Measurement forecasts of the ultra-high-energy neutrino flavor composition: unlocking new insights into astrophysics and fundamental physics

The neutrinos of cosmic origin discovered by the IceCube detector, with TeV--PeV energies, unlocked new perspectives on high-energy non-thermal astrophysical sources and high-energy fundamental physics. It is therefore captivating to extend our reach to higher energies, where neutrinos will connect us to the most extreme phenomena of our Universe. In the coming decade, a new generation of neutrino telescopes, currently under planning, will target the discovery of ultra-high-energy (UHE) neutrinos, with EeV-scale energies, predicted in the late 1960s. Discovering UHE neutrinos would shed light on the origin and production mechanism of the most energetic cosmic rays, and also allow us to probe neutrino physics at energies otherwise unattainable. A versatile tool to test both aspects is their flavor composition, i.e., the fraction of neutrinos of each flavor in the total flux. However, measuring the flavor content of UHE neutrinos would require individual UHE neutrino telescopes to have flavor-identification capabilities. This is not guaranteed, even though research is ongoing. In this work, we propose and explore a novel idea to measure the UHE neutrino flavor composition that circumvents this potential obstacle. Flavor sensitivity is manufactured from the joint detection by two telescopes, one sensitive to all flavors—the radio array of IceCube-Gen2—and one mostly sensitive to ντ —GRAND. Even under conservative choices of neutrino flux and detector size, this flavor sensitivity, predominantly to ντ , is sufficient to extract new insight. This work presents the first measurement forecasts of the UHE ντ content. Then, these forecasts are used for astrophysics, where they give meaningful constraints on the neutrino production mechanism, and for fundamental physics, improving by many orders of magnitude the constraints on Lorentz-invariance violation.