Primary HiRes (High Resolution Fly’s Eye, stereo air-fluorescence) monocular-mode measurement showing a spectral suppression at ~6x10^19 eV with 5 sigma significance, consistent with the GZK prediction and in tension with the earlier AGASA no-cutoff result. relevance_note: independent fluorescence-technique primary flux measurement that, together with Auger, resolved the AGASA/HiRes GZK controversy in favor of a real suppression — directly bears on how confidently one can state the flux above LHC-equivalent energy.
Spectrum measurement
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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O-37 - HiRes E-half energy of 10^19.73 eV matches the theoretical GZK prediction of 10^19.76 eV
Derived from the same HiRes monocular spectra and the fitted middle-segment power law; a rival team analyzing the same data would reproduce the number, while its interpretive force runs through the E1/2-method argument.
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Interpretation
A-23 - E-half agreement identifies the break as the GZK cutoff independently of the source spectral index
Reasoning:
- A break in a measured spectrum could in principle reflect source properties rather than propagation: Berezinsky et al. show the local source density changes the power law above the cutoff, and the sources’ average injection index could shift the apparent cutoff energy somewhat.
- The E1/2 statistic - the energy at which the integral spectrum falls to half the no-cutoff extrapolation - is predicted to be 10^19.76 eV for GZK proton propagation over a wide range of injection spectral slopes, i.e. it is a nearly parameter-free signature of the propagation origin of the break.
- HiRes computes E1/2 from its monocular spectra plus the fitted middle-segment power law and obtains 10^(19.73 ± 0.07) eV, statistically indistinguishable from the prediction.
- Hence the break is identified as the GZK cutoff (not a source artifact) by a test largely orthogonal to the fit that located the break - the nontrivial step connecting the measured suppression to the GZK interpretation.
Original
statement (pre-step-6): “Because the theoretical E1/2 value (10^19.76 eV, Berezinsky-Grigor’eva) is robust across a wide range of source spectral slopes, the agreement of HiRes’s measured E1/2 = 10^(19.73 ± 0.07) eV with it identifies the observed break as the GZK cutoff in a way that is insensitive to the main free parameter (the injection power law) that could otherwise mimic or shift a break.”
Validity verdict (step 6)
Verdict: corrected. Reconstruction: premise = E1/2 is insensitive to the injection spectral index for proton-GZK propagation; observation = measured 10^(19.73 +/- 0.07) vs predicted 10^19.76. As originally stated, the conclusion - the match “identifies the observed break as the GZK cutoff” - overclaims: an undercutting defeater survives, namely a heavy-composition (photodisintegration) cutoff that produces a suppression near the same energy without the proton-GZK mechanism; the E1/2 test discriminates against source-spectrum explanations but not against that alternative (the same defeater A-14’s source concedes). The weaker, evidential conclusion is immune to it, so the statement was edited to that form; original preserved below.
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H-25 - The observed suppression at 6e19 eV is the GZK cutoff from photo-pion losses on the CMB
Supporting context in the paper: the ankle at 4×10^18 eV is consistent with e+e- pair production in the same proton-CMB interactions, and both features fit the published HiRes light-composition results above 10^18 eV; Berezinsky et al. find local source density changes the above-cutoff power law but not the cutoff energy itself. For the LHC safety question this is what licenses stating the cosmic-ray flux at and above LHC-equivalent energies with confidence.
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