Bo Andersson, an unconventional strong-interaction theorist with close relations to the experimental particle physics community, died unexpectedly from a heart attack on 4 March 2002. He was returning home from the winter school in St. Petersburg, Russia, and collapsed while changing trains in Malmö, Sweden.

Bo was born in Kristianstad, Sweden, on 8 June 1937. He attended Lund University, earning his BSc in 1961, and joined the lively circle of young physicists around professor Gunnar Källén. In 1967, he defended his PhD thesis on singularities in Feynman amplitudes, which he researched under the guidance of Källén. Bo remained affiliated with Lund throughout his career. He became a professor in the theoretical physics department in 1984 and had intended on retiring in June 2002 and remaining as active as ever in research.

During his postdoctoral years, Bo continued formal studies of singularities in Feynman amplitudes, but in the early 1970s, he drastically changed his field of research. Collaborating with the nuclear emulsion experimental group at Lund, Bo worked on understanding heavy-ion reactions and became involved in the preparations for a charm-search experiment at Fermilab. Continuing along the phenomenological road, he and one of us (Gustafson) started up a close-knit collaboration at Lund for the study of strong interactions, work that continued until Bo’s death. The group that was formed fostered some 30 PhD students over the years. The coffee room at Lund was filled with lively discussions among the group’s members, blended with the rich smoke from Bo’s pipe.

Bo’s most famous contribution, in collaboration with Gustafson, was the development of what has become known as the Lund model for high-energy reactions. The model describes these reactions as a two-step process: (1) hard interactions between quarks and gluons in the initial phase of a high-energy collision, in which the effective coupling is small and the emission of gluons can be treated by perturbative calculations, and (2) the subsequent soft hadronization process, in which the coupling becomes large and nonperturbative models are needed. The description of this second step is the hallmark of the Lund model. It assumes that a stringlike confining force field is established between quark–antiquark pairs formed in the first step. Transverse excitations on the string kinks are identified as gluons. Concepts from quantum chromodynamics, such as infrared stability and color coherence, have constituted important guidelines in developing the model.

The way quarks and gluons are ordered along the string influences the color structure of events and provides unique predictions for the energy and particle flow in high-energy collisions. These features were first observed in 1980 by the JADE collaboration in three-jet events produced in electron–positron annihilation at the PETRA machine at the German Electron Synchrotron (DESY) in Hamburg, Germany, and have later been observed in multiparticle production at all kinds of high-energy events.

Color coherence implies that the color of a gluon and its corresponding anticolor in a neighboring gluon radiate softer gluons coherently as a color dipole. In a gluon emission cascade, these color dipoles form chains, which exactly match the strings stretched in the later soft phase of an interaction. This correspondence opens the possibility for a better understanding of the nature of the strong interaction, with a unified description of the two-step process and a smooth transition between the perturbative and non-perturbative regime. Bo was particularly engaged in researching this problem before his death.

The physical and mathematical properties of the string fragmentation model and its connection to the basic quantum chromodynamics field theory were continuous sources of inspiration for Bo. Practical applications such as Bose–Einstein correlations, polarization phenomena, and baryon production went hand in hand with studies of the quantization of the string and other fundamental aspects. He also thought that the understanding of heavy-ion collisions, based on the preceding concepts, was an important topic. Although Bo focused on developing a framework to describe normal collisions, he also proposed and explored the possibility of novel collective effects, giving them imaginative names such as color ropes, firecrackers, and smoke rings.

Starting in 1978, a large set of computer simulation programs have been developed by the Lund group as an essential complement to the model evolution and an important component in the effort to provide a better contact between theory and experiments. These programs with event generators for different processes in high-energy collisions—the Lund Monte Carlos, which includes Jetset, Lepto, Pythia, Fritiof, and Ariadne—have had a significant effect on the experimental community. Today, nearly all high-energy physics experimenters rely on the use of these generators to help understand data.

Bo was a dedicated researcher brimming with new ideas, which he conveyed to others with passion and intensity. Dreading conformity, he enjoyed open discussions that he felt could lead to a better understanding of the underlying physics. In particular, he sought to engage in dialogue with the experimental community to better understand existing data and to hammer out new tests. Bo enjoyed traveling. He was a frequent and appreciated lecturer and discussion partner at workshops and research schools, and spent several sabbatical periods abroad.

Bo’s interest in physics was mixed with far-ranging cultural interests and a genuine concern for fellow beings. In the early 1990s, he provided help to researchers from the decaying Eastern bloc, including a group of young mathematics students from Leningrad (now St. Petersburg) for whom he sought special funding so that they might study in Lund. In recent years, he focused his attention on China and the development of phenomenological research there. (The photo on the preceding page shows him lecturing at Shandong University, Jinan, in 1997.) He enjoyed meeting people and had friends scattered around the world. His death leaves behind a great emptiness.