Towards a resolution of the proton form factor problem: new electron and positron scattering data

There is a significant discrepancy between the values of the proton electric form factor, G(E)(p), extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G(E)(p) from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (ϵ) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ϵ at Q(2)=1.45  GeV(2). This measurement is consistent with the size of the form factor discrepancy at Q(2)≈1.75  GeV(2) and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3  GeV(2).

Probing the high momentum component of the deuteron at high Q2

The (2)H(e,e'p)n cross section at a momentum transfer of 3.5 (GeV/c)(2) was measured over a kinematical range that made it possible to study this reaction for a set of fixed missing momenta as a function of the neutron recoil angle θ(nq) and to extract missing momentum distributions for fixed values of θ(nq) up to 0.55  GeV/c. In the region of 35°≤θ(nq)≤45° recent calculations, which predict that final-state interactions are small, agree reasonably well with the experimental data. Therefore, these experimental reduced cross sections provide direct access to the high momentum component of the deuteron momentum distribution in exclusive deuteron electrodisintegration.

Probing quark-gluon interactions with transverse polarized scattering

We have extracted QCD matrix elements from our data on doubly polarized inelastic scattering of electrons on nuclei. We find the higher twist matrix element d˜2, which arises strictly from quark-gluon interactions, to be unambiguously nonzero. The data also reveal an isospin dependence of higher twist effects if we assume that the Burkhardt-Cottingham sum rule is valid. The fundamental Bjorken sum rule obtained from the a0 matrix element is satisfied at our low momentum transfer.

High-resolution spectroscopy of Lambda16N by electroproduction

An experimental study of the (16)O(e,e'K(+))(Lambda)(16)N reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e,e'K(+))Lambda, Sigma(0) exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76+/-0.16 MeV was obtained for (Lambda)(16)N with better precision than previous measurements on the mirror hypernucleus (Lambda)(16)O. Precise energies have been determined for peaks arising from a Lambda in s and p orbits coupled to the p(1/2) and p(3/2) hole states of the (15)N core nucleus.

High resolution spectroscopy of (lambda)(12)B by electroproduction

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a 12C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed (Lambda)(12)B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Lambda peak and the 11 MeV p-wave Lambda peak.

Proton spin structure in the resonance region

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085 GeV<W<1.910 GeV) at an average four momentum transfer of Q2=1.3 GeV2. Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A|| and A(perpendicular) to high precision, and extracted the asymmetries A1 and A2, and the spin structure functions g1 and g2. We found a notably nonzero A(perpendicular), significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

Thioarsenates in sulfidic waters

It has long been recognized that the formation of soluble arsenic sulfur complexes plays a key role for the mobility and toxicity of arsenic in sulfate-reducing environments. Knowledge of the exact arsenic species is essential to understand the behavior of arsenic in sulfidic aquifers and to develop remediation strategies. In the past, monomeric and trimeric thioarsenites were assumed to be the existing species in sulfidic systems. In this study, thioarsenates were identified by IC-ICP/MS in arsenite- and sulfide-containing solutions as well as in a reduced groundwater from a contaminated site. The unexpected finding of an oxidation of As(lll) to As(V) in thioarsenates in strongly reducing systems can be explained by the high affinity between As(Ill) and sulfur. In sulfide-containing solutions without oxidant, As(lll) therefore undergoes disproportionation to thioarsenates (As(V)) and elemental arsenic. It has previously been supposed that mobility as well as toxicity of arsenic increases if the redox state decreases. For sulfidic waters, the opposite is probably the case. Thus, the formation of thioarsenates could be used in connection with remediation strategies. Thioarsenates are highly sensitive to oxygen and pH. This is important for analytical procedures. A loss of soluble arsenic as well as a conversion to arsenite and arsenate may occur if water samples containing thioarsenates are analyzed with conventional methods.

