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KATRIN
By Adrian Cho
Physicists have set a new limit on the mass of the lightest particle of matter in nature. The neutrino can not weigh more than 1.1 electron volts (eV) – less than 500,000 times the mass of an electron – for example, experimenters from the Karlsruhe Tritium Neutrino experiment (KATRIN) at the Karlsruhe Institute of Technology in Germany. Reported on September 13 during a meeting in Toyama, Japan, the new result halved the previous limit of 2 eV.
Physicists have tried to measure neutrino mass for decades. However, the particle hardly interacts with ordinary matter. To deduce its mass, researchers study the radioactive "β-decay" of tritium, in which a nucleus spits an electron and a neutrino. By accurately measuring the maximum energy of electrons ejected, physicists can deduce the mass of unobserved neutrinos. KATRIN (above) takes this classic approach up to the ultimate limit using a 23-meter-long spectrometer simulator to measure the electron from tritium with unprecedented accuracy.
The new limit is based on only 28 days of data. In the end, KATRIN experimenters hope to collect data for 1,000 days and push the limit of an additional factor of 10 to 0.2 eV, or show that the neutrino weighs more than that. Cosmological measurements already suggest that the neutrino can not weigh more than about 0.1 eV, but this estimate is based on several assumptions. KATRIN's physicists claim that their best, directly measured limit on neutrino mass will probably make cosmology models more reliable.
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