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Hall C Scientific Highlights

The Qweak experiment: A precision measurement of the proton's weak charge

Neutron-weak effective coupling constants

Data constraints on the neutron-weak effective coupling constants. The thin blue line shows the improvement expected from the JLab Qweak experiment.

The Qweak experiment, now running in Hall C, will conduct the first precision measurement of the weak charge of the proton, building on the technical advances that have been made in the laboratory's parity-violation program and using the results of earlier measurements to constrain hadronic corrections. When the results from this measurement are combined with atomic parity violation and other parity violating electron scattering measurements the weak charges of the 'up' and 'down' quarks can be individually extracted.

The experiment is basically a measurement of the parity-violating longitudinal analyzing power in e-p elastic scattering at Q2 = 0.026 (GeV/c)2 employing 150μA of 85% polarized electrons on a 0.35m long liquid hydrogen target. The experiment will determine the weak charge of the proton with about 4.1% combined statistical and systematic errors. This corresponds to constraints on parity violating new physics at a mass scale of 2.3 TeV at the 95% confidence level.

R. D. Carlini, "The Qweak experiment: a precision measurement of the proton's weak charge."

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Integrals of F<sub>2</sub>

Quantifying quark-hadron duality: integrals of F2 over x from data are compared to those calculated from the QCD calculation.

Deuteron, proton and neutron structure functions

F2 structure functions of the deuteron, proton and neutron at Q2 = 5 GeV2. The p and n data are compared to the QCD calculation of Alekhin et al. (dashed), and the reconstructed deuteron function (solid) attests to the consistency of the neutron extraction.

One of the principal challenges of QCD is to bridge the small- and large-scale behavior of the strong nuclear interactions. At short distances, perturbative QCD is very successful in describing nucleon structure in terms of quarks and gluons. At large distances, the effects of confinement impose a more efficient description in terms of collective hadron degrees of freedom. Despite this apparent dichotomy, an intriguing connection has been observed between the low- and high-energy data on nucleon structure functions, which is referred to as quark-hadron duality.

A recently published study on the proton F2p structure function confirmed the presence of duality over a range of four-momentum transfer squared, Q2, from 1 to 7 GeV2 at the level of 10% or better. This analysis highlighted an important shortcoming of standard QCD fits, in which parton distribution functions at large x are basically unconstrained.

The validity of duality in the neutron has also been demonstrated recently for the first time using F2n structure functions extracted from inclusive proton and deuteron data in the nucleon resonance region, over a similar Q2 range. The confirmation of duality in both the neutron and proton F2 structure functions opens the possibility of using resonance region data to place stronger constraints on parton distributions at large x.

W. Melnitchouk, R. Ent, C. E. Keppel, Phys. Rep. 406, 127 (2005)
S. P. Malace, et al., Phys. Rev. C 80, 035207 (2009)
S. P. Malace, et al., Phys. Rev. Lett. 104, 102001 (2010)

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Nuclear Dependence of the EMC Effect

Nuclear Dependence of the EMC Effect

Visualization of the dominant 4He and 9Be configurations.

Nuclear Dependence of the EMC Effect

Slope of the isoscalar EMC ratios for 0.35 < x < 0.7 as a function of scaled nuclear density as calculated using GFMC of spatial distributions as described in Ref [1] . A clear breakdown of the simple density dependence model for the EMC effect for 9Be is observed.

Protons and neutrons are complex bound states of quarks and gluons, held together by the strong interactions of quantum chromo dynamics (QCD). Their structure may be modified inside of the dense environment of a nucleus, and such modification of hadron properties in the nuclear environment is of fundamental importance in understanding QCD. Measurements of deep inelastic scattering in nuclei show that the quark distributions in heavy nuclei are not simply the sum of the quark distributions of the constituent protons and neutron, as one might expect for a weakly bound system.

Despite much theoretical work, no unique and universally accepted explanation of this difference, known as the "EMC effect", has emerged. The nuclear effects come in part from binding and Fermi motion mechanisms with possible internal modifications of the nucleon. Many models of the EMC effect predict that it depends on the mass or density of the parent nucleus, but previous data, focusing mainly on heavier nuclei, could not determine if the EMC effect scales with mass or density. A detailed experimental study [1, 2] was performed in Hall C to test these predictions on a variety of light nuclei.

Figure 1 shows the magnitude of the EMC effect as a function of average nuclear density. The difference between 3He and 4He is much larger than predicted by an A-dependent fit from a previous experiment at SLAC, while the large EMC effect in 9Be is at odds with a density-dependent parameterization. A likely explanation is the unusual structure of 9Be, with two orbiting alpha-like clusters with an additional neutron as shown in Figure 2. The orbiting clusters yield a large radius but most of the protons and neutrons are contained within the high local densities of the clusters. Thus, the observed nuclear dependence suggests that the local environment of the nucleons may be driving the nuclear modification. Plans are being made to test this hypothesis with the 12 GeV upgrade of Jefferson Lab by including a more complete set of light nuclei.

Articles and References:
J. Seely et al., Phys. Rev. Lett. 103, 202301 (2009)
"Quarks Influenced by Their Neighborhood," Physical Review Focus, 20 November, 2009.
"I'm in ur atom, probing ur nucleus," Ars Technica, November 2009.

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Pion Form Factor


Pion form factor results from the two JLab Hall C experiments. Also shown are e-pi elastic data from CERN and earlier pion electroproduction data from DESY. The curves are from a Dyson-Schwinger equation (Maris and Tandy, 2000), QCD sum rule (Nesterenko, 1982), constituent quark model (Hwang, 2001), and a pQCD calculation (Bakulev, 2004).


The pion form factor in leading order pQCD.

In research carried out in Jefferson Lab's Hall C, the Fπ 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 (Fπ), which provides information about the distribution of charge inside the pion. By measuring Fπ 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 Fπ 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 Fπ measurement up to a value of Q2 of about 6 (GeV/c)2, which will probe the pion at double the resolution.

G.M. Huber et al., Phys. Rev. C 78, 045203 (2008)
H. P. Blok et al., Phys. Rev. C 78, 045202 (2008)
V. Tadevosyan et al., Phys. Rev. C 75, 055205 (2007)
T. Horn et al., Phys. Rev. Lett. 97, 192001 (2006)
J. Volmer et al., Phys. Rev. Lett. 86, 1713 (2001)

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Neutron Charge Distribution (GEn)

charge distribution

The world data set on GEn, including data from two Jefferson Lab experiments, E93-026 (using a polarized deuteron target) and E93-038 (using an unpolarized deuteron target and a recoil polarimeter), and other experiments that have used polarized targets and recoil polarimeters.

The Q2 dependence of the charge form factor of the neutron, GEn, can provide vital information on the origin of charge distribution in the neutron. A precise determination of GEn 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.

H. Zhu et al., Phys. Rev. Lett. 87 (2001) 081801
R. Madey et al., Phys. Rev. Lett. 91 (2003) 122002
G. Warren et al., Phys. Rev. Lett. 92 (2004) 042301
B. Plaster et al., Phys. Rev. C 73 (2006) 025205

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