Interaction Processes and Statistical Properties of the Propagation of Cosmic Rays in Photon Backgrounds

Preface 2019 (Instead of an Abstract)

On this day, 23 years ago, I received my PhD in astronomy based on this thesis. By a time when the term ‘astroparticle physics’ didn’t even exist, the purpose of this work was to lay down the fundamentals of the propagation of very- and ultra-high energy cosmic rays in photon environments as they are found in extragalactic space (e.g., the cosmic microwave background), but also in the vicinity of potential sources of such cosmic rays. It contains a (by this time) state-of-the-art treatment of photohadronic and photonuclear interactions, plus an attempt to find a simplified description of the stochastic processes governing the propagation of each individual cosmic ray. The idea behind this was to extend the simple continuous differential-equation approach common at these times to a continuous description of moments describing probability distributions, essentially an approximate solution to the Boltzmann-equation governing cosmic ray transport.

From today's perspective, I think it is fair to say that my thesis work never reached its main goal, (a) because the intended mathematical description has never been developed to a stage that it became usable, and (b) because the rapid development in computer power has rendered any mathematical approximation to stochastic processes obsolete – nowadays, as anything else, cosmic ray transport is treated in Monte-Carlo simulations. In that respect, immersion into statistics had essentially auto-didactic value, or, as Neil Armstrong didn’t say 50 years ago: a big leap for me, a small step for mankind. For the latter, a much bigger step this thesis had contributed to was that its detailed results on the interaction physics of cosmic ray protons and nuclei have strongly influenced and partly even triggered the development of todays leading Monte-Carlo codes for cosmic-ray interactions and propagation, i.e., SOPHIA [Mücke et al., CoPhC 124 p.290, 2000] and, based on this, CRPropa [Alves Batista et al., JCAP 05 (2016) 038, and references therein].

The reason for making it now, 23 years after its original small-batch publication, part of the Zenodo “Astronomy Thesis Collection” is (i) to honor the special meaning of the number 23 [Rachen & Gahlings, arXiv:1303.7476], (ii) because I feel that it had enough impact on science to belong there, and (iii) because it still contains some interesting approximate treatments of problems where Monte-Carlo methods haven’t gone yet, like photodisintegration of cosmic ray nuclei during Fermi-acceleration in a non-thermal photon-background. I therefore hope that, opened now to a wider audience, it may still catch the interest of the one or other working in astroparticle physics, and if it is only to check out how things were done in the old times.

Jörg Rachen, on September 11, 2019