PhD Student Pascal Gutjahr finds evidence for prompt muons and completes his PhD with summa cum laude

We congratulate Pascal Gutjahr on successfully completing his PhD at the Chair of Astroparticle Physics. For his dissertation, entitled “Chasing Prompt Muons: Unfolding the Atmospheric Muon Spectrum to Constrain the Prompt Flux using 12 Years of IceCube Data”, he received the highest possible grade, summa cum laude.

In his thesis, he studied atmospheric muons produced in cosmic-ray air showers. When high-energy cosmic rays from space hit the Earth’s atmosphere, large numbers of secondary particles are created, forming air showers. Some of these particles decay into muons, which can be categorized as conventional or prompt depending on the parent particle from whose decay they originate.

Conventional muons are created in the decay of charged pions and kaons, whose comparatively long lifetimes make them more likely to interact with the atmosphere before decaying. Prompt muons, by contrast, originate mainly from the decay of charmed hadrons and are called “prompt” because these parent particles have very short lifetimes and typically decay before interacting. At lower energies, the conventional component dominates, while at very high energies, the prompt component is larger. The conventional component is comparatively well understood, while the prompt component is still less well constrained and more difficult to measure. It is much smaller than the conventional component over most of the accessible energy range. Additionally, the number of cosmic-ray events decreases strongly with energy, so the high-energy region where prompt muons become more important contains only a small number of events. A measurement of this component therefore requires many years of data, a reliable estimate of the muon energy, and a careful understanding of the detector and simulation uncertainties.

The prompt muon flux is interesting for two reasons. First, it is a background for many astrophysical measurements, especially in searches for astrophysical neutrino sources. These searches aim to identify neutrinos from distant cosmic sources, and atmospheric muons and neutrinos must be understood well in order to distinguish atmospheric backgrounds from possible astrophysical signals. Second, prompt muons carry information about the production of charmed particles in air showers because they are produced in the decays of these short-lived parent particles. Measuring the prompt muon flux therefore gives indirect access to charm production in the atmosphere. This is especially relevant in the forward region of air-shower interactions, where secondary particles are produced close to the direction of the incoming cosmic ray and where accelerator experiments have no access.

In his analysis, Gutjahr used 12.12 years of data from IceCube to measure the atmospheric muon flux. He reconstructed the atmospheric muon energy spectrum at the surface from 10 TeV to 15 PeV, extending previous IceCube measurements by roughly an order of magnitude. A challenge in this measurement is that atmospheric muons often reach IceCube not as single particles, but as bundles of several muons from the same air shower. For the prompt component, the most energetic muon in such a bundle is especially important because the prompt contribution is expected to become more relevant at high energies. Gutjahr therefore used a machine-learning-based method to reconstruct the leading muon. He also produced new CORSIKA simulations, in which muons were tagged according to whether they originated from conventional or prompt processes.

Gutjahr compared the measured muon flux with several models that differ, for example, in how they describe the incoming cosmic rays and the particle interactions in the atmosphere. One of the tested models showed particularly good agreement with the data at high energies. For this model, the analysis found evidence for the prompt atmospheric muon flux with a significance of 4.4 standard deviations.

The results provide new experimental constraints on forward charm production in air showers and may contribute to resolving the so-called muon puzzle in air-shower experiments. We warmly congratulate Pascal on this excellent achievement and wish him all the best for his future scientific career.