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Register a new userEnergy-independent new physics in the flavour ratios of high-energy astrophysical neutrinos
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M. Bustamante
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We have studied the consequences of breaking the CPT symmetry in the neutrino sector, using the expected high-energy neutrino flux from distant cosmological sources such as active galaxies. For this purpose we have assumed three different hypotheses for the neutrino production model, characterised by the flavour fluxes at production phi_e^0:phi_mu^0:phi_tau^0 = 1:2:0, 0:1:0, and 1:0:0, and studied the theoretical and experimental expectations for the muon-neutrino flux at Earth, phi_mu, and for the flavour ratios at Earth, R = phi_mu/phi_e and S = phi_tau/phi_mu. CPT violation (CPTV) has been implemented by adding an energy-independent term to the standard neutrino oscillation Hamiltonian. This introduces three new mixing angles, two new eigenvalues and three new phases, all of which have currently unknown values. We have varied the new mixing angles and eigenvalues within certain bounds, together with the parameters associated to pure standard oscillations. Our results indicate that, for the models 1:2:0 and 0:1:0, it might possible to find large deviations for phi_mu, R, and S between the cases without and with CPTV, provided the CPTV eigenvalues lie within 10^{-29}-10^{-27} GeV, or above. Moreover, if CPTV exists, there are certain values of R and S that can be accounted for by up to three production models. If no CPTV were observed, we could set limits on the CPTV eigenvalues of the same order. Detection prospects calculated using IceCube suggest that for the models 1:2:0 and 0:1:0, the modifications due to CPTV are larger and more clearly separable from the standard-oscillations predictions. We conclude that IceCube is potentially able to detect CPTV but that, depending on the values of the CPTV parameters, there could be a mis-determination of the neutrino production model.
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Flavor ratios of very high energy astrophysical neutrinos, which can be studied at the Earth by a neutrino telescope such as IceCube, can serve to diagnose their production mechanism at the astrophysical source. The flavor ratios for neutrinos and antineutrinos can be quite different as we do not know how they are produced in the astrophysical environment. Due to this uncertainty the neutrino and antineutrino flavor ratios at the Earth also could be quite different. Nonetheless, it is generally assumed that flavor ratios for neutrinos and antineutrinos are the same at the Earth, in fitting the high energy astrophysical neutrino data. This is a reasonable assumption for the limited statistics for the data we currently have. However, in the future the fit must be performed allowing for a possible discrepancy in these two fractions in order to be able to disentangle different production mechanisms at the source from new physics in the neutrino sector. To reinforce this issue, in this work we show that a wrong assumption about the distribution of neutrino flavor ratios at the Earth may indeed lead to misleading interpretations of IceCube results.
We review the prospects for probing new physics with neutrino astrophysics. High energy neutrinos provide an important means of accessing physics beyond the electroweak scale. Neutrinos have a number of advantages over conventional astronomy and, in particular, carry information encoded in their flavor degree of freedom which could reveal a variety of exotic neutrino properties. We also outline ways in which neutrino astrophysics can be used to constrain dark matter properties, and explain how neutrino-based limits lead to a strong general bound on the dark matter total annihilation cross-section.
We present an in-depth analysis of the flavour and spectral composition of the 36 high-energy neutrino events observed after three years of observation by the IceCube neutrino telescope. While known astrophysical sources of HE neutrinos are expected to produce a nearly $(1:1:1)$ flavour ratio (electron : muon : tau) of neutrinos at earth, we show that the best fits based on the events detected above $E_ u ge 28$ TeV do not necessarily support this hypothesis. Crucially, the energy range that is considered when analysing the HE neutrino data can have a profound impact on the conclusions. We highlight two intriguing puzzles: an apparent deficit of muon neutrinos, seen via a deficit of track-like events; and an absence of $bar u_e$s at high energy, seen as an absence of events near the Glashow resonance. We discuss possible explanations, including the misidentification of tracks as showers, and a broken power law, in analogy to the observed HE cosmic ray spectrum.
We explore the joint implications of ultrahigh energy cosmic ray (UHECR) source environments -- constrained by the spectrum and composition of UHECRs -- and the observed high energy astrophysical neutrino spectrum. Acceleration mechanisms producing power-law CR spectra $propto E^{-2}$ are compatible with UHECR data, if CRs at high rigidities are in the quasi-ballistic diffusion regime as they escape their source environment. Both gas- and photon-dominated source environments are able to account for UHECR observations, however photon-dominated sources do so with a higher degree of accuracy. However, gas-dominated sources are in tension with current neutrino constraints. Accurate measurement of the neutrino flux at $sim 10$ PeV will provide crucial information on the viability of gas-dominated sources, as well as whether diffusive shock acceleration is consistent with UHECR observations. We also show that UHECR sources are able to give a good fit to the high energy portion of the astrophysical neutrino spectrum, above $sim$ PeV. This common origin of UHECRs and high energy astrophysical neutrinos is natural if air shower data is interpreted with the textsc{Sibyll2.3c} hadronic interaction model, which gives the best-fit to UHECRs and astrophysical neutrinos in the same part of parameter space, but not for EPOS-LHC.
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