J. Alvarez-Muñiz

Neutrinos from individual gamma-ray bursts in the BATSE catalog

Abstract We estimate the neutrino emission from individual γ-ray bursts observed by the BATSE detector on the Compton Gamma-Ray Observatory. Neutrinos are produced by photoproduction of pions when protons interact with photons in the region where the kinetic energy of the relativistic fireball is dissipated allowing the acceleration of electrons and protons. We also consider models where neutrinos are predominantly produced on the radiation surrounding the newly formed black hole. From the observed redshift and photon flux of each individual burst, we compute the neutrino flux in a variety of models based on the assumption that equal kinetic energy is dissipated into electrons and protons. Where not measured, the redshift is estimated by other methods. Unlike previous calculations of the universal diffuse neutrino flux produced by all γ-ray bursts, the individual fluxes (compiled at http://www.arcetri.astro.it/~dafne/grb/ ) can be directly compared with coincident observations by the AMANDA telescope at the South Pole. Because of its large statistics, our predictions are likely to be representative for future observations with larger neutrino telescopes.

Trigger and aperture of the surface detector array of the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory consists of 1600 water-Cherenkov detectors, for the study of extensive air showers (EAS) generated by ultra-high-energy cosmic rays. We describe the trigger hierarchy, from the identification of candidate showers at the level of a single detector, amongst a large background (mainly random single cosmic ray muons), up to the selection of real events and the rejection of random coincidences. Such trigger makes the surface detector array fully efficient for the detection of EAS with energy above 3 x 10(18) eV, for all zenith angles between 0 degrees and 60 degrees, independently of the position of the impact point and of the mass of the primary particle. In these range of energies and angles, the exposure of the surface array can be determined purely on the basis of the geometrical acceptance

Detecting microscopic black holes with neutrino telescopes

If spacetime has more than four dimensions, ultra-high energy cosmic rays may create microscopic black holes. Black holes created by cosmic neutrinos in the Earth will evaporate, and the resulting hadronic showers, muons, and taus may be detected in neutrino telescopes below the Earth’s surface. We simulate such events in detail and consider black hole cross sections with and without an exponential suppression factor. We find observable rates in both cases: for conservative cosmogenic neutrino fluxes, several black hole events per year are observable at the IceCube detector; for fluxes at the Waxman-Bahcall bound, tens of events per year are possible. We also present zenith angle and energy distributions for all three channels. The ability of neutrino telescopes to differentiate hadrons, muons, and possibly taus, and to measure these distributions provides a unique opportunity to identify black holes, to experimentally constrain the form of black hole production cross sections, and to study Hawking evaporation.

Coherent radio pulses from showers in different media: A unified parametrization

We study the frequency and angular dependences of Cherenkov radio pulses originated by the excess of electrons in electromagnetic showers in different dense media that are being given consideration as targets for ultra high energy neutrino detection such as ice, salt and the lunar regolith. By means of a simple model of shower development we rederive the scaling of the electric field spectrum with the properties of the media such as density, radiation length, Moli\`ere radius, critical energy and refraction index. We compare the predictions of the scaling relations to the numerical results obtained in our own developed GEANT4-based Monte Carlo simulation, and address in a quantitative manner the predictive power of the scaling relations. We also give a unified parametrization of the frequency spectrum and angular distribution of the electric field in ice, salt, and the lunar regolith, in terms of the relevant properties of the media to be used in practical applications. We establish the applicability of our parametrizations. Our formulas are useful for estimates of the acceptance of current and future neutrino detectors using the radio technique, and for many other practical applications.

