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1. Measurement of the depth of maximum of air-shower profiles with energies between ${10}^{18.5}$ and ${10}^{20}\mathrm{eV}$ using the surface detector of the Pierre Auger Observatory and deep learning

   https://doi.org/10.1103/PhysRevD.111.022003  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (January 2025, Physical Review D)

   We report an investigation of the mass composition of cosmic rays with energies from 3 to 100 EeV ( $1\mathrm{EeV}={10}^{18}\mathrm{eV}$) using the distributions of the depth of shower maximum $X_{\max }$. The analysis relies on $\sim 50,000$events recorded by the surface detector of the Pierre Auger Observatory and a deep-learning-based reconstruction algorithm. Above energies of 5 EeV, the dataset offers a 10-fold increase in statistics with respect to fluorescence measurements at the Observatory. After cross-calibration using the fluorescence detector, this enables the first measurement of the evolution of the mean and the standard deviation of the $X_{\max }$distributions up to 100 EeV. Our findings are threefold: (i) The evolution of the mean logarithmic mass toward a heavier composition with increasing energy can be confirmed and is extended to 100 EeV. (ii) The evolution of the fluctuations of $X_{\max }$toward a heavier and purer composition with increasing energy can be confirmed with high statistics. We report a rather heavy composition and small fluctuations in $X_{\max }$at the highest energies. (iii) We find indications for a characteristic structure beyond a constant change in the mean logarithmic mass, featuring three breaks that are observed in proximity to the ankle, instep, and suppression features in the energy spectrum. Published by the American Physical Society2025 

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2. Mass composition of ultrahigh energy cosmic rays from distribution of their arrival directions with the Telescope Array

   https://doi.org/10.1103/PhysRevD.110.022006  

   Abbasi, R U; Abe, Y; Abu-Zayyad, T; Allen, M; Arai, Y; Arimura, R; Barcikowski, E; Belz, J W; Bergman, D R; Blake, S A; et al (July 2024, Physical Review D)

   We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array (TA) experiment with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion Letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extragalactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields. The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E>100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as 2 ×10−5 Mpc−3, that is the conservative lower limit for the source number density. 

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3. Inference of the Mass Composition of Cosmic Rays with Energies from ${10}^{18.5}$ to ${10}^{20}\mathrm{eV}$ Using the Pierre Auger Observatory and Deep Learning

   https://doi.org/10.1103/PhysRevLett.134.021001  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (January 2025, Physical Review Letters)

   We present measurements of the atmospheric depth of the shower maximum $X_{\max }$, inferred for the first time on an event-by-event level using the surface detector of the Pierre Auger Observatory. Using deep learning, we were able to extend measurements of the $X_{\max }$distributions up to energies of 100 EeV ( ${10}^{20}\mathrm{eV}$), not yet revealed by current measurements, providing new insights into the mass composition of cosmic rays at extreme energies. Gaining a 10-fold increase in statistics compared to the fluorescence detector data, we find evidence that the rate of change of the average $X_{\max }$with the logarithm of energy features three breaks at $6.5\pm 0.6\left(\mathrm{stat}\right)\pm 1\left(\mathrm{syst}\right)\mathrm{EeV}$, $11\pm 2\left(\mathrm{stat}\right)\pm 1\left(\mathrm{syst}\right)\mathrm{EeV}$, and $31\pm 5\left(\mathrm{stat}\right)\pm 3\left(\mathrm{syst}\right)\mathrm{EeV}$, in the vicinity to the three prominent features (ankle, instep, suppression) of the cosmic-ray flux. The energy evolution of the mean and standard deviation of the measured $X_{\max }$distributions indicates that the mass composition becomes increasingly heavier and purer, thus being incompatible with a large fraction of light nuclei between 50 and 100 EeV. Published by the American Physical Society2025 

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4. Search for photons above ${10}^{18}\mathrm{eV}$ by simultaneously measuring the atmospheric depth and the muon content of air showers at the Pierre Auger Observatory

   https://doi.org/10.1103/PhysRevD.110.062005  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (September 2024, Physical Review D)

   The Pierre Auger Observatory is the most sensitive instrument to detect photons with energies above $10^{17}\mathrm{eV}$. It measures extensive air showers generated by ultrahigh energy cosmic rays using a hybrid technique that exploits the combination of a fluorescence detector with a ground array of particle detectors. The signatures of a photon-induced air shower are a larger atmospheric depth of 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 cascades. In this work, a new analysis technique in the energy interval between 1 and 30 EeV ( $1\mathrm{EeV}=10^{18}\mathrm{eV}$) has been developed by combining the fluorescence detector-based measurement of $X_{\max }$with the specific features of the surface detector signal through a parameter related to the air shower muon content, derived from the universality of the air shower development. No evidence of a statistically significant signal due to photon primaries was found using data collected in about 12 years of operation. Thus, upper bounds to the integral photon flux have been set using a detailed calculation of the detector exposure, in combination with a data-driven background estimation. The derived 95% confidence level upper limits are 0.0403, 0.01113, 0.0035, 0.0023, and $0.0021{\mathrm{km}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{yr}}^{-1}$above 1, 2, 3, 5, and 10 EeV, respectively, leading to the most stringent upper limits on the photon flux in the EeV range. Compared with past results, the upper limits were improved by about 40% for the lowest energy threshold and by a factor 3 above 3 EeV, where no candidates were found and the expected background is negligible. The presented limits can be used to probe the assumptions on chemical composition of ultrahigh energy cosmic rays and allow for the constraint of the mass and lifetime phase space of super-heavy dark matter particles. Published by the American Physical Society2024 

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5. Impact of the magnetic horizon on the interpretation of the Pierre Auger Observatory spectrum and composition data

   https://doi.org/10.1088/1475-7516/2024/07/094  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, GA; et al (July 2024, Journal of Cosmology and Astroparticle Physics)

   Abstract The flux of ultra-high energy cosmic rays reaching Earth above the ankle energy (5 EeV) can be described as a mixture of nuclei injected by extragalactic sources with very hard spectra and a low rigidity cutoff.Extragalactic magnetic fields existing between the Earth and the closest sources can affect the observed CR spectrum by reducing the flux of low-rigidity particles reaching Earth. We perform a combined fit of the spectrum and distributions of depth of shower maximum measured with the Pierre Auger Observatory including the effect of this magnetic horizon in the propagation of UHECRs in the intergalactic space.We find that, within a specific range of the various experimental and phenomenological systematics, the magnetic horizon effect can be relevant for turbulent magnetic field strengths in the local neighbourhood in which the closest sources lieof order B_rms≃ (50–100) nG (20 Mpc/d_s)( 100 kpc/L_coh)^1/2, with d_sthe typical intersource separation and L_cohthe magnetic field coherence length. When this is the case,the inferred slope of the source spectrum becomes softer and can be closer to the expectations of diffusive shock acceleration, i.e., ∝ E^-2.An additional cosmic-ray population with higher source density and softer spectra, presumably also extragalactic and dominating the cosmic-ray flux at EeV energies, is also required to reproduce the overall spectrum and composition results for all energies down to 0.6 EeV. 

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