Is charm the key to understanding diffraction in DIS ?

This talk concerns the production of open charm in diffractive deep inelastic scattering. This has been calculated recently in the context of the semi-classical approach to diffraction. A comparison is made to approaches in which the diffractive exchange is modelled by the exchange of two gluons in the $t$-channel. Two phenomenological test of the underlying partonic process are discussed.Comment: 6 pages, latex, 5 figures. Talk given at the diffractive working group of DIS '97, Chicago Il. The required AIP style files are provide

A fresh look at diffractive J/ψJ/\psi photoproduction at HERA, with predictions for THERA

We quantify perturbative and non-perturbative QCD effects in the exclusive $J/\psi$-photoproduction cross section, and in the shrinkage of the differential cross section with respect to momentum transfer, $t$. We predict that in the high energy THERA region there will always be a significant contribution to this process that rises quickly with energy. This implies that the taming of the rise of the cross section with energy, due to both the expansion of spatially-small fluctuations in the photon and to higher twist effects, is rather gradual.Comment: Published version, 29 pages, 16 figures, uses JHEP.cls. Substantially rewritten to better emphasize the generality of the results in response to the referee's comments. Predictions for MRST LO partons added, calculations and discussion of the real part of the amplitude and of alpha prime improved. Five of the original figures modified. Two new plots, of the dipole cross section for two different values of parameter lambda, and of energy dependence of alpha prime, added. Three additional references include

Revealing the black-body regime of small-x deep-inelastic scattering through final-state signals

We derive the major characteristics of inclusive and diffractive final states in deep-inelastic scattering off heavy nuclei for the-high energy (small-x) kinematics in which the limit of complete absorption is reached for the dominant hadronic fluctuations in the virtual photon (the black-body limit of the process). Both the longitudinal and transverse distributions of the leading hadrons are found to be strikingly different from the corresponding ones within the leading-twist approximation, and hence provide unambiguous signals for the onset of the black-body limit.Comment: 12 pages, 1 figure, final version published in PR

A data collection scheme for identification of parameters in a driver model

A high gain steering controller to compensate for limitations in a handicapped driver's range of motion is employed when adapting vehicle to his use. A driver/vehicle system can become unstable as vehicle speed is increased, therefore it is desirable to use a computer simulation of the driver/vehicle combination as a design tool to investigate the system response prior to construction of a controller and road testing. Unknown driver parameters must be identified prior to use of the model for system analysis. A means to collect the data necessary for identification of these driver model parameters without extensive instrumentation of a vehicle to measure and record vehicle states is addressed. Initial tests of the procedure identified all of the driver parameters with errors of 6% or less

Unitarity and the QCD-improved dipole picture

As a consequence of QCD factorization theorems, a wide variety of inclusive and exclusive cross sections may be formulated in terms of a universal colour dipole cross section at small $x$. It is well known that for small transverse size dipoles this cross section is related to the leading-log gluon density. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transverse size region. We point out that the observed magnitude and small $x$ rise of the gluon density from conventional fits implies that the DGLAP approximation has a restricted region of applicability. We found that `higher twist' or unitarity corrections are required in, or close to, the HERA kinematic region, even for small `perturbative' dipoles for scattering at central impact parameters. This means that the usual perturbative leading twist description, for moderate virtualities, $1 < Q^2 < 10$ GeV$^2$, has rather large `higher twist' corrections at small $x$. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles (b \gsim 0.4 fm) to $F_2$, and also to $F_L$. This also leads to deviations from the standard leading twist DGLAP results, at small $x$ and moderate $Q^2$. Our model also describes the low $Q^2$ data very well without any further tuning. We generalize the Gribov unitarity limit for the structure functions of a hadron target to account for the blackening of the interaction at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.Comment: Final version, 38 pages, 16 figures, 1 table. A successful comparison to all low Q^2 HERA data is included. The discussion has been completely rewritten and extended to include both a detailed comparison with other models for the dipole cross section and also a new section on the approach to the black limit in DIS, including various new predictions. 23 new references have been added and several figures change