A Measurement of the UHE Cosmic Ray Spectrum with the HiRes FADC Detector Andreas Zech Rutgers University for the HiRes-Fly´s Eye Collaboration

HiRes Collaboration J.A. Bellido, R.W. Clay, B.R. Dawson, K.M. Simpson University of Adelaide J. Boyer, B. Knapp, W. Lee, E.J. Mannel, M. Seman, S. Westerhoff, X. Zhang 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. MacLynn. Perera, S. Schnetzer, G.B. Thomson, A. Zech Rutgers University N. Manago, M. Sasaki, M. Sasano, M. Teshima University of Tokyo T. Abu-Zayyad, J. Albretson, G. Archbold, J. Ballling, K. Belov, Z. Cao, M. Dalton, A. Everett, J. Girard, R. Gray, W. Hanlon, C.C.H. Jui, D. Kieda, K. Kim, E.C. Loh, K. Martens, J.N. Matthews, A. McAllister, J. Meyer, S.A. Moore, A.N. Moosman, 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

Outline n UHECR Physics n The HiRes FADC Detector n Detector Simulation n Event Reconstruction n Data / M.C. Comparisons n Energy Spectrum

What’s so exciting about ultrahigh energy cosmic rays ? n Observed energies are a billion times higher than for accelerator particles. n We don’t know (yet) – where they come from (AGN ? Quasars ?...). – how they are being accelerated. – what their composition is (protons ? nuclei ?...). – if there is an interaction with the CMB (GZK cutoff ?). n UHECR physics will give new insights into Astrophysics and Particle Physics.

The HiRes FADC Detector (HiRes-2) n The newer one of the 2 HiRes air fluorescence detectors n 2 rings with 21 mirrors each n Located on Camel Back in Dugway (Utah) n Started taking data in fall 1999

The HiRes FADC Detector (HiRes-2) n 256 photomultiplier tubes per mirror. n Flash ADC electronics record at a frequency of 10 MHz.

Calibration Issues ¶ PMT calibration ; Portable Xenon Flasher for absolute calibration ; Nightly calibration with a YAG laser system ; Uncertainty... Ë Atmospheric calibration ; Steerable Laser ; Xenon Flashers, Cloud Monitor ; Uncertainty...

Detector Simulation: CORSIKA Shower Library n Gaisser-Hillas fit to the shower profile: Fit parameters scale with primary energy:

Detector Simulation: Detector Response n Choose random geometry. n Choose energy & composition according to HiRes measurements. n Take single shower from a corresponding shower library file. n Generate shower profile from parameterisation. n Light generation & propagation. n Ray tracing & optics. n Electronics & trigger simulation using info from trigger database. n Write out triggered tubes, times, FADC pulses for each event.

Event Reconstruction: Shower Geometry Trackfit > Shower Detector Plane T vs.  fit > Geometry in plane

Event Reconstruction: Profile & Energy n Reconstruct charged particle profile from recorded p.e.’s ( with known gain, fluoresc. yield, atmosphere). n Fit profile to G.H. function. n Subtract Cerenkov light. n Multiply by mean energy loss rate  => calorimetric energy n Add ‘missing energy’ (muons, neutrinos, nuclear excitations) => total energy

Data / Monte Carlo Comparisons or: Testing how well we understand our experiment... n Data shown from September March 2001 ( dataset II ) n... Hours of good weather data n Statistics: – rec. geometry: – rec. energy: – after all cuts:

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

Track Angle / Trigger Distribution

Recorded Light / Time Fit Chisquare

Reconstructed Energy and Rp

Energy and Rp Resolution

HiRes Mono Energy Spectra n HiRes-1 data from.../’97 until.../’00 n HiRes-2 data ( dataset I ) from 12/’99 until 05/’00

Defining the ‘Knee’ n HiRes-2 dataset I from 12/’99 until 05/’00 n HiRes-2 dataset II from 09/’00 until 03/’01 PRELIMINARY

Defining the ‘Knee’ HiRes-2 datasets I and II combined 12/’99 until 03/’01 PRELIMINARY

Defining the ‘Knee’ HiRes-2 datasets I and II combined and rebinned PRELIMINARY

Conclusions n We have generated air showers and detector response for the HiRes FADC detector in detail, taking changing trigger and atmosphere into account. n Our data / MC comparisons show that our simulations are realistic. n Our new energy spectrum measurement helps defining the ‘knee’ more clearly.