Search Results (26 total)

The beam-recoil double polarization Px′h and Pz′h and the recoil polarization Py′ were measured for the first time for the p(e→,e′p→)η reaction at a four-momentum transfer of Q2=0.1  GeV2/c2 and a center of mass production angle of θ=120° at the Mainz Microtron MAMI-C. With a center of mass energy range of 1500  MeV<W<1550  MeV the region of the S11(1535) and D13(1520) resonance was covered. The results are discussed in the framework of a phenomenological isobar model (Eta-MAID). While Px′h and Pz′h are in good agreement with the model, Py′ shows a significant deviation, consistent with existing photoproduction data on the polarized-target asymmetry.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ¹⁶O(e,e^′p) reaction in quasielastic, constant (q,ω) kinematics at Q²≈0.8  (GeV∕c)², q≈1  GeV∕c, and ω≈445  MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350  MeV∕c. Six-fold differential cross-section data for 0<E_miss<120  MeV were obtained for 0<p_miss<340  MeV∕c. These results have been used to extract the A_LT asymmetry and the R_L, R_T, R_LT, and R_L+TT effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q². Further, dynamical relativistic effects are strongly indicated by the observed structure in A_LT at p_miss≈300  MeV∕c. For 25<E_miss<50  MeV and p_miss≈50  MeV∕c, proton knockout from the 1s_1∕2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340  MeV∕c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120  MeV and 145<p_miss<340  MeV∕c, (e,e^′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available ¹⁶O(e,e^′p) data is also examined in detail. Finally, the Q²-dependence of the normalization factors is discussed.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

Exclusive electroproduction of π⁰ mesons on protons in the backward hemisphere has been studied at Q²=1.0  GeV² by detecting protons in the forward direction in coincidence with scattered electrons from the 4  GeV electron beam in Jefferson Lab’s Hall A. The data span the range of the total (γ*p) center-of-mass energy W from the pion production threshold to W=2.0  GeV. The differential cross sections σ_T+ϵσ_L, σ_TL, and σ_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

The ratio of the electric and magnetic form factors of the proton G_Ep/G_Mp, which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e→p→ep→ reaction. The new data presented span the range 3.5<Q²<5.6 GeV² and are well described by a linear Q² fit. Also, the ratio sqrt[Q²] F_2p/F_1p reaches a constant value above Q²  =  2 GeV².

We measured the cross section and response functions for the quasielastic ¹⁶O(e,e^′p) reaction for missing energies 25≤E_m≤120 MeV at missing momenta P_m≤340 MeV/c. For 25<E_m<50 MeV and P_m≈60 MeV/c, the reaction is dominated by a single 1s_1/2 proton knockout. At larger P_m, the single-particle aspects are increasingly masked by more complicated processes. Calculations which include pion exchange currents, isobar currents, and short-range correlations account for the shape and the transversity, but for only half of the magnitude of the measured cross section.

The first (e→,e^′p→) polarization transfer measurements on a nucleus heavier than deuterium have been carried out at Jefferson Laboratory. Transverse and longitudinal components of the polarization of protons ejected in the reaction ¹⁶O(e→,e^′p→ ) were measured in quasielastic perpendicular kinematics at a Q² of 0.8 (GeV/c)². The data are in good agreement with state of the art calculations.

Absolute differential cross sections for the reaction ep→epγ have been measured at a four-momentum transfer with virtuality Q²  =  0.33 GeV² and polarization ε  =  0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.

We have measured the cross section for quasielastic 1p-shell proton knockout in the ¹⁶O(e,e^′p) reaction at ω  =  0.439 GeV and Q²  =  0.8 (GeV/c)² for missing momentum P_miss≤355 MeV/c. We have extracted the response functions R_L+TT, R_T, R_LT, and the left-right asymmetry, A_LT, for the 1p_1/2 and the 1p_3/2 states. The data are well described by relativistic distorted wave impulse approximation calculations. At large P_miss, the structure observed in A_LT indicates the existence of dynamical relativistic effects.

The ratio of the proton's elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and the longitudinal recoil proton polarization, respectively. For elastic e→p→ep→, G_Ep/G_Mp is proportional to P_t/P_ℓ. Simultaneous measurement of P_t and P_ℓ in a polarimeter provides good control of the systematic uncertainty. The results for the ratio G_Ep/G_Mp show a systematic decrease as Q² increases from 0.5 to 3.5 GeV², indicating for the first time a definite difference in the spatial distribution of charge and magnetization currents in the proton.

The deuteron elastic structure function A(Q²) has been extracted in the range 0.7≤Q²≤6.0 (GeV/c)² from cross section measurements of elastic electron-deuteron scattering in coincidence using the Hall A Facility of Jefferson Laboratory. The data are compared to theoretical models, based on the impulse approximation with the inclusion of meson-exchange currents, and to predictions of quark dimensional scaling and perturbative quantum chromodynamics.

Measurements are reported of the proton and deuteron spin structure functions g₁^p and g₁^d at beam energies of 29.1, 16.2, and 9.7 GeV, and g₂^p and g₂^d at a beam energy of 29.1 GeV. The integrals Γ_p=∫₀¹g₁^p(x,Q²)dx and Γ_d=∫₀¹g₁^d(x,Q²)dx were evaluated at fixed Q²=3 (GeV/c)² using the full data set to yield Γ_p=0.132±0.003(stat)±0.009(syst) and Γ_d=0.047±0.003±0.006. The Q² dependence of the ratio g₁/F₁ was studied and found to be small for Q²>1 (GeV/c)². Within experimental precision the g₂ data are well described by the twist-2 contribution, g₂^WW. Twist-3 matrix elements were extracted and compared to theoretical predictions. The asymmetry A₂ was measured and found to be significantly smaller than the positivity limit sqrt[R] for both proton and deuteron targets. A₂^p is found to be positive and inconsistent with zero. Measurements of g₁ in the resonance region show strong variations with x and Q², consistent with resonant amplitudes extracted from unpolarized data. These data allow us to study the Q² dependence of the integrals Γ_p and Γ_n below the scaling region.

