Spin dependent structure function g1g_1 at small xx and small Q2Q^2

Theoretical expectations concerning the low $x$ and low $Q^2$ behaviour of $g_1$ are summarized and compared with the recent SMC data.Comment: 4 pages, plain LATEX, 1 eps figure; contribution to 3rd UK Phenomenology Workshop on HERA Physics, St. John's College, Durham, UK, September 199

Regge description of spin-spin asymmetry in photon diffractive dissociation

We explore the possibility whether the gluon helicity distribution $\Delta G(x)$ can be extracted from a comparison of experimental data on the longitudinal spin-spin asymmetry $A_{LL}$ in $\gamma p$ diffractive deep inelastic scattering with calculations performed within the framework of perturbative QCD (pQCD). The data could be obtained at the future HERA collider in scattering of polarized electrons/positrons off polarized protons. In this paper we look for such kinematical regions where contributions to $A_{LL}$ from soft processes (reggeon exchanges) are suppressed to guarantee an applicability of pQCD. It is shown that for the square of the center-of-mass energy $s_{\gamma p} \geq 10^3 GeV^2$, the hadronic diffractive mass $M_X \leq 10 GeV/c^2$, the momentum transferred to the proton $\Delta_T\leq 0.5 GeV/c$, and $Q^2\geq4 (GeV/c)^2$ the longitudinal spin-spin asymmetry due to reggeon exchanges is less than 10^{-4}. This value is presumably lower than the asymmetry which can be measured with modern experimental technique. This means that the pQCD prediction can be reliably compared with data in this kinematical region.Comment: 44 pages, LaTeX, 12 figure

Spin density matrix elements in exclusive ρ0\mathrm{\rho^0} electroproduction on 1H\mathrm{^1H} and 2H\mathrm{^2H} targets at 27.5 GeV beam energy

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive rho^0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1 GeV^2 < Q^2 < 7 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q^2 and t are presented, as well as the longitudinal-to-transverse rho^0 electroproduction cross section ratio as a function of Q^2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.Comment: 41 pages, 20 figures, 16 table; revised version corrects small typos, text clarified on 6 pages, two references added and new data added to figure 2