arXiv subjects

Thomas J. Weiler

Publications and source records attributed to Thomas J. Weiler.

At least 73 records · Page 4Linked to original sources

arXiv2007

TeV gamma-rays from photo-disintegration/de-excitation of nuclei in Westerlund 2

TeV gamma-rays can result from the photo-de-excitation of PeV cosmic ray nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HESS source at the edge of Westerlund 2. The UV background is provided by Lyman-alpha emission within the rich O and B stellar environment. The HESS flux results if there is efficient acceleration at the source of lower energy nuclei. The requirement that the Lorentz-boosted ultraviolet photons reach the Giant Dipole resonant energy (~ 20 MeV) implies a strong suppression of the gamma-ray spectrum compared to an E_γ^{-2} behavior at energies below about 1 TeV. This suppression is not apparent in the lowest-energy Westerlund 2 datum, but will be probed by the upcoming GLAST mission.

astro-ph↗

arXiv2007

TeV gamma-rays and neutrinos from photo-disintegration of nuclei in Cygnus OB2

TeV gamma-rays may provide significant information about high energy astrophysical accelerators. Such gamma-rays can result from the photo-de-excitation of PeV nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HEGRA source at the edge of the Cygnus OB2 association. The Lyman-alpha background is provided by the rich O and B stellar environment. It is found that (1) the HEGRA flux can be obtained if there is efficient acceleration at the source of lower energy nuclei; (2) the requirement that the Lorentz-boosted ultraviolet photons can excite the Giant Dipole resonance implies a strong suppression of the gamma-ray spectrum compared to an E_γ^{-2} behavior at energies \alt 1 TeV (some of these energies will be probed by the upcoming GLAST mission); (3) a TeV neutrino counterpart from neutron decay following helium photo-disintegration will be observed at IceCube only if a major proportion of the kinetic energy budget of the Cygnus OB2 association is expended in accelerating nuclei.

astro-ph↗

arXiv2007

TeV gamma-rays from photo-disintegration/de-excitation 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*) may dominate, namely the photo-disintegration of highly boosted nuclei followed by daughter de-excitation. Starbust 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 energy minimum ~ 10 TeV/(T/eV) for gamma-ray emission from a thermal region of temperature T. We also check that a diffuse gamma-ray component resulting from the interaction of a possible extreme-energy cosmic-ray nuclei with background radiation is well below the observed EGRET data. The A* mechanism described herein offers an important contribution to gamma-ray astronomy in the era of intense observational activity.

astro-ph↗

arXiv2006

Shortcuts in extra dimensions and neutrino physics

We discuss the possibility of sterile neutrinos taking shortcuts in extra dimensions, and the consequences for neutrino oscillations. This effect influences the active-sterile neutrino mixing and provides a possibility to accommodate the LSND evidence for neutrino oscillations together with bounds from accelerator and reactor experiments. We briefly comment on causality in such schemes.

hep-ph↗

arXiv2006

Radiative neutrino decay and CP-violation in R-parity violating supersymmetry

We calculate the radiative decay amplitude for Majorana neutrinos in trilinear R-parity violating supersymmetric framework. Our results make no assumption regarding the masses and mixings of fermions and sfermions. The results obtained are exemplary for generic models with loop-generated neutrino masses. Comparison of this amplitude with the neutrino mass matrix shows that the two provide independent probes of CP-violating phases.

hep-ph↗

arXiv2005

Acceptances for Space-Based and Ground-Based Fluorescence Detectors, and Inference of the Neutrino-Nucleon Cross-Section above 10^19 eV

Detection of ultra-high energy neutrinos will be useful for unraveling the dynamics of the most violent sources in the cosmos and for revealing the neutrino cross-section at extreme energy. Above ~ 10^20 eV, neutrinos may well be the only cosmic primariies. Thus, it is important to know the acceptances (event rate/flux) of proposed air-shower experiments for detecting ``horizontal'' neutrino events initiated in our atmosphere, and ``Earth-skimming'' events initiated in the Earth's surface rock or ocean. We calculate these acceptances for fluorescence detectors, both space-based as with the EUSO and OWL proposals, and ground-based, as with Auger, HiRes and Telescope Array. The neutrino cross-section sigma_nuN is not measured at energies above 5.2 x 10^13 eV. Although QCD extrapolations offer motivated guesses for sigma_nuN, new physics could intervene to provide a surprise. Therefore, we present the acceptances of horizontal (HAS) and upgoing (UAS) air showers as a function of sigma_nuN over the interesting range 10^(-34) to 10^(-30) cm^2. The dependences of acceptances on neutrino energy, shower-threshold energy, shower length and column density, and cloud layers are also studied. For UAS, we present acceptances for events over land (rock), and over the ocean (water). The latter are larger by about an order of magnitude, thus favoring space-based detectors. We revisit the idea of Kusenko and Weiler to infer sigma_nuN at E_nu ~ 10^20 eV from the ratio of HAS-to-UAS events, and find favorable results. Our calculation is mostly analytic. Included in the UAS calculation are realistic energy-losses for taus, and Earth-curvature effects.

