As a field, physicists have celebrated monumental progress at cracking the case of the elusive neutrino. This elusive fundamental particle is at the heart of everything in our universe. The Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany achieved a major milestone recently. It’s set a new upper limit on the mass of neutrinos at 0.45 electron volts, almost halving the previous limit. This development not only enhances the understanding of neutrinos but poses potential challenges to the established Standard Model of particle physics.
Neutrinos exist in three different flavor states: electron, muon, and tau neutrinos. These flavors match the three charged leptons that are the particles neutrinos can interact with. The ability of neutrinos to switch between these flavors spontaneously—a phenomenon discovered in 2015—has garnered recognition, earning the Nobel Prize in Physics for its discoverers. This fascinating flavor-changing characteristic introduces an additional layer of intrigue to neutrino studies and highlights their remarkable uniqueness in the realm of particle physics.
Neutrinos are created in many of the most energetic cosmic and terrestrial environments. They come from the explosive nuclear furnace of stars. They appear in the wake of explosive stellar death — supernovae — and are produced from radioactive decay, as well as man-made sources like particle accelerators and nuclear reactors. Remarkably, upwards of 100 billion of these ghostly particles pass through every square centimeter of the human body each second. This elusive nature makes neutrinos one of the most abundant particles in the universe.
Detecting neutrinos is a huge challenge thanks to their ghost-like nature. We know that neutrinos have mass—a fundamental property that has been verified in experiments—but directly measuring that mass is extremely difficult. Instead, physicists work backwards to infer neutrino mass from the energy they subtract from the speed of accompanying electrons during interactions.
The KATRIN experiment is an exciting new attempt to do this. By measuring the energy of the electrons emitted from tritium decay, researchers can place limits on allowed neutrino mass. KATRIN’s recent results therefore provide the most stringent upper limit on the mass of neutrinos to date. Aside from these, they deepen our knowledge of neutrino properties. The team plans to gather data through late 2025. They all look to further clarify the neutrino mass and continue to unravel the secrets of these enigmatic particles.
The consequences of this analysis go beyond the statistics, as it has the potential to reformulate basic tenets of particle physics. Neutrinos are a key ingredient in the Standard Model. Recent findings regarding their mass might upend assumptions deeply embedded in this framework. The quest to understand neutrinos will help answer some of the most fundamental questions about existence itself.
