Experiment

• Precision Test of the Standard Model
• Strange Quarks in the Proton
• Generalized Parton Distributions
• Electric and Magnetic Proton Form Factors
• Neutron Charge Distribution
• Nucleon-Delta Transition
• Pion Form Factor
• Short-Range Correlations
• The Spin Structure of the Nucleon

Theory

• Two-Photon Exchange
• Strangeness Magnetic Moment of the Proton
• Charmonium Spectrum
• Quark-Meson Coupling Model
• Color Polarizabilities

Instrumentation

• The JLab Frozen Spin Target

Nuclear Physics

Experiment

Precision Test of the Standard Model

The Electroweak Standard Model (SM) has to date been enormously successful. The search for a fundamental description of nature which goes beyond the SM is driven by two complementary experimental strategies. The first is to build increasingly energetic colliders, such as the Large Hadron Collider (LHC) at CERN, to excite matter into a new form. The second approach is to perform high precision measurements where an observed discrepancy with the SM would reveal the signature of new forms of matter. As shown in Figure 1, state-of-the-art measurements of parity-violating electron scattering (PVES) at Jefferson Lab have led to the most precise determination of the weak charges of the quarks hitherto possible. Shown in Figure 2, these measurements also constrain the possibility of new physics to an energy scale of order one TeV or higher — a factor of two above previous limits, which were dominated by atomic parity violation (APV) data. While limiting the early discovery potential for new Z' bosons, this result provides guidance as to the best windows of opportunity for discovery, both at the LHC and in the Qweak experiment now in preparation at Jefferson Lab.

Figure 1 shows the knowledge of the weak charges associated with an axial coupling to the electron and a vector coupling to the up and down quarks. All experimental limits and contours are shown at 1 standard deviation. The dashed contour displays the previous experimental limits reported by the Particle Data Group, together with the prediction of the Standard Model (the black star). The filled ellipse denotes the new constraint provided by recent high precision PVES measurements on p, D and He, while the smaller contour indicates the full constraint obtained by combining all current results. The width of the solid blue line is the anticipated uncertainty of the upcoming Qweak experiment - assuming the SM. Figure 2 shows the model independent mass limit (Lambda/g) in TeV of new physics as a function of the up and down quark flavor mixing angle, theta. The dashed red curve is the Particle Data Group limit, while the solid blue curve is the improvement from including recent PVES measurements.

References:
R. D. Young, R. D. Carlini, A. W. Thomas and J. Roche, Phys. Rev. Lett. 99 (2007) 122003
R.D. Young et al. Phys. Rev. Lett. 97 (2006) 102002
D. S. Armstrong et al. (G0 Collaboration), Phys. Rev. Lett. 95 (2005) 092001
A. Acha et al. (HAPPEX Collaboration), Phys. Rev. Lett. 98 (2007) 032301

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Strange Quarks in the Proton

While the proton is most simply described as a bound state of three quarks (2 up and 1 down), a more complete description includes a sea of gluons and virtual quark/anti-quark pairs arising from interactions between the three quarks. For instance, strange quark/anti-quark pairs are present in this quark sea even though the proton has, on average, no overall strangeness. The effect of this intrinsic strangeness on the charge and magnetism of the proton can be precisely studied by using the weak interaction (Z-boson exchange) as a probe. While the weak force is normally too slight to be detected alongside the dominant electromagnetic force, the weak interaction is required in any process which violates parity symmetry.

Researchers at Jefferson Lab and elsewhere have therefore turned to high precision measurements of the parity-violating electron scattering (PVES) asymmetry in order to study the effects of strange quarks in the proton. PVES has become an essential tool in mapping out the flavor composition of the electromagnetic form factors. Exposing the role of the strange quark with such measurements provides direct information on the underlying dynamics of non-perturbative QCD – a considerable achievement both experimentally and theoretically.