astro-ph↗

arXiv2005

Sterile-active neutrino oscillations and shortcuts in the extra dimension

We discuss a possible new resonance in active-sterile neutrino oscillations arising in theories with large extra dimensions. Fluctuations in the brane effectively increase the path-length of active neutrinos relative to the path-length of sterile neutrinos through the extra-dimensional bulk. Well below the resonance, the standard oscillation formulas apply. Well above the resonance, active-sterile oscillations are suppressed. We show that a resonance energy in the range of 30-400 MeV allows an explanation of all neutrino oscillation data, including LSND data, in a consistent four-neutrino model. A high resonance energy implies an enhanced signal in MiniBooNE. A low resonance energy implies a distorted energy spectrum in LSND, and an enhanced v_mu depletion from a stopped-pion source. The numerical value of the resonance energy may be related back to the geometric aspects of the brane world. Some astrophysical and cosmological consequences of the brane-bulk resonance are briefly sketched.

hep-ph↗

arXiv2005

Measuring Flavor Ratios of High-Energy Astrophysical Neutrinos

We discuss the prospects for next generation neutrino telescopes, such as IceCube, to measure the flavor ratios of high-energy astrophysical neutrinos. The expected flavor ratios at the sources are $ϕ_{ν_e}:ϕ_{ν_μ}:ϕ_{ν_τ} = 1:2:0$, and neutrino oscillations quickly transform these to $1:1:1$. The flavor ratios can be deduced from the relative rates of showers ($ν_e$ charged-current, most $ν_τ$ charged-current, and all flavors neutral-current), muon tracks ($ν_μ$ charged-current only), and tau lepton lollipops and double-bangs ($ν_τ$ charged-current only). The peak sensitivities for these interactions are at different neutrino energies, but the flavor ratios can be reliably connected by a reasonable measurement of the spectrum shape. Measurement of the astrophysical neutrino flavor ratios tests the assumed production mechanism and also provides a very long baseline test of a number of exotic scenarios, including neutrino decay, CPT violation, and small-$δm^2$ oscillations to sterile neutrinos.

hep-ph↗

arXiv2005

Z-bursts from the Virgo cluster

Resonant annihilation of ultra-high energy cosmic neutrinos (UHECnu) on the cosmic neutrino background (CnuB) into Z bosons--the Z-burst mechanism--and its associated absorption and emission phenomenology provide a unique, albeit indirect, probe of the CnuB in its present state. In this paper, we examine the implications of gravitational clustering of the CnuB in nearby galaxy clusters for the Z-burst phenomenology. In particular, we study the emission features of the Z-decay products originating from the Virgo cluster, and the potential of future cosmic ray experiments to observe clustering-enhanced Z-burst rates. We find that a detector with an exposure equivalent to three years of observations at the Extreme Universe Space Observatory (EUSO) will very likely measure these enhanced rates together with the associated UHECnu flux, provided that the latter saturates current observational limits and the neutrino masses are quasi-degenerate, m_nu_i < 0.1 eV. In the case of UHECnu fluxes below the electromagnetic cascade limit, or a hierarchical neutrino mass spectrum, an experimental sensitivity exceeding that of EUSO by at least two orders of magnitude is required to detect the clustering enhancements with any certainty.

