A Measurement of the Ultra-High Energy Cosmic Ray
Flux with the HiRes FADC Detector
PhD Defense Presentation
September 15th, 2004
Andreas Zech
Outline
• A Brief Introduction to UHECR Physics
• The HiRes Experiment
• Unfolding the Cosmic Ray Spectrum
• Results of the HiRes-2 Analysis
• Conclusions
Energy Spectrum
• differential flux: dN / (dE A Ω dt)
• follows roughly E-3 power law
• direct observation not possible above 1 PeV
• three features:
– ‘knee’ at ~1015.5 eV
– ‘2nd knee’ (~ 1017.5 eV)
– ‘ankle’ at ~1018.5 eV
knee
anklesecond knee
Extensive Air Showers
main channels:
π+(-) μ+(-) +νμ ( νμ )πo 2 γ
main e.m. processes:• bremsstrahlung• pair production• ionization
HiRes-2 Composition Measurement
• Indirect Composition Measurement using depth of the shower maximum.
• Shift from heavy to light composition at around 1017 eV to 1018 eV.
Prelim
inary
HiRes-2 Composition
HiRes/MIA & HR stereo Composition.
HiRes-2 Skymap
• UHECR arrival directions agree with isotropic distribution.
• AGASA claims to see small-scale anisotropies (clusters of events) at ultra-high energies.
Propagation Effects
• magnetic fields (galactic, extragalactic)
• red-shifting
• e+e-- pair production with CMBR (at ~ 1017.8 eV)
• photo-spallation of cosmic ray nuclei
• GZK effect with CMBR (at ~ 1019.8 eV)
γ (2.7 K) + p Δ(1232) π+ + n
γ (2.7 K) + p Δ(1232) π o + p Strong flux suppression for extra-galactic sources.
• Detection of longitudinal shower profile via UV fluorescence light.
• Reconstruction of geometry from recorded shower ‘track’.
• Using the atmosphere as a calorimeter.
Air Fluorescence Detectors
Fly’s Eye
HiRes-1 consists of one ringof 22 mirrors. Coverage in elevation is from 3 to 17 deg.
Sample & Hold Electronicsare used to record pulses.(5.6 μs window)
HiRes-2 has two rings of 21 mirrorseach. Coverage in elevation from 3 to 31 deg.
Flash ADC electronics record signals at a frequency of 10 MHz.
• project signal tubes onto sky
• fit tube positions to a plane through the center of the detector
• reject tubes that are off-track (and off in time) as noise
=> shower axis lies in the fitted shower-detector plane
1. Reconstruction of the shower-detector plane
3. Shower Profile & Energy Reconstruction
• Reconstruct charged particle profile from recorded p.e.
• Fit profile to G.H. function.• Subtract Čerenkov light.• Multiply by mean ionization loss rate
=> calorimetric energy• Add ‘missing energy’ (muons,
neutrinos, nuclear excitations; ~10%) => total energy
Phototube Calibration
• Relative calibration at the beginning and end of each nightly run.
– using YAG laser.
– optical fibers distribute the laser signal to all mirrors.
• Conversion of recorded FADC signals to photon flux.
• Absolute calibration using a portable light-source (“RXF”) of known intensity that is carried to both sites.
Atmospheric Calibration
• attenuation of light by two main processes:– Rayleigh (molecular)
scattering
– Mie (aerosol) scattering
• Rayleigh contribution is quite stable and well known.
• Aerosol profile of the atmosphere has to be monitored during the run.
• Monitoring with an array of vertical Xenon flashers between the two detector sites.
• Detailed monitoring with steerable lasers on both sites.
• Additional vertical laser outside of Dugway (Terra).
Atmospheric Database
Atmospheric data of the selected nights in this analysis:
<HAL> = 27 km
<VAOD> = 0.035
Deconvolution of the UHECR Spectrum
We observe the spectrum convoluted with detector acceptance and limited resolution.
Deconvolution with help of a correction factor:
D(Ei)=Σ Rij T(Ej) T(Ei)= [Gmc(Ei)/Rmc(Ei)] D(Ei)
We need M.C. to simulate acceptance (& resolution) of our detectors for the flux measurement:
This requires a simulation program that describes the shower development and detector response as realistically as possible.
CORSIKA Shower Library (proton & iron)
Gaisser-Hillas fit to the shower profile:
Fit parameters scale with primary energy:
Data / Monte Carlo Comparisons
Testing how well we simulate our experiment...
• HiRes-2 data shown from 12/99 until 09/01.
