Vector mesons in nuclear medium with small three momentum, a QCD sum rule approach

Using the QCD Operator Product Expansion, we derive the real part of the transverse and longitudinal vector vector correlation function with the $\rho,\omega$ quantum numbers to leading order in density and in ${\bf q}^2$ at $-\omega^2\to \infty$. To dimension 6, only twist-2 and 4 operators contribute. These OPE, through the energy dispersion relation, provide model independent constraints for the ${\bf q}$ dependence of the vector meson spectral density in nuclear medium. We further make a QCD sum rule type of analysis to extract the momentum dependence of the vector meson dispersion relation in medium. The contributions from twist-2 operators are added up to infinite order to check the validity of the OPE at the relevant Borel window.Comment: 6 pages, talk presented at the workshop "QCD at finite baryon density" Zif, Bielefeld, April 27-30, 1998. Submitted to Nucl. Phys.

New Parton Distribution Functions for the Photon

We present new improved parton distributions for the photon. We fit {\bf all} available data on the photon structure function, $F^{\gamma}_{2}(x,Q^2)$, with $Q^2\ge 3$ GeV$^2$, in order to determine the quark distributions. We also pay particular attention to the gluon distribution in the photon, $g^{\gamma}(x,Q^2)$, which has been poorly constrained in earlier analyses which only include structure function data. We use large $p_T$ jet production in $\gamma \gamma$ collisions from TRISTAN to constrain $g^\gamma$. We also see what information can be gleaned from $\gamma p$ collisions at HERA on $g^{\gamma}$ and on the quark distributions at large $x$, where there are no structure function data. We review future prospects of elucidating the parton distributions of the photon.Comment: 33 pages, 8 figures, uses eps