High-energy (PeV) neutrinos observed!

At June's "Neutrino 2012" conference, IceCube showed two very interesting neutrino events.  Both were cascades (electromagnetic or hadronic showers), with an apparent energy of about "PeV to 10 PeV."  This is equivalent to the mass energy of over 1,000,000 protons, or about 250 times the energy of one of the protons that are accelerated at the LHC (Large Hadron Collider).   In other words, it is far beyond anything we could imagine producing on Earth.

The events, recorded on August 9, 2011 and on January 3rd, 2012, are shown below.  

The side (top/left grahic) and top (bottom/right) view of the January 3rd event.  Each dot shows one IceCube optical module; the colored spheres show the 312 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons. About 96,000 photons were observed from the event. This is a very small fraction of the total number that were created by the shower.

The side (top/left) and top (bottom/right) view of the  August 9th event. Each dot shows one IceCube optical module; the colored spheres show the 354 optical modules that were hit in the event.  The color indicates the relative time (red = earliest, blue = latest), and the size of the sphere shows the number of observed photons.About 70,000 photons were observed


In both events, the cascades were far from the detector edges, and there was no sign of any incoming muon, so the events are unlikely to be background.  We are also confident that they are real, and not due to a detector problem of some sort.    Analysis of these events is still on-going (for example, to better determine their energies), but these are clearly far more energetic than the events previously seen by IceCube, and the estimated background from atmospheric neutrinos is about 0.14 event.    That estimate is high because it does not include at least one mitigating factor -if these were created in a roughly downward-going cosmic-ray air shower, then we would have seen evidence of the shower in the IceTop surface detection array.   Also, the 0.14 events is for all types of neutrino interactions with a lot of produced light, while these two events look like either electron neutrinos, or neutral-current (NC)  interactions of any type.  The NC interactions deposit only a fraction of the neutrino energy in the detector, so if these were NC interactions, the neutrinos would have to have been even more energetic.

For those who want more information, Aya Ishihara's talk is posted here.  n.b.  Everything above (including the event plots) is taken directly from Aya's talk.

It is too early to say what these events will mean, but this is a very very interesting development.   Stay tuned.