astro-ph↗

arXiv2005

Probing Planck scale physics with IceCube

Neutrino oscillations can be affected by decoherence induced e.g. by Planck scale suppressed interactions with the space-time foam predicted in some approaches to quantum gravity. We study the prospects for observing such effects at IceCube, using the likely flux of TeV antineutrinos from the Cygnus spiral arm. We formulate the statistical analysis for evaluating the sensitivity to quantum decoherence in the presence of the background from atmospheric neutrinos, as well as from plausible cosmic neutrino sources. We demonstrate that IceCube will improve the sensitivity to decoherence effects of ${\cal O}(E^2/M_{\rm Pl})$ by 17 orders of magnitude over present limits and, moreover, that it can probe decoherence effects of ${\cal O}(E^3/M_{\rm Pl}^2)$ which are well beyond the reach of other experiments.

hep-ph↗

arXiv2005

Neutrinos as a diagnostic of cosmic ray Galactic/extra-galactic transition

Motivated by a recent change in viewing the onset of the extra-galactic component in the cosmic ray spectrum, we have fitted the observed data down to $10^{8.6}$ GeV and have obtained the corresponding power emissivity. This transition energy is well below the threshold for resonant $pγ$ absorption on the cosmic microwave background, and thus source evolution is an essential ingredient in the fitting procedure. Two-parameter fits in the spectral and redshift evolution indices show that a standard Fermi $E_i^{-2}$ source spectrum is excluded at larger than 95% confidence level (CL). Armed with the primordial emissivity, we follow Waxman and Bahcall to derive the associated neutrino flux on the basis of optically thin sources. For $pp$ interactions as the generating mechanism, the neutrino flux exceeds the AMANDA-B10 90%CL upper limits. In the case of $pγ$ dominance, the flux is marginally consistent with AMANDA-B10 data. In the new scenario the source neutrino flux dominates over the cosmogenic flux at all energies. Thus, should data from AMANDA-II prove consistent with the model, we show that IceCube can measure the characteristic power law of the neutrino spectrum, and thus provide a window on the source dynamics.

astro-ph↗

arXiv2005

Neutrinos as a Diagnostic of High Energy Astrophysical Processes

A leading candidate for the extragalactic source of high energy cosmic rays is the Fermi engine mechanism, in which protons confined by magnetic fields are accelerated to very high energy through repeated scattering by plasma shock fronts. In the process of acceleration, collisions of trapped protons with the ambient plasma produce pions which decay to electromagnetic energy and neutrinos. For optically thin sources, a strong connection between the emerging cosmic rays and secondary neutrinos can be established. In this context, we show the feasibility of using the Glashow resonance as a discriminator between the $pp$ and $pγ$ interactions in Fermi engines as sources of neutrinos. In particular, we demonstrate how three years of observation at the km$^3$ IceCube facility can serve as a filter for the dominance of the $pp$ interaction at the source.

hep-ph↗

arXiv2004

Galactic Point Sources of TeV Antineutrinos

High energy cosmic ray experiments have identified an excess from the region of the Galactic Plane in a limited energy range around $10^{18}$ eV (EeV). This is very suggestive of neutrons as candidate primaries, because the directional signal requires relatively-stable neutral primaries, and time-dilated neutrons can reach Earth from typical Galactic distances when the neutron energy exceeds an EeV. We here point out that if the Galactic messengers are neutrons, then those with energies below an EeV will decay in flight, providing a flux of cosmic antineutrinos above a TeV which is {\it observable} at a kilometer-scale neutrino observatory. The expected event rate per year above 1 TeV in a detector such as IceCube, for example, is 20 antineutrino showers (all flavors) and a $1^\circ$ directional signal of 4 $\bar ν_μ$ events. A measurement of this flux can serve to identify the first extraterrestrial point source of TeV antineutrinos.

astro-ph↗

arXiv2004

Space-Time Foam and Cosmic-Ray Interactions

It has been proposed that propagation of cosmic rays at extreme-energy may be sensitive to Lorentz-violating metric fluctuations (``foam''). We investigate the changes in interaction thresholds for cosmic-rays and gamma-rays interacting on the CMB and IR backgrounds, for a class of stochastic models of spacetime foam. The strength of the foam is characterized by the factor (E/M_P)^a, where ``a'' is a phenomenological suppression parameter. We find that there exists a critical value of ``a'' (dependent on the particular reaction), below which the threshold energy can only be lowered, and above which the threshold energy may be raised, but at most by a factor of two. Thus, it does not appear possible in this class of models to extend cosmic-ray spectra significantly beyond their classical absorption energies. However, the lower thresholds resulting from foam may have signatures in the cosmic-ray spectrum. We also find that cosmic-ray energies cannot exceed the fundamental Planck scale, and so set a lower bound of 10^8 TeV on the context of this model for the scale of gravity. Finally, we comment in the apparent non-conservation of particle energy-momentum, and speculate on its re-emergence as dark energy in the foamy vacuum.

