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Jefferson Science Associates, LLC Managing and Operating the Thomas Jefferson National Accelerator Facility

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FY2014 JSA Initiatives Fund Proposal Summary Sheet

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A Proposal to the JSA Initiatives Fund

December 20, 2013

Tensor Spin Observables Workshop

Elena Long, Karl Slifer, Patricia SolvignonUniversity of New Hampshire

Douglas Higinbothan, Christopher KeithJefferson Lab

Misak SargsianFlorida International University

Abstract

The Jefferson Lab PAC recently approved an experiment which will use an enhanced tensorpolarized solid target. This exciting development holds the potential of initiating a new field oftensor spin physics at JLab. Experiments which utilize tensor polarized targets can help clarifyhow nuclear properties arise from partonic degrees of freedom, provide unique insight into shortrange correlations and quark angular momentum, and also help pin down the polarization ofthe quark sea.

We request support for a two day workshop focused on tensor spin observables to be held atJefferson Lab. Travel support will be provided to students and post-docs who might otherwisenot be able to participate. The workshop goals are to stimulate progress in the theoreticaltreatment of polarized spin-1 systems, foster the development of new proposals, and to reach aconsensus on the optimal polarized target configuration for the tensor spin program. Successof this workshop will be gauged by the submission of new JLab proposals to measure tensorobservables, and the creation of a whitepaper detailing the optimal next generation JLab tensorpolarized target.

1 Motivation

A deeper understanding of tensor structure will help to clarify how the gross properties of thenucleus arise from the underlying partons. This provides novel information about nuclear structure,quark angular momentum, and the polarization of the quark sea that is not accessible in spin-1/2targets.

The Jefferson Lab PAC 40 recently approved the E12-13-011 experiment to measure the leading-twist tensor structure function, b1, of the deuteron. This structure function quantifies effects notpresent in the case of spin-1/2 hadrons. However, tensor effects only exist in composite nucleartargets, so the study of b1 serves as a very interesting bridge between nucleonic and nuclear physics.On the one hand, deep inelastic scattering (DIS), clearly probes partonic degrees of freedom, i.e.quarks, but on the other hand, b1 depends solely on the deuteron (nuclear) spin state.

At low x, shadowing effects are expected to dominate b1, while at larger values, b1 provides aclean probe of exotic QCD effects, such as hidden color due to a 6-quark configuration. Since thedeuteron wave function is relatively well known, any novel effects are expected to be readily observ-able. All available models, looking at the effects of the virtual photon hitting an exchanged pion[1], convolution models with relativistic and binding energy corrections [2], covariant relativistic

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calculations [3], double-scattering effects [4], and using a virtual nucleon approximation [5], predicta small or vanishing value of b1 at moderate x. However, the first pioneer measurement of b1 atHERMES revealed a cross-over to an anomalously large negative value in the region 0.2 < x < 0.5,albeit with relatively large experimental uncertainty. Kumano [6] points out that the twist-2 struc-ture functions b1 and b2 can be used to probe orbital angular momentum. He then extracted thetensor polarized quark and anti-quark distributions from a fit to the HERMES data [7] and foundthat a non-negligible tensor polarization of the sea is necessary to reproduce the trend of the data.The new E12-13-011 measurement will provide far greater statistics than the previous HERMESresults. As an added benefit, development of an enhanced tensor polarized target at JLab will laythe foundation for future tensor spin experiments.

With the CEBAF accelerator, spectrometers, and an enhanced tensor polarized target, Jeffer-son Lab is the ideal place to investigate tensor observables at intermediate and large x. The b1

experiment has generated interest in other possible measurements, and this workshop will bringtogether experts in theory, target development, and experimental background to motivate and focusthe tensor program.

2 Workshop Goals

The workshop goals are to stimulate progress in the theoretical treatment of polarized spin-1 sys-tems, to foster the development of new proposals, and to reach a consensus on the optimal polarizedtarget configuration for the tensor spin program.

2.1 Stimulating Theory Progress

The approval of E12-13-011 has already stimulated new treatments of tensor degrees of freedom innuclei. In particular, Sargsian [5] and Miller [8] have new predictions for the tensor asymmetry inthe DIS and x > 1 region, respectively. The new experiment has also spurred a renewed discussionof the validity of the Close-Kumano Sum Rule [9]. We would like to build on this progress toencourage the continued interest of theorists, and to raise awareness in the community of thisexciting new avenue of research.

