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Stereo Spectrum of UHECR Showers at the HiRes Detector

Stereo Spectrum of UHECR Showers at the HiRes Detector. R. Wayne Springer, for the HiRes Collaboration University of Utah. The Measurement Technique Event Reconstruction Monte Carlo Simulation Aperture Determination Preliminary Results The 28 th International Cosmic Ray Conference

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Stereo Spectrum of UHECR Showers at the HiRes Detector

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  1. Stereo Spectrum of UHECR Showers at the HiRes Detector R. Wayne Springer, for the HiRes Collaboration University of Utah • The Measurement Technique • Event Reconstruction • Monte Carlo Simulation • Aperture Determination • Preliminary Results The 28th International Cosmic Ray Conference August 2, 2003

  2. Shower Development Detector Response and Calibration Atmospheric Monitoring The Air Fluorescence Technique Need to Know FluorescenceYield Need to understand • Shower Development • Atmospheric Monitoring • Detector Response and Calibration

  3. The Measurement of the Energy Spectrum Important to understand the following… • Energy Measurement • Detector Calibration • Shower Geometry (STEREO HELPS!!) • Atmospheric Conditions • Aperture • Detector Calibration • Trigger Thresholds • Reconstruction • Atmospheric Conditions Note that the atmosphere has greatest effect on the aperture at enegies below 10 EeV!!!! Need to ensure that there are no tails in Energy distribution!!!!

  4. Stereoscopic Event ReconstructionDetermination of Shower Geometry The geometry of the air shower is determined simply by finding the intersection of the shower-detector planes Reduced Uncertainty in Energy Determination

  5. Stereoscopic Event ReconstructionDetermination of shower profile • HiRes-I binning • 1.5 degree angular bins • Ray tracing to determine detector acceptance • HiRes-II binning • Time based binning • Measure intensity and direction of light spot every 100ns • Ray tracing to determine detector acceptance • Profile fit • Signal fit to shower profile function • Cerenkov correction calculated based on geometry. • dE/dX determined from fit • Primary particle total energy calculated using “standard” relationship between EM and total energy.... Signal Depth [g/cm2]

  6. Stereoscopic Event ReconstructionEnergy Resolution Entries=8758 RMS=62.5 Sigma=25.5 • Choose best of HR1 or HR2 • Basic cuts: • Profile chi2/d.o.f<15 • Nbin>3 • Xmax<Xbottom+100g/cm • Xmax>Xtop-300g/cm Entries=8828 RMS=57.9 Sigma=15.8 Num events Num events HR1 no cuts HR2 no cuts • Tight cuts • Energy uncertainty/energy <5.0 • Xbottom-Xtop>100 g/cm • Xmax>Xtop-200 g/cm • 400 g/cm <Xmax<1200 g/cm • Zenith angle<70 degrees • Cuts need to OPTIMIZE Energy and Statistics % fractional resolution % fractional resolution Entries=8006 RMS=62.3 Sigma=25.5 Entries=7002 RMS=62.3 Sigma=25.5 Num events Num events Basic Cuts Tight Cuts % fractional resolution % fractional resolution

  7. Stereoscopic Event ReconstructionData/Monte Carlo Comparison • Reweight MC events to agree with Data energy distribution • 76-24 mixture of proton and iron events to get agreement with data Xmax distribution • Still unable to model tails in Xmax distribution perfectly • Compare MC and data distributions for other observables using reweighted MC events.... Number events Number events Log Energy Log Energy MC/Data Ratio MC/Data Ratio Log Energy Log Energy

  8. Stereoscopic Event ReconstructionData/Monte Carlo ComparisonZenith and Azimuth angle distributions Number events Number events MC/Data Ratio MC/Data Ratio

  9. Determination of Aperture • Aperture=h(E)*thrown area*W , where h(E)=Nrecon/Nthrown is the efficiency to reconstruct thrown events at Energy E. • Generate Events using both a 1/E and a 1/E3 spectrum to obtain sufficient statistics at both low and high energies. • Mix Proton and Iron events to obtain xmax distribution of data • Calculate both thrown energy and reconstructed energy apertures Nsuccess v. logE Nthrown v. logE Nthrown v. logE Nsuccess v. logE Aperture v log E Aperture v log E Efficiency v log E Efficiency v log E 1/E thrown spectrum 1/E3 thrown spectrum

  10. HiRes Stereoscopic Aperture • Calculated for “average atmospheric conditions” of VSH=1.0km and HAL=25.0km • 76% Proton 24% Iron mixture • Aperture exceeds 10,000 km2-sr above 100 EeV Aperture v log Ethrown Mix Aperture v log Erecon Mix Km2-sr Km2-sr Log E(eV) Log E(eV)

  11. HiRes Stereo Flux MeasurementEnergy Distributions Number events Number events Good Weather cuts 1006 hours 1588 events NoWeather cuts 1291 hours 1944 events Log E(eV) Log E(eV)

  12. Measuring The UHECR Energy Spectrum • Count Particles vs. Energy • Observe UV fluorescence signal • Determine Geometry of shower • Bin Shower profile and reconstruct energy • “Fill histogram” Number v Energy • Determine Exposure vs. Energy • Determine detector on-time • Determine aperture for each detector configuration • Divide Count/Exposure  Energy Spectrum

  13. HiRes Stereo Flux MeasurementEnergy**3 * Flux • HiRes Stereo Spectrum is consistent With Monocular Spectrum • Change in spectral index weakly observed at an energy of 1018.6eV. • STATISTICAL ERRORS • ONLY E3 J(E) Log E(eV)

  14. Conclusion • Still evaluating sources of systematic uncertainty • Energy scale • Atmospheric effects • Fluorescence Yield • Indication of structure in spectrum • Ankle at ~1018.6eV • Need More Statistics • HiRes is still collecting data…. • GZK effect????

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