A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC Detector

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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)

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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

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• 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.

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Seeing more with one eye closed ?!?
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Measuring the Energy Spectrum with HiRes

• 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.

• Stereo observation of the cosmic ray flux
  yields a better resolution in geometry and energy
  than monocular.
• gt HiRes is a stereoscopic detector. The
  analysis of stereo events is currently under way.

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Mono versus Stereo Energy Measurements
HiRes-1 mono vs. stereo

• The HiRes monocular energy is in excellent
  agreement with stereoscopic measurements !

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HiRes FADC Event Reconstruction
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• 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
• gt the detector position and fitted line define
  the shower-detector-plane.

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2. Reconstruction of the geometry within the
s-d-plane
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Shower Profile Energy Reconstruction

• Reconstruct charged particle profile from
  recorded p.e.
• Subtract Cerenkov light.
• Fit G.H. function to the profile.
• Multiply by mean energy loss rate ? gt
  calorimetric energy
• Add missing energy (muons, neutrinos, nuclear
  excitations 10) gt total energy

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Monte Carlo Simulation Programs
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The Role of Monte Carlo Simulations in the HiRes
Experiment

• 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.

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HiRes Monte Carlo Simulation
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Varying Run Parameters

• Trigger gains
• Dead mirrors
• Livetime
• gt Nightly Database
• Light pollution
• gt Average for each data set
• Atmospheric Density
• gt Seasonal variations

• Weather
• gt strict cuts based on hourly observation
• Aerosols
• gt atmospheric database from laser shots
• gt currently, we use average values

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Data / Monte Carlo Comparisons Resolution
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Photoelectrons per degree of track

• black HiRes-2 data
• red Monte Carlo
• (5 x data statistics)
• data
• Monte Carlo

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Distance to the shower axis (Rp)
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? - Angle
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Energy Resolution

• (Erec - Etrue)
• Etrue
• ? 16

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? Resolution
?rec. - ?true
? 5 deg
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The HiRes-2 Energy Spectrum
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HiRes-2 Exposure
fit to the exposure
Flux
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HiRes-2 Energy Spectrum

• statistics
• 123 good nights,
• 536 hours live time,
• 6320 events with reconstructed geometry,
  2685 events after final cuts

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The HiRes Mono Spectra

• HiRes-1
• 97 - 04
• HiRes-2
• 99 - 01

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HiRes Mono and Flys Eye Stereo

• HiRes-1
• HiRes-2
• Flys Eye stereo

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Systematic Uncertainties
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Systematic Uncertainties

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

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Systematics due to the Input Energy Spectrum

• A fit to the Flys Eye Stereo spectrum is used
  as an
• input to the Monte Carlo.

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Flys Eye vs. E-3 input spectrum
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A bias that we are avoiding...
aperture using E-3 input spectrum aperture using
Flys Eye input spectrum

• Assuming a wrong ( E-3 ) input spectrum would
  cause us a bias of 20 in the
  aperture.

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Systematics due to the Input Composition
The input composition ( fraction of proton
and iron showers) is chosen from HiRes
Stereo and HiRes/MIA
measurements.
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Exposures for pure proton / pure iron

• lower acceptance for iron at low
  energies (lt 10 18.5 eV )
• agreement at higher energies.

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Systematic Uncertainty due to Input Composition

• We assume a /- 20 uncertainty in the proton
  fraction from HiRes / MIA HiRes Stereo
  measurements.
• This is a conservative estimate of the
  uncertainties in the composition.
• A new composition measurement is needed !
• gt HiRes , TA/TALE

black stat. errors red sys.
uncertainty
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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 ... )

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Atmospheric Database
09/00 - 03/01 clear nights

• Aerosol VAOD measurement using vertical laser
  tracks.
• Aerosol Horizontal Extinction Length from
  horizontal laser shots.

ltVAODgt 0.034
Preliminary
09/00 - 03/01 clear nights
lt1/hxlgt -1 20.8 km
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Systematic Effect on Reconstructed Energies (MC
study)
? 17.5

• Energy Resolution for
• MC with atmos. database,
• reconstructed with database

Energy Resolution for MC with atmos.
database, reconstructed with average
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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

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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.