Search Results (43 total)

Azimuthal single-spin asymmetries of leptoproduced pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton distribution function is deduced from nonvanishing Sivers effects for π⁺, π⁰, and K^±, as well as in the difference of the π⁺ and π^- cross sections.

Cross sections for the ³He(e,e^′pn)¹H reaction were measured for the first time at energy transfers of 220 and 270 MeV for several momentum transfers ranging from 300 to 450  MeV/c. Cross sections are presented as a function of the momentum of the recoil proton and the momentum transfer. Continuum Faddeev calculations using the Argonne V18 and Bonn-B nucleon-nucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming smaller at higher recoil proton momentum.

The charged pion form factor, F_π(Q²), is an important quantity that can be used to advance our knowledge of hadronic structure. However, the extraction of F_π from data requires a model of the ¹H(e,e^'π⁺)n reaction and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for F_π are presented for Q²=0.60-2.45 GeV². Above Q²=1.5 GeV², the F_π values are systematically below the monopole parametrization that describes the low Q² data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q² regime.

Cross sections for the reaction ¹H(e,e^'π⁺)n were measured in Hall C at Thomas Jefferson National Accelerator Facility (JLab) using the high-intensity Continuous Electron Beam Accelerator Facility (CEBAF) to determine the charged pion form factor. Data were taken for central four-momentum transfers ranging from Q²=0.60 to 2.45 GeV² at an invariant mass of the virtual photon-nucleon system of W=1.95 and 2.22 GeV. The measured cross sections were separated into the four structure functions σ_L,σ_T,σ_LT, and σ_TT. The various parts of the experimental setup and the analysis steps are described in detail, including the calibrations and systematic studies, which were needed to obtain high-precision results. The different types of systematic uncertainties are also discussed. The results for the separated cross sections as a function of the Mandelstam variable t at the different values of Q² are presented. Some global features of the data are discussed, and the data are compared with the results of some model calculations for the reaction ¹H(e,e^'π⁺)n.

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.

The HERMES experiment has measured the transverse polarization of Λ and Λ̅ hyperons produced inclusively in quasireal photoproduction at a positron beam energy of 27.6 GeV. The transverse polarization P_n^Λ of the Λ hyperon is found to be positive while the observed Λ̅ polarization is compatible with zero. The values averaged over the kinematic acceptance of HERMES are P_n^Λ=0.078±0.006(stat)±0.012(syst) and P_n^Λ̅ =-0.025±0.015(stat)±0.018(syst) for Λ and Λ̅ , respectively. The dependences of P_n^Λ and P_n^Λ̅ on the fraction ζ of the beam’s light-cone momentum carried by the hyperon and on the hyperon’s transverse momentum p_T were investigated. The measured Λ polarization rises linearly with p_T and exhibits a different behavior for low and high values of ζ, which approximately correspond to the backward and forward regions in the center-of-mass frame of the γ^*N reaction.

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.

We report on a study of the longitudinal to transverse cross section ratio, R=σ_L/σ_T, at low values of x and Q², as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06<Q²<2.8  GeV². Even at the lowest values of Q², R remains nearly constant and does not disappear with decreasing Q², as might be expected. We find a nearly identical behavior for hydrogen and deuterium.

The first observation of an azimuthal cross section asymmetry with respect to the charge of the incoming lepton beam is reported from a study of hard exclusive electroproduction of real photons. The data have been accumulated by the HERMES experiment at DESY, in which the HERA 27.6 GeV electron or positron beam scattered off an unpolarized hydrogen gas target. The observed asymmetry is attributed to the interference between the Bethe-Heitler process and the deeply virtual Compton scattering (DVCS) process. The interference term is sensitive to DVCS amplitudes, which provide the most direct access to generalized parton distributions.

Precise measurements of the spin structure functions of the proton g₁^p(x,Q²) and deuteron g₁^d(x,Q²) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV²≤Q²≤20  GeV². The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g₁ⁿ is extracted by combining proton and deuteron data. The integrals of g₁^p,d at Q²=5  GeV² are evaluated over the measured x range. Neglecting any possible contribution to the g₁^d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a₀ in a leading-twist next-to-next-to-leading-order analysis.

