O-36 - HiRes monocular spectra show a flux suppression at 5.6e19 eV with 5.3-sigma significance
Key data properties:
item value technique air-fluorescence, monocular mode (both stations, Dugway, Utah) energy resolution ~17% (HiRes-I, profile-constrained fit), ~12% (HiRes-II) at high energy triple-power-law fit χ²/DOF = 35.1/35 (vs 63.0/37 for single-break model) systematic uncertainties energy scale 17%, flux 30% (PMT calibration 10%, fluorescence yield 6%, missing energy 5%, aerosols 5%, mean dE/dx 10%, in quadrature) energy range covered 10^17.2 to above 10^20.5 eV The measured spectrum is in direct tension with AGASA’s earlier ground-array result of an unbroken spectrum beyond 10^20 eV.
Methodology
Nitrogen UV fluorescence from extensive air showers imaged by two stations (22 and 42 mirror-PMT telescope modules); photometric calibration via monthly Xenon flash lamps + nightly YAG laser (~10% RMS photometric accuracy); atmospheric attenuation corrected using measured vertical aerosol optical depth (mean 0.04 ± 0.02); shower profiles fit to the Gaisser-Hillas function giving near-calorimetric energy; ~10% correction for unobserved neutrino/muon energy (CORSIKA/QGSJet, Sibyll within 2%); aperture from full Monte Carlo validated by data-MC comparisons of shower distance and brightness distributions. Systematics are largely independent of AGASA’s ground-array/lateral-distribution method.
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Why this is evidence
H-9 predicts a suppression at just this energy from photo-pion losses of protons on the CMB; the instrumental branch of H-37 (and the earlier AGASA unbroken spectrum this contradicts) predicts none. H-12 also yields a steepening, but as a source property whose energy need not coincide with the GZK scale — so the location as well as the existence of the break discriminates.