Target Single-spin Asymmetries Research Articles

Accessing the gluon GTMD F1,4 in exclusive π0 production in ep collisions

We demonstrate that the longitudinal single target-spin asymmetry in exclusive π0 production in ep collisions can give access to the imaginary part of the gluon generalized transverse momentum distribution (GTMD) F1,4. Such a longitudinal spin asymmetry that results from the Coulomb-nuclear interference effect, leads to a characteristic azimuthal angular correlation of sin2ϕ, where ϕ is the azimuthal angle between the scattered lepton transverse momentum and the recoil proton’s transverse momentum. We also present a numerical estimate of the asymmetry for the kinematics accessible at EIC and EicC. Published by the American Physical Society 2024

Two-Photon Exchange in Elastic Electron Scattering on Hadronic Systems

In the present review, we discuss different aspects of the two-photon exchange (TPE) physics in elastic ep scattering at high Q2, as well as at low Q2. The imaginary part of the TPE amplitude gives rise to beam and target single-spin asymmetries. Different theoretical approaches to the calculation of these observables are considered. The real part of the TPE amplitude influences the unpolarized cross-section and double-spin observables and is, most likely, responsible for the discrepancy between two methods of measurements of the proton form factors. We review different methods of calculations of the TPE amplitudes in the framework of the “hadron” and “quark-gluon” approaches. We discuss the dispersion approach suitable for low and intermediate Q2, which includes elastic and inelastic intermediate hadronic states, as well as the connection of TPE with the proton radius puzzle. The present situation with direct experimental searches for the TPE amplitude in the e+p/e−p charge asymmetry is also discussed, as well as attempts to extract the TPE amplitudes from existing experimental data obtained by the Rosenbluth and double polarization techniques. The TPE physics in other processes such as elastic мp, e-nucleus, and еп scattering is also reviewed.

Measurement of the single-spin asymmetry [formula omitted] in quasi-elastic 3He↑(e,e′n) scattering at 0.4 < Q2 < 1.0 GeV/c2

Due to the lack of free neutron targets, studies of the structure of the neutron are typically made by scattering electrons from either 2H or 3He targets. In order to extract useful neutron information from a 3He target, one must understand how the neutron in a 3He system differs from a free neutron by taking into account nuclear effects such as final state interactions and meson exchange currents. The target single spin asymmetry Ay0 is an ideal probe of such effects, as any deviation from zero indicates effects beyond plane wave impulse approximation. New measurements of the target single spin asymmetry Ay0 at Q2 of 0.46 and 0.96 (GeV/c)2 were made at Jefferson Lab using the quasi-elastic He↑3(e,e′n) reaction. Our measured asymmetry decreases rapidly, from >20% at Q2=0.46 (GeV/c)2 to nearly zero at Q2=0.96 (GeV/c)2, demonstrating the fall-off of the reaction mechanism effects as Q2 increases. We also observed a small ϵ-dependent increase in Ay0 compared to previous measurements, particularly at moderate Q2. This indicates that upcoming high Q2 measurements from the Jefferson Lab 12 GeV program can cleanly probe neutron structure from polarized 3He using plane wave impulse approximation.

Semi-inclusive π0 target and beam-target asymmetries from 6 GeV electron scattering with CLAS

We present precision measurements of the target and beam-target spin asymmetries from neutral pion electroproduction in deep-inelastic scattering (DIS) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. We scattered 6-GeV, longitudinally polarized electrons off longitudinally polarized protons in a cryogenic 14NH3 target, and extracted double and single target spin asymmetries for ep→e′π0X in multidimensional bins in four-momentum transfer (1.0<Q2<3.2 GeV2), Bjorken-x (0.12<x<0.48), hadron energy fraction (0.4<z<0.7), transverse pion momentum (0<PT<1.0 GeV), and azimuthal angle ϕh between the lepton scattering and hadron production planes. We extracted asymmetries as a function of both x and PT, which provide access to transverse-momentum distributions of longitudinally polarized quarks. The double spin asymmetries depend weakly on PT. The sin⁡2ϕh moments are zero within uncertainties, which is consistent with the expected suppression of the Collins fragmentation function. The observed sin⁡ϕh moments suggest that quark gluon correlations are significant at large x.

Quark-photon-quark correlation and transverse target single spin asymmetry in inclusive DIS

We calculate the q q correlation function associated to transverse target lepton-nucleon inclusive deep-inelastic scattering by direct evaluation of the corresponding matrix element. We use the electromagnetic impact parameter potential for a transversely polarized nucleon to define the q q correlation function for each avor in terms of the field components of that avor. The results are compared with the two existing models for the q q correlator. Using the calculated q q correlation function, we estimated the transverse target SSA in IDIS process.

Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2&gt;1GeV2 . I. ep→eπ+n

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive $\pi^+$ electroproduction reaction $\gamma^* p \to n \pi^+$. The results were obtained from scattering of 6 GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic range covered is $1.1<W<3$ GeV and $1<Q^2<6$ GeV$^2$. Results were obtained for about 6000 bins in $W$, $Q^2$, $\cos(\theta^*)$, and $\phi^*$. Except at forward angles, very large target-spin asymmetries are observed over the entire $W$ region. Reasonable agreement is found with phenomenological fits to previous data for $W<1.6$ GeV, but very large differences are seen at higher values of $W$. A GPD-based model is in poor agreement with the data. When combined with cross section measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.4 GeV.

Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2&gt;1 GeV2 . II. ep→eπ0p

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive $\pi^0$ electroproduction reaction $\gamma^* p \to p \pi^0$, expanding an analysis of the $\gamma^* p \to n \pi^+$ reaction from the same experiment. The results were obtained from scattering of 6 GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic range covered is $1.1<W<3$ GeV and $1<Q^2<6$ GeV$^2$. Results were obtained for about 5700 bins in $W$, $Q^2$, \cthcm, and $\phi^*$. The beam-target asymmetries were found to generally be greater than zero, with relatively modest \phicmsp dependence. The target asymmetries exhibit very strong \phicmsp dependence, with a change in sign occurring between results at low $W$ and high $W$, in contrast to $\pi^+$ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for $W<1.6$ GeV, but significant differences are seen at higher $W$. When combined with cross section measurements, as well as $\pi^+$ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.4 GeV.

Target and beam-target spin asymmetries in exclusive π+ and π− electroproduction with 1.6- to 5.7-GeV electrons

Beam-target double spin asymmetries and target single-spin asymmetries in exclusive $\pi^+$ and $\pi^-$ electroproduction were obtained from scattering of 1.6 to 5.7 GeV longitudinally polarized electrons from longitudinally polarized protons (for $\pi^+$) and deuterons (for $\pi^-$) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is $1.1<W<2.6$ GeV and $0.05<Q^2<5$ GeV$^2$, with good anglular coverage in the forward hemisphere. The asymmetry results were divided into approximately 40,000 kinematic bins for $\pi^+$ from free protons and 15,000 bins for $\pi^-$ production from bound nucleons in the deuteron. The present results are found to be in reasonable agreement with fits to previous world data for $W<1.7$ GeV and $Q^2<0.5$ GeV$^2$, with discrepancies increasing at higher values of $Q^2$, especially for $W>1.5$ GeV. Very large target-spin asymmetries are observed for $W>1.6$ GeV. When combined with cross section measurements, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.3 GeV.

Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the ReactionHe3↑(e,e′ )

We report the first measurement of the target single-spin asymmetry, $A_y$, in quasi-elastic scattering from the inclusive reaction $^3$He$^{\uparrow}(e,e^\prime)$ on a $^3$He gas target polarized normal to the lepton scattering plane. Assuming time-reversal invariance, this asymmetry is strictly zero for one-photon exchange. A non-zero $A_y$ can arise from the interference between the one- and two-photon exchange processes which is sensitive to the details of the sub-structure of the nucleon. An experiment recently completed at Jefferson Lab yielded asymmetries with high statistical precision at $Q^{2}=$ 0.13, 0.46 and 0.97 GeV$^{2}$. These measurements demonstrate, for the first time, that the $^3$He asymmetry is clearly non-zero and negative with a statistical significance of (8-10)$\sigma$. Using measured proton-to-$^{3}$He cross-section ratios and the effective polarization approximation, neutron asymmetries of $-$(1-3)% were obtained. The neutron asymmetry at high $Q^2$ is related to moments of the Generalized Parton Distributions (GPDs). Our measured neutron asymmetry at $Q^2=0.97$ GeV$^2$ agrees well with a prediction based on two-photon exchange using a GPD model and thus provides a new, independent constraint on these distributions.

Target Single-Spin Asymmetry in Inclusive Hadron Production

We present the first measurement of target single-spin asymmetries (AN) in the inclusive hadron production reaction, e+3 He ↑ →h+X, using a transversely polarized 3 He target at an electron-nucleon center-of-mass energy [Formula: see text] GeV. The experiment was conducted at Jefferson Lab in Hall A using a 5.9-GeV electron beam. Several types of hadrons (π±, K± and proton) were detected with an average momentum 〈Ph〉 = 2.35 GeV/c, and an average transverse momentum 〈pT〉 = 0.64 GeV/c. The observed asymmetry strongly depends on the type of hadron. A positive asymmetry is observed for π+ and K+. A negative asymmetry is observed for π-. The π+ and π- asymmetries measured for the 3 He target and extracted the "effective- neutron" SSA. Amazingly, we found that the ratio of our observed SSA between π+ and π- productions closely resemble the ratio of up- to down-quark's contributions to neutron's anomalous magnetic moment.

