An IceCube event
display showing the latest ‘most energetic event.’ Each open circle is
an un-hit
optical module; the filled spheres show hit modules, with the radius indicating
the number of
detected photons. The colors indicate the relative time (red is first,
then orange,
yellow, green, and blue), but with the chosen scale, the timing isn’t that
useful.
Credit: Leif Radel
(August 15, 2015) The
new neutrino was found thanks to a muon trail observed by an array of 5,160
optical detectors, using electronics designed and built by Berkeley Lab
scientists and engineers. Muons are heavy relatives of electrons and are
emitted when a type of neutrino called a muon neutrino interacts with an atomic
nucleus. The recently detected muon had such a high energy—about 2600 trillion
electronvolts—that it could have only been produced by an ultra-high-energy
neutrino. The muon track was several kilometers long, too long for IceCube to
have captured the entire trace. This means the actual neutrino energy was
likely several times higher than was seen in the detector.
Similar to the way that a muon can lead scientists to a
neutrino, a neutrino can point to the origin of cosmic rays. Cosmic rays are
charged particles that are suspected to come from ultra-high-energy sources
outside the galaxy. But because they are charged particles, they arrive at
Earth only after following follow chaotic, twisted paths circling around
magnetic field lines in space.
Ultra-high-energy neutrinos are believed to come from the
same sources as cosmic rays, but differ in that they are neutral, and they
therefore travel in straight lines. So, if you catch a neutrino streaking by, the
thinking goes, just look in that direction and you can see a cosmic ray source.
But in recent years, pointing instruments in space at
neutrinos’ suspected sources hasn’t revealed obvious source candidates. Indeed,
when instruments were aimed at the sky from where this newest neutrino came, no
high-energy phenomena were found. Thus, some theorists have devised models that
propose ultra-high-energy neutrinos are actually left over from the birth of
the universe or that space might not be symmetric in the way physicists once
thought.