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Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory

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	The Pierre Auger Collaboration; Pastor Carpi, Sergio; Pinto, Teguayco
	This document is a artículoDate2009
	Este documento está disponible también en : http://hdl.handle.net/10550/48022
	From direct observations of the longitudinal development of ultra-high energy air showers performed with the Pierre Auger Observatory, upper limits of 3.8%, 2.4%, 3.5% and 11.7% (at 95% c.l.) are obtained on the fraction of cosmic-ray photons above 2, 3, 5 and 10 EeV (1 EeV = 10^18 eV) respectively. These are the first experimental limits on ultra-high energy photons at energies below 10 EeV. The results complement previous constraints on top-down models from array data and they reduce systematic uncertainties in the interpretation of shower data in terms of primary flux, nuclear composition and proton-air cross-section.
	The Pierre Auger Collaboration Pastor Carpi, Sergio Pinto, Teguayco 2009 Upper limit on the cosmic-ray photon fraction at EeV energies from the Pierre Auger Observatory Astroparticle Physics 31 6 399 406
	http://dx.doi.org/10.1016/j.astropartphys.2009.04.003