Single spin asymmetries in charged kaon production from semi-inclusive deep inelastic scattering on a transversely polarizedHe3target

We report the first measurement of target single spin asymmetries of charged kaons produced in semi-inclusive deep inelastic scattering of electrons off a transversely polarized $^3{\rm{He}}$ target. Both the Collins and Sivers moments, which are related to the nucleon transversity and Sivers distributions, respectively, are extracted over the kinematic range of 0.1$<$$x_{bj}$$<$0.4 for $K^{+}$ and $K^{-}$ production. While the Collins and Sivers moments for $K^{+}$ are consistent with zero within the experimental uncertainties, both moments for $K^{-}$ favor negative values. The Sivers moments are compared to the theoretical prediction from a phenomenological fit to the world data. While the $K^{+}$ Sivers moments are consistent with the prediction, the $K^{-}$ results differ from the prediction at the 2-sigma level.

Single spin asymmetries of inclusive hadrons produced in electron scattering from a transversely polarized3He target

We report the first measurement of target single-spin asymmetries (A$_N$) in the inclusive hadron production reaction, $e~$+$~^3\text{He}^{\uparrow}\rightarrow h+X$, using a transversely polarized $^3$He target. The experiment was conducted at Jefferson Lab in Hall A using a 5.9-GeV electron beam. Three types of hadrons ($\pi^{\pm}$, $\text{K}^{\pm}$ and proton) were detected in the transverse hadron momentum range 0.54 $<p_T<$ 0.74 GeV/c. The range of $x_F$ for pions was -0.29 $<x_F<$ -0.23 and for kaons -0.25 $<x_F<$-0.18. The observed asymmetry strongly depends on the type of hadron. A positive asymmetry is observed for $\pi^+$ and $\text{K}^+$. A negative asymmetry is observed for $\pi^{-}$. The magnitudes of the asymmetries follow $|A^{\pi^-}| < |A^{\pi^+}| < |A^{K^+}|$. The K$^{-}$ and proton asymmetries are consistent with zero within the experimental uncertainties. The $\pi^{+}$ and $\pi^{-}$ asymmetries measured for the $^3$He target and extracted for neutrons are opposite in sign with a small increase observed as a function of $p_T$.

Transverse target single-spin asymmetry in inclusive electroproduction of charged pions and kaons

Single-spin asymmetries were investigated in inclusive electroproduction of charged pions and kaons from transversely polarized protons at the Hermes experiment. The asymmetries were studied as a function of the azimuthal angle ψ about the beam direction between the target-spin direction and the hadron production plane, the transverse hadron momentum PT relative to the direction of the incident beam, and the Feynman variable xF. The sinψ amplitudes are positive for π+ and K+, slightly negative for π− and consistent with zero for K−, with particular PT but weak xF dependences. Especially large asymmetries are observed for two small subsamples of events, where also the scattered electron was recorded by the spectrometer.

Partonic description of the transverse target single-spin asymmetry in inclusive deep-inelastic scattering

The single-spin asymmetry of unpolarized leptons scattering deep-inelastically off transversely polarized nucleons is studied in a partonic picture within a collinear twist-3 framework. Since this observable is generated by multi-photon exchanges between lepton and nucleon a partonic description of the asymmetry contains essential elements of a full next-to-leading order calculation for single-spin asymmetries in perturbative Quantum Chromodynamics. In particular it is shown how nontrivial cancellations between kinematical and dynamical twist-3 contributions lead to a well-behaved and finite formula. This final result can be expressed in terms of multipartonic quark-gluon and quark-photon correlation functions. Hence, a measurement of the transverse target single-spin asymmetry may provide new constraints on these multipartonic correlations.

Consistent model forπNtransition distribution amplitudes and backward pion electroproduction

The extension of the concept of generalized parton distributions leads to the introduction of baryon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of the nonlocal three quark operator between a nucleon and a meson state. We present a general framework for modelling nucleon to pion ($\pi N$) TDAs. Our main tool is the spectral representation for \pi N TDAs in terms of quadruple distributions. We propose a factorized Ansatz for quadruple distributions with input from the soft-pion theorem for \pi N TDAs. The spectral representation is complemented with a D-term like contribution from the nucleon exchange in the cross channel. We then study backward pion electroproduction in the QCD collinear factorization approach in which the non-perturbative part of the amplitude involves \pi N TDAs. Within our two component model for \pi N TDAs we update previous leading-twist estimates of the unpolarized cross section. Finally, we compute the transverse target single spin asymmetry as a function of skewness. We find it to be sizable in the valence region and sensitive to the phenomenological input of our \pi N TDA model.

