PRECISION MEASUREMENTS OF THE NEUTRON ELECTRIC FORM FACTOR AT HIGH MOMENTUM TRANSFERS

The neutron, although electrically neutral, is composed of elementary charged particles and as a result, possesses a charge distribution within. The charge distribution can be studied by measuring a quantity called the neutron electric form factor, GnE. Experiment E02-013 at Jefferson Lab’s Hall A measured GnE at high four-momentum transfer values of Q2 = 1.2, 1.7, 2.5 and 3.4 (GeV/c)2 in double polarized semi-exclusive 3He(e, e\u27n) scattering in quasi-elestic kinematics by measuring the transverse asymmetry AT of the cross section. The neutron electric form factor is essential to know for a variety of reasons. Results from the recent Jefferson Lab experiment on the proton revealed interesting features at these momentum transfers, whereas no accurate data for the neutron is available. Also the recent development in Generalized Parton Distributions (GPDs) necessitates the need for precise values for GnE in Q2 range between 1 and 10 (GeV/c)2; they appear as limiting conditions for certain GPD functions, for example, to constrain spin-flip GPDs. The experiment used the polarized 3He target and the polarized CEBAF electron beam at energies of about 1.52, 2.08, 2.64 and 3.29 GeV. The electrons were detected in the BigBite spectrometer and the neutrons in a large array of scintillators in coincidence with the electrons. In this dissertation, we report a preliminary result, GnE = 0.03457 ± 0.007239 at Q2 = 1.7 (GeV/c)2

Observational consequences of fine structure line optical depths on infrared spectral diagnostics

It has long been known that infrared fine structure lines of abundant ions, like the [O III] 88 micron line, can become optically thick in H II regions under certain high luminosity conditions. This could mitigate their potential as diagnostic tools, especially if the source is too dusty for optical spectroscopy to otherwise determine the system's parameters. We examined a series of photoionization calculations which were designed to push the nebulae into the limit where many IR lines should be quite optically thick. We find that radiative transfer effects do not significantly change the observed emission line spectrum. This is due to a combination of grain absorption of the hydrogen ionizing continuum and the fact that the correction for stimulated emission in these lines is large. Given these results, and the likelihood that real objects have non-thermal line broadening, it seems unlikely that line optical depth presents a problem in using these lines as diagnostics of the physical conditions or chemical composition.Comment: 16 pages, 4 figures, to be published in the February 2003 issue of the PAS

Single Spin Asymmetries of Inclusive Hadrons Produced in Electron Scattering from a Transversely Polarized \u3csup\u3e3\u3c/sup\u3eHe Target

We report the first measurement of target single spin asymmetries (AN) in the inclusive hadron production reaction, e+3He↑→h+X, using a transversely polarized 3He target. The experiment was conducted at Jefferson Lab in Hall A using a 5.9-GeV electron beam. Three types of hadrons (π±, K±, and proton) were detected in the transverse hadron momentum range 0.54 \u3cpT\u3c 0.74 GeV/c. The range of xF for pions was −0.29 \u3cxF\u3c −0.23 and for kaons was −0.25 \u3cxF\u3c −0.18. The observed asymmetry strongly depends on the type of hadron. A positive asymmetry is observed for π+ and K+. A negative asymmetry is observed for π−. The magnitudes of the asymmetries follow ∣∣Aπ−|\u3c|Aπ+|\u3c|AK+∣∣. The K− and proton asymmetries are consistent with zero within the experimental uncertainties. The π+ and π− asymmetries measured for the 3He target and extracted for neutrons are opposite in sign with a small increase observed as a function of pT

Measurements of \u3cem\u3ed\u3csup\u3en\u3c/sup\u3e\u3c/em\u3e\u3csub\u3e2\u3c/sub\u3e and \u3cem\u3eA\u3csup\u3en\u3c/sup\u3e\u3c/em\u3e\u3csub\u3e1\u3c/sub\u3e: Probing the Neutron Spin Structure

We report on the results of the E06-014 experiment performed at Jefferson Lab in Hall A, where a precision measurement of the twist-3 matrix element d2 of the neutron (dn2) was conducted. The quantity dn2 represents the average color Lorentz force a struck quark experiences in a deep inelastic electron scattering event off a neutron due to its interaction with the hadronizing remnants. This color force was determined from a linear combination of the third moments of the 3He spin structure functions, g1 and g2, after nuclear corrections had been applied to these moments. The structure functions were obtained from a measurement of the unpolarized cross section and of double-spin asymmetries in the scattering of a longitudinally polarized electron beam from a transversely and a longitudinally polarized 3He target. The measurement kinematics included two average Q2 bins of 3.2  GeV2 and 4.3  GeV2, and Bjorken-x 0.25 ≤ x ≤ 0.90 covering the deep inelastic and resonance regions. We have found that dn2 is small and negative for ⟨Q2⟩ = 3.2  GeV2, and even smaller for ⟨Q2⟩ = 4.3  GeV2, consistent with the results of a lattice QCD calculation. The twist-4 matrix element fn2 was extracted by combining our measured dn2 with the world data on the first moment in x of gn1, Γn1. We found fn2 to be roughly an order of magnitude larger than dn2 . Utilizing the extracted dn2 and fn2 data, we separated the Lorentz color force into its electric and magnetic components, Fy,nE and Fy,nB, and found them to be equal and opposite in magnitude, in agreement with the predictions from an instanton model but not with those from QCD sum rules. Furthermore, using the measured double-spin asymmetries, we have extracted the virtual photon-nucleon asymmetry on the neutron An1, the structure function ratio gn1/Fn1, and the quark ratios (Δu + Δu¯)/(u + u¯) and (Δd + Δd¯)/(d + d¯). These results were found to be consistent with deep-inelastic scattering world data and with the prediction of the constituent quark model but at odds with the perturbative quantum chromodynamics predictions at large x

Precision measurements of A1N in the deep inelastic regime

We have performed precision measurements of the double-spin virtual-photon asymmetry A1A1 on the neutron in the deep inelastic scattering regime, using an open-geometry, large-acceptance spectrometer and a longitudinally and transversely polarized 3He target. Our data cover a wide kinematic range 0.277≤x≤0.5480.277≤x≤0.548 at an average Q2Q2 value of 3.078 (GeV/c)2, doubling the available high-precision neutron data in this x range. We have combined our results with world data on proton targets to make a leading-order extraction of the ratio of polarized-to-unpolarized parton distribution functions for up quarks and for down quarks in the same kinematic range. Our data are consistent with a previous observation of anA1n zero crossing near x=0.5x=0.5. We find no evidence of a transition to a positive slope in(Δd+Δd¯)/(d+d¯) up to x=0.548x=0.548

Measurement of the generalized spin polarizabilities of the neutron in the low-Q2Q^2 region

International audienceUnderstanding the nucleon spin structure in the regime where the strong interaction becomes truly strong poses a challenge to both experiment and theory. At energy scales below the nucleon mass of about 1 GeV, the intense interaction among the quarks and gluons inside the nucleon makes them highly correlated. Their coherent behaviour causes the emergence of effective degrees of freedom, requiring the application of non-perturbative techniques such as chiral effective field theory1. Here we present measurements of the neutron’s generalized spin polarizabilities that quantify the neutron’s spin precession under electromagnetic fields at very low energy-momentum transfer squared down to 0.035 GeV2. In this regime, chiral effective field theory calculations2,3,4 are expected to be applicable. Our data, however, show a strong discrepancy with these predictions, presenting a challenge to the current description of the neutron’s spin properties