2.2 Encouraging New Proposals

The approval of E12-13-011 opens the possibility of pursuing several interesting new experimentsat JLab. The polarized target installation also requires significant overhead, so it has traditionallybeen beneficial to schedule multiple experiments sequentially. With this in mind, we discuss afew of the most promising potential measurements that could be performed at JLab with a tensorpolarized target. It is our goal to better focus the physics motivation of these potential experimentsand stimulate discussion of even more novel applications of tensor polarization, so that one or morenew proposals can be submitted to the next PAC.

The Tensor Asymmetry Azz from D(e, e′) in the Quasi-Elastic and x > 1 Region

The tensor-polarized asymmetry, Azz, which is used to extract b1 in the DIS region through theD(e, e′)X channel, has also been calculated in the quasi-elastic x-region. It was first calculated in1988 by Frankfurt and Strikman, using the Hamada-Johnstone and Reid soft-core wave functions[10]. Recent calculations by M. Sargsian revisit Azz in the range using a virtual-nucleon methodand a light-cone method. Particularly in the x > 1.4 plateau region, these two calculations differ

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by approximately a factor of two [5]. An experimental determination between the two calculationscould be performed utilizing the set-up available for b1 by adding a single kinematic setting.

D(e, e′p) Azz in the Low Q2 Region

JLab’s PAC13 approved experiment E97-102, which would have measured Azz in the D(e, e′p) chan-nel in order to extract the cross-sections with the deuteron in the m = ±1 and m = 0 spin states.The collaboration intended for this measurement to be taken over the range 0.051 (GeV/c2) <Q2 < 0.185 (GeV/c2) and 0.2 GeV/c < pmiss < 0.4 GeV/c, which is dominated by the D-statewave function of the deuteron ground state. This experiment never ran, but received an A- physicsrating with the PAC determining,

“The structure of the ground state wave function in nuclei at small interparticle distanceis still an unsolved problem. The deuteron is a suitable target to start this investigationbecause its structure can be calculated with high precision using realistic NN potentials.The PAC strongly encourages effort to optimize kinematics to match standard energies.”

The Tensor Asymmetry AUT and the Orbital Angular Momentum Sum Rule

The spin-1 angular momentum sum rule, as discussed in Ref. [11], provides a test of whether theorbital angular momentum of the deuteron can be derived directly from quark contributions ofindividual protons and neutrons, or whether there are other contributions, such as from gluons. Byexamining the energy momentum tensor for the deuteron, the authors showed that it was possibleto define an additional sum rule (see Eq. 12 in Ref. [11]) where it was demonstrated that the secondmoment of this quantity is non vanishing, being related to one of the gravitomagnetic deuteronform factors.

To study this sum rule, the authors suggest a measurement of the tensor-polarized deuteron’stransverse spin asymmetry, AUT , which is derived in terms of GPDs [12, 13]. It is also important tonotice that b1 singles out the role of the D-wave component in distinguishing coherent nuclear effectsthrough tensor polarized correlations from the independent nucleon’s partonic spin structure. Asimilar role of the D-wave component was also found in the recently proposed spin sum rule whereit plays a non-trivial role producing a most striking effect through the spin flip GPD, E.

Higher Twist Structure Functions, b2, b3, and b4

The leading-twist structure function b1 has been previously measured at HERMES and has beenapproved to be measured at Jefferson Lab. However, it is only one of four additional structurefunctions present in the spin-1 hadronic tensor, Wµν . The b2 structure function is expected to berelated to b1 through a Callan-Gross-type relation, 2xb1 = b2. The b3 and b4 do not contribute atleading twist, and all three have not been directly explored experimentally.

The Polarization Moment t20

Studied previously at Jefferson Lab [14, 15] and elsewhere [16], the functions A(Q2), B(Q2), and thetensor polarization moment t20 are able to provide insight into the three form factors of the spin-1deuteron: the charge monopole GC , the charge quadruple GQ, and the magnetic dipole GM . Theseform factors can be extracted from cross-section and polarization moment measurements throughelectron scattering on tensor-polarized deuterium. In the one-photon exchange approximation, thedeuteron cross-section is defined by the Mott cross-section and a linear combination of A(Q2) and

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B(Q2). At low Q2, the tensor polarization moment t20 is dependent only on the ratio GQ/GC . Ameasurement of all three quantities allows an extraction of each of the form factors.

