From Brave New Spacetime: Quantum Gravity

Experimental confirmation of the presence of Quantum Gravity (QG), a unified theory of particle physics and general relativity, is the holy grail of modern physics. Expected signals of QG would be extremely high-energy such as the Planck energy (E_P~10¹⁹ GeV) and is inaccessible. On the other hand, QG effects may be suppressed and permeated in our spacetime with E_P^-1 (~10^-19 GeV^-1) or E_P^-2 (10^-38 GeV^-2) or higher order. This extremely weak signal in spacetime is invisible by ordinary methods but could be seen by neutrinos.

High-energy astrophysical neutrinos propagate long distances without any disturbance, and tiny new effects in spacetime may cause unexpected phase shifts, which can be encoded in the quantum mixing of neutrino states. Astrophysical neutrino flavour is a very sensitive tool to measure QG-motivated effects such as Lorentz symmetry violation.

We analyzed the 7.5-yr high-energy starting event sample in IceCube. Data flavour information is compared with simulation to look for anomalous flavour mixings. The astrophysical neutrino spectrum is marginalized to take into account its uncertainty. The analysis depends on the assumption of production neutrino flavours since the astrophysical neutrino production models are not known. Nevertheless, the sensitivity of this analysis exceeds all known methods to look at some types of QG signals, from tabletop experiments to cosmology. We will present our latest results and future prospects to search for QG signals with neutrinos.