The transfer of polarization from a high-energy positron to a Λ0 hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be DLL′Λ=0.11±0.10(stat)±0.03(syst) at an average fractional energy carried by the Λ0 hyperon ⟨z⟩=0.45. The dependence of DLL′Λ on both the fractional energy z and the fractional longitudinal momentum xF is presented.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6  GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A_zz^d and the tensor structure function b₁^d for average values of the Bjorken variable 0.01<⟨x⟩<0.45 and of the negative of the squared four-momentum transfer 0.5  GeV²<⟨Q²⟩<5  GeV². The quantities A_zz^d and b₁^d are found to be nonzero. The rise of b₁^d for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.

We studied the reaction ¹²C(e,e^'p) in quasielastic kinematics at momentum transfers between 0.6 and 1.8 (GeV/c)² covering the single-particle region. From this the nuclear transparency factors are extracted using two methods. The results are compared to theoretical predictions obtained using a generalization of Glauber theory described in this paper. Furthermore, the momentum distribution in the region of the 1s-state up to momenta of 300 MeV/c is obtained from the data and compared to the correlated basis function theory and the independent-particle shell model.

A search for an exotic baryon resonance with S=-2, Q=-2 has been performed in quasireal photoproduction on a deuterium target through the decay channel Ξ^-π^-→Λπ^-π^-→pπ^-π^-π^-. No evidence for a previously reported Ξ^--(1860) resonance is found in the Ξ^-π^- invariant mass spectrum. An upper limit for the photoproduction cross section of 2.1 nb is found at the 90% confidence level. The photoproduction cross section for the Ξ⁰(1530) is found to be between 9 and 24 nb.

Polarized deep-inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double-spin asymmetries of cross sections. Inclusive and semi-inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a reanalysis of previously published data. Inclusive and semi-inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti-up, anti-down, and strange sea quarks were computed from these asymmetries in a “leading order” QCD analysis. The polarization of the up-quark is positive and that of the down-quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data.

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (ϕ) and the target spin axis (ϕ_S) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component ⟨sin⁡(ϕ+ϕ_S)⟩_UT^π is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component ⟨sin⁡(ϕ-ϕ_S⟩_UT^π arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

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 have carried out an (e,e^′p) experiment at high momentum transfer and in parallel kinematics to measure the strength of the nuclear spectral function S(k,E) at high nucleon momenta k and large removal energies E. This strength is related to the presence of short-range and tensor correlations, and was known hitherto only indirectly and with considerable uncertainty from the lack of strength in the independent-particle region. This experiment locates by direct measurement the correlated strength predicted by theory.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.

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.

Precision measurements of the relative analyzing powers of five electron beam polarimeters, based on Compton, Møller, and Mott scattering, have been performed using the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). A Wien filter in the 100 keV beam line of the injector was used to vary the electron spin orientation exiting the injector. High statistical precision measurements of the scattering asymmetry as a function of the spin orientation were made with each polarimeter. Since each polarimeter receives beam with the same magnitude of polarization, these asymmetry measurements permit a high statistical precision comparison of the relative analyzing powers of the five polarimeters. This is the first time a precise comparison of the analyzing powers of Compton, Møller, and Mott scattering polarimeters has been made. Statistically significant disagreements among the values of the beam polarization calculated from the asymmetry measurements made with each polarimeter reveal either errors in the values of the analyzing power or failure to correctly include all systematic effects. The measurements reported here represent a first step toward understanding the systematic effects of these electron polarimeters. Such studies are necessary to realize high absolute accuracy (ca. 1%) electron polarization measurements, as required for some parity violation measurements planned at Jefferson Laboratory. Finally, a comparison of the value of the spin orientation exiting the injector that provides maximum longitudinal polarization in each experimental hall leads to an independent and very precise (better than 10^-4) absolute measurement of the final electron beam energy.

The electric form factor of the neutron was determined from measurements of the d→(e→,e^′n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (¹⁵ND₃) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G_Eⁿ=0.0526±0.0033(stat)±0.0026(sys) and 0.0454±0.0054±0.0037 at Q²=0.5 and 1.0  (GeV/c)², respectively.

Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.