[Show abstract][Hide abstract] ABSTRACT: We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d<X(max)>/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4x10(18) eV.
[Show abstract][Hide abstract] ABSTRACT: Stereo data collected by the HiRes experiment over a six year period are examined for large-scale anisotropy related to the inhomogeneous distribution of matter in the nearby Universe. We consider the generic case of small cosmic-ray deflections and a large number of sources tracing the matter distribution. In this matter tracer model the expected cosmic ray flux depends essentially on a single free parameter, the typical deflection angle theta. We find that the HiRes data with threshold energies of 40 EeV and 57 EeV are incompatible with the matter tracer model at a 95% confidence level unless theta is larger than 10 degrees and are compatible with an isotropic flux. The data set above 10 EeV is compatible with both the matter tracer model and an isotropic flux. Comment: 9 pages, 5 Postscript figures
[Show abstract][Hide abstract] ABSTRACT: The 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.
[Show abstract][Hide abstract] ABSTRACT: The High Resolution Fly's Eye (HiRes) experiment is an air fluorescence detector which, operating in stereo mode, has a typical angular resolution of 06 and is sensitive to cosmic rays with energies above 1018 eV. The HiRes cosmic-ray detector is thus an excellent instrument for the study of the arrival directions of ultra-high-energy cosmic rays. We present the results of a search for anisotropies in the distribution of arrival directions on small scales (<5°) and at the highest energies (>1019 eV). The search is based on data recorded between 1999 December and 2004 January, with a total of 271 events above 1019 eV. No small-scale anisotropy is found, and the strongest clustering found in the HiRes stereo data is consistent at the 52% level with the null hypothesis of isotropically distributed arrival directions.
The Astrophysical Journal 12/2008; 610(2):L73. · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Data taken in stereo mode by the High Resolution Fly's Eye (HiRes) air fluorescence experiment are analyzed to search for correlations between the arrival directions of ultrahigh-energy cosmic rays with the positions of BL Lacertae objects. Several previous claims of significant correlations between BL Lac objects and cosmic rays observed by other experiments are tested. These claims are not supported by the HiRes data. However, we verify a recent analysis of correlations between HiRes events and a subset of confirmed BL Lac objects from the 10th Veron Catalog, and we study this correlation in detail. Due to the a posteriori nature of the search, the significance level cannot be reliably estimated and the correlation must be tested independently before any claim can be made. We identify the precise hypotheses that will be tested with statistically independent data.
The Astrophysical Journal 12/2008; 636(2):680. · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The fluorescence yield in air is reported for wavelength and pressure ranges of interest to ultra-high energy cosmic ray detectors. A 28.5 GeV electron beam was used to excite the fluorescence. Central to the approach was the system calibration, using Rayleigh scattering of a nitrogen laser beam. In atmospheric pressure dry air, at 304 K, the yield is 20.8±1.6 photons per MeV.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2008; 175(1):472. · 1.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A key assumption in the reconstruction of extensive air showers using
the air fluorescence technique is that fluorescence is proportional to
energy deposition at all depths in the shower. This ansatz, along with
the supposition that particle distribution and energy loss can be well
modeled by modern shower simulation software, must be thoroughly
verified. We report here the results of the first direct measurement of
air fluorescence yield as a function of shower depth, as performed in
the thick-target phase of the FLASH (FLuorescence in Air from SHowers)
experimental program at the SLAC Final-Focus Test Beam facility. We
compare observed fluorescence light yields as a function of shower depth
to concurrently measured charged particle yields, to the energy
deposition predictions of the EGS and GEANT software packages, and to
empirical energy-deposition models. We also examine the extent to which
the relative yield versus shower depth is independent of wavelength
within the fluorescence spectrum. We find the proportionality hypothesis
to be well supported by the data, validating the use of fluorescence
profiles in the study of ultra high energy cosmic rays.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2008; 597:37-40. · 1.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We 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%.
[Show abstract][Hide abstract] ABSTRACT: The 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.
