Stereo Spectrum of UHECR Showers at the HiRes Detector

Stereo Spectrum of UHECR Showers at the HiRes Detector

The Measurement TechniqueThe Measurement Technique Event ReconstructionEvent Reconstruction Monte Carlo SimulationMonte Carlo Simulation Aperture DeterminationAperture Determination Preliminary ResultsPreliminary Results

The 28th International Cosmic Ray ConferenceAugust 2, 2003

R. Wayne Springer, for the HiRes CollaborationUniversity of Utah

The Air Fluorescence TechniqueThe Air Fluorescence TechniqueShower Shower

DevelopmentDevelopment

Atmospheric Atmospheric MonitoringMonitoring

Detector ResponseDetector Responseand Calibrationand Calibration

Need to understandNeed to understandShower DevelopmentShower DevelopmentAtmospheric MonitoringAtmospheric MonitoringDetector Response and CalibrationDetector Response and Calibration

Need to Need to KnowKnowFluorescenceYieldFluorescenceYield

The Measurement of the Energy SpectrumThe Measurement of the Energy Spectrum

Important to understand the following…Energy Measurement

Detector CalibrationShower Geometry (STEREO HELPS!!)Atmospheric Conditions

ApertureDetector CalibrationTrigger ThresholdsReconstructionAtmospheric Conditions

Note that the atmosphere has greatest effect on the apertureat enegies below 10 EeV!!!!

Need to ensure that there are no tails in Energy distribution!!!!

Stereoscopic Event ReconstructionStereoscopic Event ReconstructionDetermination of Shower GeometryDetermination of Shower Geometry

The geometry of the air shower is determinedsimply by finding the intersection of the shower-

detector planes

Reduced Uncertainty in Energy Determination

Stereoscopic Event ReconstructionStereoscopic Event ReconstructionDetermination of shower profileDetermination 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.... Depth [g/cm2]

Sig

nal

Stereoscopic Event ReconstructionStereoscopic Event ReconstructionEnergy ResolutionEnergy Resolution

➢Choose best of HR1 or HR2 ➢Basic cuts:

➢Profile chi2/d.o.f<15➢Nbin>3➢Xmax<Xbottom+100g/cm➢Xmax>Xtop-300g/cm

➢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 Cuts need to OPTIMIZE OPTIMIZE Energy and Energy and

StatisticsStatistics

HR2 no cuts

Entries=8828RMS=57.9Sigma=15.8




HR1 no cuts

Basic CutsTight Cuts

Num

eve

nts

Num

eve

nts

Num

eve

nts

Num

eve

nts

% fractional resolution

% fractional resolution % fractional resolution

% fractional resolution

Stereoscopic Event ReconstructionStereoscopic Event ReconstructionData/Monte Carlo ComparisonData/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....

Log Energy Log Energy

Log Energy Log Energy

Nu

mb

er e

ven

ts

Nu

mb

er e

ven

ts

MC

/Dat

a R

atio

MC

/Dat

a R

atio

Stereoscopic Event ReconstructionStereoscopic Event ReconstructionData/Monte Carlo ComparisonData/Monte Carlo Comparison

Zenith and Azimuth angle distributions Zenith and Azimuth angle distributions

MC

/Dat

a R

atio

MC

/Dat

a R

atio

Nu

mb

er e

ven

ts

Nu

mb

er e

ven

ts

Determination of ApertureDetermination of Aperture➢Aperture=thrown area*where (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

Nsuccessv. logE

Efficiencyv log E

Aperturev log E

Nthrownv. logE

Nsuccessv. logE

Nthrownv. logE

Aperturev log E

Efficiencyv log E

1/E thrown spectrum 1/E3 thrown spectrum

HiRes Stereoscopic ApertureHiRes 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

Km

2-sr

Aperture v log Ethrown Mix Aperture v log Erecon Mix

Log E(eV) Log E(eV)

Km

2-sr

HiRes Stereo Flux MeasurementHiRes Stereo Flux MeasurementEnergy DistributionsEnergy Distributions

Good Weather cuts1006 hours1588 events

NoWeather cuts1291 hours1944 events

Nu

mb

er e

ven

ts

Nu

mb

er e

ven

ts

Log E(eV) Log E(eV)

Measuring The UHECR Energy SpectrumMeasuring 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

ΔtAΩ

ΔEΔN=J(E)

/

HiRes Stereo Flux MeasurementHiRes Stereo Flux MeasurementEnergy**3 * FluxEnergy**3 * Flux

•HiRes Stereo Spectrum is consistent With Monocular Spectrum •Change in spectral index weakly observed at an energy of 1018.6eV.

•STATISTICAL ERRORSONLY

Log E(eV)

E3 J

(E)

ConclusionConclusion● 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????

Stereo Spectrum of UHECR Showers at the HiRes Detector

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