Polarized Nucleon Target Research Articles

Longitudinal Solid Polarized Target for CLAS12

The Run Group C suite of experiments measure multiple spin-dependent observables by scattering an 11 GeV electron beam from longitudinally polarized nucleon targets inside the CLAS12 spectrometer in Hall B at Jefferson Lab. The dynamically polarized target built for these experiments has been extensively tested by the JLab Target Group in the Experimental Equipment Lab using an auxiliary 5 T magnet. We report on the operational experience with the target, the benchmarks achieved so far (using various polarizable materials) as well as the complete target setup, experimental readiness, and its present status. Our results show that all target components work well and the project has started successfully with the first beam on June 11, 2022.

The sin 2phi _h azimuthal asymmetry of pion production in SIDIS within TMD factorization

We study the sin 2phi _h azimuthal asymmetry of charged and neutral pion productions off the longitudinally polarized nucleon targets in semi-inclusive deeply inelastic scattering (SIDIS) process within the transverse momentum dependent (TMD) factorization. The asymmetry is contributed by the convolution of the TMD distribution h_{1L}^{perp } and the Collins fragmentation function H_{1}^perp . We adopt the Wandzura–Wilczek-type (WW-type) approximation for h_{1L}^{perp } and two different parametrizations on the nonperturbative Sudakov form factor for h_{1L}^{perp } and H_{1}^perp . We estimate the sin 2phi _h asymmetry at HERMES and CLAS and compare them with the corresponding experimental data. We also provide the prediction at kinematical configuration of CLAS12. It is shown that our theoretical calculation can describe the HERMES and CLAS data, except the asymmetry of the pi ^- production off proton target at HERMES and CLAS. We also find that different choices of the nonperturbative part of TMD evolution lead qualitatively similar results.

Double spin asymmetry in dihadron production in SIDIS off a longitudinally polarized nucleon target

In this paper we study the double longitudinal spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering (SIDIS). We calculate a unknown twist-3 dihadron fragmentation function $\widetilde{D}^\sphericalangle$ within a spectator model which has been used successfully in describing the dihadron production in both the unpolarized and the single polarized processes. The collinear picture, in which the transverse momentum of the final state hadron pair is integrated out, has been considered. The $\cos\phi_R$ azimuthal asymmetry arises from the coupling $e_L H_1^\sphericalangle$ and the coupling $g_1 \widetilde{D}^\sphericalangle$ is studied. We estimate the $\cos\phi_R$ asymmetry at the kinematics of COMPASS and compare with the data. The prediction at the future Electron Ion Collider (EIC) has also been presented.

Transversity distributions from difference asymmetries in semi-inclusive DIS

In recent years information on the transversity distribution $h_1$ has been obtained combining the Collins asymmetry results from semi-inclusive deep inelastic scattering (SIDIS) data on transversely polarized nucleon targets with the information on the fragmentation function of a transversely polarized quark from the asymmetries measured in $e^+e^-$ annihilation into hadrons. An alternative method was proposed long time ago, which does not require the $e^+e^-$ data, but allows one to get ratios of the $u$ and $d$ quark transversity distributions from the SIDIS data alone. The method utilizes the ratio of the difference of the Collins asymmetries of positively and negatively charged hadrons produced on transversely polarized proton and deuteron targets. We have applied this method to the COMPASS proton and deuteron data, and extracted the ratio $h_1^d/h_1^u$. The results are compared to those obtained in a previous point--by--point extraction based both on SIDIS and $e^+e^-$ data.

Single-spin asymmetry in dihadron production in SIDIS off longitudinally polarized nucleon targets

We study the single longitudinal-spin asymmetry of dihadron production in semi-inclusive deep inelastic scattering process. We consider the collinear picture in which the transverse momentum of the final-state hadron pair is integrated out, such that the $\sin \phi_R$ azimuthal asymmetry arises from the coupling $h_L\, H_{1}^{\sphericalangle}$ as well as the coupling $g_1 \,\widetilde{G}^{\sphericalangle}$. We calculate the unknown twist-3 dihadron fragmentation function $\widetilde{G}^{\sphericalangle}$ using a spectator model which is successful in describing the dihadron production in the unpolarized process. Using the spectator model results for the quark distributions and dihadron fragmentation functions, we estimate the $\sin \phi_R$ asymmetry of dihadron production in SIDIS at the kinematics of COMPASS and compare it with the COMPASS preliminary data. In addition, the prediction on the $\sin \phi_R$ asymmetry at the typical kinematics of the future Electron Ion Collider is also presented. In order to test the reliability of the spectator model estimate, we compare the model result for the distribution $h_L$ with the Wandzura-Wilczek approximation for that distribution, and compare $H_{1}^{\sphericalangle}$ with the existing parametrization. Although the asymmetry is dominated by the $h_L H_{1}^{\sphericalangle}$ term, we find that the contribution from the $g_1\, \widetilde{G}^{\sphericalangle}$ term should also be taken into account in certain kinematical region.