2.3 Target Development

The Hall A/C solid polarized target operates on the principle of Dynamic Nuclear Polarization(DNP) to enhance the low temperature, high magnetic field polarization of solid materials bymicrowave pumping. The target material, consisting of small beads of irradiated, frozen ammonia(NH3 or ND3), is contained in a permeable container that is immersed in liquid helium and cooledto 1K by a powerful evaporation refrigerator. The superconducting Helmholtz coils have a conicallyshaped aperture along the beam axis which permits unobstructed forward scattering. Microwavesare used to drive electronic spin flips in the target sample that transfer the polarization of unpairedelectrons to the nucleon spins via dipolar coupling. Maximum (beam-averaged) deuteron vectorpolarizations of up to 60% (30%) have been obtained in ND3, corresponding to a maximum (beam-averaged) tensor polarization of approximately 35% (9%). Higher polarizations (70% vector, 40%tensor) and greater resistance to radiation damage have been observed in 6LiD, making it anattractive possible alternative to ND3. However, the polarization of the 6Li nucleus is substantialin this material, and careful consideration must be given to the background signal from electron -6Li scattering.

Both positive and negative vector polarizations can be achieved, depending on the portion ofthe ESR spectrum that is exposed to microwaves. However, the standard DNP technique –simplemicrowave saturation of the ESR line at one frequency– leads only to a positive tensor polarizationthat is independent of the sign of the vector polarization. Techniques to enhance and perhapsreverse the tensor polarization are under consideration.

In the case of ND3, the electric quadrupole interaction splits the deuteron NMR spectra into twoquasi-distinct lines, and this opens the door to two possible techniques. In the first, RF saturationof one of the deuteron NMR lines disturbs the relative populations of the three magnetic sublevels,resulting in higher (and possibly negative) tensor polarizations. This “hole-burning” method hasbeen demonstrated by deBoer [17], Meyer [18, 19], and Bueltmann et al. [20]. The second optionuses independent microwave sources to simultaneously drive the transitions for positive and negativevector polarization. Here again, higher and possibly negative values of tensor polarization mightbe possible. Initial, unpublished studies [20] indicate that this technique is feasible, with the caveatthat resonant coupling of the two microwave sources must be avoided. In either case, extractionof the tensor polarization from the resultant NMR spectrum may not be straightforward, and willrequire considerable R&D work.

The existing polarized target underwent significant renovation prior to the gp2/Gp

E experimentsin Hall A [21]. Most notably, the superconducting Helmholtz coils quenched catastrophically priorto the experiment and were replaced by a slightly smaller set of coils scavenged from the decom-missioned Hall B polarized target. Informal discussion has begun within the community on thecharacteristics desirable in the next generation polarized target for 12 GeV. While the most eco-nomical approach would be to simply procure a replacement magnet for the existing target, theapproved b1 experiment might benefit from a new, specialized solenoid coil with multiple, in-linetarget samples. This approach has been utilized for many years by the EMC, SMC, and COMPASScollaborations. Unfortunately, a solenoidal design limits the potential for large angle scattering,and precludes the multiple field orientations that are possible with a more generalized design. It isimportant that we have a thorough, open discussion of the optimal configuration before investingresources and manpower into either option.

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Theory Session

Speaker Institution Topic

Gerald Miller U. Washington One-Pion Exchange Contribution to b1

Misak Sargsian F.I.U. Azz in the Virtual Nucleon ApproximationMark Strikman F.I.U. Light Cone Calculation of b1

Simonetta Liuti U. Virginia Spin-1 Angular Momentum Sum RuleShunzo Kumano KEK Tensor Polarized Distribution FunctionsMarc Vanderhaeghen U. Mainz Spin-1 FormalismDieter Drechshel U. Mainz Sum Rules for Spin-1 TargetsRobert Jaffe M.I.T. Tensor Spin-Dependent ObservablesStan Brodsky SLAC Hidden Color at Large xChueng-Ryong Ji N.C.S.U. Novel QCD Phenomena from Tensor ObservablesLeonard Gamberg Penn State Orbital Angular MomentumW. Melnitchouk Jefferson Lab Parity Violating Electron ScatteringAlessandro Bacchetta U. Pavia Positivity Constraints on b1

Rocco Schiavilla O.D.U. Azz in the Q.E. RegionWally Van Orden O.D.U. Tensor Observables in the x > 1 RegionFronz Gross W&M Connections to SRC

Panel Discussion All New Directions in Tensor Spin Physics

Target Development Session

Speaker Institution Topic

Chris Keith Jefferson Lab Tensor Target DevelopmentDon Crabb U. Virginia Next Generation Polarized TargetJosh Pierce U. Virginia Status of Existing TargetDustin Keller U. of Virginia Enhancing Tensor PolarizationStephen Bueltmann O.D.U. The CLAS12 DNP TargetGreg Smith Jefferson Lab Direct Measurement of Pzz