World data at the lowest momentum transfer Q², which most directly relates to the “static” strange magnetic moment and charge radius, is shown in figure 1 as constraints on the fractional strange quark contributions to the proton form factors. Superimposed are results from global fits of the low Q2 data , which differ in treatment of the theoretically challenging correction term from the anapole moment of the proton. The ellipses represent allowed regions at 95% statistical confidence level. As is evident from these fits, the strange charge radius is very small, while the strange quark contribution to the proton magnetic moment contribution is less than 10%.

(See also Strange Magnetic Moment entry in the Theory section)

References:
A. Acha et al. (HAPPEX Collaboration), Phys. Rev. Lett. 98 (2007) 032301
D. S. Armstrong et al. (G0 Collaboration), Phys. Rev. Lett. 95 (2005) 092001
R.D. Young et al., Phys. Rev. Lett. 97 (2006) 102002
Jianglai Liu et al., Phys. Rev. C 76 (2007) 025202

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Generalized Parton Distributions (GPDs)

Ten years ago, a unifying concept for the description of nucleon structure was introduced, now commonly known as Generalized Parton Distributions (GPDs). These functions contain both the usual form factors and parton distributions but in addition include correlations between states of different longitudinal and transverse momenta. GPDs therefore give three-dimensional pictures of the nucleon, providing information on such things as the transverse spatial distribution of quarks as a function of their longitudinal momentum fraction and the total angular momentum carried by quarks in the nucleon.

Deeply Virtual Compton Scattering (DVCS) is the simplest process which allows extraction of GPDs from data and provides the cornerstone of their exploration at Jefferson Lab. First results of DVCS Beam Spin Asymmetries (BSA) in Hall B in 2001 suggested that the GPD framework can be applied at 6 GeV energies, and two dedicated experiments have since taken data: the Hall A E00-110 and the Hall B E01-113 experiments. The former measured helicity-dependent cross sections and provided the best check so far of the Bjorken-type scaling which is expected of DVCS in the factorization regime. The latter performed BSA measurements over a large kinematic domain, scanning this observable as a function of X_B, t and Q² and is currently in the final stages of the analysis. Recently, new results on the first measurements of the target spin asymmetry were published by the CLAS collaboration, confirming again that the factorization is likely to be applicable at Q² values as low as 2 GeV².

Future experiments are planned to measure the target spin asymmetries at 6 GeV, such as the E05-114 experiment in CLAS. Further down the line, the advent of an 11 GeV beam will provide even larger kinematical coverage with much improved statistics and enable one to explore fully the (X_B, t, Q²) space with its large acceptance, imposing severe constraints on theoretical models of nucleon structure.

References:
A.V. Radyushkin, Phys. Lett. B380 (1996) 417
X.-D. Ji, Phys Rev. Lett. 78 (1997) 610
S. Stepanyan et al. (CLAS), Phys. Rev. Lett. 87 (2001) 182002
P. Bertin et al. (Hall A), JLab Experiment E00-110 (2000)
V. Burkert et al. (CLAS) JLab Experiment E01-113 (2001)
S. Chen et al. (CLAS), hep-ex/0605012

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Electric and Magnetic Elastic Proton Form Factors (G_E^p/G_M^p)

Data from experiments measuring the ratio of the electric and magnetic elastic form factors of the proton, G_E^p/G_M^p, have shown an unexpected and significantly different dependence on the four-momentum transfer squared, Q², for G_E^p than for G_M^p. This has been interpreted as indicating a difference between the spatial distributions of charge and magnetization at short distances. The results from two JLab experiments using the new polariztion transfer method were surprising, as they disagree with the ratios μ_pG_E^p/G_M^p, where μ_P is the proton magnetic moment, obtained by measuring cross sections (Rosenbluth method). The latter appear to be near unity up to about 6 GeV², whereas the polarization results show a ratio value around 0.3 at Q² of 5.6 GeV².

