快猫短视频

IT鈥橲 called DONUT, and it has filled a stubborn hole in particle physics. The
experiment, conducted with the help of a 15-metre-long detector at Fermilab near
Chicago, has provided the first direct evidence of the existence of the tau
neutrino, the last of the matter particles, or fermions, predicted by the
standard model of fundamental particles. 鈥淭his is a tremendous pioneering
experiment,鈥� says physicist Martin Perl, who received a Nobel prize for
discovering the related tau lepton.

According to the standard model, 12 fermions鈥攕ix quarks and six
leptons鈥攑rovide the building blocks for all the matter in the Universe.
Particle physicists had little doubt that the chargeless and, possibly, massless
tau neutrino was out there. Now it can take its place alongside its siblings,
the electron and muon neutrinos, first detected in 1956 and 1962,
respectively.

An international team at Fermilab carried out the Direct Observation of the
Nu Tau (DONUT) experiment in 1997, using the lab鈥檚 Tevatron accelerator to
produce a beam of neutrinos that they believed would contain tau neutrinos. They
passed the beam through a series of iron plates at the heart of the detector,
hoping that neutrinos would collide with the metal nuclei. Layers of a
photographic-like emulsion sandwiched between the plates recorded the tracks of
collision debris.

Any colliding tau neutrinos would produce tau leptons, which leave a
characteristic kinked trail in the emulsion. Team members at Nagoya University
in Japan have spent the past three years making three-dimensional scans of the
emulsion layers to look for these telltale trails. Out of around a thousand
candidates, just four turned out to be the trails of tau leptons. 鈥淚t鈥檚 a hard
game in that sense,鈥� says Regina Rameika, a physicist on the DONUT team.