The search for photons at EeV energies and beyond has considerable
astrophysical interest and will remain one of the key challenges for ultra-high
energy cosmic ray (UHECR) observatories in the near future. Several upper
limits to the photon flux have been established since no photon has been
unambiguously observed up to now. An improvement in the reconstruction
efficiency of the photon showers and/or better discrimination tools are needed
to improve these limits apart from an increase in statistics. Following this
direction, we analyze in this work the ability of the surface parameter Sb,
originally proposed for hadron discrimination, for photon search.
Semi-analytical and numerical studies are performed in order to optimize Sb
for the discrimination of photons from a proton background in the energy range
from 10^18.5 to 10^19.6 eV. Although not shown explicitly, the same analysis
has been performed for Fe nuclei and the corresponding results are discussed
when appropriate. The effects of different array geometries and the
underestimation of the muon component in the shower simulations are analyzed,
as well as the Sb dependence on primary energy and zenith angle.Comment: 9 pages, 19 Figures. Accepted in Astroparticle Physics on May 31th,
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