The large discrepancy between the ratios obtained with the Rosenbluth and the recoil polarization method was confirmed by recent precision measurements of G_E^p/G_M^p in Hall A using the traditional Rosenbluth method. These demonstrated that the discrepancy is due to missing physics in the extraction of G_E^p/G_M^p from the data, rather than systematic problems in either data set. A likely explanation is the two-photon exchange process, which affects both cross section and polarization transfer components at the level of a few percent. However, because the Rosenbluth method is very sensitive to small variations in the angular dependence of the cross section, the two-photon effects have a much more dramatic impact on the results from Rosenbluth separation, while modifying the ratios obtained with the polarization method by a few percent only.

(See also Two-Photon Exchange entry in the Theory section)

The ratios μpGEp/GMp from two JLab recoil polarization experiments, compared to the Rosenbluth separation data (left) and with several theoretical calculations (right).

References:
M. K. Jones et al., Phys. Rev. Lett. 84 (2000) 1398
V. Punjabi et al., Phys. Rev. C 71 (2005) 055202
O. Gayou et al., Phys. Rev. Lett. 88 (2002) 092301
I.A. Qattan et al., Phys. Rev. Lett. 94 (2005) 142301

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Neutron Charge Distribution (G_Eⁿ)

The Q² dependence of the charge form factor of the neutron, G_Eⁿ, can provide vital information on the origin of charge distribution in the neutron. A precise determination of G_Eⁿ has challenged physicists for more than 40 years, primarily from the lack of a free neutron target and the fact that the charge form factor is so small.

The application of new techniques and technologies at Jefferson Lab has allowed decisive steps to be made toward rectifying the situation, with two experiments providing a precise measurement of the charge form factor out to large Q². These unique experiments have put the neutron charge form factor on nearly equal footing with the other nucleon form factors. For the first time, data are available which constrain any modern theory which attempts to describe all four nucleon form factors: the proton electric and magnetic form factors and the neutron electric and magnetic form factors.

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Nucleon-Delta Transition

One of the many success stories of JLab's resonance physics program has been what we have learned about the Delta resonance, which is the lowest energy quantum excitation of the nucleon. There are several ways the nucleon can be electromagnetically excited to the Delta. One, denoted M1, or magnetic dipole moment, gives us information about the distribution of the quarks' electric current within the nucleon and Delta. Another, denoted E2, or electric quadrupole moment, describes the deviation from sphericity of the quarks' electric charge distribution.

In the simplest quark picture, both the nucleon and Delta are spherical objects, so that E1=0, and the excitation proceeds completely by M1. Experiments at JLab have shown, however, that the E2/M1 ratio is small but non-zero, suggesting that the nucleon and Delta are not perfectly spherical. The physical picture which this reveals is that the nucleon is a "core" of three quarks enveloped by a non-spherical "cloud" of pions, and photons often interact with this cloud rather than the core. The expectation is that at very high Q², or short wavelengths, one penetrates the cloud and observes the core, which looks very different, so that E2/M1 should significantly change, even approaching unity at very high Q².

The E2/M1 ratio has been measured at JLab up to the very highest Q² ever recorded, corresponding to a resolution of less than 0.05 femtometers. Remarkably, E2/M1 doesn't change very much, but remains approximately constant at a small negative value. At this time, we do not yet understand what properties of the nucleon or Delta give rise to this behavior. There is evidence that further development of the theoretical tool called Lattice QCD may supply the answer.

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Pion Form Factor

In research carried out in Jefferson Lab's Hall C, the F_π collaboration is studying how the strong force combines nature's fundamental building blocks into the lightest particle built of quarks: the pion – which is arguably the most important of the mesons due to its Goldstone nature (it has an unusually small mass). We can naively picture the pion as consisting of one each of the lightest quarks and anti-quarks. As with all quark-based particles, however, a more realistic description of the pion also includes the quark-gluon sea: a strong-force driven bevy of quarks, anti-quarks and gluons popping into and out of existence and providing the foundation of the pion's structure.

This structure is mapped out by a single form factor (F_π), which provides information about the distribution of charge inside the pion. By measuring F_π at ever shorter distances, it is possible to study its transition from a particle where the quark-gluon sea plays a significant role in its structure to what looks like a simple quark-antiquark system.

