The Angantyr model for heavy-ion collisions in Pythia8

Abstract

We present a new model for building up complete exclusive hadronic final states in high energy nucleus collisions. It is a direct extrapolation of high energy pp collisions (as described by PYTHIA), and thus bridges a large part of the existing gap between heavy ion and high energy physics phenomenology. The model is inspired by the old Fritiof model and the notion of wounded nucleons. Two essential features are the treatment of multi-parton interactions and diffractive excitation in each N N sub-collision. Diffractive excitation is related to fluctuations in the nucleon partonic sub-structure, and fluctuations in both projectile and target are here included for the first time. The model is able to give a good description of general final-state properties such as multiplicity and transverse momentum distributions, both in pA and AA collisions. The model can therefore serve as a baseline for understanding the non-collective background to observables sensitive to collective behaviour. As PYTHIA does not include a mechanism to reproduce the collective effects seen in pp collisions, such effects are also not reproduced by the present version of Angantyr. Effects of high string density, shown to be able to reproduce e.g. higher strangeness ratios and the ridge in pp, will be added in future studies.

Collective Effects: the viewpoint of HEP MC codes

“…Studies of soft physics have always been central, like MPIs and CR [12]. The Fritiof model for AA collisions [23,24] was a separate offshoot, but recently the related Angantyr pA/AA model is a fully integrated part of Pythia [25]. Many other event generators are built on top of Pythia code, and so are most of the alternative scenarios to be discussed here.…”

The Pythia event generator: Past, present and future

“…The model was quite successful for fixed-target energies, but perturbative parton-parton scatterings were difficult to include, and therefore it could not reliably be used at higher energies. A few years ago heavy-ion activities were started up again, with the Angantyr model [101]. It is based on the conventional MPI model for pp collisions, and allows for a nucleon to undergo successive collisions, where one is of the conventional pp type and the rest of them can be viewed as a Pomeron taken out of the nucleon colliding with a nucleon from the other nucleus.…”

Dipole evolution: perspectives for collectivity and γ*A collisions

“…The method described above can be directly applied to heavy ion collisions as they are modelled in the Angantyr framework for heavy ion collisions in Pythia 8 (see appendix B for a brief review, and refs. [35,36] for a full description). In the Angantyr model, sub-collisions are chosen using a Glauber-like approach.…”

“…cross sections) in cases where little guidance from data exists. In this paper (see section 7) we will also apply the formalism to extract Glauber-Gribov (GG) colour fluctuations in γ * p collisions, in order to take the initial steps towards a generation of electron-ion (eA) collisions within the Angantyr framework [35,36] -a possibility which is foreseen to aid the preparation of an eA program currently being planned [37].…”