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These are the proceedings of the first Tensor Spin Observables Workshop that was held in March 2014 at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. The conference was convened to study the physics that can be done with the recently approved E12-13-011 polarized target. A tensor polarized target holds the potential of initiating a new generation of tensor spin physics at Jefferson Lab. Experiments which utilize tensor polarized targets can help clarify how nuclear properties arise from partonic degrees of freedom, provide unique insight into short-range correlations and quark angular momentum, and also help pin down the polarization of the quark sea with a future Electron Ion Collider.

This three day workshop was focused on tensor spin observables and the associated tensor target development. The workshop goals were to stimulate progress in the theoretical treatment of polarized spin-1 systems, foster the development of new proposals, and to reach a consensus on the optimal polarized target configuration for the tensor spin program. The workshop was sponsored by the University of New Hampshire, the Jefferson Science Associates, Florida International University, and Jefferson Lab. It was organized by Karl Slifer (chair), Patricia Solvignon, and Elena Long of the University of New Hampshire, Douglas Higinbotham and Christopher Keith of Jefferson Lab, and Misak Sargsian of the Florida International University. These proceedings represent the effort put forth by the community to begin exploring the possibilities that a high-luminosity, high-tensor polarized solid target can offer.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We explain spin structure for a spin-one hadron, in which there are new structure functions, in addition to the ones (F₁, F₂, g₁, g₂) which exist for the spin-1/2 nucleon, associated with its tensor structure. The new structure functions are b₁, b₂, b₃, and b₄ in deep inelastic scattering of a charged-lepton from a spin-one hadron such as the deuteron. Among them, twist- two functions are related by the Callan-Gross type relation b₂ = 2xb₁ in the Bjorken scaling limit. First, these new structure functions are introduced, and useful formulae are derived for projection operators of b_1-4 from a hadron tensor W_μν. Second, a sum rule is explained for b₁, and possible tensor-polarized distributions are discussed by using HERMES data in order to propose future experimental measurements and to compare them with theoretical models. A proposal was approved to measure b₁ at the Thomas Jefferson National Accelerator Facility (JLab), so that much progress is expected for b₁ in the near future. Third, formalisms of polarized proton-deuteron Drell-Yan processes are explained for probing especially tensor- polarized antiquark distributions, which were suggested by the HERMES data. The studies of the tensor-polarized structure functions will open a new era in 2020's for tensor-structure studies in terms of quark and gluon degrees of freedom, which are very different from ordinary descriptions in terms of nucleons and mesons.

The b₁ deep-inelastic structure function is an observable feature of a spin-1 system that is sensitive to non-nucleonic components of the target nuclear wave function. The contributions of exchanged pions in the deuteron are estimated and found to be of measurable size for values of x of about 0.1. A simple model for a hidden-color, six-quark configuration (with only about 0.15% probability to exist in the deuteron) is proposed and found to give substantial contributions for values of x greater than about 0.2. Good agreement with the only existing (HERMES) data is obtained. Predictions are made for an upcoming JLab experiment. The Close and Kumano sum rule is investigated and found to be a useful guide to understanding various possible effects that may contribute.

We describe Jefferson Lab E12-13-011, an inclusive deep inelastic scattering experiment to measure the leading twist deuteron tensor structure function b₁ in the region 0.16 < x < 0.49 for 0.8 < Q² < 5.0 GeV². The experiment has been conditionally approved, contingent on target performance, with A⁻ physics rating to run with 30 days of 11 GeV incident beam on a tensor polarized solid target in Jefferson Lab's Hall C.

With the acceptance of QCD as the fundamental theory of strong interactions, one of the basic problems in the analysis of nuclear phenomena became how to consistently account for the effects of the underlying quark/gluon structure of nucleons and nuclei. Besides providing more detailed understanding of conventional nuclear physics, QCD may also point to novel phenomena accessible by new or upgraded nuclear experimental facilities. We discuss a few interesting applications of QCD to nuclear physics with an emphasis on the hidden color degrees of freedom.

We formulate the generalized deep inelastic tensor spin structure of the deuteron which can be obtained from deeply virtual Compton scattering and meson production experiments. We discuss its connection to the total quark angular momentum sum rule for a spin-one hadronic system within a gauge invariant decomposition of hadronic spin.

Deep-inelastic scattering (DIS) from a tensor polarized deuteron is sensitive to possible non-nucleonic components of the deuteron wave function. To accurately estimate the size of the nucleonic contribution, final-state interactions (FSIs) need to be accounted for in calculations. We outline a model that, based on the diffractive nature of the effective hadron-nucleon interaction, uses the generalized eikonal approximation to model the FSIs in the resonance region, taking into account the proton-neutron component of the deuteron. The calculation uses a factorized model with a basis of three resonances with mass W < 2 GeV as the relevant set of effective hadron states entering the final-state interaction amplitude for inclusive DIS. We present results for the tensor asymmetry observable A_zz for kinematics accessible in experiments at Jefferson Lab and Hermes. For inclusive DIS, sizeable effects are found when including FSIs for Bjorken x > 0.2, but the overall size of A_zz remains small. For tagged spectator DIS, FSIs effects are largest at spectator momenta around 300 MeV and for forward spectator angles.