Coherent Radiation from Extensive Air Showers in the Ultra-High Frequency Band

Using detailed Monte Carlo simulations we have characterized the features of the radio emission of inclined air showers in the ultrahigh frequency band (300 MHz\char21{}3 GHz). The Fourier spectrum of the radiation is shown to have a sizable intensity well into the GHz frequency range. The emission is mainly due to transverse currents induced by the geomagnetic field and the excess charge produced by the Askaryan effect. At these frequencies only a significantly reduced volume of the shower around the axis contributes coherently to the signal observed on the ground. The size of the coherently emitting volume depends on frequency, shower geometry and observer position, and is interpreted in terms of the relative time delays at observation dominated by the curvature of the shower front. At ground level, the maximum emission at high frequencies is concentrated in an elliptical ringlike region around the intersection of a Cherenkov cone with its vertex at shower maximum and the ground. The frequency spectrum of inclined showers when observed at positions that view shower maximum in the Cherenkov direction, is shown to be in broad agreement with the pulses detected by the Antarctic Impulsive Transient Antenna experiment, making the interpretation that they are due to ultrahigh energy cosmic ray atmospheric showers consistent with our simulations. These results are also of great importance for experiments aiming to detect molecular bremsstrahlung radiation in the GHz range as they present an important background for its detection.

A Search for Microwave Emission From Ultra-High Energy Cosmic Rays

We present a search for microwave emission from air showers induced by ultrahigh energy cosmic rays with the microwave detection of air showers experiment. No events were found, ruling out a wide range of power flux and coherence of the putative emission, including those suggested by recent laboratory measurements.

Practical and accurate calculations of Askaryan radiation

An in-depth characterization of coherent radio Cherenkov pulses from particle showers in dense dielectric media, referred to as the Askaryan effect, is presented. The time-domain calculation developed in this article is based on a form factor to account for the lateral dimensions of the shower. It is computationally efficient and able to reproduce the results of detailed particle shower simulations with high fidelity in most regions of practical interest including Fresnel effects due to the longitudinal development of the shower. In addition, an intuitive interpretation of the characteristics of the Askaryan pulse is provided. We expect our approach to benefit the analysis of radio pulses in experiments exploiting the radio technique.

Radio pulses from ultra-high energy atmospheric showers as the superposition of Askaryan and geomagnetic mechanisms

Abstract Radio emission in atmospheric showers is currently interpreted in terms of radiation due to the deviation of the charged particles in the magnetic field of the Earth and to the charge excess (Askaryan effect). Each of these mechanisms has a distinctive polarization. The complex signal patterns can be qualitatively explained as the interference (superposition) of the fields induced by each mechanism. In this work we explicitly and quantitatively test a simple phenomenological model based on this idea. The model is constructed by isolating each of the two components at the simulation level and by making use of approximate symmetries for each of the contributions separately. The results of the model are then checked against full ZHAireS Monte Carlo simulations of the electric field calculated from first principles. We show that the simple model describes radio emission at a few percent level in a wide range of shower-observer geometries and on a shower-by-shower basis. As a consequence, this approach provides a simple method to reduce the computing time needed to accurately predict the electric field of radio pulses emitted from air showers, with many practical applications in experimental situations of interest.

Atmospheric shower fluctuations and the constant intensity cut method

We explore the constant intensity cut method that is widely used for the derivation of the cosmic ray energy spectrum, for comparisons of data obtained at different atmospheric depths, for measuring average shower profiles, and for estimates of the proton-air cross section from extensive air shower data. The constant intensity cut method is based on the selection of air showers by charged particle or muon size and therefore is subject to intrinsic shower fluctuations. We demostrate that, depending on the selection method, shower fluctuations can strongly influence the characteristics of the selected showers. Furthermore, a mixture of different primaries in the cosmic ray flux complicates the interpretation of measurements based on the method of constant intensity cuts. As an example we consider data published by the Akeno Collaboration. The interpretation of the Akeno measurements suggests that more than 60-70% of cosmic ray primaries in the energy range 10^{16}-19^{17} eV are heavy nuclei. Our conclusions depend only weakly on the hadronic interaction model chosen to perform the simulations, namely SIBYLL and QGSjet.

Characterisation of the electromagnetic component in ultra-high energy inclined air showers

Abstract Inclined air showers – those arriving at ground with zenith angle with respect to the vertical θ > 60 ° – are characterised by the dominance of the muonic component at ground which is accompanied by an electromagnetic “halo” produced mainly by muon decay and muon interactions. By means of Monte Carlo simulations we give a full characterisation of the particle densities at ground in ultra-high energy inclined showers as a function of primary energy and mass composition, as well as for different hadronic models assumed in the simulations. We also investigate the effect of intrinsic shower-to-shower fluctuations in the particle densities.