Session C4: Invited Session: Astrophysical and Cosmological Neutrinos

Theoretical Implications of IceCube Neutrinos

The IceCube Collaboration has recently found evidence for an astrophysical flux of neutrinos. The flux is consistent with an isotropic and equal-flavor $E^{-2}$ power-law spectrum from 60 TeV to 2 PeV. There are also indications that the neutrino spectrum beyond 2 PeV requires a spectral break or cutoff. The origin of the IceCube excess is not known, but its multi-messenger context can already provide some theoretical orientation. For instance, the production of PeV neutrinos require hadronic interactions of cosmic rays (CRs) with energies of a few 10 PeV, extending into the poorly understood transition region between Galactic and extra-Galactic CRs. A local contribution to the neutrino flux from Galactic accelerators is hence feasible and could show up as arrival direction clustering towards Galactic structures. In this context, a possible association of the PeV neutrino sources with unidentified TeV gamma-ray sources, peculiar supernovae or the Fermi Bubbles has been speculated. In addition, a local hadronic neutrino production would predict an observable PeV gamma-ray flux. Spectral features of the neutrino flux, in particular a break or cutoff, serve as additional hints for candidate CR sources and astrophysical environments for neutrino production. Possible scenearios include starburst galaxies, low-luminosity gamma-ray bursts and the cores of active galactic nuclei. I will outline general theoretical implications of the IceCube excess and summarize various source candidates.   

Searching for the neutrino flux from cosmic GZK interactions

There is expected to be an observable flux of ultra-high energy neutrinos from interactions between the highest energy cosmic rays and cosmic microwave background photons through what is known as the GZK process. Once observed, this GZK-induced neutrino flux will be the key to answering questions about the highest energy universe at cosmic distances that cannot be probed with cosmic rays. I will review the status of searches for ultra-high energy neutrinos and what the results mean for constraining the GZK-induced neutrino flux. I will then outline the implications of current and future constraints on neutrino flux models for understanding the nature of the highest energy astrophysics sources as well as fundamental physics at extreme energy and distance scales.   

Cosmological constraints on number of neutrinos and neutrino masses

The number of species of neutrinos ($N_{\rm eff}$) and the neutrino masses ($\Sigma m_\nu$) has been constrained by the measurement of cosmological signals, including the power spectrum of cosmic microwave background (CMB), baryon acoustic oscillations (BAO) and the expansion rate of local Universe ($H_0$). I will report the recent constraints on $N_{\rm eff}$ and $\Sigma m_\nu$ by CMB measurements from WMAP, SPT and especially Planck satellite and its combination with BAO or $H_0$ measurement. The physical stories of how $N_{\rm eff}$ and $\Sigma m_\nu$ are constrained are different. They come from the different features on different angular scales of CMB power spectrum, which will be presented given the current precision of Planck data. I will show how $N_{\rm eff}$ and $\Sigma m_\nu$ are further constrained by adding BAO and $H_0$ data. The impact of $N_{\rm eff}$ and $\Sigma m_\nu$ to the consistency between CMB, BAO and $H_0$ data will also be discussed.