$. The possibility of DIS from light and heavy nuclei would allow investigations of high gluon density sub-nuclear matter.

I will describe briefly the EIC project, its physics motivations and goals: both in polarized and unpolarized DIS and a tentative schedule.

Markus Diehl

Generalized parton distributions

I give an overview on the physics of generalized parton distributions. After motivating their general interest and discussing their relations with other non-perturbative quantities, I focus on their roles in spin physics and in the physics of small x, on progress in understanding twist 3, and on the extraction of relevant information from experimental data.

Gail Dodge

Spin Structure Measurements in the Resonance Region with CLAS

Measurement of the Deeply Virtual Compton Scattering at HERA

A measurement is presented of Deeply Virtual Compton Scattering at HERA. The cross section is measured as a function of $Q^2$ and $W$. The data are compared to LO QCD calculations based on the two gluon exchange model.

Andreas Freund

Deeply Virtual Compton Scattering: A NLO QCD Analysis

We present a complete NLO QCD analysis of DVCS for the whole kinematical range of HERA, HERMES and JLAB using various input models to give a therorectical uncertainty band. The sensitivity of the real part of the DVCS amplitude to the input distributions is invetsigated in detail!

abstract in form of a postscript file

Delia Hasch

Single-spin azimuthal asymmetries from electroproduction of neutral pions at HERMES

The first measurement of a single-spin asymmetry for semi-inclusive neutral pion production in deep-inelastic scattering will be presented. A significant target-spin asymmetry amplitude of about $2 \%$ in the azimuthal distribution of neutral pions relative to the lepton scattering plane has been observed. This result is compared to single-spin asymmetries for charged pion production measured in the same kinematic range. Also shown are predictions of a model calculation for the $\pi^0$ single-spin asymmetry which base on a fragmentation function that represents sensitivity to the transverse polarisation of the struck quark.

Fritz-Herbert Heinsius

Spin physics with COMPASS

The COMPASS experiment at CERN investigates the hadron structure by deep inealistic muon scattering and hadronic production processes. The main goal of the experiment is to measure the gluon contribution to the nucleon spin via open charm production and hadron pairs with high transverse momentum. An overview of the spin physics program and perspectives for COMPASS running in 2001 and beyond will be presented.

Experimental check of the GDH sum rule at ELSA and MAMI

The GDH-Collaboration measures the total photo absorption cross section of circularly polarized real photons on longitudinally polarized nucleons. The GDH sum rule verification is currently pursued in the energy range 0.14 - 3.1 GeV at ELSA and MAMI. Results for the proton up to 1.4 GeV are presented.

Ed Kinney

Polarized Quark and Gluon Distributions from Semi-inclusive Asymmetries

Semi-inclusive lepton scattering asymmetries offer a valuable tool todirectly probe the contributions to the nucleon spin structure from the different quark flavors. One can also learn about the gluon polarization if events arising from the photon-gluon fusion reaction can be isolated. After a brief introduction to the methods of analyzing semi-inclusive asymmetries and some of the questions regarding systematic and model uncertainties in these methods, I will summarize the latest results of the published measurements from SMC and HERMES. These polarized distributions can be compared with the results of fits to the Q^2 dependence of the inclusive measurements. Finally, I will give an overview of measurements underway at HERMES and planned at COMPASS and RHIC.

Yuji Koike

Single Spin Asymmetries and Soft Gluon Poles

I will report the cross section formulas for the single transverse spin asymmetry in the pion production, and the hyperon polarization in the unpolarized NN collision. In particular, I will identify all the soft-gluon pole contributions with twist-3 fragmentation and distribution functions and present the estimate of each contribution.

The Generalized Drell-Hearn-Gerasimov Sum and Spin Structure of the Neutron at Low Q^2

We have measured inclusive cross sections and asymmetries in the $^3{\vec {\rm He}}({\vec {\rm e}}, {\rm e}^{\prime})$ reaction in the quasi-elastic and resonance regions over a wide range of Q$^2$ (0.03 $\lesssim$ Q$^2$ $\lesssim$ 1 GeV$^2$/c$^2$).

Longitudinally polarized electrons in an energy range from 0.86 GeV to 5.1 GeV were scattered off a high pressure polarized $^3$He target in Hall A. The polarized beam had rapid helicity flips, and both longitudinal and transverse polarizations of the target, with respect to the incident beam, were used. The data were collected with both High Resolution Spectrometers.

This measurement allows the evaluation of the Drell-Hearn-Gerasimov (DHG) sum on $^3$He and the neutron as a function of Q$^2$ in the resonance region. Furthermore, we extracted the spin structure functions g$_1^n$(x, Q$^2$) and g$_2^n$(x, Q$^2$) in this region. These results, together with similar measurements on the proton, will allow us to study the Bjorken sum rule in the region of small Q$^2$ and will improve our understanding of the transition from perturbative to non-perturbative QCD.

