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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. 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

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  1. 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

  2. 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

  3. Pull Distribution – Data and MC (XmaxI-XmaxII)/((XmaxI+XmaxII)/2)

  4. Comparison of Hadronic Interaction Models

  5. Xmax Distribution Width Predictions

  6. Resolution After All Cuts 27 gm/cm2 12% Energy Xmax

  7. 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

  8. Two-Component Mixing Model

  9. Optimal Two-Component Fit Solid Line: Data Heavy Dots: QGSJet Light Dots: SIBYLL # Events Xmax

  10. Stereo HiRes Elongation Rate

  11. 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

  12. 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

  13. Uncertainty in the Atmosphere vary by 1s Log(Energy) (eV) Xmax

  14. Effect of Atmospheric Uncertainty on Elongation Rate

  15. Effect of Detector Acceptance

  16. 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

  17. 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

  18. Determination of shower profile

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