According to a new study submitted for publication in Physical Review Letters (arXiv.org version), two rare events observed in 2011 and 2012 by the IceCube Neutrino Telescope at the South Pole are the highest energy neutrinos ever detected.
The record events have an estimated energy of about 1.04 and 1.14 PeV respectively, hundreds of times larger than the energy of a proton at the Large Hadron Collider.
Most of the neutrinos reaching the Earth originate either in the Sun or in our atmosphere through the interaction of incoming cosmic rays. However, if we look at neutrino energy, once we reach the PeV scale, neutrinos coming from far off in our galaxy or from more distant places in the visible Universe become dominant.
The scientists working with the IceCube have estimated that the probability that these two events are not background – anything else in the detector besides astrophysical neutrinos – is at the 2.8 sigma level. Therefore, the signal observed so far does not allow claiming a first observation of astrophysical neutrinos. Nevertheless, it is clearly pointing to IceCube as the place to look for them.
Inspired by the observation of these two PeV events, IceCube team is already working on an analysis with an optimized technique that will be substantially more sensitive to similar events at lower energies.
Results of that analysis are expected soon and could provide more evidence of what is now a very intriguing hint of an astrophysical signal.
Bibliographic information: Aartsen MG et al. 2013. First observation of PeV-energy neutrinos with IceCube. IceCube Collaboration. Physical Review Letters, submitted for publication; arXiv: 1304.5356