Generalized Parton Distributions at Large Momentum Transfer

The role of generalized parton distributions in wide-angle exclusive reactions will be discussed. In contrast to deep virtual exclusive reactions the wide angle processes offer the possibility of investigating the generalized parton distributions at large momentum transfer.

Sebastian Kuhn

Inclusive Asymmetries and Spin Structure Functions of the Nucleon

Over the past 20+ years, an impressive data set on the inclusive double spin asymmetries $A_1(Q^2,x)$ and $A_2(Q^2,x)$ has been collected at many labs worldwide, including SLAC, CERN, HERA, JLab, and MAMI. Many more experiments are underway or in various stages of preparation. The large kinematic range of the existing and anticipated data, from the real photon point $Q^2 = 0$ to the scaling region and from elastic scattering to the DIS regime, allows us to study many questions at the forefront of hadronic physics. Among these are the valence quark, sea quark and gluon structure of the nucleon, the transition from quark degrees of freedom to hadronic ones (including quark-hadron duality), perturbative and higher twist scaling violations, and the validity and convergence of several fundamental sum rules. I will present an overview of the status and results from existing experiments and give an outlook of what we can expect from new programs now underway.

Masanori Miyama

Flavor asymmetry in polarized antiquark distributions

We discuss ubar/dbar asymmetry in polarized parton distributions in a meson-cloud picture. A polarized nucleon splits into a rho meson and a baryon, then the virtual photon interacts with the polarized rho meson. This contribution produces ubar/dbar asymmetry in the polarized antiquark distributions of the nucleon. This topic was investigated by Fries and Schafer in 1998. We extend their studies to discuss more details.

Theoretical aspects of azimuthal and transverse spin asymmetries

I discuss the operator structure for observables that appear in azimuthal single or double spin asymmetries. This enables the study of the Q^2 evolution of these functions. Some examples, e.g. the evolution of the Collins fragmentation function, will be discussed.

Francesco Murgia

Transverse Lambda polarization in unpolarized semi-inclusive DIS

The long-standing problem of transverse Lambda polarization in high-energycollisions of unpolarized hadrons can be tackled by considering new, spin and k_T-dependent quark fragmentation functions for an unpolarized quark into a polarized, spin-1/2 hadron. Simple phenomenological parameterizations of these new "polarizing fragmentation functions", which describe quite well the experimental data on Lambda and Lambda-bar produced in p-N processes, are utilized here to give predictions for transverse Lambda polarization in semi-inclusive DIS.

u-quark dominance in sin(\phi) azimuthal asymmetry in \gamma* \vec{p} -> \pi^{\pm, 0} X

I would like to present the talk on single spin asymmetries in semi-inclusive neutral and charged pion electroproduction on longitudinally polarized protons.

Gerry Peterson

Gluon Spin Distribution and GDH Sum Rule Measurements at SLAC

A quasi-monochromatic circularly-polarized photon beam with energies up to 45 GeV will be developed at SLAC to measure: (1) the gluon spin density by photoproduction of open-charm from a polarized target, and (2) the spin-dependent cross section on longitudinally polarized protons and neutrons to test the Gerasimov-Drell-Hearn sum rule.

Marco Radici

Interference Fragmentation Functions and Spin Asymmetries

A new class of fragmentation functions, arising from the interference of different hadron production channels, is analyzed in detail. Their symmetry properties with respect to naive time-reversal transformations allow for the exploration of final-state interactions occurring after the hadronization. Their symmetry properties with respect to chiral transformations allow envisaging spin asymmetry measurements to factorize out the quark transversity. Explicit calculations of interference fragmentation functions arising from final-state interactions of two hadrons (proton-pion or pion-pion pairs) detected in the same current jet for the case of semi-inclusive Deep-Inelastic Scattering, will be shown.
 

John Ralston

Localizing Quark Orbital Angular Momentum Gauge Invariantly

Certain exclusive reactions permit the measurement of quark orbital angular momentum. Theory in the context of generalized (skew) parton distributions permits description in a gauge-invariant way. A prediction based on this approach of the ratios of proton electromagnetic form factors, $F_2(Q^2)/F_1(Q^2)$ at large $Q^2$ has recently been confirmed at CEBAF.

Oscar Rondon

The transverse spin structure of the nucleon measured in SLAC Experiments 155/155x

Possible future fixed-target DIS experiments at DESY

Polarized Parton Densities in the Nucleon

We present a critical assessment of what can be learned about the polarized parton densities from the present data on inclusive polarized DIS.

Eric Thomas
 

Jochen Volmer

The first measurement of the azimuthal single spin (beam) asymmetry from the Deeply Virtual Compton Scattering process (DVCS) is reported by the HERMES experiment. The azimuthal asymmetry arises from the interference between the DVCS and the Bethe-Heitler amplitudes and allows the probing of the real and imaginary parts of the DVCS helicity amplitudes. This may help in accessing Skewed Parton distributions in a theoretically clean way, since no soft-QCD contributions from meson wave functions are involved in the final state.