In 2001, Jefferson Lab provided the first high precision pion electroproduction data for F_π between Q² values of 0.6 and 1.6 (GeV/c)². The new result, at Q²=2.45 (GeV/c)², is still far from the transition to the Q² region where the pion looks like a simple quark-antiquark pair and is providing a stringent test for models that attempt to incorporate the important "softer" quark sea contributions. Plans are now being made to access the transition region with the higher-energy electron beam proposed for the 12 GeV Upgrade at Jefferson Lab. The Upgrade will allow an extension of the F_π measurement up to a value of Q² of about 6 (GeV/c)², which will probe the pion at double the resolution.

References:
T. Horn et. al., Phys. Rev. Lett. 97 (2006) 192001
V. Tadevosyan et al., Phys. Rev. C 75 (2007) 055205
J. Volmer et al., Phys. Rev. Lett. 86 (2001) 1713

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Short-Range Correlations

One fundamental issue of nuclear physics is why nuclei are dilute bound systems of nucleons. The long-range attraction between nucleons would lead to a collapse of a heavy nucleus into an object the size of a proton if there was no short-range repulsion. Including a repulsive interaction at distances where nucleons come close together, 0.7 fm, leads to a reasonable prediction of the present description of the low energy properties of nuclei, such as binding energy and saturation of nuclear densities. The price is the prediction of significant short-range correlations (SRCs) in the high-momentum component of the nuclear wave function, where the high momentum of a nucleon is balanced by just one nucleon and not by the nucleus as a whole. For many decades, a direct observation of SRCs was considered an important challenge for nuclear physics.

A Jefferson Lab experiment measured the ratio of the cross section for inclusive scattering off several nuclei to that off a ³He nucleus. These ratios should exhibit scaling; namely, they should be constant and independent of x and Q² at kinematics where SRCs are dominant. Scaling regions were observed at 1.5 < x < 2 and x > 2.25, which correspond to 2- and 3-nucleon SRC-dominant regions (see figure). Using the measured scaling factors and scaling onsets, combined with well known wave functions of deuterons and ³He, the probabilities of these SRCs were extracted for ⁴He, ¹²C and ⁵⁶Fe nuclei, thus providing a more complete picture of these nuclei.

References:
K. S. Egiyan et al., Phys. Rev. C 68 (2003) 014313
K. S. Egiyan et al., Phys. Rev. Lett. 96 (2006) 082501

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The Spin Structure of the Nucleon

Nucleon spin is being extensively studied at JLab [1-4], addressing fundamental questions unanswered by earlier generations of experiments. While previous experiments made important contributions to our understanding of the nucleon spin (for example that quark spins alone cannot explain it), crucial questions such as the role of gluon polarization and quark orbital angular momentum remained unresolved. With its unique capabilities, JLab has investigated these questions with high-precision measurements. Our knowledge of the gluon polarization ∆ significantly improved after the JLab polarized structure function data were included in the world data set and reanalyzed recently in Ref. [5], see Fig. 1. Furthermore, the effect of the quark orbital momentum ∆, which has been difficult to measure in experiments, can be seen in the large-x data in Fig. 2, where the predictions based on perturbative QCD (dashed curves) disagree with JLab large-x measurements if quark orbital momenta are neglected. The perturbative QCD results and data are reconciled only after quark orbital momentum components are added to the nucleon wave function [6], (solid line in Fig. 2).

Figure 1: Improvement on the gluon polarization ∆. Solid (dashed) lines: uncertainty on ∆ before (after) the JLab data.	Figure 2: Large-x JLab data on quark polarizations. The solid lines include quark orbital anglar momentum while the dashed lines do not.