We report on a precision measurement of the neutron spin structure function g₁ⁿ using deep inelastic scattering of polarized electrons by polarized ³He. For the kinematic range 0.014<x<0.7 and 1<Q²<17 (GeV/c)², we obtain ∫0.014^0.7g₁ⁿ(x) dx  =  -0.036±0.004(stat)±0.005(syst) at an average Q²  =  5 (GeV/c)². We find relatively large negative values for g₁ⁿ at low x. The results call into question the usual Regge theory method for extrapolating to x  =  0 to find the full neutron integral ∫1g₁ⁿ(x) dx, needed for testing the quark-parton model and QCD sum rules.

We have measured the proton and deuteron spin structure functions g₁^p and g₁^d in the region of the nucleon resonances for W²<5 GeV² and Q²≃0.5 and Q²≃1.2 GeV² by inelastically scattering 9.7 GeV polarized electrons off polarized ¹⁵NH₃ and ¹⁵ND₃ targets. We observe significant structure in g₁^p in the resonance region. We have used the present results, together with the deep-inelastic data at higher W², to extract γ(Q²)≡∫1 g₁(x,Q²)dx. This is the first information on the low- Q² evolution of γ toward the Gerasimov-Drell-Hearn limit at Q²  =  0.

The neutron longitudinal and transverse asymmetries A₁ⁿ and A₂ⁿ have been extracted from deep inelastic scattering of polarized electrons by a polarized ³He target at incident energies of 19.42, 22.66, and 25.51 GeV. The measurement allows for the determination of the neutron spin structure functions g₁ⁿ(x, Q²) and g₂ⁿ(x, Q²) over the range 0.03<x<0.6 at an average Q² of 2 (GeV/c)². The data are used for the evaluation of the Ellis-Jaffe and Bjorken sum rules. The neutron spin structure function g₁ⁿ(x, Q²) is small and negative within the range of our measurement, yielding an integral ∫0.03^0.6g₁ⁿ(x)dx=-0.028±0.006 (stat)±0.006 (syst). Assuming Regge behavior at low x, we extract Γ₁ⁿ=∫0¹g₁ⁿ(x)dx=-0.031±0.006 (stat)±0.009 (syst). Combined with previous proton integral results from SLAC experiment E143, we find Γ₁^p-Γ₁ⁿ=0.160±0.015 in agreement with the Bjorken sum rule prediction Γ₁^p-Γ₁ⁿ=0.176±0.008 at a Q² value of 3 (GeV/c)² evaluated using α_s=0.32±0.05.

We have measured proton and deuteron virtual photon-nucleon asymmetries A₂^p and A₂^d and structure functions g₂^p and g₂^d over the range 0.03<x<0.8 and 1.3<Q²<10 (GeV/c)² by inelastically scattering polarized electrons off polarized ammonia targets. Results for A₂ are significantly smaller than the positivity limit sqrt[R] for both targets. Within experimental precision the g₂ data are well described by the twist-2 contribution, g₂^WW. Twist-3 matrix elements have been extracted and are compared to theoretical predictions.

We report on a high-statistics measurement of the deuteron spin structure function g₁^d at a beam energy of 29 GeV in the kinematic range 0.029<x<0.8 and 1<Q²<10 (GeV /c)². The integral γ₁^d  =  ∫1g₁^d dx evaluated at fixed Q²  =  3 (GeV /c)² gives 0.042±0.003(stat)±0.004(syst). Combining this result with our earlier measurement of g₁^p, we find γ₁^p-γ₁ⁿ  =  0.163±0.010(stat)±0.016(syst), which agrees with the prediction of the Bjorken sum rule with O(α_s³) corrections, γ₁^p-γ₁ⁿ  =  0.171±0.008. We find the quark contribution to the proton helicity to be Δq  =  0.30±0.06.

We have measured the ratio g₁^p / F₁^p over the range 0.029<x<0.8 and 1.3<Q²<10 (GeV/c)² using deep-inelastic scattering of polarized electrons from polarized ammonia. An evaluation of the integral ∫0¹g₁^p(x, Q²) dx at fixed Q²=3 (GeV/c)² yields 0.127±0.004(stat)±0.010(syst), in agreement with previous experiments, but well below the Ellis-Jaffe sum rule prediction of 0.160±0.006. In the quark-parton model, this implies Δq=0.27±0.10.

The cross section for exclusive π⁺ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (ɛ∼0.2 and ɛ∼0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors G_A and G_P at ‖t‖=0.073, 0.139, and 0.179 (GeV/c)². The slope of G_A agrees with the value obtained in neutrino experiments. G_P satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.

The spin structure function of the neutron g₁ⁿ has been determined over the range 0.03<x<0.6 at an average Q² of 2 (GeV/c)² by measuring the asymmetry in deep inelastic scattering of polarized electrons from a polarized ³He target at energies between 19 and 26 GeV. The integral of the neutron spin structure function is found to be F₀¹g₁ⁿ(x)dx=-0.022±0.011. Earlier reported proton results together with the Bjorken sum rule predict F₀¹g₁ⁿ(x)dx=-0.059±0.019.