GPDs and Orbital Angular Momentum

Generalized Parton Distributions provide information on the distribution of quarks in impact parameter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. This force when acting along the whole trajectory of the active quark leads to transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark Orbital Angular Momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe–Manohar and Ji OAMs. In the context of the scalar diquark model we demonstrate that the effect from that torque contributes at the same order as single-spin asymmetries.

Inclusive single-spin asymmetries, quark-photon, and quark-quark correlations

Generalized Parton Distributions (GPDs) provide information on the distribution of quarks in impact paarmeter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. %The strength of that force can be related to twist-3 PDFs. This force when acting along the whole trajectory of the active quark leads to a transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark Orbital Angular Momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe-Manohar and Ji OAMs.

Quark Orbital Angular Momentum

Generalized parton distributions provide information on the distribution of quarks in impact parameter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. This force when acting along the whole trajectory of the active quark leads to transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark orbital angular momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe–Manohar and Ji OAMs.

Single-Spin Asymmetries AULsinϕh in Semi-Inclusive Pions Production

The single-spin asymmetry [Formula: see text] of charged and neutral pion production in semi-inclusive deep-inelastic scattering on longitudinally polarized nucleon targets is studied. We particularly consider the effects of the twist-3 transverse-momentum dependent distribution functions [Formula: see text] and [Formula: see text], which are calculated in two different spectator-diquark models. We estimate the asymmetry for [Formula: see text], [Formula: see text] and [Formula: see text] produced off the proton target at HERMES and compare the results with the HERMES measurements. We also predict the same asymmetric moment for different pions at the kinematics of CLAS 5.5 GeV on a proton target, as well as at COMPASS on a deuteron target for comparison.

Overview of HERMES results

The HERMES experiment has collected a wealth of deep-inelastic scattering data using the 27.6 GeV polarized lepton beam at HERA and various pure gas targets, both unpolarized and polarized. This allowed for a series of diverse and unique measurements. Among them are measurements that provide information on the threedimensional structure of the nucleon, both in momentum space and in position space. Results of measurements of exclusive ω production on an unpolarized and transversely polarized nucleon target, sensitive to the distribution in transverse-position and longitudinalmomentum space, are discussed as well as the three-dimensional extraction of azimuthal asymmetries measured in semi-inclusive deep-inelastic scattering, sensitive to twist-2 and twist-3 distributions in three-dimensional momentum space.

Single-spin asymmetries in electroproduction of pions on the longitudinally polarized nucleon targets

We study the single-spin asymmetries of pions produced in semi-inclusive deep-inelastic scattering on the longitudinally polarized nucleon targets. We particularly consider the effects of the twist-3 transverse-momentum dependent distribution functions ${f}_{L}^{\ensuremath{\perp}}$ and ${h}_{L}$ to the asymmetry. We calculate the asymmetric moment ${A}_{UL}^{\mathrm{sin}{\ensuremath{\phi}}_{h}}$ for ${\ensuremath{\pi}}^{+}$, ${\ensuremath{\pi}}^{\ensuremath{-}}$, and ${\ensuremath{\pi}}^{0}$ produced off the proton target at HERMES and compare the results with the HERMES data. We also present the prediction of the same asymmetries for different pions at the kinematics of CLAS 5.5 GeV on a proton target, as well as at COMPASS on a deuteron target.

Studies of partonic transverse momentum and spin structure of the nucleon

The investigation of the partonic degrees of freedom beyond collinear approximation (3D description) has been gained increasing interest in the last decade. The Thomas Jefferson National Laboratory, after the CEBAF upgrade to 12 GeV, will become the most complete facility for the investigation of the hadron structure in the valence region by scattering of polarized electron off various polarized nucleon targets. A compendium of the planned experiments is here presented.

JLAB12 AND THE STRUCTURE OF HADRONS

The investigation of the partonic degrees of freedom beyond collinear approximation (3D description) has been gained increasing interest in the last decade. The Thomas Jefferson National Laboratory, after the CEBAF upgrade to 12 GeV, will become the most complete facility for the investigation of the hadron structure in the valence region by scattering of polarized electron off various polarized nucleon targets. A compendium of the planned experiments is here presented.