References:
[1] For a review of older JLab data, see e.g. J.P. Chen, A. Deur et Z.-E. Meziani, Mod. Phys. Lett. A20 (2005) 2745
[2] Hall A results : K. Slifer et al. Phys. Rev. Lett. 101 (2008) 022303; P. Solvignon et al., arXiv: 0803.3845 (2008)
[3] Hall B results : K. V. Dharmawardane et al., Phys. Lett. B 641 (2006) 11 ; P. E. Bosted et al., Phys Rev C 75 (2007) 
035203 ; A. Deur et al., Phys. Rev. D 78 (2008) 032001; Y. Prok et al., arXiv: 0802.2232 (2008)
[4] Hall C results : F. R. Wesselmann et al., Phys. Rev. Lett. 98 (2007) 132003
[5] E. Leader, A. Sidorov, D. Stamenov, Phys. Rev. D 75 (2007) 074027
[6] H. Avakian, S. Brodsky, A. Deur, F. Yuan, Phys. Rev. Lett. 99 (2007) 082001

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Theory

Two-Photon Exchange in Elastic Electron-Proton Scattering

The ratio of the electric to magnetic proton form factors has traditionally been determined using the "Rosenbluth" or longitudinal-transverse (LT) separation method, in which the ratio is extracted from the angular dependence of the cross section at fixed momentum transfer, Q². Recent measurements at JLab using the alternative, polarization transfer (PT) technique have found a dramatically different behavior of the ratio compared with the Rosenbluth results, leading to much discussion about the possible origin of the discrepancy.

In a series of recent papers, JLab Theory Center staff and users have analyzed in detail the effects of two-photon exchange (TPE) in elastic ep scattering. In particular, contributions from elastic and excited nucleon intermediate states have been found to have a strong angular dependence when the finite size of the nucleon is taken into account, largely reconciling the LT and PT measurements. A complementary approach, in which the TPE contributions have been calculated at the partonic level in terms of generalized parton distributions, was also found to reduce the Rosenbluth cross sections, bringing them closer to the PT results.

The TPE calculations have subsequently been used in a global reanalysis of all elastic ep data, with corrected values of the proton's electric and magnetic form factors extracted over the full Q² range of the existing data. The analysis combined the corrected Rosenbluth cross section and PT data and was the first extraction of G_E^p and G_M^p including explicit TPE corrections and their associated uncertainties.

References:
P. G. Blunden, W. Melnitchouk, and J. A. Tjon, Phys. Rev. C 72  (2005) 034612 
A.V. Afanasev, S.J. Brodsky, C.E. Carlson, Y.C. Chen and M. Vanderhaeghen, Phys. Rev. D 72 (2005) 013008
J. Arrington, W. Melnitchouk and J.A. Tjon, Phys. Rev. C 76 (2007) 035205 

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Strangeness Magnetic Moment of the Proton

A theoretical calculation has produced a prediction of the strange quark contribution to the magnetic moment of the proton, G_M^P, that is more than 10 times more precise than currently accessible in experiment. This result provides vital information on the hidden-flavor structure of the nucleon. Nucleons are primarily composed of three quarks, two 'up' and one 'down' type quark for a proton, and the reverse for a neutron. Quantum fluctuations can cause brief appearances of any number of quark-antiquark pairs. These pairs can either be the familiar up or down quarks, or possibly a third species of quark, called the strange quark.

These short-lived strange quark pairs certainly contribute to nucleon properties; however, the role the strange quark plays in nucleon structure is controversial and poorly understood. For instance, there have been claims that the strange quark accounts for as much as 20% of the mass of the proton and carries about 10% of the total spin. The present calculation, performed by theorists at Jefferson Lab in collaboration with the CSSM in Adelaide, Australia, has focused on the contribution of strange quarks to the proton's magnetic moment. This calculation has been produced by combining charge symmetry, modern lattice QCD simulation results, and improved chiral extrapolation techniques. This work has produced a prediction for the proton's strange magnetic moment, G_M^P=-0.046 ± 0.019 nuclear magnetons. In light of the large strange-quark influence on the mass and spin, the latest result is a shockingly small value, representing only half a percent of the total proton magnetic moment.

Previously, there has been large theoretical uncertainty, with model calculations predicting values for G_M^P over the range -0.4 up to +0.7. The level of precision in the new calculation is unprecedented — greater than 10 times more precise than currently resolved in measurements at MIT-Bates, JLab and Mainz. At the present level of experimental precision, theory and experiment are in agreement. The next generation of experimental programs, such as the 2005 HAPPEx run and the next phase of the G0 experiment at JLab, should provide further insight and a strong test of the theoretical prediction.

