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Guenter Sigl

Publications and source records attributed to Guenter Sigl.

At least 91 records · Page 5Linked to original sources
arXiv2000

Particle and Astrophysics Aspects of Ultrahigh Energy Cosmic Rays

The origin of cosmic rays is one of the major unresolved astrophysical questions. In particular, the highest energy cosmic rays observed possess macroscopic energies and their origin is likely to be associated with the most energetic processes in the Universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond the Standard Model and processes taking place at the earliest moments of our Universe. Furthermore, many new experimental activities promise a strong increase of statistics at the highest energies and a combination with $γ-$ray and neutrino astrophysics will put strong constraints on these theoretical models. Detailed Monte Carlo simulations indicate that charged ultra-high energy cosmic rays can also be used as probes of large scale magnetic fields whose origin may open another window into the very early Universe. We give an overview over this quickly evolving research field.

astro-ph↗
arXiv2000

Testing the Correlation of Ultra-High Energy Cosmic Rays with High Redshift Sources

We study the correlation between compact radio quasars or 3EG gamma-ray blazars and the arrival directions of cosmic rays above 10^19 eV using an updated list of air shower detections. Our Monte Carlo simulations reveal no significant correlations above random and some previous positive results appear to be an effect of the small sample size. Consequently, unless somehow severely deflected, there is no evidence for ultra-high energy cosmic ray primaries being new particles or particles with new interactions beyond the electroweak scale, produced in high-redshift active galactic nuclei.

astro-ph↗
arXiv1999

Origin and Propagation of Extremely High Energy Cosmic Rays

Cosmic ray particles with energies in excess of 10**(20) eV have been detected. The sources as well as the physical mechanism(s) responsible for endowing cosmic ray particles with such enormous energies are unknown. This report gives a review of the physics and astrophysics associated with the questions of origin and propagation of these Extremely High Energy (EHE) cosmic rays in the Universe. After a brief review of the observed cosmic rays in general and their possible sources and acceleration mechanisms, a detailed discussion is given of possible "top-down" (non-acceleration) scenarios of origin of EHE cosmic rays through decay of sufficiently massive particles originating from processes in the early Universe. The massive particles can come from collapse and/or annihilation of cosmic topological defects (such as monopoles, cosmic strings, etc.) associated with Grand Unified Theories or they could be some long-lived metastable supermassive relic particles that were created in the early Universe and are decaying in the current epoch. The highest energy end of the cosmic ray spectrum can thus be used as a probe of new fundamental physics beyond Standard Model. We discuss the role of existing and proposed cosmic ray, gamma-ray and neutrino experiments in this context. We also discuss how observations with next generation experiments of images and spectra of EHE cosmic ray sources can be used to obtain new information on Galactic and extragalactic magnetic fields and possibly their origin.

astro-ph↗
arXiv1999

On Metric Preheating

We consider the generation of super-horizon metric fluctuations during an epoch of preheating in the presence of a scalar field χquadratically coupled to the inflaton. We find that the requirement of efficient broad resonance is concomitant with a severe damping of super-horizon δχquantum fluctuations during inflation. Employing perturbation theory with backreaction included as spatial averages to second order in the scalar fields and in the metric, we argue that the usual inflationary prediction for metric perturbations on scales relevant for structure formation is not strongly modified.

hep-ph↗
arXiv1999

Supercluster Magnetic Fields and Anisotropy of Cosmic Rays above 10**(19) eV

We predict energy spectra and angular distributions of nucleons above 10**(19) eV that originate from sources distributed in the Local Supercluster, which is also supposed to contain a large scale magnetic field of strength between 0.05 and 0.5 micro Gauss. We show that this model can explain all present-day features of ultra-high energy cosmic rays, at least for field strengths close to 0.5 micro Gauss. The large-scale anisotropy and the clustering predicted by this scenario will allow strong discrimination against other models with next generation experiments.

astro-ph↗
arXiv1998

Ultra-High Energy Cosmic Ray Propagation in the Local Supercluster

We present detailed numerical simulations and analytical approximations of the propagation of nucleons above 10**(19) eV in the Local Supercluster, assuming that the ambient magnetic field is turbulent, and its strength 0.01 < B_rms < 1 micro-Gauss. In such strong magnetic fields, protons in the low energy part of the spectrum, 10**(19) eV < E < E_C diffuse, while the higher energy particles, with E > E_C propagate along nearly straight lines. The magnitude of the transition energy E_C depends mainly on the strength of the magnetic field, the coherence length, and the distance to the source; for B_rms=0.1 micro-Gauss, a largest eddy of length 10 Mpc, and a distance to the source of 10 Mpc, E_C=100 EeV. Our numerical treatment substantially improves on previous analytical approximations, as it allows to treat carefully the transition between the two propagation regimes, as well as the effects due to inhomogeneities expected on scales of a few Mpc. We show that a turbulent magnetic field B_rms=0.1 micro-Gauss, close to equipartition, would allow to reproduce exactly the observed spectrum of ultra high energy cosmic rays, up to the highest energy observed, for a distance to the source below 10 Mpc, for the geometry of the Local Supercluster, i.e. a sheet of thickness 10 Mpc. Diffusion, in this case, allows to reproduce the high flux beyond the Greisen Zatsepin Kuzmin cut-off, with a soft injection spectrum proportional to E**(-2.4). Moreover, the large deflection angles at the highest energies observed, typically 10 degrees for the above values, would explain why no close-by astrophysical counterpart could be associated with these events.

