A search for ultra-high-energy photons at the Pierre Auger Observatory exploiting air-shower universality
Pierpaolo Savina, P. Abreu, M. Aglietta, Justin M. Albury, I. Allekotte, A. Almela, Jaime Álvarez-Muñiz, Rafael Alves Batista, Gioacchino Alex Anastasi, Luis A. Anchordoqui, Belén Andrada, S. Andringa, Carla Aramo, Paulo Ricardo Araújo Ferreira, Juan Carlos Arteaga Velázquez, Hernán Gonzalo Asorey, P. Assis, Gualberto Avila, Alina Mihaela Badescu, Alena Bakalová, Alexandru Balaceanu, Felicia Barbato, Ricardo Jorge Barreira Luz, Karl‐Heinz Becker, Jose A. Bellido, Corinne Bérat, Mario E. Bertaina, X. Bertou, Peter L. Biermann, Virginia Binet, Kathrin Bismark, Teresa Bister, Jonathan Biteau, Jiří Blažek, C. Bleve, Martina Boháčová, Denise Boncioli, C. Bonifazi, Luan Bonneau Arbeletche, Nataliia Borodai, Ana Martina Botti, Jeffrey Brack, T. Bretz, P. Gabriel Brichetto Orchera, Florian Lukas Briechle, P. Buchholz, A. Bueno, S. Buitink, Mario Buscemi, Max Büsken, K. S. Caballero‐Mora, Lorenzo Caccianiga, Fabrizia Canfora, Ioana Caracas, Juan Miguel Carceller, R. Caruso, Antonella Castellina, Fernando Catalani, G. Cataldi, Lorenzo Cazon, Marcos Cerda, J. A. Chinellato, J. Chudoba, L. Chytka, R. W. Clay, Agustín Cobos Cerutti, Roberta Colalillo, Alan Coleman, Maria Rita Coluccia, R. Conceição, Antonio Condorelli, Giovanni Consolati, F. Contreras, Fabio Convenga, Diego Correia dos Santos, C. E. Covault, S. Dasso, K. Daumiller, B. R. Dawson, Jarryd A. Day, Rogerio M. de Almeida, Joaquin de Jesus, S. J. de Jong, G. De Mauro, João de Mello Neto, I. De Mitri, Jaime de Oliveira, Danelise de Oliveira Franco, Francesco de Palma, Vitor de Souza, Emanuele De Vito, Mariano del Río, Olivier Deligny, Luca Deval, Armando di Matteo, C. Dobrigkeit, Juan Carlos D’Olivo, Luis Miguel Domingues Mendes, Rita Cassia dos Anjos, Diego dos Santos
Abstract
The Pierre Auger Observatory is the most sensitive detector to primary photons with energies above $\sim 0.2$ EeV. It measures extensive air showers using a hybrid technique that combines a fluorescence detector (FD) with a ground array of particle detectors (SD). The signatures of a photon-induced air shower are a larger atmospheric depth at the shower maximum ($X_{max}$) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced background. Using observables measured by the FD and SD, three photon searches in different energy bands are performed. In particular, between threshold energies of 1-10 EeV, a new analysis technique has been developed by combining the FD-based measurement of $X_{max}$ with the SD signal through a parameter related to its muon content, derived from the universality of the air showers. This technique has led to a better photon/hadron separation and, consequently, to a higher search sensitivity, resulting in a tighter upper limit than before. The outcome of this new analysis is presented here, along with previous results in the energy ranges below 1 EeV and above 10 EeV. From the data collected by the Pierre Auger Observatory in about 15 years of operation, the most stringent constraints on the fraction of photons in the cosmic flux are set over almost three decades in energy.