As a further check of this calculation, the magnetic moments of the ground state baryon magnetic moments have also been evaluated. The excellent agreement with the experimental values are shown in the figure.

References:
D. B. Leinweber et al., Phys. Rev. Lett. 94 (2005) 212001
D. B. Leinweber, A. W. Thomas and R. D. Young, Phys. Rev. Lett. 92 (2004) 242002
R. D. Young, D. B. Leinweber, A. W. Thomas and S. V. Wright, Phys. Rev. D 66 (2002) 094507

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Charmonium Spectrum and Quark Confinement

Hall D of the 12 GeV upgraded CEBAF will house the GlueX experiment, which intends to map the spectrum of mesons, the hybrid mesons in particular, through photoproduction off protons. Knowledge of the spectrum of hybrid mesons will aid us in understanding the nature of the confinement of quarks within hadrons, since within hybrids the gluonic field binding the meson is excited. Just as the excited states of hydrogen taught us about QED, we hope the excited states of glue in mesons will teach us about the non-trivial aspects of QCD.

An essential unknown in the GlueX proposal is the coupling strength of hybrid mesons to photons interacting with the meson cloud surrounding the proton target. Such quantities have been calculated within models such as the flux-tube model, in which the gluonic field between quarks forms itself into a tube, but no attempt has yet been made to make the computation directly from QCD. The first stage of a program to do this is underway in the Jefferson Lab Theory Center, where we are applying the powerful technique of lattice QCD to the problem.

Simulations within lattice QCD of hadrons made of realistically light quarks currently require either excessive computing time or a well-controlled theory with which to extrapolate data computed at un-physically heavy quark masses. Since neither of these are available for the problem at hand, the closely related problem of charmonium radiative transitions was addressed. Charmonia are mesons composed of a charm quark and an anti-charm quark, where the charm quark is a heavier cousin of the up quark, having the same color and electric charge, but a much larger mass. Charmonia are a subject of much interest at experimental facilities such as CLEO, BES, Belle, Babar and Fermilab, where despite thirty years of intense study, they continue to spring surprises. They provide a set of states analogous to the light mesons, but over which we have much tighter theoretical control, in lattice QCD and in models.

We have computed, for the first time within a lattice QCD simulation, the radiative transition rates between many of the lightest charmonium states, observing good agreement with experimental measurements. In addition, we are able to compute in regions inaccessible to experiment — and it is here that we have the power to test phenomenological models, which have the advantage of allowing computation "with paper and pencil" but the disadvantage of not being as closely related to the full theory of QCD.

References:
J. J. Dudek, R. G. Edwards and D. G. Richards, Phys. Rev. D73 (2006) 074507
F. E. Close and J. J. Dudek, Phys. Rev. Lett. 91 (2003) 142001

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The Quark-Meson Coupling Model

The Quark-Meson Coupling (QMC) model, a theory which takes the radical step of incorporating self-consistent changes in the quark structure of a nucleon when it is bound in matter, has been transformed into a theory of quasi-nucleons interacting through many-body forces. This adjustment allows the QMC model to be related to the time-honored descriptions of the nucleus where nucleon structure was supposed to play no role. Of course, in experiments conducted at very high energies, it is customary to see the nucleus as a collection of quarks interacting via the exchange of gluons. At lower energies, where the spatial resolution is lower, one is apt to view the nucleus in terms of nucleons interacting via the exchange of mesons.

