Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Search for photons with energies above 10$^{18}$ eV using the hybrid
  detector of the Pierre Auger Observatory   A search for ultra-high energy photons with energies above 1 EeV is performed
using nine years of data collected by the Pierre Auger Observatory in hybrid
operation mode. An unprecedented separation power between photon and hadron
primaries is achieved by combining measurements of the longitudinal air-shower
development with the particle content at ground measured by the fluorescence
and surface detectors, respectively. Only three photon candidates at energies 1
- 2 EeV are found, which is compatible with the expected hadron-induced
background. Upper limits on the integral flux of ultra-high energy photons of
0.038, 0.010, 0.009, 0.008 and 0.007 km$^{-2}$ sr$^{-1}$ yr$^{-1}$ are derived
at 95% C.L. for energy thresholds of 1, 2, 3, 5 and 10 EeV. These limits bound
the fractions of photons in the all-particle integral flux below 0.14%, 0.17%,
0.42%, 0.86% and 2.9%. For the first time the photon fraction at EeV energies
is constrained at the sub-percent level. The improved limits are below the flux
of diffuse photons predicted by some astrophysical scenarios for cosmogenic
photon production. The new results rule-out the early top-down models $-$ in
which ultra-high energy cosmic rays are produced by, e.g., the decay of
super-massive particles $-$ and challenge the most recent super-heavy dark
matter models.

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The categories of this paper are: quant-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: eess
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: stat
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The categories of this paper are: cmp-lg
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The categories of this paper are: alg-geom
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The categories of this paper are: nlin
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The categories of this paper are: q-fin
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The categories of this paper are: hep-ph
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The categories of this paper are: nucl-ex