Search Results (36 total)

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We have studied the quasielastic ³He(e,e^′p)²H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502  MeV/c, respectively. The ³He(e,e^′p)²H cross section was measured for missing momenta up to 1000  MeV/c, while the A_TL asymmetry was extracted for missing momenta up to 660  MeV/c. For missing momenta up to 150  MeV/c, the cross section is described by variational calculations using modern ³He wave functions. For missing momenta from 150 to 750  MeV/c, strong final-state interaction effects are observed. Near 1000  MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A_TL asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.

Results of the Jefferson Lab Hall A quasielastic ³He(e,e^′p)pn measurements are presented. These measurements were performed at fixed transferred momentum and energy, q=1502  MeV/c and ω=840  MeV, respectively, for missing momenta p_m up to 1  GeV/c and missing energies in the continuum region, up to pion threshold; this kinematic coverage is much more extensive than that of any previous experiment. The cross section data are presented along with the effective momentum density distribution and compared to theoretical models.

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 report on a new measurement of the parity-violating asymmetry in quasielastic electron scattering from the deuteron at backward angles at Q²=0.038 (GeV/c)². This quantity provides a determination of the neutral weak axial vector form factor of the nucleon, which can potentially receive large electroweak corrections. The measured asymmetry A=-3.51±0.57  (stat)±0.58  (syst)  ppm is consistent with theoretical predictions. We also report on updated results of the previous experiment at Q²=0.091 (GeV/c)², which are also consistent with theoretical predictions.

We report measurements of cross sections for the reaction ¹H(e,e^′K⁺)Y, for both the Λ and Σ⁰ hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q²<2 (GeV/c)². Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ⁰ results have not been previously reported.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

The ¹H(e,e^′K⁺)Λ reaction was studied as a function of the squared four-momentum transfer, Q², and the virtual photon polarization, ɛ. For each of four Q² settings, 0.52, 0.75, 1.00, and 2.00 (GeV/c)², the longitudinal and transverse virtual photon cross sections were extracted in measurements at three virtual photon polarizations. The Q² dependence of the σ_L/σ_T ratio differs significantly from current theoretical predictions. This, combined with the precision of the measurement, implies a need for revision of existing calculations.

The analyzing power A_y0 of the reaction H(p→,d)π⁺ has been measured at a fixed value of the Mandelstam variable u_d=-0.17 GeV² for nine proton energies between 1000 and 1300 MeV. The experiment was performed at SATURNE with the SPES1 spectrometer. The data exhibit structure around sqrt[s]≃2.37 GeV. The origin of this structure could be related to a resonancelike behavior of the ¹S₀P or ¹G₄F partial amplitudes.

Heavy residual nuclei are shown to result from the most violent (i.e., central) collisions for ⁴⁰Ar + Ag reactions of up to 1.36 GeV; their average velocities are ≳80% of the c.m. velocity. Angular and energy distributions for ^1,2,3H, 3,4He, and Li are measured in coincidence with these heavy nuclei. The dominant light particle components are nearly isotropic in a frame of reference having the velocity of the heavy residues. In addition there are forward-peaked high-energy components of the H, He, and Li emission attributable to prethermalization emission. Fractional abundances of these prethermalization components increase markedly with increasing incident energy. Mass and momentum balance preclude the presence of a projectilelike fragment and thus indicate fusionlike reactions with large but incomplete linear momentum transfer. The remainder of the momentum is carried away by the spray of forward-peaked ejectiles. For 1.36 GeV ⁴⁰Ar∼½ of the 900 MeV available is completely thermalized, and ∼½ goes into prethermalization emission after strong collisional mixing.

For central collisions of 1360 MeV ⁴⁰Ar + Ag we report correlations in relative momentum and in velocity difference for ^1,2,3H ejectiles at 32° and 68°. Comparison to trajectory calculations gives a measure of the mean emission times, and, for unlike pairs, the average emission orders. There is a strong variation of the average emission times as a function of ejectile energy that is very similar for each H isotope; they change from ≳1000 fm/c for ∼10 MeV in the c.m. to ≲50 fm/c for ≳30 MeV. This indicates a broad spectrum of emission sources and associated characteristics. The longer times suggest evaporative emission from thermalized systems for ^1,2,3H of energies ∼10 MeV (i.e., those near the emission barrier). The shorter times, along with the observed energy spectra, suggest extensive prethermalization or direct emission from the central collision zone for ^1,2,3H ejectiles of much higher energy.