Actually, even in the lower energy range, one should keep the quarks in mind, because their motion inside a nucleon may change when it resides in a nucleus. That is, a nucleon is one thing when on its own and another thing when inside a nucleus, in which case it becomes a "quasi-nucleon". The QMC model takes this dichotomy into account by describing the interactions between a quark in one nucleon with a quark in another nucleon by meson exchange (see illustration at www.aip.org/png/2004/220.htm). The quarks in that nucleon are in turn interacting with the quarks in another and so on. The nucleus is now seen as quasi-nucleons interacting through forces which involve 2, 3, or even 4 bodies. The necessity of such many-body forces was empirically known from traditional nuclear physics, and the merit of the QMC model is that it explains their origin and predicts their intensity. The newer version of the QMC model will enable one to pursue more dramatic impacts of the change of hadron properties in medium, including the modification of weak and electromagnetic form factors.

References:
P. A. M. Guichon and A. W. Thomas, Phys. Rev. Lett. 93 (2004) 132502
K. Saito, K. Tsushima and A. W. Thomas, Prog. Part. Nucl. Phys. 58 (2007) 1 

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Quark-Hadron Duality and Color Polarizabilities

Within the theoretical framework of the operator product expansion, the moments, or integrals over Bjorken-x, of structure functions can be expanded in inverse powers of the momentum scale Q². The leading term in the expansion corresponds to scattering from free quarks and is responsible for the scaling of the structure functions. The higher order terms involve mixed quark-gluon operators and contain information on long-range correlations between partons, which are related to quark confinement. Quark-hadron duality (equivalence of quark and hadron descriptions of structure function moments) occurs when the higher-order interaction terms are small.

An example of a quantity which reflects such quark-gluon interactions that was extracted recently from moments of the spin dependent structure functions of the nucleon is the "color polarizability." This describes how the color electric and magnetic fields respond when the nucleon is polarized. The results indicate small but positive color electric polarizabilities for both the proton and neutron, but a negative magnetic polarizability for the proton, and one consistent with zero for the neutron. The negative value for the proton magnetic polarizability suggests that, on average, the induced color magnetic field in a proton is oriented in a direction opposite to that of its spin.

Also see Quark-Hadron Duality entry.

References:
M. Osipenko et al., Phys. Lett. B 609 (2005) 259
Z. -E. Meziani et al., Phys. Lett. B 613 (2005) 148
W. Melnitchouk, R. Ent and C.E. Keppel, Physics Reports 406 (2005) 127

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Instrumentation

The JLab Frozen Spin Target

Nuclear-spin polarized targets play a key role in experimental nuclear and particle physics. They are essential for understanding how the proton and neutron get their spins from their constituent quarks and gluons and for measuring the electromagnetic structure of these nucleons in both their ground and excited states. While the Jefferson Lab Frozen Spin Target (FROST) is the fourth and latest polarized target to be used at JLab, it is the first to be entirely designed and built here. FROST is designed to be used inside the CEBAF Large Acceptance Spectrometer (CLAS) with beams of real photons.

Scientists in the JLab Target Group use a microwave-based technique called dynamic nuclear polarization, or DNP, to polarize free protons (hydrogen nuclei) within a sample of frozen butanol.  However, to polarize the free protons, DNP also requires a powerful superconducting magnet, which obscures a large fraction of particles scattering from the target.

To overcome this problem, the Target Group constructed one of the world's most powerful 3He-4He dilution refrigerators to cool the target to less than three hundredths of a degree above absolute zero.  At such low temperatures, the polarization of the protons decays very slowly (in other words, the spins are "frozen"), and both the microwave source and the polarizing magnet can be switched off.

The sample is then removed from the polarizing magnet, and a smaller, 0.56 tesla magnet is used to "hold" the polarization during the scattering experiment. This holding magnet, integrated into the FROST cryostat, is thin enough for scattered particles to pass through and be detected by the CLAS spectrometer. Under these field and temperature conditions, the polarization decay is only about 1% per day.

FROST has already been used in Hall B for experiments with the target polarized along the direction of the photon beam. In its next use (2010), the polarization must be perpendicular to the beam. To accomplish this, the Target Group has also built a 0.54 tesla dipole magnet to rotate the spins 90º after polarization and hold them in the perpendicular direction.

References:
St. Goertz, W. Meyer, and G. Reicherz, Progr Part Nucl Phys 49 (2002) 403-489
C.D. Keith, FROST webpage

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