My field of research is high energy particle physics phenomenology. Since the end of the 1970s I have mainly worked with quark- and gluon dynamics in high energy collisions, for a long time in collaboration with late Bo Andersson. Together with students we developed what became known as "the Lund model" for high energy reactions. A particularly important ingredient was here the string model for hadronization in quark and gluon jets (see refs. 1-3 and the review paper 4). (This model is an important ingredient in the Pythia event generator by Torbjörn Sjöstrand et al.)

Later important results are in particular the dipole formulation of quark and gluon cascades, implemented in the Ariadne MC simulation program by Leif Lönnblad (refs. 5-6) and later the dipole model DIPSY for high energy collisions (refs. 7-8 and subsequent papers). The latter is based on BFKL evolution, including non-leading-log corrections, colour suppressed effects, and saturation.

More recently I have been mainly working on high energy nucleus collisions. In these reactions experiments show long range collective effects, which traditionally have been interpreted as results of a transition to a high temperature plasma phase, causing a high outward pressure. However, recent results from the LHC show similar effects also in proton-proton and proton-nucleus collisions, where they were not expected. In refs. 9-10, we showed that in high multiplicity pp collisions a similar pressure can be expected from strings or colour flux tubes, without assuming a hot thermalized medium.

Together with Leif Lönnblad and PhD students I am now working on generalizing these ideas to nucleus collisions. This work is part of the "CLASH" project, which is a collaboration between theorists and experimentalists, supported by Knut and Alice Wallenbergs foundation. Our aim is to test if the collective effects seen in nucleus collisions also could be obtained without assuming a hot medium. As a first step we have developed a model, called Angantyr (ref. 11), which generalizes the basic features of proton-proton collisions to nucleus collisions. The model is able to reproduce basic features like particle multiplicities, and how they depend on the centrality of the collisions.

As a second step we are now working on including the collective effects from overlapping strings, discussed in refs. 9-10, in the Angantyr model, to study if such a non-thermal model could also describe the collective effects observed in nucleus collisions. If this will turn out to be the case, it would be necessary to find improved observables, which could distinguish between the effects of a hot or a cold medium.

1. A semiclassical model for quark jet fragmentation
B. Andersson, G. Gustafson, C. Peterson, Z.Phys. C1 (1979) 105

2. Semiclassical models for gluonjets and leptoproduction based on the massless relativistic string
B. Andersson, G. Gustafson, Z.Phys. C3 (1980) 223

3. A general model for jet fragmantation
B. Andersson, G. Gustafson, B. Söderberg, Z.Phys. C20 (1983) 317

4. Parton fragmentation and string dynamics
B. Andersson, G. Gustafson, G. Ingelman, T. Sjöstrand, Phys.Rept. 97 (1983) 31-145

5. Dual description of a confined color field
G. Gustafson, Phys.Lett. B175 (1986) 453

6. Dipole formulation of QCD cascades
G. Gustafson, U. Pettersson, Nucl.Phys. B306 (1988) 746

7. Energy concervation and saturation in small-x evolution
E. Avsar, G. Gustafson, L. Lönnblad, JHEP 0507 (2005) 062

8. Small-x dipole evolution beyond the large-N(c) limit
E. Avsar, G. Gustafson, L. Lönnblad, JHEP 0701 (2007) 012

9. Effects of overlapping strings in pp collisions
C. Bierlich, G. Gustafson, L. Lönnblad, A. Tarasov, JHEP 1503 (2015) 148

10. Collectivity without plasma in hadronic collisions
C. Bierlich, G. Gustafson, L. Lönnblad, Phys.Lett. B779 (2018) 58

11. The Angantyr model for heavy-ion collisions in PYTHIA8
C. Bierlich, G. Gustafson, L. Lönnblad, H. Shah, JHEP 1810 (2018) 134