Exploring the structure of the proton through polarization observables inlp→jetX

We present results for a complete set of polarization observables for jet production in lepton proton collision, where the final state lepton is not observed. The calculations are carried out in collinear factorization at the level of Born diagrams. For all the observables we also provide numerical estimates for typical kinematics of a potential future Electron Ion Collider. On the basis of this numerical study, the prospects for the transverse single target spin asymmetry are particularly promising. This observable is given by a certain quark-gluon correlation function, which has a direct relation to the transverse momentum dependent Sivers parton distribution.

Measurement of double-spin asymmetries associated with deeply virtual Compton scattering on a transversely polarized hydrogen target

Double-spin asymmetries in exclusive electroproduction of real photons from a transversely polarized hydrogen target are measured with respect to the product of target polarization with beam helicity and beam charge, and with respect to the product of target polarization with beam helicity alone. The asymmetries arise from the deeply virtual Compton scattering process and its interference with the Bethe–Heitler process. They are related to the real part of the same combination of Compton form factors as that determining the previously published transverse target single-spin asymmetries through the imaginary part. The results for the double-spin asymmetries are found to be compatible with zero within the uncertainties of the measurement, and are not incompatible with the predictions of the only available GPD-based calculation.

Single Spin Asymmetries in Charged Pion Production from Semi-Inclusive Deep Inelastic Scattering on a Transversely PolarizedHe3Target atQ2=1.4–2.7 GeV2

We report the first measurement of target single spin asymmetries in the semi-inclusive (3)He(e,e'π(±))X reaction on a transversely polarized target. The experiment, conducted at Jefferson Lab using a 5.9 GeV electron beam, covers a range of 0.16 < x < 0.35 with 1.4 < Q(2) < 2.7 GeV(2). The Collins and Sivers moments were extracted from the azimuthal angular dependence of the measured asymmetries. The π(±) Collins moments for (3)He are consistent with zero, except for the π(+) moment at x = 0.35, which deviates from zero by 2.3σ. While the π(-) Sivers moments are consistent with zero, the π(+) Sivers moments favor negative values. The neutron results were extracted using the nucleon effective polarization and measured cross section ratios of proton to (3)He, and are largely consistent with the predictions of phenomenological fits and quark model calculations.

Transverse spin structure of the nucleon through target single-spin asymmetry in semi-inclusive deep-inelastic (e, e’ $ \pi^{\pm}_{}$ ) reaction at Jefferson Lab

Jefferson Lab (JLab) 12 GeV energy upgrade provides a golden opportunity to perform precision studies of the transverse spin and transverse-momentum-dependent structure in the valence quark region for both the proton and the neutron. In this paper, we focus our discussion on a recently approved experiment on the neutron as an example of the precision studies planned at JLab. The new experiment will perform precision measurements of target Single Spin Asymmetries (SSA) from semi-inclusive electro-production of charged pions from a 40-cm long transversely polarized $^3$He target in Deep-Inelastic-Scattering kinematics using 11 and 8.8 GeV electron beams. This new coincidence experiment in Hall A will employ a newly proposed solenoid spectrometer (SoLID). The large acceptance spectrometer and the high polarized luminosity will provide precise 4-D ($x$, $z$, $P_T$ and $Q^2$) data on the Collins, Sivers, and pretzelocity asymmetries for the neutron through the azimuthal angular dependence. The full 2$\pi$ azimuthal angular coverage in the lab is essential in controlling the systematic uncertainties. The results from this experiment, when combined with the proton Collins asymmetry measurement and the Collins fragmentation function determined from the e$^+$e$^-$ collision data, will allow for a quark flavor separation in order to achieve a determination of the tensor charge of the d quark to a 10% accuracy. The extracted Sivers and pretzelocity asymmetries will provide important information to understand the correlations between the quark orbital angular momentum and the nucleon spin and between the quark spin and nucleon spin.

Measurement of azimuthal asymmetries associated with deeply virtual Compton scattering on a longitudinally polarized deuterium target

Azimuthal asymmetries in exclusive electroproduction of a real photon from a longitudinally polarized deuterium target are measured with respect to target polarization alone and with respect to target polarization combined with beam helicity and/or beam charge. The asymmetries appear in the distribution of the real photons in the azimuthal angle ϕ around the virtual photon direction, relative to the lepton scattering plane. The asymmetries arise from the deeply virtual Compton scattering process and its interference with the Bethe–Heitler process. The results for the beam-charge and beam-helicity asymmetries from a tensor polarized deuterium target with vanishing vector polarization are shown to be compatible with those from an unpolarized deuterium target, which is expected for incoherent scattering dominant at larger momentum transfer. Furthermore, the results for the single target-spin asymmetry and for the double-spin asymmetry are found to be compatible with the corresponding asymmetries previously measured on a hydrogen target. For coherent scattering on the deuteron at small momentum transfer to the target, these findings imply that the tensor contribution to the cross section is small. Furthermore, the tensor asymmetry is found to be compatible with zero.