Cosmic Particle Showers in PYTHIA

Abstract

The event generator PYTHIA can be used to simulate the hadronic part of cosmic particle showers in the atmosphere. Until now, the collision between particles and atmospheric nuclei has been modeled in a simplified way by the PythiaCascade add-on, approximating the nuclear effects. However, at present it is possible to include the Angantyr module, which constructed a detailed description of the nuclear geometry accounting for fluctuations of the nucleons in nuclei.
The aim of this scientific thesis work is to compare and investigate the differences in the evolution of atmospheric cascades, depending on whether PythiaCascade or Angantyr is used, to analyze the impact of the nuclear effects caused by Angantyr’s geometric modeling. At the end, a short discussion on the muon puzzle is included, which is one of the inconsistencies between our models and reality and describes that an excess of muons is measured on Earth compared to what is predicted in the simulations. Strangeness enhancement, the effect of rope string hadronization, cannot completely explain the muon puzzle, but seems to contribute to the abundance of high-energy muons

Popular Abstract

In the early 20th century, Victor Francis Hess measured the radiation in the atmosphere to investigate the assumption that it would decrease with distance to the ground. He took himself and his measuring equipment on balloon flights up to five kilometers into the sky. These experiments allowed him to discover that the radiation increased considerably the higher he went, contradicting the assumption. This radiation is a stream of incoming cosmic rays, which bombard the atmosphere of the Earth at all times. Some of these particles were found to originate from our Sun, whereas others stem from cosmic acceleration events occurring inside or outside our galaxy, such as supernova explosions of stars. There are also particles with such high energies that there is not even an explanation as to what kind of event might be able to produce them. These particles from space give rise to many questions in the field of particle astrophysics, which aims to explain the extreme phenomena happening in our universe. Research facilities such as the Pierre Auger Observatory in Argentina are used to collect data about the highest-energy particle showers hitting the Earth's surface. The easiest way to imagine such a particle shower is by taking the 'shower'-part literally. When a particle from space hits an atomic nucleus in our atmosphere, many new particles are created, similar to a stream of water drops coming from a shower head. On their way downwards, the particles can decay into more particles, collide with more atmospheric nuclei, or just propagate directly to the surface of the Earth. In the 'shower' context, one could imagine the water droplets splitting up into a spray of fine mist, similar to a cosmic ray shower where the original momentum is split between more and more particles. The cone-like shape from the shower head to the floor is just like the shape of a particle shower.
In a ground-based experiment only the particle shower remnants can be measured, so it is important to have computer simulations based on the Standard Model of particle physics in combination with phenomenological models. In my project, I use an event generator called PYTHIA to simulate how a particle shower evolves, including intermediate particles that were present during the evolution, but decayed or interacted before they could be measured in experiments. Until now, the collision between particles and atmospheric nuclei has been modeled in a simplified way, so my goal is to include a module which models the nuclear geometry in more detail. By comparing and investigating the differences, I can analyze the impact of nuclear effects on the evolution of atmospheric particle showers.
This knowledge is helpful in improving the templates of what to expect, which could then be compared with existing and future experimental data as well as be used to find and investigate inconsistencies between our theoretical models and reality. One inconsistency is called the muon puzzle, as we measure more muons hitting the Earth than we would expect based on our simulations. In my project, I will also include a brief discourse on this, to see whether a more detailed simulation could explain the abundance of muons measured in experiments.

• author: Feuerhahn, Verena Sophia
• supervisor: Leif Lönnblad; Torbjörn Sjöstrand
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Cosmic Particle Showers, Atmospheric Cascades, Angantyr, Pythia
• language: English
• id: 9196696
• date added to LUP: 2025-06-17 08:20:27
• date last changed: 2025-06-17 08:20:27