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Todor Stanev: Cosmogenic Neutrinos

March 15, 2013

Very high energy neutrinos have always been predicted to come from the sources of UHE cosmic rays, as first stated by Waxman and Bahcall. The limit was challenged byt nobody doubts the relation between the cosmic ray sources and astrophysical neutrinos.  The upper limits of W&B are stronger than those of Manheim, Protheroe et al.

Cosmogenic neutrinos however do not come directly from the source, but are diffuse. They are produced by the interaction of UHECR with the photons in the universe. This suggestion was made in 1969. Often called GZK neutrinos, these should instead be called BZS neutrinos (Berezinsky, Zatsepin, Stecker (1973)). What happens is that there is a minimum proton energy that can interact with the photon background. If one takes the average energy of the microwave background, the threshoudl is about 3*10^19 eV. There is also production in the infrared and optical background.

Of course as always, when protons interact and produce pions, we have gamma rays and neutrinos. The spectra one expects have still quite high energy. These are cosmogenic neutrinos and cosmogenic gamma rays. The pizero decays in two gammas. A charged pion decays and produces three neutrinos and an electron, and the neergy of two peaks is lower.

The fluxes of neutrinos depend on flavor. THere is a two-peak system in the flux vs energy plot. Electron neutrinos and antineutrinos peak at much lower energy, slightly above 10^16 eV. It is very important for the shape and flux of cosmogenic neutrinos that we establish their emission in UHECR sources, the maximum acceleration energy, their energy spectrum at acceleration, the composition, and the distribution of these sources in the Universe. These parameters are of course correlated with one another.

The maximum acceleration energy of UHE protons is one of the most important astrophysical parameters for the estimate of the cosmogenic neutrino flux. If this is below 10^20 eV, then the total flux is very low. What is important is the max energy per nucleon: if a nucleus has larger energy still what matters is the energy of each proton individually, of course.

Using the current measurement of the UHECR flux, particularly auger fits, one obtains predictions for neutrinos. Different neutrino flavours in a standard cosmogenic neutrino calculation show different peak values. If coming from neutron decay (antineutrinos of electron kind) their energy is smaller.

CMB is not the only universal photon field in the universe. The infrared and optical background is much more energeitc but the number density is much smaller. This  means that steeper proton spectra generate more neutrinos in interactions with this background than flatter ones.

The highest possible cross section of processes useful to detect these neutrinos is at the Glashow resonance: a electron antineutrino interacts with an electron to produce a W-. Some of the decay products of the W- would produce a visible signature in the detector.  Detection of the Glashow resonance would support a heavy composition of the cosmic rays.

Two neutrino events with very high energy were detected by IceCube. Is it possible that these events be cosmogenic neutrinos ? There are several processes we have to look at. The Glashow resonance is one. One must also thing at CC interaction of an electron neutrino above 1PeV, or tau neutrinos with the same energy. It is difficult to see the tau track because the decay length is of 50 meters. Then there are neutral current interactions of any neutrino type. So the responsible neutrinos could be either extraterrestrial or atmospheric, with a strong prompt (charm) contribution. For atmospheric background neutrinos, the calculated flux is 0.086 events in two years of IceCube, so these are not likely atmospheric.

The current limits on the flux of high-energy cosmogenic neutrinos are higher than the W&B limit. In conclusion, it is likely that the two neutrinos come from sources than by UHECR in propagation.

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