Search Results (84 total)

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e→,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

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.

High-precision measurements of the proton elastic form-factor ratio, μ_pG_E^p/G_M^p, have been made at four-momentum transfer, Q², values between 0.2 and 0.5  GeV². The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q² range the deviation from unity is primarily due to G_E^p being smaller than expected.

We investigated simultaneously the ¹²C(e,e^′p) and ¹²C(e,e^′pp) reactions at Q²=2  (GeV/c)², x_B=1.2, and in an (e, e^′p) missing-momentum range from 300 to 600  MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5±2)% of the ¹²C(e,e^′p) events, a recoiling partner proton was observed back-to-back to the ¹²C(e,e^′p) missing-momentum vector, an experimental signature of correlations.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q²=1.0 (GeV/c)² in 10 bins of 1.17≤W≤1.35 GeV across the Δ resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near the physical mass, W=M_Δ≈1.232 GeV, but the variation among models is large for response functions governed by imaginary parts, and for both types of response functions, the variation increases rapidly with W>M_Δ. We performed a multipole analysis that adjusts suitable subsets of ℓ_π≤2 amplitudes with higher partial waves constrained by baseline models. This analysis provides both real and imaginary parts. The fitted multipole amplitudes are nearly model independent—there is very little sensitivity to the choice of baseline model or truncation scheme. By contrast, truncation errors in the traditional Legendre analysis of N→Δ quadrupole ratios are not negligible. Parabolic fits to the W dependence around M_Δ for the multiple analysis gives values for Re(S₁₊/M₁₊)=(-6.61±0.18)% and Re(E₁₊/M₁₊)=(-2.87±0.19)% for the pπ⁰ channel at W=1.232 GeV and Q²=1.0 (GeV/c)² that are distinctly larger than those from the Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S₀₊/M₁₊)=(+7.1±0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe.

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.

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets with ⟨θ_lab⟩≈6.0°. The ⁴He result is A_PV=(+6.40±0.23(stat)±0.12(syst))×10^-6. The hydrogen result is A_PV=(-1.58±0.12(stat)±0.04(syst))×10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s=0.002±0.014±0.007 at ⟨Q²⟩=0.077  GeV², and G_E^s+0.09G_M^s=0.007±0.011±0.006 at ⟨Q²⟩=0.109  GeV², providing new limits on the role of strange quarks in the nucleon charge and magnetization 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.

We present the first measurements of the e→p→epγ cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q² dependence (from 1.5 to 2.3  GeV²) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q². The helicity-independent cross section is also measured at Q²=2.3  GeV². We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

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.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ⁴He at an average scattering angle ⟨θ_lab⟩=5.7° and a four-momentum transfer Q²=0.091  GeV². From these data, for the first time, the strange electric form factor of the nucleon G_E^s can be isolated. The measured asymmetry of A_PV=(6.72±0.84_(stat)±0.21_(syst))×10^-6 yields a value of G_E^s=-0.038±0.042_(stat)±0.010_(syst), consistent with zero.

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 measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W=1.23  GeV at Q²=1.0  (GeV/c)², obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S₁₊/M₁₊)=-(6.84±0.15)% and Re (E₁₊/M₁₊)=-(2.91±0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M₁₊ dominance and ℓ_π≤1 truncation.

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.

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.

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 differential cross sections for the γn→π^-p and the γp→π⁺n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50^° to 110^°. The π^- and π⁺ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π⁺ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π^-p)/dσ/dt(γp→π⁺n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.

We report the results of a new Rosenbluth measurement of the proton electromagnetic form factors at Q² values of 2.64, 3.20, and 4.10  GeV². Cross sections were determined by detecting the recoiling proton, in contrast to previous measurements which detected the scattered electron. Cross sections were determined to 3%, with relative uncertainties below 1%. The ratio μ_pG_E/G_M was determined to 4%–8% and showed μ_pG_E/G_M≈1. These results are consistent with, and much more precise than, previous Rosenbluth extractions. They are inconsistent with recent polarization transfer measurements of similar precision, implying a systematic difference between the techniques.

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.

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.

We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x=0.33, 0.47, and 0.60 and Q²=2.7, 3.5, and 4.8  (GeV∕c)², respectively. These measurements were performed using a 5.7  GeV longitudinally polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x=0.33 are consistent with previous world data and, at the two higher-x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x=0.47 and the value at x=0.60 is significantly positive. These results agree with a next-to-leading-order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (PQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of 2. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ∕F₁ⁿ values based on the quark-parton model. While results for Δu∕u agree well with predictions from various models, results for Δd∕d disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.