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Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004)

A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC Detector. Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004). Outline. Monocular vs. Stereoscopic Observation HiRes FADC Event Reconstruction

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Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004)

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  1. A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC Detector Andreas Zech Rutgers University for the HiResCollaboration CRIS ‘04 (May 31st , 2004)

  2. Outline • Monocular vs. Stereoscopic Observation • HiRes FADC Event Reconstruction • Monte Carlo Simulation Programs • Data / Monte Carlo Comparisons • The HiRes-2 Energy Spectrum • Studies of Systematic Effects on the Aperture

  3. The two HiRes Detectors HiRes-1: • taking data since 1997 • 1 ring with 21 mirrors ( elev. 3o to 17o) • Sample & Hold Electronics ( 5.6 s ) HiRes-2: • started data taking in 1999 • 2 rings with 42 mirrors (elev. 3o to 31o) • FADC electronics recording at 10 MHz.

  4. Seeing more with one eye closed ?!?

  5. Stereo observation of the cosmic ray flux yields a better resolution in geometry and energy than monocular. => HiRes is a stereoscopic detector. The analysis of stereo events is currently under way. Analyzing our data in monocular mode has some advantages: better statistics at the high energy end due to longer lifetime of HiRes-1. extension of the spectrum to lower energies due to greater elevation coverage and better time resolution of HiRes-2. Measuring the Energy Spectrum with HiRes

  6. Mono versus Stereo Energy Measurements The HiRes monocular energy is in excellent agreement with stereoscopic measurements ! HiRes-1 mono vs. stereo

  7. HiRes FADC Event Reconstruction

  8. 1. Reconstruction of the • shower-detector-plane • project signal tubes onto the sky • fit tube positions to a line • reject tubes that are off-track (and off in time) as noise • => the detector position and fitted line define the shower-detector-plane.

  9. 2. Reconstruction of the geometry within the s-d-plane

  10. Reconstruct charged particle profile from recorded p.e. Subtract Čerenkov light. Fit G.H. function to the profile. Multiply by mean energy loss rate  =>calorimetric energy Add ‘missing energy’ (muons, neutrinos, nuclear excitations; ~10%) => total energy Shower Profile & Energy Reconstruction

  11. Monte Carlo Simulation Programs

  12. We need M.C. to calculate the acceptance of our detectors for the flux measurement: M.C. is also a powerful tool for resolution studies. This requires a simulation program that describes the shower development and detector response as realistically as possible. The Role of Monte Carlo Simulations in the HiRes Experiment

  13. HiRes Monte Carlo Simulation

  14. Trigger gains Dead mirrors Livetime => Nightly Database Light pollution => Average for each data set Atmospheric Density => Seasonal variations Weather => strict cuts based on hourly observation Aerosols => atmospheric database from laser shots => currently, we use average values Varying Run Parameters

  15. Data / Monte Carlo Comparisons& Resolution

  16. Photoelectrons per degree of track black: HiRes-2 data red: Monte Carlo (5 x data statistics) data Monte Carlo

  17. m Distance to the shower axis (Rp)

  18.  - Angle

  19. Energy Resolution (Erec - Etrue) Etrue  ~ 16 %

  20. deg  Resolution rec. - true  ~ 5 deg

  21. The HiRes-2 Energy Spectrum

  22. HiRes-2 Exposure fit to the exposure Flux:

  23. HiRes-2 Energy Spectrum statistics: 123 good nights, 536 hours live time, 6320 events with reconstructed geometry, 2685 events after final cuts

  24. The HiRes Mono Spectra • HiRes-1 ‘97 - ‘04 • HiRes-2 ‘99 - ‘01

  25. HiRes Mono and Fly’s Eye Stereo • HiRes-1 • HiRes-2 • Fly’s Eye stereo

  26. Systematic Uncertainties

  27. Systematic Uncertainties Systematic uncertainties in the energy scale: • absolute calibration of phototubes: +/- 10 % • fluorescence yield: +/- 10 % • correction for unobserved energy: +/- 5 % • aerosol concentration: < 9 % + atmospheric uncertainty in aperture =>totaluncertainty in the flux: +/- 31 % What uncertainties in the aperture are introduced with our inputs to the Monte Carlo ? (i.e. input spectrum, composition, atmosphere)

  28. A fit to the Fly’s Eye Stereo spectrumis used as an input to the Monte Carlo. Systematics due to the Input Energy Spectrum

  29. red: MC withE-3 input spectrum black: data set 2 red: MC withFly’s Eye input spectrum black: data set 2 Fly’s Eye vs. E-3 input spectrum

  30. A bias that we are avoiding... aperture using E-3 input spectrum aperture using Fly’s Eye input spectrum Assuming a wrong ( E-3 ) input spectrum would cause us a bias of ~ 20 % in the aperture.

  31. Systematics due to the Input Composition The inputcomposition ( = fraction of proton and iron showers) is chosen from HiRes Stereoand HiRes/MIAmeasurements.

  32. red: proton exposure blue:iron exposure log E (eV) Exposures for pure proton / pure iron • lower acceptance for iron at low energies (< 10 18.5 eV ) • agreement at higher energies.

  33. Systematic Uncertainty due to Input Composition • We assume a +/- 20 % uncertainty in the proton fractionfrom HiRes / MIA & HiRes Stereo measurements. • This is a conservative estimate of the uncertainties in the composition. • A new composition measurement is needed ! => HiRes , TA/TALE black: stat. errors red: sys. uncertainty

  34. Systematics due to Aerosol • We are currently using a measurement of the average aerosol content of the atmosphere for our analysis. • What is the systematic effect on the energy resolution and aperture due to this assumption? • ( This is work in progress ... )

  35. Atmospheric Database 09/00- 03/01 clear nights • Aerosol VAOD measurement using vertical laser tracks. • Aerosol Horizontal Extinction Length from horizontal laser shots. <VAOD> ~ 0.034 Preliminary 09/00- 03/01 clear nights <1/hxl> -1 ~ 20.8 km

  36.  ~ 15.9 % Systematic Effect on Reconstructed Energies (MC study) Energy Resolution for MC with atmos. database, reconstructed with database  ~ 17.5 % Energy Resolution for MC with atmos. database, reconstructed with average

  37. log (E) Systematic Effects on the Aperture Ratio of Apertures: • numerator: using MC with atmos. db. , reconstructed with atmos. db. • denominator: using MC with atmos. db. , reconstructed with average

  38. Conclusions • Measurements of the Cosmic Ray Flux in monocular mode cover a wider energy range than in stereoscopic mode while providing very good energy resolution. • Our Monte Carlo Programs simulate all aspects of our experiment in a realistic way. • We have investigated systematic uncertainties related to the input spectrum, input composition and the aerosol content of the atmosphere. Further studies of atmospherics are under way.

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