hep-ph↗

arXiv2004

Neutrino Bursts from Fanaroff-Riley I Radio Galaxies

On the basis of existing observations (at the 4.5 σlevel) of TeV gamma-ray outbursts from the Fanaroff-Riley I (FRI) radio galaxy Centaurus A, we estimate the accompanying neutrino flux in a scenario where both photons and neutrinos emerge from pion decay. We find a neutrino flux on Earth dF_ν/dE_ν= 4.5\times 10^{-11} (E_ν/TeV)^{-2} TeV^{-1} cm^{-2} s^{-1}, equally spread in flavor as a result of maximal mixing. Such a flux will trigger at the IceCube facility about 10 showers/burst, with negligible background from atmospheric muons, and primary neutrino energies in excess of 100 TeV. The only other FRI radio galaxy observed in the TeV photon energy range at the 4σlevel is M87. The burst nature of this activity is not established; however, we show that the intrinsic neutrino luminosity during the active period is the same as the Centaurus A burst. On the assumption that Centaurus A typifies the FRI population, we show that IceCube should collect 10 showers (all neutrino flavors) in 3 years, attaining a 95% CL sensitivity to the diffuse neutrino flux from FRI radio galaxies in one year of observation.

astro-ph↗

arXiv2004

Relic Neutrino Absorption Spectroscopy

Resonant annihilation of extremely high-energy cosmic neutrinos on big-bang relic anti-neutrinos (and vice versa) into Z-bosons leads to sizable absorption dips in the neutrino flux to be observed at Earth. The high-energy edges of these dips are fixed, via the resonance energies, by the neutrino masses alone. Their depths are determined by the cosmic neutrino background density, by the cosmological parameters determining the expansion rate of the universe, and by the large redshift history of the cosmic neutrino sources. We investigate the possibility of determining the existence of the cosmic neutrino background within the next decade from a measurement of these absorption dips in the neutrino flux. As a by-product, we study the prospects to infer the absolute neutrino mass scale. We find that, with the presently planned neutrino detectors (ANITA, Auger, EUSO, OWL, RICE, and SalSA) operating in the relevant energy regime above 10^{21} eV, relic neutrino absorption spectroscopy becomes a realistic possibility. It requires, however, the existence of extremely powerful neutrino sources, which should be opaque to nucleons and high-energy photons to evade present constraints. Furthermore, the neutrino mass spectrum must be quasi-degenerate to optimize the dip, which implies m_{nu} >~ 0.1 eV for the lightest neutrino. With a second generation of neutrino detectors, these demanding requirements can be relaxed considerably.

hep-ph↗

arXiv2004

Pseudo-Dirac Neutrinos, a Challenge for Neutrino Telescopes

Neutrinos may be pseudo-Dirac states, such that each generation is actually composed of two maximally-mixed Majorana neutrinos separated by a tiny mass difference. The usual active neutrino oscillation phenomenology would be unaltered if the pseudo-Dirac splittings are $δm^2 \alt 10^{-12}$ eV$^2$; in addition, neutrinoless double beta decay would be highly suppressed. However, it may be possible to distinguish pseudo-Dirac from Dirac neutrinos using high-energy astrophysical neutrinos. By measuring flavor ratios as a function of $L/E$, mass-squared differences down to $δm^2 \sim 10^{-18}$ eV$^2$ can be reached. We comment on the possibility of probing cosmological parameters with neutrinos.

hep-ph↗

arXiv2003

Sensitivity to $θ_{13}$ and $δ$ in the Decaying Astrophysical Neutrino Scenario

We have previously shown that the decay of high-energy neutrinos from distant astrophysical sources would be revealed by flavor ratios that deviate strongly from the $ϕ_{ν_e}:ϕ_{ν_μ}:ϕ_{ν_τ} = 1:1:1$ expected from oscillations alone. Here we show that the deviations are significantly larger when the mixing angle $θ_{13}$ and the CP phase $δ$ are allowed to be nonzero. If neutrinos decay, this could allow measurement of $θ_{13}$ and $δ$ in IceCube and other near-term neutrino telescopes.

hep-ph↗