A previously unexploited experimental observable is used to explore emission times for intermediate mass fragments relative to directly emitted ²H and ³H particles. Small-angle correlations are reported in central collisions for 34A  MeV ⁴⁰Ar+^natAg. High-velocity ³H and ²H particles follow a direct emission scenario with mean lifetime τ∼30-60  fm/c. Fragmentation to Li is characterized by τ∼120 fm/c. Current model calculations suggest a delay time of ∼100-200  fm/c for expansion of the central collision zone prior to the onset of “freeze-out” into fragments. But the observed velocity difference spectra limit the delay time to ≲50 fm/c for expansion between direct emission and fragmentation.

We report on the first measurement of tensor polarization transfer coefficients for the excitation of the 12.7 MeV 1⁺ state of ¹²C by (d→,d→) inelastic scattering at 393 MeV. The experiment has been performed at Saturne with a tensor polarimeter. The data have allowed the determination of the spin-flip probability S₁, found to be in good agreement with the approximate signature S_d^y determined previously with a vector polarimeter. The relationships between measured polarizations and analyzing powers are discussed.

Intermediate mass fragments have been studied from the reaction ⁸⁶Kr+⁶³Cu for ⁸⁶Kr beam energies of 486, 550, 640, and 730 MeV. Average center-of-mass (c.m.) energies are nearly constant with the c.m. angle and vary little with incident energy. Furthermore, the angular distributions are well approximated by 1/sinθ_c.m.. From this and other evidence we conclude that equilibration has occurred prior to fissionlike asymmetric binary breakup of the composite nucleus in the predominant mechanism for IMF production.

Light charged particles from the reaction 640 MeV ⁸⁶Kr+⁶³Cu have been measured in singles and in coincidence with intermediate mass fragments, fissionlike fragments, and other light charged particles. Multiplicities for ¹H and ⁴He in association with the evaporation residues, fragments, and intermediate mass fragments have been determined. Composite nuclei, most of which decay to evaporation residues, are the major sources of evaporative light-charged particle emission. Average multiplicities for prescission ¹H and ⁴He can be associated with the composite nucleus en route to scission; they are relatively large and thus suggest a time scale for fission longer than that for evaporation. The multiplicities for ¹H and ⁴He in association with the intermediate mass fragments indicate that these fragments are usually born with a significant quantity of excitation energy. These multiplicities are used to estimate the primary masses and kinetic energies of the intermediate mass fragments.

Particle-particle correlations are studied for large- and small-angle separations for the reactions 312 and 680 MeV ⁴⁰Ar+^natAg. The spectral shapes and angular ansiotropies allow a characterization of the effective temperatures and spin zones of the hot nuclear emission sources. Small-angle correlations are compared to reaction simulations that employ various parametrizations for the emission time scales. We conclude that the initial lifetime scale for ^1,2,3H particle evaporation is of the order of 10^-22 s. These times are so short as to suggest a breakdown of the concept of ‘‘sequential evaporation,’’ for which a new ansatz is explored.

Large and small angle correlation measurements are reported for ¹-¹H, ⁴-⁴He, and ²-⁴He pairs from 7.8A and 17A MeV ⁴⁰Ar+^natAg. These measurements are interpreted with the aid of trajectory calculations designed to simulate evaporationlike emission of particles and fragments. The most important physical parameter needed to fit the correlation function peaks is the fractional yield of unstable fragments (⁸Be or ⁶Li) compared to independently evaporated particles (⁴He-⁴He) or (²H-⁴He). In this approach, the peaks in the simulated correlation functions contain essentially no information concerning the space-time extent of the emitter source. Nevertheless, these simulations can account for the data very well.

