Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004)
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Transcript of Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004)
A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC
Detector
Andreas Zech
Rutgers University
for the HiRes CollaborationCRIS ‘04 (May 31st , 2004)
Outline
• Monocular vs. Stereoscopic Observation
• HiRes FADC Event Reconstruction
• Monte Carlo Simulation Programs
• Data / Monte Carlo Comparisons
• The HiRes-2 Energy Spectrum
• Studies of Systematic Effects on the Aperture
The two HiRes Detectors
HiRes-1:
• taking data since 1997
• 1 ring with 21 mirrors ( elev. 3o to 17o)
• Sample & Hold Electronics ( 5.6 s )
HiRes-2:
• started data taking in 1999
• 2 rings with 42 mirrors (elev. 3o to 31o)
• FADC electronics recording at 10 MHz.
Seeing more with one eye closed ?!?
Measuring the Energy Spectrum with HiRes
Stereo observation of the cosmic ray flux yields a better resolution in geometry and energy than monocular.
=> HiRes is a stereoscopic detector. The analysis of stereo events is currently under way.
Analyzing our data in monocular mode has some advantages:
• better statistics at the high energy end due to longer lifetime of HiRes-1.
• extension of the spectrum to lower energies due to greater elevation coverage and better time resolution of HiRes-2.
Mono versus Stereo Energy Measurements
The HiRes monocular energy is in excellent agreement with stereoscopic measurements !
HiRes-1 mono vs. stereo
HiRes FADC Event Reconstruction
1. Reconstruction of the shower-detector-plane
• project signal tubes onto the sky• fit tube positions to a line• reject tubes that are off-track (and off in time) as noise
=> the detector position and fitted line define the shower-detector-plane.
2. Reconstruction of the geometry within the
s-d-plane
Shower Profile & Energy Reconstruction
• Reconstruct charged particle profile from recorded p.e.
• Subtract Čerenkov light.• Fit G.H. function to the profile.• Multiply by mean energy loss rate
=> calorimetric energy• Add ‘missing energy’ (muons,
neutrinos, nuclear excitations; ~10%) => total energy
Monte Carlo Simulation Programs
The Role of Monte Carlo Simulations in the HiRes Experiment
We need M.C. to calculate the acceptance of our detectors for the flux measurement:
M.C. is also a powerful tool for resolution studies.
This requires a simulation program that describes the shower development and detector response as realistically as possible.
HiRes Monte Carlo Simulation
Varying Run Parameters
• Trigger gains
• Dead mirrors
• Livetime
=> Nightly Database
• Light pollution
=> Average for each data set
• Atmospheric Density
=> Seasonal variations
• Weather
=> strict cuts based on hourly observation
• Aerosols
=> atmospheric database from laser shots
=> currently, we use average values
Data / Monte Carlo Comparisons&
Resolution
Photoelectrons per degree of track
black: HiRes-2 data
red: Monte Carlo
(5 x data statistics)
data
Monte Carlo
Distance to the shower axis (Rp)
m
- Angle
Energy Resolution
(Erec - Etrue)
Etrue
~ 16 %
Resolution
deg
~ 5 deg
rec. - true
The HiRes-2 Energy Spectrum
Flux:
fit to the exposure
HiRes-2 Exposure
)]}logexp(1[exp{ Ecba
HiRes-2 Energy Spectrum
statistics:
123 good nights,
536 hours live time,
6320 events with reconstructed geometry, 2685 events after final cuts
The HiRes Mono Spectra
• HiRes-1
‘97 - ‘04
• HiRes-2
‘99 - ‘01
HiRes Mono and Fly’s Eye Stereo
• HiRes-1
• HiRes-2
• Fly’s Eye stereo
Systematic Uncertainties
Systematic Uncertainties
Systematic uncertainties in the energy scale:• absolute calibration of phototubes: +/- 10 %
• fluorescence yield: +/- 10 %
• correction for unobserved energy: +/- 5 %
• aerosol concentration: < 9 %
+ atmospheric uncertainty in aperture
=> total uncertainty in the flux: +/- 31 %
What uncertainties in the aperture are introduced with our inputs to the Monte Carlo ? (i.e. input spectrum, composition, atmosphere)
Systematics due to the Input Energy Spectrum
A fit to the Fly’s Eye Stereo spectrum is used as an
input to the Monte Carlo.
Fly’s Eye vs. E-3 input spectrum
red: MC with Fly’s Eye input spectrum black: data set 2
red: MC with E-3 input spectrum black: data set 2
A bias that we are avoiding...
Assuming a wrong ( E-3 ) input spectrum would cause us a bias of ~ 20 % in the aperture.
aperture using E-3 input spectrum
aperture using Fly’s Eye input spectrum
Systematics due to the Input Composition
The input composition ( = fraction of proton and iron showers)is chosen from HiRes Stereo and HiRes/MIA measurements.
Exposures for pure proton / pure iron
• lower acceptance for iron at low energies (< 10 18.5 eV )
• agreement at higher energies.
red: proton exposure blue: iron exposure
log E (eV)
Systematic Uncertainty due to Input Composition
• We assume a +/- 20 % uncertainty in the proton
fraction from HiRes / MIA & HiRes Stereo
measurements. • This is a conservative
estimate of the uncertainties in the composition.
• A new composition measurement is needed !
=> HiRes , TA/TALE
black: stat. errors red: sys.
uncertainty
Systematics due to Aerosol
• We are currently using a measurement of the average aerosol content of the atmosphere for our analysis.
• What is the systematic effect on the energy resolution and aperture due to this assumption?
• ( This is work in progress ... )
• Aerosol VAOD measurement using vertical laser tracks.
• Aerosol Horizontal Extinction Length from horizontal laser shots.
Atmospheric Database
09/00 - 03/01 clear nights
09/00 - 03/01 clear nights
PreliminaryPreliminary
<VAOD> ~ 0.034
<1/hxl> -1 ~ 20.8 km
Systematic Effect on Reconstructed Energies (MC study)
Energy Resolution for
MC with atmos. database,
reconstructed with database
Energy Resolution for
MC with atmos. database,
reconstructed with average
~ 15.9 % ~ 17.5 %
Systematic Effects on the Aperture
Ratio of Apertures:
• numerator: using MC with atmos. db. , reconstructed with atmos. db.
• denominator: using MC with atmos. db. , reconstructed with averagelog (E)
Conclusions
• Measurements of the Cosmic Ray Flux in monocular mode cover a wider energy range than in stereoscopic mode while providing very good energy resolution.
• Our Monte Carlo Programs simulate all aspects of our experiment in a realistic way.
• We have investigated systematic uncertainties related to the input spectrum, input composition and the aerosol content of the atmosphere. Further studies of atmospherics are under way.