Primary Telescope Array (Utah, Northern-Hemisphere ground array, surface detector) energy-spectrum measurement above 1.6x10^18 eV from 4 years of data; finds a dip at 4.6x10^18 eV and a steepening at 5.4x10^19 eV consistent with GZK, cross-checking Auger (Southern Hemisphere) with an independent Northern-Hemisphere detector. relevance_note: third, geographically/technique-independent primary flux measurement, closing the loop on whether the extreme-energy suppression is a real physical feature vs. an instrumental artifact of any one experiment.
Methodology
Telescope Array surface detector (scintillator ground array, Utah, Northern Hemisphere): 4 years of data (May 2008 - May 2012), 13,100 events above 10^18.2 eV, exposure 3690 km^2 sr yr. The SD energy scale is fixed to the calorimetric fluorescence-detector scale via a 27% renormalization derived from FD-SD common events. Dominant systematic: 22% energy-scale uncertainty, propagating to 37% in flux; aperture systematic ~3%.
Spectrum (§4)
O-10 - Telescope Array SD spectrum shows an ankle at 4.6e18 eV and a 5.5-sigma flux suppression at 5.4e19 eV
Spectral indices: -3.34 +/- 0.04 (below ankle), -2.67 +/- 0.03 (between breaks), -4.6 +/- 0.6 (above suppression). Dominant systematic: 22% energy-scale uncertainty (SD scale fixed to the calorimetric fluorescence-detector scale via a 27% renormalization from FD-SD common events), propagating to 37% in flux; aperture systematic ~3%.
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Telescope Array surface detector (scintillator ground array, Utah, Northern Hemisphere): 4 years of data (May 2008 - May 2012), 13,100 events above 10^18.2 eV, exposure 3690 km^2 sr yr. The SD energy scale is fixed to the calorimetric fluorescence-detector scale via a 27% renormalization derived from FD-SD common events. Dominant systematic: 22% energy-scale uncertainty, propagating to 37% in flux; aperture systematic ~3%.
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Interpretation (§1, §4-5)
H-9 - The UHECR ankle and suppression are caused by GZK interactions of extragalactic protons with the CMB
Rival reading noted in the paper itself: Auger’s Xmax results may indicate a heavy composition at the highest energies, under which the suppression would be caused by spallation of heavy nuclei and the ankle would need a separate explanation.
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A-14 - The measured E-half energy matches the parameter-insensitive GZK-proton prediction
Reasoning
- The GZK mechanism (photo-pion production on the CMB) fixes an absolute energy scale set by the CMB photon field and the pion production threshold, nearly independent of source spectra, evolution, or distribution; Berezinsky et al. computed the summary statistic E_1/2 = 10^19.72 eV for protons across a wide range of such assumptions.
- An instrumental artifact or an intrinsic source cutoff carries no reason to reproduce this particular energy: a priori it could fall anywhere. The measured value 19.72 +/- 0.05 landing exactly on the parameter-insensitive prediction is therefore evidence specifically for the propagation-loss (GZK, protonic) interpretation of the break rather than a coincidental cutoff.
- Same for the ankle: the dip at ~5e18 eV coincides with the predicted e+e- pair-production dip for protons on the CMB, while heavier nuclei would not produce such a dip.
- Caveat contained in the same paper: Auger’s composition measurements (Xmax) suggest heavy primaries in the South, under which the suppression energy would instead reflect photodisintegration of nuclei; the E_1/2 match is strong but not decisive.
Validity verdict (step 6)
Reconstruction: premise = E_1/2 = 10^19.72 eV is a near-parameter-free prediction of proton-GZK propagation; observation = TA measures 19.72 +/- 0.05. Load-bearing step: an alternative cause (instrument artifact, intrinsic source cutoff) has no reason to land on that particular energy, so the match is evidence specifically for the propagation-loss interpretation - a valid likelihood-ratio-shaped inference. Undercutting probe: heavy-composition photodisintegration can suppress near a similar energy; the body concedes this (“strong but not decisive”) and the statement claims a quantitative location match with specificity against generic source physics, not proof - it survives as stated.
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Comparison with other experiments (§4-5, Fig. 5)
O-11 - TA spectrum agrees with HiRes, and with Auger up to a 20 percent energy-scale shift except a 3-sigma difference in break energy
The comparison rests on the TA dataset together with the published AGASA, HiRes, and Auger spectra shown in Fig. 5. Its significance: three geographically and technique-independent experiments (HiRes fluorescence, TA surface array in the Northern hemisphere, Auger hybrid in the Southern hemisphere) all see a high-energy suppression, so the feature is physical rather than an instrumental artifact of any single detector; the residual 3-sigma discrepancy concerns the break’s exact energy, not its existence.
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