Boyer, J H

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29th International Cosmic Ray Conference Pune (2005) 7, 351354Preliminary Results on the High Energy Cosmic Ray Spectrum fromthe Stereo Analysis of the HiRes Fly’s Eye DataJ.H. Boyer , W.H. Hanlon, B.C. Knapp , E.J. Mannel , M. Seman  and R.W. Springerfor the HiRes Collaboration(a) Department of Physics, Columbia University, New York, New York, USA(b) Department of Physics and High Energy Astrophysics Institute, University of Utah, Salt Lake City, Utah, USA(c) Currently at IEP SAS, Kosic, SlovakiaPresenter: Eric J. Mannel (mannel@nevis.columbia.edu), usa-mannel-EJ-abs1-he14-oralThe HiRes Fly’s Eye experiment, operating in Dugway Utah, has been collecting stereo data on extensive airshowers from cosmic rays since October 1999. In this paper we describe the stereo reconstruction technique,including daily gain and atmospheric corrections, along with a discussion of the Monte Carlo simulation usedto determine the detector aperture.1. IntroductionInterest in the high energy cosmic ray spectrum continues since the Fly’s Eye Experiment first reported theobservation of a cosmic ray induced extensive air shower with an energy in excess of 	 eV [1] and subse-quent reports from the AGASA ground array experiment of the observation of multiple extensive air showersproduced by cosmic rays with energies in excess of  eV [2]. The GZK[3] mechanism implies that obser-vations of cosmic rays above 	 eV should be suppressed unless the source of the cosmic rays is nearby,making the confirmation of the observation of events above GZK cutoff interesting.The High Resolution Fly’s Eye experiment (HiRes) started data taking in October 1999 and has now collectedover 3000 hours of stereo data. We will present preliminary results of the analysis of data collected throughApril 2005. The results are based on a full stereo reconstruction of events and includes corrections for hourlyvariations in detector response and atmospheric conditions.2. The HiRes DetectorThe HiRes experiment employs a second generation air fluorescence detector located on two sites separatedby 12.6 Km on the Dugway Proving Grounds, Dugway Utah. The detector has an aperture greater then km  sr for extensive air showers generated by cosmic rays at energies of 	 eV, more then 10 times the Fly’sEye aperture. Both sites consist of optical telescopes instrumented with a camera consisting of 256 hexagonalphoto-multiplier tubes (PMT’s) close packed and located at the focal plane of the telescope to record the imageof the extensive air shower as it passes through the field of view of the telescope. The first detector (HiRes-1)is located at the site of the original Fly’s Eye experiment. The HiRes-1 detector consists of 20 telescopescovering 320 degrees in azimuth and  to ﬀﬁ above the horizon. These telescopes are instrumented with asample-and-hold data acquisition system that integrates signals from the PMT’s for a fixed gate time. A fulldescription of the HiRes-1 detector electronics can be found in [4]. The second detector (HiRes-2), locatedon Camelsback Mountain, consists of 42 telescopes covering 336 degrees in azimuth and   to ﬃﬂ  above thehorizon. These telescopes are instrumented with a Flash ADC (FADC) data acquisition system allowing theintegration window to be adjusted during the analysis stage. A full description of the HiRes-2 data acquisitionsystem can be found in [5].352 J.H. Boyer et al.3. Photometric ResponseIn order to understand the energy scale of the HiRes detectors, it is necessary to both calibrate and monitor theelectronic gain of the detectors and measure the photometric response to a known light source. The electroniccalibration is done periodically using a roving xenon light source (RXF) which can be placed at at the center ofthe mirror. This RXF provides a stable uniform calibrated light source which can be transported to all mirrorsat each site to provide a site-wide and an inter-site calibration. The RXF data is analyzed using photo-statisticsfor each channel to determine the individual channel gains. This method takes into account variations in theactive area and quantum efficiency for the PMT’s.Since the RXF calibration procedure is both labor and time intensive, a monitoring system was developed totrack the individual channel gains on a nightly basis. The nightly monitoring uses YAG laser systems with thelaser light distributed to each detector via optic fibers. Variations in the light intensity seen by each camera donot allow for an absolute calibration, however used in conjunction with the RXF gain calibrations, allows thegains to be measured on a daily basis. A full description of the calibration systems can be found in [6].The total photometric response is determined periodically using a mobile laser system that is placed 4Km infront of each telescope to record the response of the detector to a known light source. These roving lasercalibration studies are done during regular data taking and are reconstructed using the standard HiRes recon-struction package. Corrections for gain and atmospheric variations as described above are incorporated and thereconstructed laser