In order to understand the initial partonic state in proton-nucleus and electron-nucleus collisions, we investigate the total, inelastic, and (quasi-)elastic cross sections in pA and gamma-A collisions, as these observables are insensitive to possibl
e collective effects in the final state interactions. We used as a tool the DIPSY dipole model, which is based on BFKL dynamics including non-leading effects, saturation, and colour interference, which we have extended to describe collisions of protons and virtual photons with nuclei. We present results for collisions with O, Cu, and Pb nuclei, and reproduce preliminary data on the pPb inelastic cross section at LHC by CMS and LHCb. The large NN cross section results in pA scattering that scales approximately with the area. The results are compared with conventional Glauber model calculations, and we note that the more subtle dynamical effects are more easily studied in the ratios between the total, inelastic and (quasi-)elastic cross sections. The smaller photon interaction makes the gamma-A collisions more closely proportional to A, and we see here that future electron-ion colliders would be valuable complements to the pA collisions in studies of dynamical effects from correlations, coherence and fluctuations in the initial state in high energy nuclear collisions.
The Sudakov veto algorithm for generating emission and no-emission probabilities in parton showers is revisited and some reweighting techniques are suggested to improve statistics by oversampling in specific cases.
In this paper we describe a formalism for generating exclusive final states in diffractive excitation, based on the optical analogy where diffraction is fully determined by the absorption into inelastic channels. The formalism is based on the Good--W
alker formalism for diffractive excitation, and it is assumed that the virtual parton cascades represent the diffractive eigenstates defined by a definite absorption amplitude. We emphasize that, although diffractive excitation is basically a quantum-mechanical phenomenon with strong interference effects, it is possible to calculate the different interfering components to the amplitude in an event generator, add them and thus calculate the reaction cross section for exclusive diffractive final states. The formalism is implemented in the DIPSY event generator, introducing no tunable parameters beyond what has been determined previously in studies of non-diffractive events. Some early results from DIS and proton-proton collisions are presented, and compared to experimental data.
We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes
using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.
