A Measurement of the Ultra-High Energy Cosmic Ray

Flux with the HiRes FADC Detector

PhD Defense PresentationSeptember 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

A Brief Introduction to Ultra-High Energy Cosmic Ray Physics

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.

The HiRes Experiment

• 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.

• Mirror area ~ 5 m^2 .• 256 (16x16) PMT per mirror.• One PMT sees ~ 1 degree of the sky.

• 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

2. Reconstruction of the geometry within the

s-d-plane (HiRes-2)

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

Unfolding the Cosmic Ray Spectrum

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.

HiRes Monte Carlo Simulation

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

Rp (Distance to shower axis)

# of p.e. / degree of track

Energy Distribution & Resolution

σ = 18 %

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).

Results of the HiRes-2 Analysis

The HiRes-2 UHECR Spectrum

The HiRes monocular UHECR Spectra

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

Fits to the HiRes-2 Spectrum

J ~ E -3.33+/-0.01 J ~ E -2.81+/-0.02

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

HiRes and Fly’s Eye

HiRes and Haverah Park

HiRes and Yakutsk

HiRes and AGASA

Conclusions

• 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 CollaborationJ.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 !

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The Truth about Ultra-High Energy Cosmic Rays