Search Results (32 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.

The strength of the neutron-neutron correlation function from the E=45A MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions depends on the neutron parallel velocity. This indicates the presence of multiple sources of neutron emission. We find these sources consistent with a dissipative, binary reaction mechanism as it is described by, e.g., Boltzmann-Uehling-Uhlenbeck calculations.

Small angle neutron-neutron correlations have been measured for the E/A=45 MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions. Two-neutron correlation functions, both integrated and gated on the total momentum of the neutron pair, have been constructed. In order to explain these data, a fraction of fast “dynamical” emission is needed in addition to slower evaporative emission. The overall emission time scale is shorter for the symmetric system, indicating that the dynamical component is stronger in this case.

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 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.

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.

Two-neutron correlations at small relative momentum in ²⁰Ne+C and ²⁰Ne+Co reactions at 30A MeV exhibit correlation functions dominated by the attractive final-state interaction. The results are consistent with two-proton data. The data suggest large apparent emission sources for both reactions with contributions from both pre-equilibrium emission and evaporation.

Particle-particle coincidence measurements have been made for the reactions 312 and 680 MeV ⁴⁰Ar+^natAg. The large-angle correlation data indicate essentially complete energy thermalization even for 680 Mev ⁴⁰Ar+^natAg. They also allow for an assignment of ≊64ħ for the rms spin and ≊4.3 MeV for the average initial temperature of the composite nuclei formed by incomplete fusion. The small-angle correlation data for ¹-¹H and ²-²H are consistent with phase-space, statistical-model predictions of the evaporation lifetimes. This result implies that energy mixing and evaporation particle emission steps occur with intervals of ≊10^-22 s, a time period comparable to the traversal time for a projectile velocity of 17 MeV/nucleon.

A 4π charged-particle multidetector has been used to study the reaction ⁴⁰Ar+^natAg from 280–1356 MeV. Charged-particle multiplicity distributions show a low-multiplicity group associated with peripheral collisions and a high-multiplicity group associated with central collisions. Average multiplicities for central collisions increase with increasing projectile energy, indicating ever-increasing collision violence. Angular distributions of emitted protons are essentially isotropic for θ≥80° in a reference frame characterized by the empirical systematics of linear momentum transfer (i.e., ≊100% to ≊70% from 7–34 MeV/nucleon). Spectra of these protons at side angles are evaporationlike in shape and indicate relative effective temperatures of 3, 6, 8, and 12 MeV for beam energies of 7, 17, 27, and 34A MeV, respectively. Azimuthal angular correlations between various particle pairs are consistent with spin-driven emission from emitter sources of reasonable spin values. In short, these results support a classical picture of extensively thermalized emitter nuclei even for initial excitation energies of ≊5 MeV per system nucleon and spins of ≥100ħ.

The associated phenomena of preequilibrium emission and incomplete fusion in heavy ion collisions at low impact parameter are studied in the context of inverse kinematics and of a symmetrical system. The ⁴⁰Ar+¹³C, ²⁴Mg, and ⁴⁵Sc reactions performed at E_? MeV/nucleon are discussed. Heavy fragments and light particles have been detected inclusively and exclusively. Salient features of preequilibrium and incomplete fusion are presented and discussed in the framework of theoretical models. Characteristics of nuclear species formed in incomplete fusion are studied and possible limits to fusion are examined. Various possible origins of preequilibrium emission are suggested. The ⁴⁰Ar+¹³C reaction appears to be a particular case as concerns the preequilibrium emission.