Double-Polarization Observables in Pion-Photoproduction from Polarized HD at Legs (original) (raw)

Review of Polarization Observables in Incoherent Pion Photoproduction on the Deuteron

Polarization observables in incoherent pion photoproduction from the deuteron are investigated in the energy region from π-threshold up to the ∆ (1232)-resonance with inclusion of all leading πNN effects. Formal expressions for polarization observables are derived and described by various beam, target and beam-target asymmetries for polarized photons and/or polarized deuterons. For the elementary pion photoproduction operator on the free nucleon, a realistic effective Lagrangian approach is used which includes seven nucleon resonances, in addition to Born and vector-meson exchange terms. The interactions in the final two-body subsystems are taken from separable representations of realistic potentials. Results are given for the unpolarized cross sections, the doubly polarized cross sections for parallel and antiparallel helicity states, the linear photon asymmetry, the double polarization E-asymmetry and, the vector and tensor deuteron asymmetries for the γd → π − pp, γd → π + nn, and γd → π 0 np channels. The contributions to the spin asymmetry and the Gerasimov-Drell-Hearn (GDH) integral from separate channels are evaluated by explicit integration up to a photon lab-energy of 350 MeV. Effects of final-state interaction are investigated and their role in these observables are found to be significant, specially for π 0 production. The extracted results are compared with available experimental data and predictions of other works, and a satisfactory agreement is obtained. The sensitivity of the γd → πNN results to the elementary γN → πN operator is also investigated and a considerable dependence is found. This indicates that it can serve as a filter for different elementary operators. We expect that the results presented here may be useful to interpret the recent measurements from the high-intensity and high duty-factor electron accelerators MAMI, ELSA, Jefferson Lab, LEGS, and MAX-Lab.

Polarized parton distributions of the nucleon

Nuclear Physics B, 1990

Spin-dependent (polarized) parton distributions are presented satisfying the available experimental and theoretical constraints. In particular we present radiatively generated predictions for AG(x, Q)2 compatible with recent data, and for the gluon asymmetry AG(x, Q2)/G(x, Q2). It is furthermore shown that the data on gP(x, Q~) do not provide unique information on AG(x, Q~) in contrast to its unambiguous first moment .0 AG(x, Qii)dx. In addition, a prediction for the structure function g~(x, Qo ~-10 GeV 2) is given for the case of lowest twist dominance. Present measurements [1] of the spin-dependent (polarized) structure function gP(x, Q2) at Qz = Q0-10 GeV 2 suggest a large negative polarization for the sea quarks [2, 3] or alternatively [4, 5] a large positive polarization for the gluons. Up to now most attention has been paid to the first moments of the polarized parton distributions. The detailed x-dependencies implied by the data were recently discussed in refs. [6, 7] within the framework of a specific non-perturbative model [6] or using some mathematically consistent but otherwise unmotivated parametrizations [7]. As will be shown, the results of ref. [6] have some shortcomings in addition to their dependence on somewhat unreliable non-perturbative assumptions and it seems therefore appropriate to perform an unbiased study of the polarized parton distributions. We shall start with the scenario of refs. [2, 3] which is essentially a straightforward leading-order approach implying 2gP(x, Q2)=~Auv(x, QZ)+ ~Adv(x, Q2)+~AFl(x, Q2), (1) where we have taken, for simplicity and with no significant loss of accuracy, an SU(3)-flavor symmetric polarized sea AFt-Ah = Ad= A~ = As and where Au v and Ad v are the corresponding polarized valence distributions. Our notation here is as follows: Aq(x, Q2) _= q+ (x, Q2) _ q_(x, Q2), (2) Aq(Q2) =_ flAq(x ' Q2) dx, (3) at?

Incoherent pion photoproduction on the deuteron including polarization effects

Annals of Physics, 2011

Azimuthal spin asymmetries of pion electroproduction

Physical Review D, 2002

Azimuthal spin asymmetries, both for charged and neutral pion production in semiinclusive deep inelastic scattering of unpolarized charged lepton beams on longitudinally and transversely polarized nucleon targets, are analyzed and calculated. Various assumptions and approximations in the quark distributions and fragmentation functions often used in these calculations are studied in detail. It is found that different approaches to the distribution and fragmentation functions may lead to quite different predictions on the azimuthal asymmetries measured in the HERMES experiments, thus their effects should be taken into account before using the available data as a measurement of quark transversity distributions. It is also found that the unfavored quark to pion fragmentation functions must be taken into account for π − production from a proton target, although they can be neglected for π + and π 0 production. Pion production from a proton target is suitable to study the u quark transversity distribution, whereas a combination of pion production from both proton and neutron targets can measure the flavor structure of quark transversity distributions.

Double-Polarization Observables in Pion-Photoproduction from Polarized HD at Legs (original) (raw)

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