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Karlsruhe Tritium Neutrino Experiment (KATRIN)
Sensitive German experiment sets new limit on maximum neutrino mass
Context: The Karlsruhe Tritium Neutrino Experiment (KATRIN) collaboration recently published an upper limit on the sum of the masses of the three known neutrino types.
Key Findings
- KATRIN’s latest result was based on 259 days of data collected from five measurement runs between March 2019 and June 2021.
- The result: the combined mass of the three known neutrino types cannot exceed 8.8 × 10⁻⁷ times the mass of the electron.
- This is twice as precise as the previous best result — a major experimental milestone in neutrino physics.
What are Neutrinos?
- Neutrinos are tiny, nearly massless subatomic particles. Nicknamed “ghost particles” because they rarely interact with other matter, making them extremely difficult to detect.
- Neutrinos come in three types (flavors): Electron neutrino (νₑ), Muon neutrino (νμ), Tau neutrino (ντ).
- Neutrinos are produced in various high-energy processes, including:
- Nuclear reactions in the Sun (billions pass through your body every second!)
- Supernova explosions (massive star deaths)
- Radioactive decay (such as beta decay)
- Particle accelerators (scientific experiments)
- Cosmic rays interacting with Earth’s atmosphere
What is the KATRIN Experiment?
- KATRIN is a high-precision experiment designed to measure the mass of neutrinos, which are among the hardest-to-detect subatomic particles.
- The core instrument, a 200-tonne spectrometer, was constructed in Deggendorf, Germany, in 2006.
- Due to its massive size, land transport was deemed unsafe, leading to an 8,600-km detour to Karlsruhe via waterways, including: Danube River → Black Sea → Mediterranean Sea → Atlantic Ocean → Rhine River.
- The experiment closely observes the disintegration of molecular tritium, focusing on the maximum energies of electrons emitted during tritium decay, which carry information about neutrino mass.
- KATRIN collected data from 36 million electrons to set the latest constraint.
Why is the KATRIN Result Significant?
- The new upper limit on neutrino mass is 20 times stronger than the first constraints set in 1991 by experiments in Los Alamos (USA) and Tokyo (Japan).
- Unlike other methods, KATRIN’s result is robust and assumption-free, making it a reliable benchmark in neutrino physics.
- Other approaches, such as cosmological observations, set a tighter upper limit at 1.4 × 10⁻⁷ times the electron mass, but rely on assumptions about the early universe’s evolution, weakening their validity.
- Another method involves neutrinoless double beta decay, but it assumes neutrinos are their own antiparticles from the outset.
- KATRIN’s direct measurement approach avoids such assumptions, making its findings more reliable.
What are the Challenges in Measuring Neutrino Mass?
- Neutrino Mass Mystery: Neutrinos exist in three types and undergo particle oscillations, proving that at least two types have nonzero mass. However, oscillations only measure mass differences, not absolute values. Measuring actual neutrino masses is extremely challenging, requiring advanced experiments like KATRIN.
- Standard Model Breakdown: The Standard Model of particle physics predicts neutrinos to be massless, contradicting experimental data. This discrepancy suggests the existence of new, undiscovered forces and particles, hinting at physics beyond the Standard Model.
- Antiparticle Puzzle: Neutrinos are electrically neutral, making them potential self-conjugate particles. Unlike neutrons (which consist of charged quarks), neutrinos appear to be elementary particles.
- To confirm this, physicists need to determine whether neutrinos have a Majorana mass or Dirac mass.
- The neutrinoless double beta decay experiment aims to settle this by detecting whether two neutrinos can annihilate each other.
How to Overcome These Challenges?
- Precision experiments like KATRIN use highly sensitive spectrometers to analyse electron emissions from tritium decay.
- Cosmological observations study the role of neutrinos in shaping galactic structures to infer their mass.
- Neutrinoless double beta decay experiments aim to determine whether neutrinos are their own antiparticles, which could provide indirect mass constraints.
- Advanced detector technologies are being developed to improve neutrino detection efficiency.
