"The proposed project aims at probing the relativistic particle content of the most numerous, persistent population of high energy cosmic sources in the Universe: jets of active galactic nuclei (AGN). This undertaking is leveraged by today's synergy of high performance ground- and space-based photon detecting instruments as well as key advances in astroparticle physics experiments, yielding dense high quality data over a broad energy range from a large collection of AGN. Such powerful objects are among the prime candidate sources of the enigmatic ultrahigh energy cosmic rays, however, conclusive evidence is still missing. The development of diagnostic tools that probe the composition of the material in AGN jets, that go beyond the so far used broadband modeling methodology, is key to this project, and allows a systematic search for observational features of hadronic interactions from high energy cosmic jet sources. Meaningful constraints on their relativistic particle content will have far-reaching implications for the physics of cosmic rays, particle acceleration to energies beyond reach for any terrestrial laboratory, and our physical understanding of jet formation, collimation and mass loading. Furthermore, significant impact for searches of extragalactic neutrino sources is expected. This project will promote the professional reintegration of the proposer, who has a strong background in broadband modeling including hadronic processes, into the European astroparticle physics community, will complement the activities of the center of experimental astroparticle physics at Innsbruck University, and strengthen the scientific return of neutrino astrophysics experiments with its strong presence in Europe."