Search Results (35 total)

We report a high statistics measurement of Υ production with an 800  GeV/c proton beam on hydrogen and deuterium targets. The dominance of the gluon-gluon fusion process for Υ production at this energy implies that the cross section ratio, σ(p+d→Υ)/2σ(p+p→Υ), is sensitive to the gluon content in the neutron relative to that in the proton. Over the kinematic region 0<x_F<0.6, this ratio is found to be consistent with unity, in striking contrast to the behavior of the Drell-Yan cross section ratio σ(p+d)_DY/2σ(p+p)_DY. This result shows that the gluon distributions in the proton and neutron are very similar. The Υ production cross sections are also compared with the p+d and p+Cu cross sections from earlier measurements.

Kaon electroproduction from light nuclei and hydrogen, using ¹H, ²H, ³He, ⁴He, and carbon targets has been measured at the Thomas Jefferson National Accelerator Facility. The quasifree angular distributions of Λ and Σ hyperons were determined at Q²=0.35 (GeV/c)² and W=1.91 GeV. Electroproduction on hydrogen was measured at the same kinematics for reference.

We report a measurement of the angular distributions of Drell-Yan dimuons produced using an 800  GeV/c proton beam on a deuterium target. The muon angular distributions in the dilepton rest frame have been measured over the kinematic range 4.5<m_μμ<15  GeV/c², 0<p_T<4  GeV/c, and 0<x_F<0.8. No significant cos⁡2ϕ dependence is found in these proton-induced Drell-Yan data, in contrast with the situation for pion-induced Drell-Yan data. The data are compared with expectations from models which attribute the cos⁡2ϕ distribution to a QCD vacuum effect or to the presence of the transverse-momentum-dependent Boer-Mulders structure function h₁^⊥. Constraints on the magnitude of the sea-quark h₁^⊥ structure functions are obtained.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

The _Λ^3,4H and _Λ⁴H hypernuclear bound states have been observed for the first time in kaon electroproduction on ^3,4He targets. The production cross sections have been determined at Q²=0.35   GeV² and W=1.91   GeV. For either hypernucleus the nuclear form factor is determined by comparing the angular distribution of the ^3,4He(e,e^′K⁺)_Λ^3,4H processes to the elementary cross section ¹H(e,éK⁺)Λ on the free proton, measured during the same experiment.

We report on precision measurements of the elastic cross section for electron-proton scattering performed in Hall C at Jefferson Lab. The measurements were made at 28 distinct kinematic settings covering a range in momentum transfer of 0.4<Q²<5.5  (GeV∕c)². These measurements represent a significant contribution to the world’s cross section data set in the Q² range, where a large discrepancy currently exists between the ratio of electric to magnetic proton form factors extracted from previous cross section measurements and that recently measured via polarization transfer in Hall A at Jefferson Lab. This data set shows good agreement with previous cross section measurements, indicating that if a heretofore unknown systematic error does exist in the cross section measurements, then it is intrinsic to all such measurements.

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 present measurements of the polarization of the J/ψ produced in 800-GeV proton interactions with a copper target. Polarization of the J/ψ is sensitive to the cc̅ production and hadronization processes. A longitudinal polarization is observed at large x_F, while at small x_F the state is produced essentially unpolarized or slightly transversely polarized. No significant variation of the polarization is observed versus p_T.

The quasielastic (e,e^′p) reaction was studied on targets of deuterium, carbon, and iron up to a value of momentum transfer Q² of 8.1 (GeV/c)². A nuclear transparency was determined by comparing the data to calculations in the plane-wave impulse approximation. The dependence of the nuclear transparency on Q² and the mass number A was investigated in a search for the onset of the color transparency phenomenon. We find no evidence for the onset of color transparency within our range of Q². A fit to the world’s nuclear transparency data reflects the energy dependence of the free-proton–nucleon cross section.

The coherent ³He(e,e^′π⁺)³H reaction was measured at Q²=0.4 (GeV/c)² and W=1.6 GeV for two values of the virtual photon polarization, ε, allowing the separation of longitudinal and transverse cross sections. The results from the coherent process on ³He were compared to H(e,e^′π⁺)n data taken at the same kinematics. This marks the first direct comparison of these processes. At these kinematics (p_π=1.1 GeV/c), pion rescattering from the spectator nucleons in the ³He(e,e^′π⁺)³H process is expected to be small, simplifying the comparison to π⁺ production from the free proton.

Separated longitudinal and transverse cross sections for charged pion electroproduction from ¹H, ²H, and ³He were measured at Q²  =  0.4 (GeV/c)² for two values of the invariant mass, W  =  1.15 GeV and W  =  1.60 GeV, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in ³He at W  =  1.60 GeV. The W  =  1.15 GeV results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production.

The first measurements of the d(γ,p)n differential cross section at forward angles and photon energies above 4 GeV were performed at the Thomas Jefferson National Accelerator Facility (JLab). The results indicate evidence of an angular dependent scaling threshold. Results at θ_cm  =  37° are consistent with the constituent counting rules for E_γ≳4 GeV, while those at 70° are consistent with the constituent counting rules for E_γ≳1.5 GeV.

