Measuring polarized gluon and quark distributions with meson photoproduction

Abstract

We calculate polarization asymmetries in photoproduction of high transverse momentum mesons, focusing on charged pions, considering the direct, fragmentation, and resolved photon processes. The results at very high meson momentum measure the polarized quark distributions and are sensitive to differences among the existing models. The results at moderate meson momentum are sensitive to the polarized gluon distribution and can provide a good way to measure it. Suitable data may come as a by-product of deep inelastic experiments to measure g1 or from dedicated experiments. I. MOTIVATIONIn this article, we will discuss photoproduction of high transverse momentum pions from polarized initial states. There are three motivations for doing so. One is the opportunity to learn the polarization distribution of quarks and gluons in nucleons. We will show that pion photoproduction with polarized initial states has, over a wide kinematic region of moderate transverse momentum pions, a large sensitivity to the polarized gluon distribution functions of the target. Within this wide kinematic region there are broad circumstances where the known polarized quark distributions all give similar pion photoproduction contributions, so that differences among the results are due mainly to the polarized gluon distributions. Hence data where rather ordinary mesons are produced can select among the various models for this quantity.Another motivation, which we have written something about earlier [1], comes from the kinematic region of very high transverse momentum pions where the gluon contributions are small but the differences among the various models for the polarized quark distributions are significant. Hence examining different kinematic regions of pion photoproduction yields information about both polarized quark and polarized gluon distributions.A third motivation, also dependent upon the highest transverse momentum pions, is the possibility of learn-0 * On leave from Kharkov Institute of Physics and Technology, Kharkov, Ukraine.ing something about the pion distribution amplitude. In this region, ratios of cross sections determine the target's quark distributions, but the magnitude of the cross section depends upon the same integral involving the pion distribution amplitude that enters the pion electromagnetic form factor of the π 0 γγ * vertex. Hence if one looks at the unpolarized case, where the target distributions are fairly well known, one has another measure of this integral. (Pion photoproduction in the unpolarized case has been well and successfully studied theoretically and experimentally [2], but not at the highest transverse momenta where the pions will be dominantly produced in a short distance process rather than via fragmentation [1,3,4].)Presently, information on polarized quark distributions comes from deep inelastic electron or muon scattering with polarized beams and targets [5]. Single arm measurements of g 1 give information about a charge-squared weighted combination of polarized quark distributions. Obtaining p...

Double spin asymmetries of inclusive hadron electroproduction from a transversely polarizedHe3target

“…Asymmetries from the 3 He target were observed to be non-zero for π ± production when the target was polarized transversely in the horizontal plane. The π + and π − asymmetries have opposite signs, analogous to the behavior of ALT in semi-inclusive deep-inelastic scattering.…”

“…In this paper, we report a measurement of beamtarget double-spin asymmetries in inclusive chargedhadron production using a longitudinally polarized electron beam scattered from a transversely polarized 3 He target. The measured asymmetry is defined as…”

“…The mechanism of inclusive hadron photoproduction was studied in [3,4]. The production of hadrons arises mainly from four types of processes: fragmentation processes, direct processes, resolved photon processes and soft contributions.…”

“…We report the measurement of beam-target double-spin asymmetries (ALT) in the inclusive production of identified hadrons, e + 3 He ↑ → h + X, using a longitudinally polarized 5.9 GeV electron beam and a transversely polarized 3 He target. Hadrons (π ± , K ± and proton) were detected at 16…”

Soft-gluon resummation for high-pTinclusive-hadron production at COMPASS

“…In the polarized case, very little is known about the photon's parton content. Estimates based on next-to-leading order calculations have shown here [5,6] (see also [7]) that the process remains a good probe of ∆g even in the presence of resolved contributions.…”