Perspectives for probing the Earth core composition with atmospheric neutrino detectors: the case of hydrogen

Abstract:

Our knowledge about the Earth's interior is mainly based on seismic measurements, cosmochemical and petrological constraints, and theories of Earth formation. Whereas the matter density profile is determined with a high precision, the chemical composition of the deep Earth can only be estimated indirectly. In particular the amount and nature of light elements in the Core remain controversial.

Here we present a new approach using elementary particles - neutrinos - which provides the theoretical basis for a measurement of the electron density inside the Earth, and a complementary way of probing the composition of the deep Earth. The key ingredient is the phenomenon of neutrino flavor oscillations. For atmospheric neutrinos of ~GeV energy crossing the Earth, such oscillations are distorted due to coherent forward scattering on electrons along their path. Measuring the flavor, energy and angular distributions of these neutrinos therefore provides sensitivity to the electron density in the layers of matter traversed. Combining this measurement with a known matter density profile directly constrains the proton-to-nucleon (Z/A) ratio in the corresponding layers. Since this parameter varies among chemical elements this technique has the potential to provide insights into the chemical composition of the core.

The goal of the presented study is to identify detector configurations that would be required to constrain the Earth core composition. The case of hydrogen, that has been put forward recently as an element that could account for the different seismic constraints on density and seismic velocity in the core, is of specific interest as its Z/A ratio is the most different from pure Fe. We show that larger and more densely instrumented versions of the already existing detector types would provide sufficient detection efficiency to exclude an outer core model containing 1% wt of hydrogen with respect to a pure FeNi-core with 95% confidence within 20 years. Models that predict different abundancies of Si and O (slightly different Z/A values) appear more challenging to disentangle and would likely require a different experimental approach, such as the combination of several detectors.