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1. Arrival Directions of Cosmic Rays above 32 EeV from Phase One of the Pierre Auger Observatory

   https://doi.org/10.3847/1538-4357/ac7d4e  

   Abreu, P.; Aglietta, M.; Albury, J. M.; Allekotte, I.; Almeida Cheminant, K.; Almela, A.; Alvarez-Muñiz, J.; Alves Batista, R.; Ammerman Yebra, J.; Anastasi, G. A.; et al (August 2022, The Astrophysical Journal)

   Abstract A promising energy range to look for angular correlations between cosmic rays of extragalactic origin and their sources is at the highest energies, above a few tens of EeV (1 EeV ≡ 10 18 eV). Despite the flux of these particles being extremely low, the area of ∼3000 km 2 covered at the Pierre Auger Observatory, and the 17 yr data-taking period of the Phase 1 of its operations, have enabled us to measure the arrival directions of more than 2600 ultra-high-energy cosmic rays above 32 EeV. We publish this data set, the largest available at such energies from an integrated exposure of 122,000 km 2 sr yr, and search it for anisotropies over the 3.4 π steradians covered with the Observatory. Evidence for a deviation in excess of isotropy at intermediate angular scales, with ∼15° Gaussian spread or ∼25° top-hat radius, is obtained at the 4 σ significance level for cosmic-ray energies above ∼40 EeV. 

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2. Search for photons above 10 ¹⁹ eV with the surface detector of the Pierre Auger Observatory

   https://doi.org/10.1088/1475-7516/2023/05/021  

   Abreu, P.; Aglietta, M.; Allekotte, I.; Almeida Cheminant, K.; Almela, A.; Alvarez-Muñiz, J.; Ammerman Yebra, J.; Anastasi, G.A.; Anchordoqui, L.; Andrada, B.; et al (May 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract We use the surface detector of the Pierre Auger Observatory to search for air showers initiated by photons with an energy above 10 19 eV. Photons in the zenith angle range from 30 ∘ to 60 ∘ can be identified in the overwhelming background of showers initiated by charged cosmic rays through the broader time structure of the signals induced in the water-Cherenkov detectors of the array and the steeper lateral distribution of shower particles reaching ground. Applying the search method to data collected between January 2004 and June 2020, upper limits at 95% CL are set to an E -2 diffuse flux of ultra-high energy photons above 10 19 eV, 2 × 10 19 eV and 4 × 10 19 eV amounting to 2.11 × 10 -3 , 3.12 × 10 -4 and 1.72 × 10 -4  km -2  sr -1  yr -1 , respectively. While the sensitivity of the present search around 2 × 10 19 eV approaches expectations of cosmogenic photon fluxes in the case of a pure-proton composition, it is one order of magnitude above those from more realistic mixed-composition models. The inferred limits have also implications for the search of super-heavy dark matter that are discussed and illustrated. 

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3. A Search for Photons with Energies Above 2 × 10 ¹⁷ eV Using Hybrid Data from the Low-Energy Extensions of the Pierre Auger Observatory

   https://doi.org/10.3847/1538-4357/ac7393  

   Abreu, P.; Aglietta, M.; Albury, J. M.; Allekotte, I.; Cheminant, K. Almeida; Almela, A.; Alvarez-Muñiz, J.; Batista, R. Alves; Yebra, J. Ammerman; Anastasi, G. A.; et al (July 2022, The Astrophysical Journal)

   Abstract Ultra-high-energy photons with energies exceeding 10 17 eV offer a wealth of connections to different aspects of cosmic-ray astrophysics as well as to gamma-ray and neutrino astronomy. The recent observations of photons with energies in the 10 15 eV range further motivate searches for even higher-energy photons. In this paper, we present a search for photons with energies exceeding 2 × 10 17 eV using about 5.5 yr of hybrid data from the low-energy extensions of the Pierre Auger Observatory. The upper limits on the integral photon flux derived here are the most stringent ones to date in the energy region between 10 17 and 10 18 eV. 

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4. Testing the Compatibility of the Depth of the Shower Maximum Measurements performed at Telescope Array and the Pierre Auger Observatory: Auger-TA Mass Composition Working Group Report

   https://doi.org/10.1051/epjconf/202328302008  

   Bergman, Douglas; Bellido, Jose; de Souza, Vitor; Engel, Ralph; Gerber, Zane; Kim, JiHyun; Mayotte, Eric; Tkachenko, Olena; Unger, Michael; Yushkov, Alexey (January 2023, EPJ Web of Conferences)

   De Mitri, I.; Barbato, F.C.T.; Boncioli, D.; Evoli, C.; Pagliaroli, G.; Salamida, F. (Ed.)

   The Telescope Array and the Pierre Auger Observatory estimate the composition of ultra-high-energy cosmic rays by observing the distribution of depths of air-shower maxima, X max . Both experiments directly observe the longitudinal development of air showers using fluorescence telescopes with surface particle detectors used in conjunction to provide precision in determining air-shower geometry. The two experiments differ in the details of the analysis of events, so a direct comparison of X max distributions is not possible. The Auger – Telescope Array Composition Working Group presents their results from a technique to compare X max measurements from Auger with those of Telescope Array. In particular, the compatibility of the first two moments of the X max distributions of Auger with the data from the Black Rock Mesa and Long Ridge detectors of the Telescope Array is tested for energies above 10 18.2 eV. Quantitative comparisons are obtained using air-shower simulations of four representative species made using the Sibyll 2.3d high-energy interaction model. These are weighted to fit the fractional composition seen in Auger data and reconstructed using the Telescope Array detector response and analysis methods. 

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5. Direct measurement of the muonic content of extensive air showers between $$\mathbf { 2\times 10^{17}}$$ and $$\mathbf {2\times 10^{18}}~$$eV at the Pierre Auger Observatory

   https://doi.org/10.1140/epjc/s10052-020-8055-y  

   Aab, A.; Abreu, P.; Aglietta, M.; Albury, J. M.; Allekotte, I.; Almela, A.; Alvarez Castillo, J.; Alvarez-Muñiz, J.; Anastasi, G. A.; Anchordoqui, L.; et al (August 2020, The European Physical Journal C)

   null (Ed.)

   Abstract The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between $$2\times 10^{17}$$ 2 × 10 17 and $$2\times 10^{18}$$ 2 × 10 18 eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at $$10^{{17.5}}\, {\mathrm{eV}} $$ 10 17.5 eV and $$10^{{18}}\, {\mathrm{eV}} $$ 10 18 eV . We find that, for the models to explain the data, an increase in the muon density of $$38\%$$ 38 % $$\pm 4\% (12\%)$$ ± 4 % ( 12 % ) $$\pm {}^{21\%}_{18\%}$$ ± 18 % 21 % for EPOS-LHC , and of $$50\% (53\%)$$ 50 % ( 53 % ) $$\pm 4\% (13\%)$$ ± 4 % ( 13 % ) $$\pm {}^{23\%}_{20\%}$$ ± 20 % 23 % for QGSJetII-04 , is respectively needed. 

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