CONTEXT

• Neutrinos are elusive subatomic particles, discovered in 1938, posing deep questions in particle physics.
• Their mass is incredibly small and difficult to measure, yet it has vast implications in modern physics.
• Standard Model predicts neutrinos as massless — but neutrino oscillation experiments proved they have mass, winning the 2015 Nobel Prize.
• Since 1991, global efforts have aimed to precisely determine their mass via different experiments.
• The KATRIN experiment in Germany is the latest and most sensitive direct attempt to measure the absolute neutrino mass.

CONCEPT

• Neutrino Types: There are three known types (or “flavors”) of neutrinos.
• Oscillation: Neutrinos change types during travel, implying they have mass — though only the mass differences are measurable this way.
• Direct Mass Measurement: Requires tracking beta decay electrons, as neutrino mass affects their energy spectrum.
• Tritium Decay: KATRIN uses radioactive tritium, observing emitted electrons to infer neutrino mass.
• Spectrometer Challenge: KATRIN’s massive 200-tonne detector was transported from Deggendorf to Karlsruhe via a complex 8,600-km journey.
• Majorana vs Dirac Neutrinos: Whether neutrinos are their own antiparticles remains unanswered; neutrinoless double beta decay could provide the answer.
• Detection Difficulty: Neutrinos interact so weakly with matter that they pass through nearly all substances unimpeded.

CURRENT

• KATRIN has analyzed 36 million electrons over 259 days between 2019–2021.
• The latest upper limit on the sum of the masses of all three neutrino types is 0.8 eV (8.8 × 10⁻⁷ times electron mass) — a 2x improvement from earlier limits.
• It improves upon past experiments in Tokyo and Los Alamos by about 20-fold.
• Unlike cosmological estimates or double beta decay experiments, KATRIN’s result is model-independent and doesn’t rely on cosmological assumptions or unverified neutrino properties.
• Cosmological studies suggest a tighter limit (0.14 eV), but those depend on early universe models.
• KATRIN stands out for its robustness, representing a landmark achievement in neutrino physics.
• The experiment is ongoing, with further data expected to refine the neutrino mass limit.