Diffractive and non-diffractive wounded nucleons and final states in pA collisions

We review the state-of-the-art of Glauber-inspired models for estimating the distribution of the number of participating nucleons in pA and AA collisions. We argue that there is room for improvement in these models when it comes to the treatment of diffractive excitation processes, and present a new simple Glauber-like model where these processes are better taken into account. We also suggest a new way of using the number of participating, or wounded, nucleons to extrapolate event characteristics from pp collisions, and hence get an estimate of basic hadronic final-state properties in pA collisions, which may be used to extract possible nuclear effects. The new method is inspired by the Fritiof model, but based on the full, semi-hard multiparton interaction model of Pythia 8.Comment: 50 pages, 13 figure

Development Strategies for Pythia version 7

This document describes the strategies for the development of the Pythia7 program. Both the internal and external structure of the program is discussed. Some comments on relationship to other software is given as well as some comments on coding conventions and other technical details.Comment: 27 pages, 3 eps figure

Pythia version 7-0.0 - a proof-of-concept version

This document describes the first proof-of-concept version of the Pythia7 program. Pythia7 is a complete re-write of the Pythia program in C++. It is mainly intended to be a replacement for the `Lund' family of event generators, but is also a toolkit with a structure suitable for implementing any event generator model. In this document, the structure of the program is presented both from the user and the developer point of view. It is not intended to be a complete manual, but together with the documentation provided in the distribution, it should be sufficient to start working with the program.Comment: 39 pages, 3 figure

PYTHIA 6.3 Physics and Manual

The PYTHIA program can be used to generate high-energy-physics `events', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, beam remnants and underlying events, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest.Comment: 8 + 454 page

Baryon correlations in Pythia

We present the results from our investigation of angular correlations between baryon pairs in the PYTHIA8 event generator. We show how colour reconnection models and hadronization mechanisms influence such angular correlations and in particular address the effect of gluons on the baryon production mechanism in the Lund string fragmentation model. We conclude by discussing the new theoretical ideas in comparison with the ALICE pp collision results for the baryon angular correlations. We propose a hypothesis for suppressing baryons produced in gluon jets and show how that may influence the angular correlations.Comment: 19 pages, 8 figure

A spatially constrained QCD colour reconnection in pp, pA, and AA collisions in the PYTHIA8/Angantyr model

We present an updated version of the QCD-based colour reconnection model in PYTHIA8, where we constrain the range in impact parameter for which reconnections are allowed. In this way, we can introduce more realistic colour reconnections in the Angantyr model for heavy ion collisions, where previously only reconnections within separate nucleon sub-collisions have been allowed. We investigate how the new impact parameter constraint influences final states in pp collisions, and retune parameters of the multi-parton interaction parameters in PYTHIA to compensate so that minimum bias data are reproduced. We also study multiplicity distributions in pA collisions and find that, in order to counteract the loss in multiplicity due to the introduction of global colour reconnections, we need to modify some parameters in the Angantyr model while keeping the parameters tuned to pp fixed. With Angantyr we can then extrapolate to AA collisions without further parameter tuning and retaining a reasonable description of the basic multiplicity distributions.Comment: 26 pages, 20 figures, Updated in accordance with the journal version of the articl

HZTool and Rivet: Toolkit and Framework for the Comparison of Simulated Final States and Data at Colliders

A common problem in particle physics is the requirement to reproduce comparisons between data and theory when the theory is a (general purpose) Monte Carlo simulation and the data are measurements of final state observables in high energy collisions. The complexity of the experiments, the obervables and the models all contribute to making this a highly non-trivial task. We describe an existing library of Fortran routines, HZTool, which enables, for each measurement of interest, a comparable prediction to be produced from any given Monte Carlo generator. The HZTool library is being maintained by CEDAR, with subroutines for various measurements contributed by a number of authors within and outside the CEDAR collaboration. We also describe the outline design and current status of a replacement for HZTool, to be called Rivet (Robust Independent Validation of Experiment and Theory). This will use an object-oriented design, implemented in C++, together with standard interfaces (such as HepMC and AIDA) to make the new framework more flexible and extensible than the Fortran HZTool.Comment: Contribution to CHEP06 conferenc

Merging high energy with soft and collinear logarithms using HEJ and PYTHIA

We present a method to combine the all-order treatment of the High Energy Jets exclusive partonic Monte Carlo (HEJ) with the parton shower of Pythia8, while retaining the logarithmic accuracy of both. This procedure enables the generation of fully realistic and hadronised events with HEJ. Furthermore, the combination of the two allorder treatments leads to improvements in the quality of the description of observables, in particular for regions with disparate transverse scales

Effects of Overlapping Strings in pp Collisions

In models for hadron collisions based on string hadronization, the strings are usually treated as independent, allowing no interaction between the confined colour fields. In studies of nucleus collisions it has been suggested that strings close in space can fuse to form "colour ropes". Such ropes are expected to give more strange particles and baryons, which also has been suggested as a signal for plasma formation. Overlapping strings can also be expected in pp collisions, where usually no phase transition is expected. In particular at the high LHC energies the expected density of strings is quite high. To investigate possible effects of rope formation, we present a model in which strings are allowed to combine into higher multiplets, giving rise to increased production of baryons and strangeness, or recombine into singlet structures and vanish. Also a crude model for strings recombining into junction structures is considered, again giving rise to increased baryon production. The models are implemented in the DIPSY MC event generator, using PYTHIA 8 for hadronization, and comparison to pp minimum bias data, reveals improvement in the description of identified particle spectra.Comment: 48 pages, 22 figure

Total, inelastic and (quasi-)elastic cross sections of high energy pA and γ⋆^{⋆} A reactions with the dipole formalism

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 possible 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