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Stereo Composition Measurement with the High Resolution Fly’s Eye. J.N. Matthews, G. Archbold, and P. Sokolsky University of Utah For the HiRes collaboration. UHECR Composition Measurement. Measure depth of shower maximum (X max ) using stereo HiRes data.

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stereo composition measurement with the high resolution fly s eye

Stereo Composition Measurement with the High Resolution Fly’s Eye

J.N. Matthews, G. Archbold, and P. Sokolsky

University of Utah

For the HiRes collaboration

uhecr composition measurement
UHECR Composition Measurement
  • Measure depth of shower maximum (Xmax) using stereo HiRes data.
  • Monte Carlo study of Xmax & E resolution
  • Compare Xmax vs. Energy with Monte Carlo predictions from CORSIKA, using QGSjet and Sibyll hadronic generators.
  • Measure Elongation Rate (DXmax/DE)
  • Systematic Uncertainties
  • Conclusions
pull distribution data and mc
Pull Distribution – Data and MC

(XmaxI-XmaxII)/((XmaxI+XmaxII)/2)

resolution after all cuts
Resolution After All Cuts

27 gm/cm2

12%

Energy

Xmax

all energy x max distribution
All-Energy Xmax Distribution

Data v Proton Models

Data v Iron Nuclei Models

Solid Line: Data

Heavy Dots: QGSJet

Light Dots: SIBYLL

Stereo Data:

~800 Events

11/1999 – 9/2001

optimal two component fit
Optimal Two-Component Fit

Solid Line: Data

Heavy Dots: QGSJet

Light Dots: SIBYLL

# Events

Xmax

elongation rate result
Elongation Rate Result
  • HiRes Stereo:
    • Range 1 x 1018 to 3 x 1019 eV
    • ER = 54.5 +/- 6.5 (stat) +/- 4.5 (syst) gm/cm2 per decade energy
  • HiRes Prototype/MIA:
    • Range 1x1017 to 1x1018 eV
    • ER = 93.0 +/- 8.5 (stat) +/- 10.5 (syst) gm/cm2 per decade energy
systematic uncertainties
Systematic Uncertainties:
  • Atmospheric attenuation: vary atmosphere by 1s
  • Detector trigger bias: study acceptance with MC
  • Cherenkov light subtraction: vary angular distribution by 1s
  • Detector pointing
systematic uncertainty in x max
Systematic Uncertainty in Xmax
  • Atmospheric attenuation- 10 gm/cm2
  • Cherenkov subtraction - negligible
  • Detector bias - 5 gm/cm2
  • Pointing Direction - 15 gm/cm2
  • Sum in Quadrature – 18.5 gm/cm2
conclusions
Conclusions
  • Systematic uncertainties are significantly smaller than the proton/Fe separation
  • HiRes Stereo measurement consistent with unchanging, light composition above 1018 eV
  • Stereo HiRes and HiRes Prototype-MIA measurements are consistent in overlap region
  • HiRes Prototype-MIA Hybrid result consistent with changing composition (Heavy to Light) between 1017 and 1018 eV
  • No significant information near GZK region yet
  • Come back to 29th ICRC
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