• ~ 531 hours of good weather data.
• average atmosphere used for consistency with HiRes-1.
• Statistics: – rec. geometry: 6262 events
– after all cuts: 2666 events
• M.C. : ~ 5 x data statistics
Acceptances & Exposure
Rmc(Ei) / Gmc(Ei)
Acceptances from simulations broken up into 3 datasets.
A* Ω * t * Rmc(Ei) / Gmc(Ei)
‘Smoothed’ exposure (in 104 km2 sr s).
Systematic Uncertainties
Systematic uncertainties in the energy scale:• absolute calibration of phototubes: +/- 10 %
• fluorescence yield: +/- 10 %
• correction for ‘misssing’ energy: +/- 5 %
• aerosol concentration: < 6 %
=> uncertainty in energy scale: +/- 16 % + atmospheric uncertainty in aperture
=> total uncertainty in the flux: +/- 31 %
What are the uncertainties due atmospheric variations and due to the MC input composition ?
Systematics due to MC Input Composition
• Detector acceptance at low energies depends on c.r. composition.
• MC uses HiRes/MIA measurement as input composition.
• Relevant uncertainties :
– detector calibration
– atmosphere
– fit to HiRes/MIA data
=> +/-5 % uncertainty in proton fraction
Systematics due to Atmospheric Variations
• Repeated HiRes-2 analysis using the atmospheric database.
• Regular Analysis:– <HAL>=25 km,
<VAOD>=0.04
– in MC generation
– in data & MC reconstr.
• Systematics Check:– HAL & VAOD from
database (hourly entries)
– in MC generation
– in data & MC reconstr.
spectrum with database
spectrum with average
Spectrum Fit
• Fit to the HiRes monocular spectra assuming– galactic & extragalactic
components
– all propagation effects (e+e-, red-shift, GZK)
• Details of the fit procedure– Float normalization, input
spectral slope () and m
– Extragalactic component• 45% protons at 1017 eV
• 80% protons at 1017.85 eV
• 100% protons at 1020 eV
– Use binned maximum likelihood method
Galactic
Extragalactic
• We have measured the UHECR spectrum from 1017.2 eV to the highest energies with HiRes-2 in monocular mode.
• A simulation of the exact data taking conditions was used to determine the acceptance and resolution of the detector.
• The simulation was tested in detailed data-MC comparisons and proven to be realistic.
• Systematic uncertainties due to atmospheric variations and MC input composition were shown to be small.
• We observe the ‘ankle’ in the HiRes-2 spectrum at 1018.5 eV.
• The HiRes-2 result is in close agreement with HiRes-1 and Fly’s Eye.
• The HiRes-2 spectrum is consistent with the ‘second knee’ and GZK flux suppression.
• The combined monocular HiRes spectra show evidence for a break above 1019.8 eV. The Poisson probability for continuation of the spectrum with unchanged slope from the HiRes monocular data is 1 x 10-4 .
The HiRes CollaborationThe HiRes Collaboration
J.A. Bellido, R.W. Clay, B.R. Dawson, K.M. Simpson
University of Adelaide
J. Boyer, S. Benzvi, B. Connolly, C. Finley, B. Knapp, E.J. Mannel,
A. O’Neil, M. Seman, S. Westerhoff
Columbia University
J. Belz, M. Munro, M. Schindel
Montana State University
G. Martin, J.A.J. Matthews, M. Roberts
University of New Mexico
D. Bergman, L. Perera, G. Hughes,S. Stratton, D. Ivanov,
S. Schnetzer, G.B. Thomson, A. Zech
Rutgers University
N. Manago, M. Sasaki
University of Tokyo
T. Abu-Zayyad, J. Albretson, G. Archbold, J. Balling, K. Belov, Z. Cao, M. Dalton,
A. Everett, J. Girard, R. Gray, W. Hanlon, P. Hüntemeyer, C.C.H. Jui, D. Kieda, K. Kim, E.C. Loh, K. Martens, J.N. Matthews, A. McAllister, J. Meyer,
S.A. Moore, P. Morrison, J.R. Mumford, K. Reil,R. Riehle, P. Shen, J. Smith,
P. Sokolsky, R.W. Springer, J. Steck, B.T. Stokes, S.B. Thomas,
T.D. Vanderveen, L. Wiencke
University of Utah
J. Amann, C. Hoffman, M. Holzscheiter, L. Marek, C. Painter, J. Sarracino,
G. Sinnis, N. Thompson, D. Tupa
Los Alamos National Laboratory
THANK YOU !THANK YOU !