astro-ph↗
arXiv1998

Probing Grand Unified Theories with Cosmic Ray, Gamma-Ray and Neutrino Astrophysics

We explore scenarios where the highest energy cosmic rays are produced by new particle physics near the grand unification scale. Using detailed numerical simulations of extragalactic nucleon, gamma-ray, and neutrino propagation, we show the existence of an interesting parameter range for which such scenarios may explain part of the data and are consistent with all observational constraints. A combination of proposed observatories for ultra-high energy cosmic rays, neutrino telescopes of a few kilometer scale, and gamma-ray astrophysics instruments should be able to test these scenarios. In particular, for neutrino masses in the eV range, exclusive neutrino decay modes of superheavy particles can give rise to neutrino fluxes comparable to those predicted in models of active galactic nuclei.

hep-ph↗
arXiv1998

Numerical Toy-Model Calculation of the Nucleon Spin Autocorrelation Function in a Supernova Core

We develop a simple model for the evolution of a nucleon spin in a hot and dense nuclear medium. A given nucleon is limited to one-dimensional motion in a distribution of external, spin-dependent scattering potentials. We calculate the nucleon spin autocorrelation function numerically for a variety of potential densities and distributions which are meant to bracket realistic conditions in a supernova core. For all plausible configurations the width of the spin-density structure function is found to be less than the temperature. This is in contrast with a naive perturbative calculation based on the one-pion exchange potential which overestimates the width and thus suggests a large suppression of the neutrino opacities by nucleon spin fluctuations. Our results suggest that it may be justified to neglect the collisional broadening of the spin-density structure function for the purpose of estimating the neutrino opacities in the deep inner core of a supernova. On the other hand, we find no indication that processes such as axion or neutrino pair emission, which depend on nucleon spin fluctuations, are substantially suppressed beyond the multiple-scattering effect already discussed in the literature. Aside from these practical conclusions, our model reveals a number of interesting and unexpected insights. For example, the spin-relaxation rate saturates with increasing potential strength only if bound states are not allowed to form by including a repulsive core. There is no saturation with increasing density of scattering potentials until localized eigenstates of energy begin to form.

hep-ph↗
arXiv1998

Extremely High Energy Neutrinos, Neutrino Hot Dark Matter, and the Highest Energy Cosmic Rays

Extremely high energy (up to 10**(22) eV) cosmic neutrino beams initiate high energy particle cascades in the background of relic neutrinos from the Big Bang. We perform numerical calculations to show that such cascades could contribute more than 10% to the observed cosmic ray flux above 10**(19) eV if neutrinos have masses in the electron volt range. The required intensity of primary neutrinos could be consistent with astrophysical models for their production if the maximum neutrino energy reaches to 10**(22) eV and the massive neutrino dark matter is locally clustered. Future observations of ultra high energy cosmic rays will lead to an indirect but practical search for neutrino dark matter.

hep-ph↗
arXiv1997

Angle-Time-Energy Images of Ultra-High Energy Cosmic Ray Sources

Substantial amount of information both on the source and on characteristics of intercepting magnetic fields is encoded in the distribution in arrival times, directions, and energies of charged ultra-high energy cosmic rays from discrete sources. We present a numerical approach that allows to extract such information from data from next generation experiments.

astro-ph↗
arXiv1997

Reconstruction of Source and Cosmic Magnetic Field Characteristics from Clusters of Ultra-High Energy Cosmic Rays

We present a detailed Monte Carlo study coupled to a likelihood analysis of the potential of next generation ultra-high energy cosmic ray experiments to reconstruct properties of the sources and the extra-galactic magnetic field. Such characteristics are encoded in the distributions of arrival time, direction, and energy of clusters of charged cosmic rays above a few 10**(19) eV. The parameters we consider for reconstruction are the emission timescale, total fluence (or power), injection spectrum, and distance of the source, as well as the r.m.s. field strength, power spectrum, and coherence length of the magnetic field. We discuss five generic situations which can be identified relatively easily and allow a reasonable reconstruction of at least part of these parameters. Our numerical code is set up such that it can easily be applied to the data from future experiments.