Oscar Rondon U. Virginia Luminosity Improvements in DNP Targets

Panel Discussion All The Ideal 12 GeV Polarized Target

New Proposals Session

Speaker Institution Topic

Caroline Reidl DESY The HERMES b1 ExperimentElena Long U. New Hampshire Tensor Asymmetry Azz in the x > 1 RegionKarl Slifer U. New Hampshire The JLab b1 ExperimentDonal Day U. Virginia The Vector Response Function t11Roy Holt Argonne Measuring t20 at Large Q2

Ron Gilman Rutgers Deuteron Elastic Form Factors at Large Q2

Werner Boeglin F.I.U. Measuring Ayy in (e,e’p) ScatteringDoug Higinbothan Jefferson Lab Tensor Analyzing Powers for QE ScatteringXiadong Jiang Los Alamos Polarized Drell Yan

Panel Discussion All Prioritizing New Physics Proposals

Table 1: Potential Workshop Speakers

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3 Workshop Organization

The workshop will be held at Jefferson Lab in order to take advantage of its central locationin the community and high concentration of potential workshop participants. We anticipate theneed for two full days of invited talks, which will be roughly split into three topics: Theory, TargetDevelopment, and New Proposals as outlined in Table 1, which is a very rough draft of the scientificprogram, and lists the core speakers that we anticipate would likely participate in this workshop.

The participating institutions have agreed to contribute $3500 to cover the costs of workshopcatering, materials and to produce a workshop proceedings. We request $5000 from the JSAInitiatives Fund in order to provide full travel and lodging support for one foreign invited speakerand partial support for several additional domestic invited speakers. Preference for travel supportwill be given to junior researchers, particularly students and post-docs, who might otherwise notbe able to participate.

4 Summary

We request support for a two day workshop focused on tensor spin observables to be held at JeffersonLab. The workshop goals are to stimulate progress in the theoretical treatment of polarized spin-1systems, foster the development of new proposals, and to reach a consensus on the optimal polarizedtarget configuration for the tensor spin program.

References

[1] G. A. Miller, Stanford Proceedings 1989, Electronuclear Physics with Internal Targets 30-33 .

[2] H. Khan and P. Hoodbhoy, Phys. Rev. C44, 1219 (1991).

[3] A. Y. Umnikov, Phys. Lett. B391, 177 (1997).

[4] K. Bora and R. L. Jaffe, Phys. Rev. D57, 6906 (1998).

[5] M. Sargsian, private communication, to be published.

[6] S. Kumano, Phys. Rev. D82, 017501 (2010).

[7] A. Airapetian et al., Phys. Rev. Lett. 95, 242001 (2005).

[8] G. A. Miller, Pionic and Hidden-Color, Six-Quark Contributions to the Deuteron b1 Structure Function, 2013,arXiv 1311.4561.

[9] F. E. Close and S. Kumano, Phys. Rev. D42, 2377 (1990).

[10] L. Frankfurt and M. Strikman, Phys.Rept. 160, 235 (1988).

[11] S. K. Taneja, K. Kathuria, S. Liuti, and G. R. Goldstein, Phys.Rev. D86, 036008 (2012).

[12] E. R. Berger, F. Cano, M. Diehl, and B. Pire, Phys.Rev.Lett. 87, 142302 (2001).

[13] E. R. Berger, M. Diehl, and B. Pire, Eur.Phys.J. C23, 675 (2002).

[14] D. Abbott et al., Phys. Rev. Lett. 82, 1379 (1999).

[15] D. Abbott et al., Eur.Phys.J. A7, 421 (2000).

[16] M. E. Schulze et al., Phys. Rev. Lett. 52, 597 (1984); V. Dmitriev et al., Phys. Lett. B157, 143 (1985); R. A.Gilman et al., Phys. Rev. Lett. 65, 1733 (1990); B. Boden et al., Z. Phys. C49, 175 (1991); M. Garcon, Nucl.Phys. A508, 445C (1990); I. The et al., Phys. Rev. Lett. 67, 173 (1991); M. Garcon et al., Phys. Rev. C49,2516 (1994).

[17] W. de Boer, Cern Report CERN-74-11 (1974).

[18] W. Meyer and E. Schilling, BONN-HE-85-06, C84-09-03.1 (1985).

[19] W. Meyer et al., Nucl. Instrum. Meth. A244, 574 (1986).

[20] S. Bueltmann, D. Crabb, Y. Prok. UVa Target Studies, UVa Target Lab technical note, 1999.

[21] J. Pierce et al., Dynamically polarized target for the g2p and GEp experiments at Jefferson Lab, arXiv 1305.3295.In press. Nucl. Instr. Meth., 2013.

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