An important step towards understanding neutrino masses

Neutrinos are used to investigate a broad spectrum of physics topics, ranging from the extreme universe to the underlying symmetries of nature. These intriguing particles may have the answer to a few, long-standing open questions in physics and astronomy. In particular, neutrinos themselves are the origin of still unresolved and maybe totally new physics.

One of the main open questions in neutrino physics is the relative mass of the three neutrino types, a property known as the neutrino mass ordering. Is the third neutrino more massive than the other neutrinos, in what scientists call the normal ordering (NO), or is it lighter, referred to as inverted ordering (IO)? In a new paper by the IceCube Collaboration, physicists use the inner and denser DeepCore detector within IceCube to try to answer this question. A weak preference is shown for NO, a result that is complementary to and in agreement with results from other experiments. This paper has been submitted to the European Physical Journal.

Mass ordering
The negative log-likelihood (LLH) as a function of sin2(θ23) for Analysis A, relative to the global minimum LLHmin. The preference for NO over IO is visible over all the range of sin2 (θ23) with the best-fit for both orderings being in the lower octant (sin2 (θ23) < 0.5). Image: IceCube Collaboration

When neutrinos travel through space and matter, they oscillate, meaning they change their flavor (electron, muon or tau) depending on their energy and the propagation distance. This quantum effect is explained by the fact that neutrino mass states, i.e. those for which the mass is a well-defined property, are not the same as the neutrino flavor states, the states in which neutrinos interact. These mass states are called neutrinos 1, 2 and 3.

But we know very little about neutrino mass, except that for all neutrinos it is very small and that nature may work fairly differently depending on which neutrino is more massive. Some unification theories, for example, predict a normal mass ordering. Also depending on this mass ordering, the outcomes of a supernova explosion might be different.

Several current and future long-baseline accelerator experiments, as well as experiments with atmospheric neutrinos and reactor neutrinos, are targeting a precise measurement of the mass ordering. For atmospheric neutrinos, the propagation through Earth induces a small modulation of the oscillation of neutrinos below 15 GeV, at about the lowest neutrino energies detected in IceCube. Interestingly, this modulation depends on the mass ordering. The high statistics of detected neutrinos within IceCube allows us to search for this small effect.

In this study, researchers performed two independent analyses, both using three years of IceCube data and targeting this challenging measurement of the neutrino mass ordering with low-energy atmospheric neutrinos in IceCube. “When we embarked on this new analysis we were not aware of all the experimental challenges that we had to solve to measure the faint signals of these low-energy neutrinos with a sufficient precision,” says Martin Leuermann, a main analyzer of this study who worked on this analysis as a PhD candidate at RWTH Aachen University.

Both analyses obtain a consistent result within their uncertainties and a small preference for NO. Although the ordering signature is very weak, it provides a complimentary measurement. Unlike beam experiments, this result is independent of the CP-violating phase, another important parameter for characterizing neutrino oscillations. Another valuable outcome of this study is the successful implementation and verification of analysis methods that have been prototyped for future extensions of IceCube such as PINGU or the imminent IceCube Upgrade.

The IceCube Upgrade is already underway and is expected to be completed by 2023. It will deploy new sensors within DeepCore, which will greatly enhance the accuracy of detecting these lowest energy neutrinos in IceCube that are most critical for this measurement. “We have now proven that the concept of future extensions of IceCube do work in practice and thus we can look forward to unprecedented measurements of neutrino properties,” says Steven Wren, also a main analyzer of this study who worked on this analysis as a PhD candidate at the University of Manchester.

info “Probing the Neutrino Mass Ordering with Atmospheric Neutrinos from

Three Years of IceCube DeepCore Data,” The IceCube Collaboration: M. G. Aartsen et al. Submitted to the European Physical Journal