Stereo Composition Measurement with the

High Resolution Fly’s Eye

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

University of UtahFor the HiRes collaboration

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 (Xmax/E)• Systematic Uncertainties• Conclusions

Pull Distribution – Data and MC

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

Comparison of Hadronic Interaction Models

Xmax Distribution Width Predictions

Resolution After All Cuts

EnergyEnergy XXmaxmax

12% 27 gm/cm2

All-Energy Xmax Distribution

Solid Line: DataSolid Line: DataHeavy Dots: QGSJet Heavy Dots: QGSJet Light Dots: SIBYLLLight Dots: SIBYLL

Data v Proton Models Data v Iron Nuclei Models

Stereo Data:Stereo Data:~800 Events ~800 Events 11/1999 – 9/200111/1999 – 9/2001

Two-Component Mixing Model

Optimal Two-Component Fit

Xmax

# E

vent

sSolid Line: DataSolid Line: DataHeavy Dots: QGSJet Heavy Dots: QGSJet Light Dots: SIBYLLLight Dots: SIBYLL

Stereo HiRes Elongation Rate

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:• Atmospheric attenuation:

vary atmosphere by 1• Detector trigger bias:

study acceptance with MC• Cherenkov light subtraction:

vary angular distribution by 1

• Detector pointing

Uncertainty in the Atmosphere vary by 1

Log(Energy) (eV)Log(Energy) (eV) XXmaxmax

Effect of Atmospheric Uncertainty on Elongation Rate

Effect of Detector Acceptance

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• 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

Determination of shower profile