Test of Lorentz invariance: exploiting the potential of air showers at ultra-high energy

• Applicant: Professor Dr. Markus Risse
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 408049454

As a fundamental pillar of modern physics, Lorentz invariance is a key target of experimental precision tests. This holds even more since Lorentz invariance (LI) may actually be violated at some level as suggested by theoretical considerations when trying to combine general relativity and quantum mechanics. The discovery of a violation of Lorentz invariance (LV) would constitute an essential breakthrough in fundamental physics; confirming the validity of Lorentz invariance with improving accuracy gives important guidance towards a more fundamental theory. Some of the world-best tests of Lorentz invariance in the photon sector make use of astroparticles such as cosmic rays. The photon sector allows for clean probes of LV physics, and cosmic rays reach ultra-high energies (UHE) far beyond the capabilities of current technology. Within the framework of modified Maxwell theory and regarding extensive air showers initiated by UHE cosmic rays, limits were obtained that significantly improved previous ones (see, e.g., [1]). This was based on including the new LV processes of photon decay and vacuum Cherenkov radiation - both are forbidden in standard physics - in the shower simulation and comparing the expectations to data. So far, the analyses used the depth of shower maximum as an observable, a quantity from the longitudinal shower profile. In the renewal project, the focus is put on the number of ground muons. This is a quantity accessible for ground detectors which is known to carry information about the primary type complementary to the depth of shower maximum. Based on our simulation tools and simulated data sets in hand, we want to explore the impact of LV on the muon number and how it can be exploited to test LI. It is expected that including the muon number leads to better constraints on the allowed primary composition and, thus, to an improved sensitivity to LV. Inspection of muons is also motivated by indications of several air shower experiments of a "muon excess" in data as compared to simulations. We will check to what extent LV could contribute to these excess muons.

• DFG Programme: Research Grants