energy is compared to the laser energy as monitored during data taking. This front-to-backcalibration allows for a cross check on the calibration of the HiRes Detector.Steerable laser systems are located at each site and observable by the opposite site. Sweeps of the lasersprobe the atmosphere throughout the field of view of the two detectors allowing atmospheric conditions to bemonitored on an hourly basis. These laser events are reconstructed using the standard HiRes reconstructiontechniques to determine the hourly atmospheric conditions which are entered into a database and used for datareconstruction and Monte Carlo simulation. A summary of the atmospheric monitoring can be found in [7].In addition to the hourly sweeps, the lasers also fire along the trajectories of high energy shower candidatesshortly after the events are recorded allowing for additional checks on the atmospheric conditions shortly afterthe event occurred.4. ReconstructionThe first step in the reconstruction of extensive air showers is to time match the events observed at each of thetwo sites. Time matched stereo candidates are then filtered to accept shower like events and laser events. Theshower geometry is reconstructed by performing a simultaneous fit using the pointing vectors of the photo-tubes, the observed signals, and signal timing from both sites. The detector shower plans may calculatedfrom the results of the fit. The second step of the reconstruction is to determine the shower profile. We usean iterative approach to simulate the longitudinal and lateral profile of the shower which includes isotropicemission of fluorescence light that is propagated to the detector and wavelength dependent attenuation in theatmosphere. Wavelength dependent detector responses, mirror reflectivity, UV-filter transmission, and photo-multiplier response are also included. If all of the details of the shower development, light emission, transportand detection are correct and enough of the profile is observed to determine the total energy deposition in theatmosphere, then the parameters of the shower have been measured: energy and shower geometry.Preliminary Results on the High Energy Cosmic Ray Spectrum from the Stereo Analysis of the HiRes Fly.. 3535. Monte Carlo and Aperture DeterminationIn order to properly describe the HiRes detector aperture as a function of energy, it is necessary to simulate notonly the detector electronics, but the atmospheric conditions which affect the transmission of the fluorescencelight to the detector. A detailed Monte Carlo has been developed for the HiRes detector that incorporates thenightly gain measurements and the hourly atmospheric conditions. Cosmic ray showers are generated usingthe CORSIKA [8] generator for both proton and iron primaries with energies ranging from  to !" eV.The showers are parameterized and entered into a shower library that is used as input to the detector simulationportion of the Monte Carlo.Cosmic Ray showers from the library are chosen and assigned random geometries. The fluorescence andCerenkov light from the shower is simulated and photons are ray traced to the detector. Variations in thetransmission of the photons due to differences in the nightly atmospheric conditions are accounted for by usingthe atmospheric database. The detector response is simulated for photons traced to the camera, accountingfor variations in mirror reflectivity and UV filter transmission differences of the individual telescopes. Mirrorand channel dead-times, variations in gain, and non-linearity effects are also included in the simulation of theelectronic response to the showers.The resulting Monte Carlo event sample is reconstructed using the same reconstruction programs used toreconstruct the raw data to determine the effective stereo aperture of the Hires detector. As a cross check, theparameters of reconstructed shower candidates, geometry and energy, are used as input to the Monte Carlo tosimulate the detector response for observed shower candidates.6. AcknowledgmentsThis work is supported by US NSF grants PHY-9321949, PHY-9322298, PHY-9904048, PHY-9974537, PHY-0098826, PHY-0140688, PHY-0245428, PHY-0305516, PHY-0307098, and by the DOE grant FG03-92ER40732. We gratefully acknowledge the contributions from the technical staffs of our home institutions. Thecooperation of Colonels E. Fischer and G. Harter, the US Army, and the Dugway Proving Ground staff isgreatly appreciated.References[1] D.J. Bird et al., Astrophys J, 441, 144 (1995)[2] M. Takeda et al., Phys. Rev. Lett. 81, 1163 (1998)[3] K. Greisen, Phys. Rev. Lett. 16, 748 (1966)G.T. Zatsepin and V.A. K’usmin, Pis’ma Zh. Eksp, Teo. Fiz., 4, 144 (1966)[4] T. Abu-Zayyad et al., NIM A450, 253, (2000)[5] J. Boyer et al., NIM A482, 457, (2002)[6] J.H.V Girard et al., NIM A460, (2001)[7] These proceedings, HE 1.5 (2005)R.Abassi et al., Submitted to Astroparticle Phys. (2005)[8] D. Heck, J. Knapp, J.N. Capdevielle, G. Schatz, and T. Thout, Report FZKA 6019 (1998).354 J.H. Boyer et al.

Preliminary Results on the High Energy Cosmic Ray Spectrum from the Stereo Analysis of the HiRes Fly's Eye Data

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