Flavor composition of the high-energy neutrino events in IceCube.

The IceCube experiment has recently reported the observation of 28 high-energy (>30 TeV) neutrino events, separated into 21 showers and 7 muon tracks, consistent with an extraterrestrial origin. In this Letter, we compute the compatibility of such an observation with possible combinations of neutrino flavors with relative proportion (αe:αµâˆ¶ατ)⊕. Although the 7∶21 track-to-shower ratio is naively favored for the canonical (1∶1∶1)⊕ at Earth, this is not true once the atmospheric muon and neutrino backgrounds are properly accounted for. We find that, for an astrophysical neutrino E(-2) energy spectrum, (1∶1∶1)⊕ at Earth is disfavored at 81% C.L. If this proportion does not change, 6 more years of data would be needed to exclude (1∶1∶1)⊕ at Earth at 3σ C.L. Indeed, with the recently released 3-yr data, that flavor composition is excluded at 92% C.L. The best fit is obtained for (1∶0∶0)⊕ at Earth, which cannot be achieved from any flavor ratio at sources with averaged oscillations during propagation. If confirmed, this result would suggest either a misunderstanding of the expected background events or a misidentification of tracks as showers, or even more compellingly, some exotic physics which deviates from the standard scenario.

TeV gamma rays from photodisintegration and daughter deexcitation of cosmic-ray nuclei.

It is commonly assumed that high-energy gamma rays are made via either purely electromagnetic processes or the hadronic process of pion production, followed by decay. We investigate astrophysical contexts where a third process (A*) would dominate: namely, the photodisintegration of highly boosted nuclei followed by daughter deexcitation. Starburst regions such as Cygnus OB2 appear to be promising sites for TeV gamma-ray emission via this mechanism. A unique feature of the A* process is a sharp flattening of the energy spectrum below approximately 10 TeV/(T/eV) for gamma-ray emission from a thermal region of temperature T. The A* mechanism described herein offers an important contribution to gamma-ray astronomy in the era of intense observational activity.

Low reheating temperature and the visible sterile neutrino.

We present here a scenario, based on a low reheating temperature T(R)<<100 MeV at the end of (the last episode of) inflation, in which the coupling of sterile neutrinos to active neutrinos can be as large as experimental bounds permit (thus making this neutrino "visible" in future experiments). In previous models this coupling was forced to be very small to prevent a cosmological overabundance of sterile neutrinos. Here the abundance depends on how low the reheating temperature is. For example, the sterile neutrino required by the Liquid Scintillator Neutrino Detector result may not have any cosmological problem within our scenario.