Cosmogenic neutrinos the guaranteed signal
Secondary neutrinos and photons can be produced by ultrahigh energy cosmic rays (UHECRs) when they interact with ambient baryonic matter and radiation fields inside the source or during their propagation from source to Earth. These particles travel in geodesics unaffected by magnetic fields and bear valuable information of the birthplace of their progenitors. The quest for sources of UHECRs has thus long been associated with the detection of neutrinos and gamma rays that might pinpoint the position of the accelerators in the sky.
The detection of these particles is not straightforward however: first, the propagation of gamma rays with energy exceeding several TeV is affected by their interaction with CMB and radio photons. These interactions lead to the production of high energy electron and positron pairs which in turn up-scatter CMB or radio photons by inverse Compton processes, initiating electromagnetic cascades. As a consequence, one does not expect to observe gamma rays of energy above ∼ 100 TeV from sources located beyond a horizon of a few Mpc (Wdowczyk et al. 1972; Protheroe 1986; Protheroe & Stanev 1993). Above EeV energies, photons can again propagate over large distances, depending on the radio background, and can reach observable levels around tens of EeV (Lee 1998). Secondary neutrinos are very useful because, unlike cosmic-rays and photons, they are not absorbed by the cosmic backgrounds while propagating through the Universe. In particular, they give a unique access to observing sources at PeV energies. However, their small interaction cross-section makes it difficult to detect them on the Earth requiring the construction of km3 detectors (see, e.g., Anchordoqui & Montaruli 2010).
A number of authors have estimated the cosmogenic neutrino flux with varying assumptions (e.g., Engel et al. 2001; Ave et al. 2005; Seckel & Stanev 2005; Hooper et al. 2005; Berezinsky 2006; Stanev et al. 2006; Allard et al. 2006; Takami et al. 2009; Kotera et al. 2010). Figure 1 summarizes the effects of different assumptions about the UHECR source evolution, the Galactic to extragalactic transition, the injected chemical composition, and the maximum acceleration energy Emax, on the cosmogenic neutrino flux. It demonstrates that the parameter space is currently poorly constrained with uncertainties of several orders of magnitude in the predicted flux. UHECR models with large proton Emax(> 100 EeV), source evolution corresponding to the star formation history or the GRB rate evolution, dip or ankle transition models, and pure proton or mixed ‘Galactic’ compositions are shaded in grey in Figure 1 and give detectable fluxes in the EeV range with 0.06 − 0.2 neutrino per year at IceCube and 0.03 − 0.06 neutrino per year for the Auger Observatory. If EeV neutrinos are detected, PeV information can help select between competing models of cosmic ray composition at the highest energy and the Galactic to extragalactic transition at ankle energies. With improved sensitivity, ZeV (=10^21 eV) neutrino observatories, such as ANITA and JEM-EUSO could explore the maximum acceleration energy.
Figure 1 - [Updated experimental sensitivities 2012, please cite Kotera et al. (2010), and email me for the latest version, or for predicted neutrino flux data files.]
Cosmogenic neutrino flux for all flavors, for different UHECR parameters compared to instrument sensitivities. Pink solid line corresponds to a strong source evolution case (FRII evolution, see Wall et al. 2005) with a pure proton composition, dip transition model, and Emax = 3 ZeV. Blue lines correspond to uniform source evolution with: iron rich (30%) composition and EZ,max < Z 10 EeV (dotted line) and pure iron injection and EZ,max = Z 100 EeV (solid). Grey shaded range brackets dip and ankle transition models, with evolution of star formation history for z < 4, pure proton and mixed ‘Galactic’ compositions, and large proton Emax(> 100 EeV)). Including the uniform source evolution would broaden the shaded area down to the black solid line. Current experimental limits (solid lines) assume 90% confidence level and full mixing neutrino oscillation. The differential limit and the integral flux limit on a pure E−2 spectrum (straight line) are presented for IceCube 22 lines (pale blue, Abbasi et al. 2010), ANITA-II (green, Gorham et al. 2010) and Auger South (red, Abraham et al. 2009a). For future instruments, we present the projected instrument sensitivities (dashed lines) for IceCube 80 lines (pale blue, acceptances from S. Yoshida, private communication, see also Karle 2010), and for JEM-EUSO (purple, Medina-Tanco et al. 2009).
While propagating from their source to the observer, ultrahigh energy cosmic rays interact with cosmological photon backgrounds and generate to the so-called “cosmogenic neutrinos”. In Kotera, Allard & Olinto (2010), we scanned over the whole parameter space of ultrahigh energy cosmic ray scenarios, and showed the corresponding expected fluxes.