[Show abstract][Hide abstract] ABSTRACT: Air-fluorescence detectors such as the High Resolution Fly's Eye (HiRes) detector are very sensitive to upward-going, Earth-skimming ultrahigh energy electron-neutrino-induced showers. This is due to the relatively large interaction cross sections of these high-energy neutrinos and to the Landau-Pomeranchuk-Migdal (LPM) effect. The LPM effect causes a significant decrease in the cross sections for bremsstrahlung and pair production, allowing charged-current electron-neutrino-induced showers occurring deep in the Earth's crust to be detectable as they exit the Earth into the atmosphere. A search for upward-going neutrino-induced showers in the HiRes-II monocular dataset has yielded a null result. From an LPM calculation of the energy spectrum of charged particles as a function of primary energy and depth for electron-induced showers in rock, we calculate the shape of the resulting profile of these showers in air. We describe a full detector Monte Carlo simulation to determine the detector response to upward-going electron-neutrino-induced cascades and present an upper limit on the flux of electron-neutrinos. Comment: 13 pages, 3 figures. submitted to Astrophysical Journal
The Astrophysical Journal 03/2008; · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Measurements are reported of the yield and spectrum of fluorescence, excited by a 28.5 GeV electron beam, in air at a range of pressures of interest to ultra-high energy cosmic ray detectors. The wavelength range was 300 - 420 nm. System calibration has been performed using Rayleigh scattering of a nitrogen laser beam. In atmospheric pressure dry air at 304 K the yield is 20.8 +/- 1.6 photons per MeV
[Show abstract][Hide abstract] ABSTRACT: We report the results of a search for point-like deviations from
isotropy in the arrival directions of ultra-high energy cosmic rays in
the northern hemisphere. In the monocular data set collected by the
High-Resolution Fly’s Eye, consisting of 1525 events with energy
exceeding 1018.5 eV, we find no evidence for point-like
excesses. We place a 90% c.l. upper limit of 0.8 hadronic cosmic
rays/km2 yr on the flux from such sources for the northern
hemisphere and place tighter limits as a function of position in the
[Show abstract][Hide abstract] ABSTRACT: We have studied several sources of systematic uncertainty in calculating
the aperture of the High Resolution Fly’s Eye experiment (HiRes)
in monocular mode, primarily as they affect the HiRes-II site. The
energy dependent aperture is determined with detailed Monte Carlo
simulations of the air showers and the detector response. We have
studied the effects of changes to the input energy spectrum and
composition used in the simulation. A realistic shape of the input
spectrum is used in our analysis in order to avoid biases in the
aperture estimate due to the limited detector resolution. We have
examined the effect of exchanging our input spectrum with a simple
E-3 power law in the “ankle” region.
Uncertainties in the input composition are shown to be significant for
energies below ˜1018 eV for data from the HiRes-II
detector. Another source of uncertainties is the choice of the hadronic
interaction model in the air shower generator. We compare the aperture
estimate for two different models: QGSJet01 and SIBYLL 2.1. We also
describe the implications of employing an atmospheric database with
hourly measurements of the aerosol component, instead of using an
average as has been used in our previously published measurements of the
[Show abstract][Hide abstract] ABSTRACT: The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff. HiRes' mea-surement of the flux of cosmic rays shows a sharp suppression at an energy of 6 × 10 19 eV, exactly the expected cutoff energy. We observe the "Ankle" of the cosmic ray spectrum as well, at an energy of 4 × 10 18 eV. We describe the experiment, data collection, analysis, and estimate the systematic uncertainties. The results are presented and the calculation of a ∼ 5 standard deviation observation of the GZK cutoff is described.
[Show abstract][Hide abstract] ABSTRACT: In a test experiment at the Final Focus Test Beam of the Stanford Linear Accelerator Center, the fluorescence yield of 28.5 GeV electrons in air and nitrogen was measured. The measured photon yields between 300 and 400 nm at 1 atm and 29 °C arephotons per electron per meter. Assuming that the fluorescence yield is proportional to the energy deposition of a charged particle traveling through air, good agreement with measurements at lower particle energies is observed.
[Show abstract][Hide abstract] ABSTRACT: Measurements are reported on the fluorescence of air as a function of depth in electromagnetic showers initiated by bunches of 28.5 GeV electrons. The light yield is compared with the expected and observed depth profiles of ionization in the showers. It validates the use of atmospheric fluorescence profiles in measuring ultra high energy cosmic rays.
[Show abstract][Hide abstract] ABSTRACT: We have measured the cosmic ray spectrum at energies above using the two air fluorescence detectors of the High Resolution Fly's Eye experiment operating in monocular mode. We describe the detector, PMT and atmospheric calibrations, and the analysis techniques for the two detectors. We fit the spectrum to models describing galactic and extragalactic sources. Our measured spectrum gives an observation of a feature known as the “ankle” near , and strong evidence for a suppression near .