The Unreasonable Effectiveness of the Air-Fluorescence Technique in Determining the EAS Shower Maximum

We review all existing air-fluorescence measurements of the elongation rate of extensive air showers (slope of mean EAS shower maximum (Xmax) vs log of shower energy E) above 1017 eV. We find remarkable agreement for all current and historic experiments over a 30 year period for the energy range from 1017 to 3x1018 eV. The mean elongation rate in this energy interval is near 80 gm/cm2/decade Above this energy, experiments in the Northern hemisphere are in good agreement with an average elongation rate of 48 +/- 10 gm/cm2/decade while Southern hemisphere experiments have a flatter elongation rate of 26 +/- 2 gm/cm2/decade We point out that, given the agreement at lower energies, possible systematic reasons for this difference are unlikely. Given this, the world elongation rate data alone may indicate a composition difference of UHECR in the Northern and Southern hemisphere and thus a diversity of UHECR sources in the Northern and Southern sky.Comment: Accepted by JET

Search for correlations between HiRes stereo events and active galactic nuclei

ManuscriptWe have searched for correlations between the pointing directions of ultrahigh energy cosmic rays observed by the High Resolution Fly's Eye experiment and Active Galactic Nuclei (AGN) visible from its northern hemisphere location. No correlations, other than random correlations, have been found.We report our results using search parameters prescribed by the Pierre Auger collaboration. Using these parameters, the Auger collaboration concludes that a positive correlation exists for sources visible to their southern hemisphere location. We also describe results using two methods for determining the chance probability of correlations: one in which a hypothesis is formed from scanning one half of the data and tested on the second half, and another which involves a scan over the entire data set. The most significant correlation found occurred with a chance probability of 24%

First observation of the Greisen-Zatsepin-Kuzmin suppression

Journal ArticleThe High Resolution Fly's Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes' measurement of the flux of ultrahigh energy cosmic rays shows a sharp suppression at an energy of 6 x 10^19 eV, consistent with the expected cutoff energy. We observe the ankle of the cosmic-ray energy spectrum as well, at an energy of 4 x 10^18 eV. We describe the experiment, data collection, and analysis and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described

Measurement of the flux of ultrahigh energy cosmic rays from monocular observations by the High Resolution Fly's Eye experiment

Journal ArticleWe have measured the cosmic ray spectrum above 1017.2 eV using the two air-fluorescence detectors of the High Resolution Fly's Eye observatory operating in monocular mode.We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors.We fit the spectrum to a model consisting of galactic and extragalactic sources

Evidence for a high-energy cosmic-ray spectrum cutoff

Journal ArticleWe report a measurement of the ultrahigh-energy cosmic-ray spectrum using an atmospheric fluorescence technique for extensive-air-shower detection. The differential spectrum between 0.1 and 10 EeV (1 EeV = 10^18 eV) is well fitted by a power law with slope 2.94 ±0.02. Above 10 EeV evidence is presented for the development of a spectral " bump " followed by a cutoff at 70 EeV

Limits on deeply penetrating particles in the >10^17 eV cosmic-ray flux

Journal ArticleWe report on a search for deeply penetrating particles in the > 10^17 eV cosmic-ray flux using the University of Utah Fly's Eye detector. No such events have been found in 6 x 106 sec of running time. We consequently set limits on the following: quark matter in the primary cosmic-ray flux, high-energy long-lived weakly interacting particles produced in proton-air interactions, such as τ's; astrophysical neutrino flux; and other hypothetical high-energy weakly interacting components of the cosmic-ray flux such as photinos

Disappointing model for ultrahigh-energy cosmic rays

Data of Pierre Auger Observatory show a proton-dominated chemical composition of ultrahigh-energy cosmic rays spectrum at (1 - 3) EeV and a steadily heavier composition with energy increasing. In order to explain this feature we assume that (1 - 3) EeV protons are extragalactic and derive their maximum acceleration energy, E_p^{max} \simeq 4 EeV, compatible with both the spectrum and the composition. We also assume the rigidity-dependent acceleration mechanism of heavier nuclei, E_A^{max} = Z x E_p^{max}. The proposed model has rather disappointing consequences: i) no pion photo-production on CMB photons in extragalactic space and hence ii) no high-energy cosmogenic neutrino fluxes; iii) no GZK-cutoff in the spectrum; iv) no correlation with nearby sources due to nuclei deflection in the galactic magnetic fields up to highest energies.Comment: 4 pages, 7 figures, the talk presented by A. Gazizov at NPA5 Conference, April 3-8, 2011, Eilat, Israe

Measurement of the flux of ultra high energy cosmic rays by the stereo technique

ManuscriptThe High Resolution Fly's Eye (HiRes) experiment has measured the flux of ultrahigh energy cosmic rays using the stereoscopic air fluorescence technique. The HiRes experiment consists of two detectors that observe cosmic ray showers via the fluorescence light they emit. HiRes data can be analyzed in monocular mode, where each detector is treated separately, or in stereoscopic mode where they are considered together. Using the monocular mode the HiRes collaboration measured the cosmic ray spectrum and made the first observation of the Greisen-Zatsepin-Kuzmin cutoff. In this paper we present the cosmic ray spectrum measured by the stereoscopic technique. Good agreement is found with the monocular spectrum in all details

The structure of EAS at E 0.1 EeV

The ratio of extensive air showers (EAS) total shower energy in the electromagnetic channel (E em) to the size of the shower at maximum development (N max) from a direct measurement of shower longitudinal development using the air fluorescence technique was calculated. The values are not inconsistent with values based upon track length integrals of the Gaisser-Hillas formula for shower development or the known relation between shower energy and size at maximum for pure electromagnetic cascades. Using Linsley's estimates for undetected shower energy based on an analysis of a wide variety of cosmic ray data, the following relation for total shower energy E vs N max is obtained. The Gaisser Hillas implied undetected shower energy fractions