Measurement of the electric form factor of the neutron at Q2=0.5 and 1.0 GeV2/c2

The electric form factor of the neutron was determined from measurements of the d-->(e-->,e'n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (15ND3) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G(n)(E)=0.0526+/-0.0033(stat)+/-0.0026(sys) and 0.0454+/-0.0054+/-0.0037 at Q(2)=0.5 and 1.0 (GeV/c)(2), respectively.

Measurements of GnE/GnM from the 2H(e-->,en-->)1H Reaction to Q2=1.45 (GeV/c)2

We report new measurements of the ratio of the electric form factor to the magnetic form factor of the neutron, G(n)(E)/G(n)(M), obtained via recoil polarimetry from the quasielastic 2H(e-->,e(')n-->)1H reaction at Q2 values of 0.45, 1.13, and 1.45 (GeV/c)(2) with relative statistical uncertainties of 7.6% and 8.4% at the two higher Q2 points, which points have never been achieved in polarization measurements.

[Protein and amino acid metabolism in laying hens receiving N-15 labeled casein. 6. Patterns of N-15 incorporation into the N fraction and amino acids of the follicles and residual ovaries]

4 experimental hens were fed labelled casein as sole protein source for a period of 6 days. The dietary nitrogen contained a 15N excess of 0.69 atom%. Hens 1 and 2 were slaughtered 2 days and hens 3 and 4 four days after receiving the last 15N' dose. 10 follicles with diameters more than 6 mm were investigated for their content of N, for the TCE-soluble portion of N (TCE=trichloracetic acid) and for the content of different amino acids in protein. The corresponding 15N' data were established and the same analyses were carried out with the residual ovaries. The mean proportion of the TCE soluble N fraction in the total amount of N was 90% in the follicles and 88% in the residual ovaries. The content of certain kinds of amino acids in follicular protein depends on the degree of maturity of the follicles. There were differences in the percentage composition of amino acids between the follicles and residual ovaries. The mean percentage of lysine contained in follicular protein was 7,2%, that in the residual ovaries 7,9%. Corresponding data for arginine were 6,9% and 7,9%, those for glutamic acid 11,5% and 13,8%, for glycine 3,1% and 5,7%, for alanine 5,0% and 6,1%, for methionine 2,8% and 1,7% and for cystine 3,7% and 2,4%. Less than half of the 15N' atom % and less than half of the percentage of 15N' in the total amount of N and of the amount of amino acids in both the follicles and the residual ovaries were found in the hens killed 144 hrs after the last 15N dose than in the hens slaughtered 48 hrs after the last 15N intake. The atom % of 15N' in the N contained in the amino acids serine, threonine, glutamic acid, proline and leucine decreased with the process of ageing of the follicles. On an average, 88% of the total quantity of 15N' contained in the follicle samples were found as amino acid N, while in the residual ovaries 90% were found.

[Studies on the protein and amino acid metabolism of laying hens using 15N labeled casein. 15N concentration in the intestinal content and intestinal tissue in relation to the time of the last 15N administration]

Four laying hens with anus praeternaturalis were fed 15N casein for a period of 6 experimental days. Each experimental bird thus received a 15N excess (15N') of 154.44 mg. 11.54 mg of 15N' were found in lysine 3.99 mg in histidine and 9.58 mg in arginine. 2 hens were slaughtered 48 hours after the last 15N dose; another 2 were sacrificed after 144 hrs. In this investigation the N content of the intestinal contents and in the proteins of the small and large intestine were determined. The average N content of the small intestine per hen 2.65% that of the large intestine, 2.60% and, correspondingly, the TCE soluble proportion of N was 21.8% and 20.4%. The percentage of amino acids in the contents of the small intestine was found to change relative to that of the dietary proteins, approximating the protein composition of the intestine. In the first two hens the atom % 15N' was found in the different sections of the intestine and in the intestinal contents varied between 0.14 and 0.28. The data obtained from the two hens slaughtered 144 hours after the last 15N' dose were nearly the same ranging from 0.08 to 0.10 atom % 15n'.