Compton scattering: From deeply virtual to quasi-real

We address the question of interpolation of the virtual Compton scattering process off a polarized nucleon target between the deeply virtual regime for the initial-state photon and its near on-shell kinematics making use of the photon helicity-dependent Compton Form Factors (CFFs) as a main ingredient of the formalism. The five-fold differential cross section1 for the reaction with all possible polarization options for the lepton and nucleon spins is evaluated in terms of CFFs in the rest reference frame of the initial-state nucleon. We suggest a rather simple parametrization of the Compton hadronic tensor in terms of CFFs which are free from kinematical singularities and are directly related, at large photon virtualities, to generalized parton distributions. We also provide a relation of our basis spanned by a minimal number of Dirac bilinears to the one introduced by Tarrach for the parametrization of the virtual Compton tensor and utilize the former to establish a set of equalities among our CFFs and generalized polarizabilities. As a complementary result, we express Compton scattering in the Born approximation in terms of CFFs as well.

Model-independent determination of the parity ofΞhyperons

Based on reflection symmetry in the reaction plane, it is shown that measuring the transverse spin-transfer coefficient $K_{yy}$ in the $\bar{K}N \to K\Xi$ reaction directly determines the parity of the produced cascade hyperon in a model-independent way as $\pi_\Xi =K_{yy}$, where $\pi_\Xi =\pm 1$ is the parity. This result based on Bohr's theorem provides a completely general, universal relationship that applies to the entire hyperon spectrum. A similar expression is obtained for the photoreaction $\gamma N \to K K \Xi$ by measuring both the double-polarization observable $K_{yy}$ and the photon-beam asymmetry $\Sigma$. Regarding the feasibility of such experiments, it is pointed out that the self-analyzing property of the $\Xi$'s can be invoked, thus requiring only a polarized nucleon target.

Single spin asymmetries in hadronic processes

Single-spin asymmetry (SSA) phenomena in hadron physics are studied. The SSAs in semi-inclusive Deep Inelastic Scattering (DIS) is understood by the final-state interactions from gluon exchange between the outgoing quark and the target spectator system. The existence of such SSAs for transversely polarized nucleon targets requires a phase difference between two amplitudes coupling the proton target with J p z = ± 1 2 to the same final-state. The asymmetry of the angular distribution in Drell-Yan processes is investigated by the SSA of the quark spin which is induced by the initial-state interactions.

Spin Physics at SPring-8 – Recent Results

We have been carrying out hadron photoproduction experiments at SPring-8 since the completion of the LEPS beam-line in 2000. The photoproduction of the ϕ, K, η and π0 mesons is studied by using linearly polarized photon beams with energies of Eγ = 1.5–2.9 GeV. Introducing a polarized nucleon target is expected to upgrade the LEPS experiments to a new level. The measurement of double spin polarization asymmetries provides important information to investigate the hidden nucleon structure, hadron photoproduction dynamics, and exotic hadron properties. We started developing a polarized HD target in 2005. Five large refrigerators necessary for the polarized target have been already obtained. We plan to carry out an experiment using polarized photon beams and the polarized HD target in 2011–2012. We also started constructing a new beam-line (LEPS2) at SPring-8 in 2010. The large acceptance E949 spectrometer, which has been used at BNL, will be transported to the LEPS2 beam-line in 2011. With these new developments, advanced studies of spin physics are expected in future experiments.

TRANSVERSE SPIN AND TRANSVERSE MOMENTUM EFFECTS AT COMPASS

The study of transverse spin and transverse momentum effects is part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. For these studies, a 160 GeV/c momentum muon beam is scattered on a transversely polarized nucleon target, and the scattered muon and the forward going hadrons produced in DIS processes are reconstructed and identified in a magnetic spectrometer. The measurements have been performed on a deuteron target in 2002, 2003 and 2004, and on a proton target in 2007. The main results obtained measuring single spin asymmetries are reviewed, with particular emphasis on the most recent proton measurements. After two years of spectroscopy measurements with hadron beams, in 2008 and 2009, the Collaboration will resume measurements with the muon beam and a transversely polarized target in 2010.

Left-right asymmetry for meson production in SIDIS process

We analyze the left-right asymmetry in the semi-inclusive deep inelastic scattering (SIDIS) process using a method where no weighting function are used. Considering all flavor of quarks, we reanalyze the π± production and extend our calculation on the K± production. The predictions on HERMES, COMPASS and JLab kinematics with transversely polarized nucleon target are shown in this paper.

Left–right asymmetry in semi-inclusive deep inelastic scattering process

We analyze the left–right asymmetry of pion production in semi-inclusive deep inelastic scattering (SIDIS) process of unpolarized charged lepton on transversely polarized nucleon target. Unlike available treatments, in which some specific weighting functions are multiplied to separate theoretically motivated quantities, we do not introduce any weighting function following the analyzing method by the E704 experiment. The advantage is that this basic observable is free of any theoretical bias, although we can perform the calculation under the current theoretical framework. We present numerical calculations at both HERMES kinematics for the proton target and JLab kinematics for the neutron target. We find that with the current theoretical understanding, Sivers effect plays a key role in our analysis.