Measurements of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360 000 Drell-Yan muon pairs remained after all cuts on the data. From these data, the ratio of down antiquark (d̅ ) to up (ū) antiquark distributions in the proton sea is determined over a wide range in Bjorken-x. These results confirm previous measurements by E866 and extend them to lower x. From these data, (d̅ -ū) and ∫(d̅ -ū)dx are evaluated for 0.015<x<0.35. These results are compared with parametrizations of various parton distribution functions, models and experimental results from NA51, NMC and HERMES.

We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q²=0.1 (GeV/c)² and large scattering angles. This quantity must vanish in the single virtual photon exchange, plane-wave impulse approximation for this reaction, and can therefore provide information on two photon exchange amplitudes for electromagnetic interactions with hadronic systems. The observable we have measured is driven by the imaginary part of the two photon exchange amplitude, the hadronic side of which is simply the Compton amplitude for the proton with two virtual photons. We find a small but nonzero value of A=-15.4±5.4 ppm.

We present a measurement of the polarization observed for bottomonium states produced in p-Cu collisions at sqrt[s]  =  38.8  GeV. The angular distribution of the decay dimuons of the ϒ(1S) state shows no polarization at small values of the fractional longitudinal momentum x_F and transverse momentum p_T but significant positive transverse production polarization for either p_T>1.8  GeV/c or for x_F>0.35. The ϒ(2S+3S) (unresolved) states show a large transverse production polarization at all values of x_F and p_T measured. These observations challenge NRQCD calculations of the polarization expected in the hadronic production of bottomonium states.

Separated longitudinal and transverse structure functions for the reaction ¹H(e,e^′π⁺)n were measured in the momentum transfer region Q²  =  0.6–1.6 (GeV/c)² at a value of the invariant mass W  =  1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q² elastic data.

Measurements of the suppression of the yield per nucleon of J/ψ and ψ^′ production for 800 GeV/ c protons incident on heavy nuclear targets, relative to light nuclear targets, have been made with very broad coverage in x_F and p_T. The observed suppression is smallest at x_F values of 0.25 and below, and increases at larger values of x_F. It is also strongest at small p_T. Substantial differences between ψ^′ and J/ψ production are observed for the first time in p-A collisions. The suppression for ψ^′ production is stronger than that for J/ψ for x_F near zero, but becomes comparable to that for J/ψ for x_F>0.6.

We report a new measurement of the parity-violating asymmetry in elastic electron scattering from the proton at backward scattering angles. This asymmetry is sensitive to the strange magnetic form factor of the proton as well as electroweak axial radiative corrections. The new measurement of A  =  -4.92±0.61±0.73 ppm provides a significant constraint on these quantities. The implications for the strange magnetic form factor are discussed in the context of theoretical estimates for the axial corrections.

A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe, and W targets is reported. The behavior of the Drell-Yan ratios at small target-parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross-section ratios as a function of the incident-parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus.

The Drell-Yan cross section ratios, σ(p+d)/σ(p+p), measured in Fermilab E866, have led to the first determination of d̅ (x)/ū(x), d̅ (x)-ū(x), and the integral of d̅ (x)-ū(x) for the proton over the range 0.02<~x<~0.345. The E866 results are compared with predictions based on parton distribution functions and various theoretical models. The relationship between the E866 results and the NMC measurement of the Gottfried integral is discussed. The agreement between the E866 results and models employing virtual mesons indicates that these non-perturbative processes play an important role in the origin of the d̅ ,ū asymmetry in the nucleon sea.

A precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on hydrogen and deuterium targets is reported. Over 140 000 Drell-Yan muon pairs with dimuon mass M_μ⁺μ^-≥4.5 GeV/c² were recorded. From these data, the ratio of antidown ( d̅ ) to antiup ( u̅ ) quark distributions in the proton sea is determined over a wide range in Bjorken x. A strong x dependence is observed in the ratio d̅ /u̅ , showing substantial enhancement of d̅ with respect to u̅ for x<0.2. This result is in fair agreement with recent parton distribution parametrizations of the sea. For x>0.2, the observed d̅ /u̅ ratio is much nearer unity than given by the parametrizations.

The projections of surface reconstructions are normally solved from the interatomic vectors found in two-dimensional Patterson maps computed with the intensities of the in-plane superstructure reflections. Since for difficult reconstructions this procedure is not trivial, an alternative automated one based on the “direct methods” sum function [Rius, Miravitlles, and Allmann, Acta Crystallogr. A52, 634 (1996)] is shown. It has been applied successfully to the known c(4×2) reconstruction of Ge(001) and to the so-far unresolved In_0.04Ga_0.96As (001) p(4×2) surface reconstruction. For this last system we propose a modification of one of the models previously proposed for GaAs(001) whose characteristic feature is the presence of dimers along the fourfold direction.

We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value G_M^Z  =  0.34±0.09±0.04±0.05 n.m. at Q²  =  0.1 (GeV/c)².

The inclusive A(e,e^′) cross section for x≃1 was measured on ²H, C, Fe, and Au for momentum transfers Q² from 1 to 6.8 (GeV/c)². The scaling behavior of the data was examined in the region of transition from y scaling to x scaling. Throughout this transitional region, the data exhibit ξ scaling, reminiscent of the Bloom-Gilman duality seen in free nucleon scattering. © 1996 The American Physical Society.