C.F. Perdrisat

Cross sections and transverse single-spin asymmetries in forward neutral-pion production from proton collisions at sqrt[s]=200 GeV.

Abstract Measurements of the production of forward jets from transversely polarized proton collisions at s = 500 xa0GeV conducted at the Relativistic Heavy Ion Collider (RHIC) are reported. Our measured jet cross section is consistent with hard scattering expectations. Our measured analyzing power for forward jet production is small and positive, and provides constraints on the Sivers functions that are related to partonic orbital angular momentum through theoretical models.

Polarization transfer in the 4He(e-->,e'p-->)3H reaction up to Q2=2.6 (GeV/c)2.

We have measured the proton recoil polarization in the 4He(e-->,e()p-->)4H reaction at Q(2)=0.5, 1.0, 1.6, and 2.6 (GeV/c)(2). The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.

Measurement of GEp/GMp in ep → ep to Q2 = 5.6 GeV2

The ratio of the electric and magnetic form factors of the proton G(Ep)/G(Mp), which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic (e) over right arrowp --> e (p) over right arrow reaction. The new data presented span the range 3.5 < Q(2) < 5.6 GeV2 and are well described by a linear Q(2) fit. Also, the ratio rootQ(2) F-2p/F-1p reaches a constant value above Q(2) = 2 GeV2.The ratio of the electric and magnetic form factors of the proton G(E(p))/G(M(p)), which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e(-->)p---> e(-->)p reaction. The new data presented span the range 3.5< Q(2)< 5.6 GeV(2) and are well described by a linear Q(2) fit. Also, the ratio sqrt[Q(2)] F(2(p))/F(1(p)) reaches a constant value above Q(2) = 2 GeV(2).

Two-body pion production d(p, π+)t AT 470 and 590 MeV

Abstract The differential cross section for the reaction d(p, π + )t has been measured for c.m. angles between 37° and 160°, at incident proton energies of 470 and 590 MeV. The results are in qualitative agreement with the predictions of a two-nucleon model of the reaction when the D-state contribution is included, except for the existence of a backward peak at 470 MeV. Other models of the reaction are discussed; none of them gives a rationale for the backward peak at 470 MeV. A similar peak is present in older data at 325 MeV.

Induced Proton Polarization for pi0 Electroproduction at Q2 = 0.126 GeV2/c2 Around the Delta(1232) Resonance

We present a measurement of the induced proton polarization P{sub n} in {pi}{sup 0} electroproduction on the proton around the {Delta} resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W = 1231 MeV and Q{sup 2} = 0.126 GeV{sup 2}/c{sup 2}, respectively. We measured a large induced polarization, P{sub n} = -0.397 {+-} 0.055 {+-} 0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

The 6Li(α, 2α)2H reaction at 700 MeV and the α-d momentum distribution

Abstract The results of an experimental study of the reaction 6Li(α, 2α)2H at 700 MeV are given. A comparison with the data from seven other reactions pertaining to the α+d cluster structure of 6Li is presented. The amplitude for the virtual transition 6Li→α+d obtained three of these experiments and the present one are comparable. The internal momentum distributions extracted with the plane-wave impulse approximation in four of the experiments are compatible with a pole form, the width parameter being given by the α-separation energy in 6Li. The remarkable consistency among these results is seen as evidence for the validity of the simple models used for both the reaction mechanism (PWIA) and to describe the α d-vertex (pole approximation) in 6Li.

Quasielastic He-3(e,e(')p)H-2 reaction at Q(2)=1.5 GeV2 for recoil momenta up to 1 GeV/c

We have studied the quasielastic He-3(e,e()p)H-2 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 He-3(e,e()p)H-2 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 He-3 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.

Q^2 Dependence of the Neutron Spin Structure Function g~2^n at Low Q^2

We present the first measurement of the Q2 dependence of the neutron spin structure function g2(n) at five kinematic points covering 0.57 (GeV/c)2 < or = Q2 < or = 1.34 (GeV/c)2 at x approximately = 0.2. Though the naive quark-parton model predicts g2 = 0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g2(n) can be predicted using next-to-leading order fits to world data for g1(n). Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q2, indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g1(n) are consistent with next-to-leading order fits to world data.

180° production of protons in high-energy p, d, and α reactions in nuclei

Abstract Measurement of the differential cross sections for the 180° production of protons from 6 Li, C, and Ta by protons (600 MeV), duetrons (361 MeV), and α-particles (726 MeV) have been carried out. The results are shown to be in accord with “quasi-two body scaling”. The data follow a structure function distribution G(k)≈ e − k k 0 up to internal nucleon momenta, k = 1.3 GeV/ c . The measurements in 6 Li extend the validity of this functional form up to values of k close to the elastic scattering limit.

Differential cross section and analyzing power for backward pions in 2H(p, π+)3H☆

Abstract Cross sections and analyzing powers are presented for the reaction 2 H ( p , π + ) 3 H at 400, 425, 443 and 470 MeV, and laboratory pion angles of 120°, 130°, 140°, 150° and 160°. The cross sections have flat angular distributions, are in contradiction with an earlier experiment which indicated a backward peak at 470 MeV, but are in quantitative agreement with a calculation based on the two-nucleon model.