The neutron's deep-inelastic structure functions provide essential information for the flavor separation of the nucleon parton densities, the nucleon spin decomposition, and precision studies of QCD phenomena in the flavor-singlet and nonsinglet sectors. Traditional inclusive measurements on nuclear targets are limited by dilution from scattering on protons, Fermi motion and binding effects, final-state interactions, and nuclear shadowing at x ≪ 0.1. An Electron-Ion Collider (EIC) would enable next-generation measurements of neutron structure with polarized deuteron beams and detection of forward-moving spectator protons over a wide range of recoil momenta (0 < p_R < several 100MeV in the nucleus rest frame). The free neutron structure functions could be obtained by extrapolating the measured recoil momentum distributions to the on-shell point. The method eliminates nuclear modifications and can be applied to polarized scattering, as well as to semi-inclusive and exclusive final states. We review the prospects for neutron structure measurements with spectator tagging at EIC, the status of R&D efforts, and the accelerator and detector requirements.

The deuteron is a simple, spin-1 nuclear system. This makes it an effective testbed for investigating nuclear physics. In addition, it is also relatively easy to polarize. Tensor polarization provides new opportunities to study properties of the nucleus. This is a significant part of the motivation for a polarized deuteron beam for an electron-ion collider envisioned for the future. This paper will discuss the motivation and a useful physics starting point for such a facility.

The possibility of using a tensor polarized deuteron target in electroproduction reactions creates new opportunities for studying different phenomena related to the short-range hadronic and nuclear physics. The use of the tensor polarized deuteron allows us to isolate smaller than average inter-nucleon distances for the bound two-nucleon system. In this report we consider several high Q² reactions which are particularly sensitive to the short-range two- nucleon configurations in the deuteron. One is the relativistic dynamics of electron-bound- nucleon scattering, which can be studied in both inclusive and exclusive reactions, and the other is the strong final state interaction in close proximity of two nucleons that can be used as a sensitive probe for color-transparency phenomena.

The tensor asymmetry A_zz in the quasi-elastic region through the tensor polarized D(e, e')X channel is sensitive to the nucleon-nucleon potential. Previous measurements of A_zz have been used to extract b₁ in the DIS region and T₂₀ in the elastic region. In the quasielastic region, A_zz can be used to compare light cone calculations with variation nucleon- nucleon calculations, and is an important quantity to determine for understanding tensor effects, such as the dominance of pn correlations in nuclei. In the quasi-elastic region, A_zz was first calculated in 1988 by Frankfurt and Strikman using the Hamada-Johnstone and Reid soft-core wave functions [1]. Recent calculations by M. Sargsian revisit A_zz in the x > 1 range using virtual-nucleon and light-cone methods, which differ by up to a factor of two [2]. Discussed in these proceedings, a study has been completed that determines the feasibility of measuring A_zz in the quasi-elastic x > 1 region at Jefferson Lab's Hall C.

A tensor polarized target in Hall A at Jefferson Lab would offer the possibility to measure the D(e, e'p)n cross section for the M_s = 0 and the M_s = ±1 states separately (the quantization axis is along the momentum transfer). These data would serve as a new, stringent test of our current understanding of the deuteron structure for missing momenta up to 450 MeV/c, a region where the deuteron wave function is dominated by the D-state. No data exist to date for missing momenta above 150 MeV/c. The technique to separate these cross sections, possible kinematic settings, and a rough estimate of the achievable precision is presented.

A class of spin observables can be obtained from the relative difference of or asymmetry between cross sections of different spin states of beam or target particles. Such observables have the advantage that the normalization factors needed to calculate absolute cross sections from yields often divide out or cancel to a large degree in constructing asymmetries. However, normalization factors can change with time, giving different normalization factors for different target or beam spin states, leading to systematic errors in asymmetries in addition to those determined from statistics. Rapidly flipping spin orientation, such as what is routinely done with polarized beams, can significantly reduce the impact of these normalization fluctuations and drifts. Target spin orientations typically require minutes to hours to change, versus fractions of a second for beams, making systematic errors for observables based on target spin flips more difficult to control. Such systematic errors from normalization drifts are discussed in the context of the proposed measurement of the deuteron b₁ structure function at Jefferson Lab.

Studies of spin-dependent observables generally rely on calculating the difference between scattering yields for opposite spin states of the beam helicity or the target polarization or both. While the beam helicity can be changed at rates of up to many times per second, target polarization flips involve times on the order of hours or longer. To measure observables that depend only on the differences for opposite target polarizations with minimal systematic effects caused by changes in the beam, target, and detector systems, special methods need to be followed. One option is to use targets which have two or more sections in the beam path, each with materials polarized independently. While this approach does not remove all sources of systematic effects, it can be combined with frequent alternation of spin states to improve the control of time dependent and configuration uncertainties.

The first measurements of tensor observables in π scattering experiments were performed in the mid-80's at TRIUMF, and later at SIN/PSI. The full suite of tensor observables accessible in π elastic scattering were measured: T₂₀, T₂₁, and T₂₂. The vector analyzing power iT₁₁ was also measured. These results led to a better understanding of the three-body theory used to describe this reaction. A direct measurement of the target tensor polarization was also made independent of the usual NMR techniques by exploiting the (nearly) model-independent result for the tensor analyzing power at 90°_cm in the π → 2p reaction. This method was also used to check efforts to enhance the tensor polarization by RF burning of the NMR spectrum. A brief description of the methods developed to measure and analyze these experiments is provided.

A discussion of tensor polarization of solid targets produced by Dynamic Nuclear Polarization and Dynamic Nuclear Orientation in the use of nuclear physics experiments is presented. Techniques in the deuteron tensor polarization enhancement and enhanced tensor polarization measurement uncertainty are also discussed in the interest and preparation of future experiments.