Neutrino signal is a mystery

Does an extremely energetic particle trace in the IceCube detector come from a tau particle?

Traces of a high-energy neutrino in the IceCube detector. © IceCube Collaboration
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Mysterious particle trail: The neutrino detector IceCube has captured a signal that puzzles physicists. Because this track is much more energetic than any previously registered. A possible explanation for this is provided by two US physicists. They suspect that the signal was caused by a tau lepton - the collision product of a tau neutrino with an atom. But that could mean that cosmic neutrinos are much more energetic than previously thought.

Every second, billions of neutrinos race through our bodies without us noticing. Because these almost massless "ghost particles" seldom interact with other matter - so it's hard to track them down. In addition, the three neutrino varieties - electron, muon and tau neutrino - can convert to another species in flight.

"Cosmic Messengers"

To detect the elusive neutrinos, physicists use detectors that register the particles and flashes of light left by the neutrinos in their rare collisions with atoms. The IceCube detector consists of more than 5, 000 basketball-size photosensors sunk deep into the Antarctic ice. With him, some years ago, researchers were able to detect the particularly high-energy cosmic neutrinos.

"These high-energy neutrinos are new cosmic messengers, and it's extremely important that we understand their message accurately, " says Ranjan Laha of the US National Accelerator Laboratory in California. Because these particles are commonly believed to occur in some of the most dramatic events in the cosmos, such as supernovae and gamma-ray bursts

Enigmatic particle trail

But a particle trace in the IceCube detector still gives puzzles to physicists: In June 2014, the sensors recorded an event with extraordinarily high energy. The signal reached 2.6 Petaelectron Volts (PeV) - much more than any previously known event. IceCube has only ever caught signals of more than one PeV three times - and none of them has reached that energy, as reported by Laha and his colleague Matthew Kistler of Stanford University. display

"This track from June 2014 immediately raises questions, " say the researchers. "Which neutrino variety has produced this track? What does this mean for the nature of cosmic neutrinos? "So far, physicists have assumed that this extremely energetic particle trace comes from a muon an elementary particle that results from the collision of a muon neutrino with an atomic nucleus.

Spectrum of ascending dews as a function of the energy of the dew entering IceCube: The vertical band indicates the energy needed by the tau particle to deposit 2.6 PeV in IceCube. Matthew D. Kistler and Ranjan Laha

Tau instead of muon?

Now, however, new analyzes by the two physicists raise doubts about this interpretation. According to their calculations, the particle track for a muon is too high-energy and the distance to the remaining muon signals too large. "It is unlikely that the spectrum of neutrinos responsible for the IceCube events between 40 TeV and 2 PeV can produce such a trace, " the researchers said.

Instead, scientists are considering the possibility that the trace could be from a high-energy tau lepton. This particle is the heaviest "brother" of the electron and, according to some theory, can be formed in collisions of tau neutrinos with atoms. If this proves true, this would be a first. Because: "A clear Tau signal has never been identified by IceCube, " the researchers said.

Top of an iceberg?

But this means that if the rattling signal really comes from a tau lepton, it sheds a whole new light on the population of cosmic neutrinos. Because in order to give off the large energy of 2.6 PeV in the detector, the appropriate Tau neutrino burned an initial energy of at least 67 PeV, as the physicists calculated. This is, however, proven or assumed significantly more than previously for cosmic neutrinos.

"This would open a window to a stream of astrophysical neutrinos with unexpectedly high energies, " say Kistler and Laha. The 2.6 PeV signal in IceCube would then be the first indication of this novel, high-energy component of the cosmic neutrino spectrum. "According to our models, this IceCube event could even be the tip of an iceberg." The two physicists therefore plead for further analysis of this event. (Physical Review Letters, 2018; doi: 10.1103 / PhysRevLett.120.241105)

(Johannes Gutenberg University Mainz, 20.06.2018 - NPO)