astro-ph↗
arXiv1997

Ultra-High Energy Neutrino-Nucleon Cross Section and Radiative Corrections

A cubic kilometer scale experiment has been proposed to detect cosmic neutrinos of energy from tens of GeV up to the highest energies observed for cosmic rays, around 10**20 eV, or possibly even beyond. Detection efficiencies depend crucially on the neutrino-nucleon cross section at these energies at which radiative corrections beyond the lowest order approximation could become non-negligible. The differential cross sections can be modified by more than 50% in some regions of phase space. Estimates of corrections to the quantities most relevant for neutrino detection at these energies give, however, less dramatic effects: The average inelasticity in the outgoing lepton is increased from about 0.19 to 0.24. The inclusive cross section is reduced by roughly half a percent. The dominant uncertainty of the standard model ultra-high energy neutrino-nucleon cross section therefore still comes from uncertainties of the parton distributions in the nucleon at very low momentum fractions.

hep-ph↗
arXiv1997

Ultra-high energy cosmic ray sources and large scale magnetic fields

Protons of energies up to 10**20 eV can be subject to significant deflection and energy dependent time delay in lage scale extragalactic or halo magnetic fields of strengths comparable to current upper limits. By performing 3-dimensional Monte Carlo simulations of nucleon propagation, we show how observations of arrival direction and time distributions can be used to measure the structure and strength of large-scale magnetic fields, and constrain the nature of the source of ultra-high energy cosmic rays.

astro-ph↗
arXiv1997

Maximum Likelihood Analysis of Clusters of Ultra-High Energy Cosmic Rays

We present a numerical code designed to conduct a likelihood analysis for clusters of nucleons above 10**19 eV originating from discrete astrophysical sources such as powerful radio galaxies, gamma-ray bursts or topological defects. The code simulates the propagation of nucleons in a large-scale magnetic field and constructs the likelihood of a given observed event cluster as a function of the average time delay due to deflection in the magnetic field, the source activity time scale, the total fluence of the source, and the power law index of the particle injection spectrum. Other parameters such as the coherence length and the power spectrum of the magnetic field are also considered. We apply it to the three pairs of events above 4X10**19 eV recently reported by the Akeno Giant Air Shower Array (AGASA) experiment, assuming that these pairs were caused by nucleon primaries which originated from a common source. Although current data are too sparse to fully constrain each of the parameters considered, and/or to discriminate models of the origin of ultra-high energy cosmic rays, several tendencies are indicated. If the clustering suggested by AGASA is real, next generation experiments with their increased exposure should detect more than 10 particles per source over a few years and our method will put strong constraints on both the large-scale magnetic field parameters and the nature of these sources.

astro-ph↗
arXiv1997

Nucleon Spin Fluctuations and Neutrino-Nucleon Energy Transfer in Supernovae

The formation of neutrino spectra in a supernova depends crucially on strength and inelasticity of weak interactions in hot nuclear matter. Neutrino interactions with nonrelativistic nucleons are mainly governed by the dynamical structure function for the nucleon spin density which describes its fluctuations. It has recently been shown that these fluctuations give rise to a new mode of energy transfer between neutrinos and nucleons which inside the neutrinosphere is of comparable or greater importance than ordinary recoil. We calculate numerically the spin density structure function in the limit of a dilute, non-degenerate medium from exact two-nucleon wave functions for some representative nuclear interaction potentials. We show that spectrum and magnitude of the energy transfer can deviate significantly from those based on the Born approximation. They are, however, rather insensitive to the particular nuclear potential as long as it reproduces experimental nucleon scattering phase shifts at energies up to a few tens of MeV. We also compare with calculations based on a one-pion exchange potential in Born approximation and briefly comment on their applicability near the center of a supernova core. Our study is relevant for numerical simulations of the neutrino spectra emerging from type-II supernovae.

astro-ph↗
arXiv1996

A Fresh Look at Axions and SN 1987A

We re-examine the very stringent limits on the axion mass based on the strength and duration of the neutrino signal from SN 1987A, in the light of new measurements of the axial-vector coupling strength of nucleons, possible suppression of axion emission due to many-body effects, and additional emission processes involving pions. The suppression of axion emission due to nucleon spin fluctuations induced by many-body effects degrades previous limits by a factor of about 2. Emission processes involving thermal pions can strengthen the limits by a factor of 3-4 within a perturbative treatment that neglects saturation of nucleon spin fluctuations. Inclusion of saturation effects, however, tends to make the limits less dependent on pion abundances. The resulting axion mass limit also depends on the precise couplings of the axion and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.

astro-ph↗
arXiv1996

Topological Defect Models of Ultra-High Energy Cosmic Rays

We give an overview over models in which cosmic rays above 1 EeV = 10**(18) eV are produced by the decay of supermassive "X" particles released from topological defects possibly created in cosmological phase transitions. We note that, for an interesting particle physics parameter range, these models are still consistent with current data, and discuss signatures for the topological defect mechanism which can be tested by the next generation experiments.

astro-ph↗