A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC Detector (HiRes-2) Andreas Zech (for the HiRes Collaboration) Rutgers University APS meeting April ‘04

Measuring the Energy Spectrum with HiRes Stereo observation of the cosmic ray flux yields a better resolution in geometry and energy compared to mono. => the analysis of HiRes stereo events is currently under way. Analyzing our data in monocular mode has some advantages as well: better statistics at the high energy end due to longer lifetime of HiRes-1. extension of the spectrum to lower energies due to longer tracks (2 rings!) of HiRes-2.

Effective mirror area ~ 5 m^ (16x16) PMT per mirror. One PMT sees ~ 1 degree of the sky. FADC electronics recording at 10MHz.

Shower Geometry Reconstruction fit to tubes on the track time vs. angle fit

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

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. è This requires a simulation program that describes the shower development and detector response as realistically as possible.

HiRes Monte Carlo Simulation

Data / MC Comparisons: a Test of our Simulation black: HiRes-2 data red: Monte Carlo data Monte Carlo

Energy Resolution (E rec - E true ) E true  ~ 17 %

HiRes-2 Exposure Flux:

HiRes-2 Energy Spectrum statistics: 123 good nights, 536 hours live time, 7011 events with reconstructed geometry, 2636 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 in the energy scale: absolute calibration of phototubes: +/- 10 % fluorescence yield: +/- 10 % correction for unobserved energy: +/- 5 % aerosol concentration: < 9 % + atmospheric uncertainty in aperture => uncertainty in the flux: +/- 31 % What other uncertainties in the aperture are introduced with our inputs to the Monte Carlo ?

M.C. Input Energy & Composition The composition is chosen from our HiRes Stereo and HiRes/MIA measurement. A fit to the Fly’s Eye Stereo spectrum is used as an input for the M.C.

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

Apertures for pure proton / iron poor acceptance for iron at low energies (< eV ) agreement at higher energies.

Systematic Uncertainty due to Input Composition Our systematic uncertainty assuming a +/- 20 % uncertainty in the proton fraction from HiRes / MIA & HiRes Stereo measurements. data set 2 (~1/3 of our data) black: stat. errors red: sys. errors

Conclusions The HiRes-2 spectrum confirms the ankle at ~ eV. The HiRes-2 measurement is consistent with the GZK flux suppression observed by HiRes-1. We cannot claim detection of the second knee due to low statistics and systematic uncertainties at the low energy end.

Cuts Track length Number of ‘good’ tubes Zenith angle Track angle Psi angle Error in Psi angle Good weather conditions Time tangent fit Chisquare Profile fit Chisquare Cerenkov light contribution ‘Bracketing’ cut