Azimuthal angle correlations have been measured for Li-Li pairs from ⁴⁰Ar + ¹⁹⁷Au, ^natAg, ^natCu, ²⁷Al (17A, 27A, and 34A MeV). Many of these correlations exhibit enhancements at Δcphi of 0° and 180°, the classical pattern for evaporation from a hot, high-spin source. A very different pattern is predicted by a simple multifragmentation model, i.e., a peak at Δcphi≊60°. This peak is driven by the rapid Coulomb explosion of a nonrotating nucleus. The latter pattern is not observed experimentally, however, if collective rotation is included in the multifragmentation model, its predictions are more consistent with the observations. Such comparisons can give a promising test for sequential emission from a rotating source versus instantaneous explosive multifragmentation, but one needs a very good selection of collision centrality to reduce the role of the collective rotation. For most of these data the dominant driving forces seem to be rotational motion perturbed by final-state Coulomb repulsions for time delays of the order of 10^-22 s between successive emissions of Li fragments.

Average transverse momenta have been carefully measured for α-particle pairs at laboratory angles of 21°, 31°, 47°, and 67°. They exhibit small but distinct shifts for different relative azimuthal angles Δcphi. Detection on the same side of the beam (Δcphi≊24°) yields a smaller average tranverse momentum or energy than on the opposite side (Δcphi≊168°). For α-α pairs detected at ≊67° to the beam these small shifts are analyzed with a recoil model to extract the average emitter mass for the first α particle from the coincident pairs. The results are A=130±10 to 115±10 for reactions of ^107,109Ag with ⁴⁰Ar for 7A and 34A MeV, respectively. The particle pairs emitted at 21° are produced from more complex source mixtures. There is evidence from p-Li and α-Li pairs that Li fragments are emitted after protons or α particles in ≊1/2 of the events. The overall pattern is consistent with incomplete fusion leading to the formation of a hot, essentially thermalized composite nucleus and its associated statistical decay.

Two proton correlation functions have been measured at 30 MeV/nucleon for various systems (²⁰Ne+¹²C,²⁷Al,⁵⁹Co,¹⁹⁷Au) in the forward hemisphere. Source radii extracted by comparisons to the final state interaction model of Koonin turn out to be independent of the mean emission angle of the protons as well as of the size of the studied system, and little affected by energy cuts on the detected proton pairs. Typical size (∼5.5–6 fm) exceeds the radius of the compound nucleus for almost all studied systems, suggesting significant time delays between proton emissions. However, similarity of transverse and longitudinal correlation functions seems to eliminate the existence of a long lifetime. Due to the poor sensitivity of the proton-proton correlation function, this method seems not well suited for source size measurements in this energy domain.

In-plane and out-of-plane angular correlations have been measured between fragments of Z>3, Li fragments, ^3,4He, and ^1,2,3H. The changing patterns for ⁴⁰Ar induced reactions of 7A, 17A, 27A, and 34A MeV give an overview of the decreasing importance of mass-symmetric fissionlike reactions at the expense of a broad range of more mass-asymmetric breakups. Evidence is given that these fragments come from a central collision group of reactions that have similar violence and from which many combinations of fragments and particles are ejected. Very similar azimuthal angular correlations are observed for particles with a Li fragment and for particles with a pair of heavier fragments (Z>3). This similarity suggests comparable strengths of association with the reaction plane for single Li fragments and for fragment pairs of Z>3. Azimuthal angular correlations for Li-Li pairs exhibit distinct asymmetries; their interpretation via trajectory-model calculations indicates mean delay times of ≊5×10^-22 s.

Systematic results are presented for multiplicity distributions of ^1,2,3H and ^3,4,6,8He produced in reactions of ^natAg with ⁴⁰Ar beams of 280, 680, 1080, and 1356 MeV. A variety of gating conditions is explored and classified in connection with the more peripheral or the more central collisions. Correlations between H-He multiplicity and Z or angle of projectilelike fragments follow qualitative expectations for deeply inelastic scattering. Pairs of light charged particles or intermediate-mass fragments at side angles are well correlated with the most violent collisions, leading to incomplete fusion. The cross sections for these more central collisions imply entrance-channel orbital angular momenta up to ≊300ħ in the reactions that form very highly excited composite nuclear systems.