giant air shower

Ultra-High Energy Cosmic Rays:
Neutrino Component

In some production scenarios such as topological defect sources, but possibly also some more conventional sources such as powerful active galactic nuclei, the ultra-high energy fluxes of hadrons and gamma rays are accompanied by even larger fluxes of ultra-high energy neutrinos. The prospects for their observation in the defect scenarios were discussed by Sangjin Lee, David Schramm (U of C), Paolo Coppi (Yale University), and myself in this paper. There we demonstrate that the constraint imposed by requiring that top down scenarios do not overproduce the measured universal gamma ray background around 1GeV implies a model independent upper limit on these neutrino fluxes which only depends on the ratio r of energy injected into the neutrino versus electromagnetic channel. The corresponding upper limit on predicted event rates above energy E in a detector of acceptance A(E) in units of volume times solid angle is 0.3 r [A(E)/(1 km**3 x 2pi x sr)] (E/10 EeV)**(-0.6) events per year. For r comparable or greater than 1, neutrino fluxes near this upper limit are therefore potentially detectable by the proposed km**3 scale neutrino observatories (see also the International Neutrino Astrophysical Observatory Consortium and links there). For such an observatory several prototypes now exist such as AMANDA (with ICECUBE its planned extension) in the South Pole ice, and ANTARES, NESTOR, and the BAIKAL experiment in the deep sea or lake. Furthermore, ultra-high energy cosmic and gamma ray detectors such as the Japanese Telescope Array and the Pierre Auger Project have some sensitivity to horizontal air showers created by neutrinos. In addition, part of the motivation for the space based detector projects represented by the Orbiting Wide-angle Light collectors (OWL) and the Extreme Universe Observatory (EUSO) is the search for deeply penetrating air showers caused by neutrinos.

A detection of the ultra-high energy neutrino flux might establish an experimental lower limit on r and thus allow important insights into new fundamental physics near the grand unification scale. Even a non-detection with more stringent upper limits would also be useful since it could eliminate large classes of top down models of ultra-high energy cosmic ray origin. For example, failing to detect neutrinos above 10 EeV with an exposure A x t would rule out such scenarios of the type shown in the figure above for r > [100 km**3 x 2pi x sr x yr/(A x t)].

TD spectrum for exclusive quark decay, < 10**(-11) Gauss The figure on the left shows a top down scenario with special focus on the expected neutrino flux, as compared to contributions from other potential sources. Shown are predictions for the differential fluxes of muon neutrinos and anti-neutrinos (solid blue lines) from the atmospheric background for different zenith angles (hatched region marked "atmospheric"), from proton blazars that are photon optically thick to nucleons and are therefore not subject to the "Waxman Bahcall bound", but contribute to the diffuse gamma-ray flux (``proton blazar''), from ultra-high energy cosmic ray interactions with the cosmic microwave background ("cosmogenic"), and for the same top down model (marked "SLBY98a", for details see this paper) as discussed overleaf for the "visible" gamma ray and nucleon fluxes that are shown again as dashed red line and dotted black line, respectively. The data shown for the cosmic ray flux and the diffuse gamma ray flux from EGRET are as overleaf. Points with arrows represent approximate upper limits on the diffuse neutrino flux from the Frejus, the EAS-TOP, and the Fly's Eye experiments, as indicated. The projected limit for the proposed Pierre Auger Project is for non-detection over a five year period, and similarly for the Orbiting Wide-angle Light collectors (OWL). The neutrino flux prediction from this TD scenario produces an integral flux of about 0.15 events per year above 10 EeV in a 2 pi km**3 sr size detector, which is close to the model independent upper limit discussed above.

One of the major unresolved questions in particle physics is whether neutrinos have mass. If they have masses m of several electron volts, the relic neutrino background would constitute a significant fraction of the energy density of the present day universe in the form of hot dark matter. At the same time an ultra-high energy neutrino of energy E = 4 x 10**(21) eV (eV / m) would be able to resonantly produce a Z boson by interacting with the neutrino background. The decay products might be detectable and provide interesting signatures to indirectly detect the presence of this hot dark matter by ultra-high energy cosmic ray experiments! To exactly quantify such signatures in collaboration with Shigeru Yoshida (ICRR, Tokyo), Pijush Bhattacharjee (Indian Institute for Astrophysics, Bangalore), and Sangjin Lee (U of C) I therefore developed a code that follows this neutrino cascading in detail and couples the secondary gamma-rays and nucleons from neutrino interactions to the electromagnetic and nucleon propagation code we developed earlier.

TD spectrum for exclusive neutrino decay, < 10**(-11) Gauss As an example, the figure on the left shows resulting neutrino fluxes in a TD scenario (marked "SLBY98b", for details see this paper) where heavy X particles of mass 10**14 GeV decay into two neutrinos, using the same line key as above. The neutrino fluxes are maximal in the sense that the secondary cascade gamma rays produced mostly by neutrino interactions with the uniform relic neutrino background is compatible with the EGRET limit. All neutrinos have been assumed to have a mass of 1 eV. The secondary gamma-rays and nucleons at ultra high energies are mostly produced on the local clustered (hot dark matter) component of massive neutrinos and can account for the highest energy events if the product of the clustering length scale and the overdensity over this scale is larger than 150 Mpc. On a Supercluster scale of about 5 Mpc, this would correspond to an overdensity of 30, not inconceivable values. As can be seen, the neutrino fluxes are even comparable with predictions by more conventional models of active galactic nuclei down to 1 PeV, and would give rise to event rates of 0.64 above 10 EeV, and 2.8 above 1 PeV per year in a 2 pi km**3 sr size detector.

AGN neutrino spectrum and secondary production As a further example (for details, see this paper), the figure on the left shows fluxes from a homogeneous distribution of AGNs exclusively emitting neutrinos with an E**(-1) spectrum up to 10**(22) eV and a redshift of 3 (assuming a bright phase exponent of 3). The neutrinos of mass 1 eV were assumed to have an overdensity of 20 on the scale of the local Supercluster. Again, the gamma ray and nucleon fluxes are all secondaries of the neutrino interactions. See also Physical Review Focus 2, story 33 for a popular account of this scenario.

The detection of ultra-high energy neutrinos not only depends on the astrophysical fluxes but also on the neutrino-nucleon cross section which is not measured above 100 TeV. For example, radiative corrections can have an important influence on the character of the air showers created by ultra-high energy neutrinos interacting in ice, water, or the atmosphere; I have discussed Standard Model predictions in this paper.

In addition, new interactions beyond the electroweak scale could significantly modify cross sections which in turn can be probed by astrophysical ultra-high energy neutrinos.

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