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.