Measuring the mass of the neutrino remains one of the most important challenges in contemporary physics. Among the lightest and most elusive particles in the Universe, neutrinos pass through matter almost without interacting, yet they hold fundamental clues to the evolution of the cosmos.
A new study by Valentina Tozzini of Cnr Nano, with Andrea Casale of Columbia University, Angelo Esposito of Sapienza University of Rome and INFN, and Guido Menichetti of the University of Pisa, investigates in detail the role of a graphene substrate in tritium decay, one of the key processes used to determine the neutrino mass. The work has been carried out within the PTOLEMY project and has been published in Physical Review C.
Tritium is a radioactive isotope of hydrogen that decays by emitting an electron and a neutrino. By precisely measuring the energy of the emitted electrons, researchers can obtain indirect information about the neutrino mass. However, this approach is now approaching its experimental limits.
To improve measurement sensitivity, the PTOLEMY project aims to use tritium adsorbed on graphene, a two-dimensional material consisting of a single layer of carbon atoms. Under these conditions, interactions between the tritium and the graphene substrate can modify the energy of the emitted electrons, giving rise to so-called “solid-state effects.”
“Inside a material, unlike in a vacuum, electronic and vibrational excitations alter the energy spectrum of the emitted electrons—the very signal used to determine the neutrino mass. Quantifying these effects with high precision is therefore essential for the correct interpretation of experimental data,” explains Valentina Tozzini senior reseracher at Cnr Nano.
To address this challenge, the researchers developed a theoretical framework combining simulations of graphene’s electronic structure—based on an unconventional application of the computational method known as Density Functional Theory—with a quantum-mechanical description of nuclear decay. This approach realistically reproduces the conditions experienced by tritium in experimental devices. For the first time, it enabled the calculation of the electron energy distribution while consistently accounting for both nuclear and solid-state effects.
“This result perfectly reflects the multidisciplinary nature of the PTOLEMY project,” says Marcello Messina, Senior Researcher at the INFN Gran Sasso National Laboratories, Technical Coordinator and one of the founders of PTOLEMY. “The project spans cosmology, particle physics, condensed matter physics and theoretical physics. In each of these fields, it offers outstanding opportunities to achieve results at the frontiers of knowledge.”
“This type of analysis provides a deeper understanding of how the material affects the decay signal, making it possible to distinguish features in the electron energy distribution that have so far been difficult to isolate,” the researchers explain. “It also provides guidance for designing the optimal medium for neutrino mass measurements and, in the longer term, for detecting relic neutrinos from the cosmic neutrino background through neutrino capture on tritium.”
The study explores a highly interdisciplinary field at the interface of condensed matter physics, nuclear physics and cosmology, highlighting how two-dimensional materials such as graphene are not only advanced technological platforms but also powerful tools for addressing some of the most fundamental questions in modern physics.
A feature article, A Solid-State Pathway to Neutrino Mass, is available on the APS Physics website.
Reference article: A. Esposito, G. Menichetti, and V. Tozzini, 𝛽-decay spectrum of tritiated graphene: Combining nuclear quantum mechanics with density functional theory. Phys. Rev. C 113, 054607. DOI: https://doi.org/10.1103/gr8x-lf9f
[IMAGE: artistic rendering of 𝛽-decay spectrum of tritiated graphene]
