Neutrino Phenomena in Particle Physics

Neutrinos are fundamental leptons that interact only via the weak force and gravity, making them extraordinarily elusive. The discovery of neutrino flavour oscillations demonstrated that these particles possess non-zero masses and mix between three flavours—electron, muon and tau—while propagating. Oscillation experiments have measured two independent mass-squared differences and three mixing angles, yet the ordering of mass eigenstates and the presence of CP violation in the lepton sector remain open questions. Neutrinos also play a pivotal role in astrophysics and cosmology: they influence supernova dynamics, contribute to the evolution of large-scale structure and offer a unique window into physics beyond the Standard Model. Searches for neutrinoless double beta decay seek to determine whether neutrinos are Majorana particles, which would have profound implications for the origin of their masses and the matter–antimatter asymmetry of the Universe. Recent advances in detector technology, background reduction and global data analyses are driving the field towards precision measurements of the remaining unknown parameters.

Research from Nature Portfolio

Recent studies have focused on imaging Ba2+ ions in high-pressure xenon gas as a step towards background-free sensitivity to neutrinoless double beta decay. Using molecular chemosensors and high-resolution optical microscopy within a ten-bar xenon environment, researchers have resolved individual barium ions at a gas–solid interface, paving the way for barium tagging in 136Xe experiments. This development addresses a critical challenge in establishing the Majorana nature of the neutrino by reducing backgrounds in next-generation detectors.

A seminal investigation earlier demonstrated the first clear evidence for neutrino flavour oscillations in atmospheric neutrinos. By analysing muon and electron neutrino fluxes traversing the Earth, this work confirmed that neutrinos change flavour en route, implying non-zero mass differences and mixing angles. These findings provided the first direct proof that neutrinos have mass and laid the foundation for modern oscillation physics.

Neutrino Phenomena in Particle Physics publication trend

The graph below shows the total number of articles in neutrino phenomena in particle physics across all publications each year (not limited to Nature Index journals).

Technical terms

Flavour oscillation: The quantum phenomenon whereby a neutrino of one flavour (electron, muon or tau) transforms into another as it propagates.

Neutrinoless double beta decay: A hypothetical nuclear decay process in which two neutrons convert into two protons and two electrons with no neutrinos emitted, indicating Majorana neutrino properties.

Majorana neutrino: A neutrino that is its own antiparticle, whose existence would allow lepton number violation.

Cherenkov detector: An instrument that observes charged particles via the characteristic light emitted when they travel faster than the speed of light in a medium.

CP violation: A disparity in the behaviour of particles and antiparticles under the combined operations of charge conjugation and parity inversion, here manifest in the neutrino sector.

Inverse beta decay: A reaction in which an antineutrino interacts with a proton to produce a positron and a neutron, used to detect low-energy electron antineutrinos.

References

1. Fluorescence imaging of individual ions and molecules in pressurized noble gases for barium tagging in 136Xe. Nature Communications (2024).
2. Search for Astrophysical Electron Antineutrinos in Super-Kamiokande with 0.01% Gadolinium-loaded Water. The Astrophysical Journal Letters (2023).