Colour Screening, Quark Propagation in Nuclear Matter and the Broadening of the Momentum Distribution of Drell-Yan Pairs

We calculate the broadening of the transverse momentum distribution of a quark propagating through nuclear matter. Colour screening plays a fundamental role in that it cuts off quark-nucleon interactions with soft gluons. The mean transverse momentum of the quark acquired along its trajectory, observed via Drell-Yan pairs, is related to it the ratio of the total inelastic meson-nucleon cross section it to the meson mean squared radius. Parameter-free calculations agree with the data.Comment: LaTex file, 7 pages + 2 figs (not included, available by fax) Heidelberg , HD-TVP-93-

Charmonium suppression in p-A collisions

The new high precision data on charmonium production in proton-nucleus collisions by the E866/NuSea collaboration at Fermilab allow - together with older data at lower energies - to fix a unique set of parameters for the standard production and absorption scenario of charmonium in a proton-nucleus reaction. In this scenario the c-cbar pair is formed in an octet state, emits a gluon and continues its radial expansion in a singlet state until it has reached the charmonium radius. In all three phases it can interact with the nuclear environment. We find that the lifetime of the octet state is much shorter than acceptable on physical grounds. This challenges the physical reality of the first phase in the standard scenario.Comment: 8 pages, 10 figure

Color mixing in high-energy hadron collisions

The color mixing of mesons propagating in a nucleus is studied with the help of a color-octet Pomeron partner present in the two-gluon model of the Pomeron. For a simple model with four meson-nucleon channels, color mixings are found to be absent for pointlike mesons and very small for small mesons. These results seem to validate the absorption model with two independent color components used in recent analyses of the nuclear absorption of $J/\psi$ mesons produced in nuclear reactions.Comment: 3 journal-style page

Color-Octet Fraction in J/Psi Production and Absorption

The cross section between a $c \bar c$ pair and a nucleon is small and sensitive to the $c - \bar c$ separation if the pair is in a color-singlet state, but very large and insensitive to the separation if it is in a color-octet state. We use this property in an absorption model involving both color components to deduce the color structure of $c \bar c$ pairs produced in $p(B)A \to \psi X$ reactions. Our analysis shows that the NA3, NA38 and E772 data are not inconsistent with the theoretical picture that color-octet and color-singlet precursors are produced in roughly equal proportions if the produced color-singlet precursors are pointlike and transparent. However, if the color-singlet precursors are not transparent but have a cross section of a few mb, these data do show a definite preference for a larger fraction of color-singlet precursors. In either case, the color-octet fraction increases with $x_F$, approaching unity as $x_F$ becomes large.Comment: 9 pages, updated to include new result

Channel-Coupling Effects in High-Energy Hadron Collisions

The Two-Gluon Model of the Pomeron predicts strongly size-dependent high-energy hadron cross sections. Yet experimental cross sections for radially excited mesons appear surprisingly close in value. The strong coupling of these mesons in hadron collisions also predicted by the model permits a qualitative understanding of this puzzling behavior in terms of eigenmode propagation with a common eigen-$\sigma$. A detailed semiempirical coupled-channel model of the Pomeron is constructed to elucidate this and other features of high-energy hadron cross sections.Comment: 13 pages, latex, no figure