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1. Testing effects of Lorentz invariance violation in the propagation of astroparticles with the Pierre Auger Observatory

   https://doi.org/10.1088/1475-7516/2022/01/023  

   Abreu, P.; Aglietta, M.; Albury, J.M.; Allekotte, I.; Almeida Cheminant, K.; Almela, A.; Alvarez-Muñiz, J.; Alves Batista, R.; Anastasi, G.A.; Anchordoqui, L.; et al (January 2022, Journal of Cosmology and Astroparticle Physics)

   Abstract Lorentz invariance violation (LIV) is often described by dispersion relations of the form E i 2  =  m i 2 + p i 2 +δ i,n E 2+n with delta different based on particle type i , with energy E , momentum p and rest mass m . Kinematics and energy thresholds of interactions are modified once the LIV terms become comparable to the squared masses of the particles involved. Thus, the strongest constraints on the LIV coefficients δ i,n tend to come from the highest energies. At sufficiently high energies, photons produced by cosmic ray interactions as they propagate through the Universe could be subluminal and unattenuated over cosmological distances. Cosmic ray interactions can also be modified and lead to detectable fingerprints in the energy spectrum and mass composition observed on Earth. The data collected at the Pierre Auger Observatory are therefore possibly sensitive to both the electromagnetic and hadronic sectors of LIV. In this article, we explore these two sectors by comparing the energy spectrum and the composition of cosmic rays and the upper limits on the photon flux from the Pierre Auger Observatory with simulations including LIV. Constraints on LIV parameters depend strongly on the mass composition of cosmic rays at the highest energies. For the electromagnetic sector, while no constraints can be obtained in the absence of protons beyond 10 19 eV, we obtain δ γ,0  > -10 -21 , δ γ,1  > -10 -40 eV -1 and δ γ,2  > -10 -58 eV -2 in the case of a subdominant proton component up to 10 20 eV. For the hadronic sector, we study the best description of the data as a function of LIV coefficients and we derive constraints in the hadronic sector such as δ had,0  < 10 -19 , δ had,1  < 10 -38 eV -1 and δ had,2  < 10 -57 eV -2 at 5σ CL. 

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2. The Cosmic-Ray Composition between 2 PeV and 2 EeV Observed with the TALE Detector in Monocular Mode

   https://doi.org/https://doi.org/10.3847/1538-4357/abdd30  

   Abbasi, R.U. (March 2021, The astrophysical journal)

   We report on a measurement of the cosmic-ray composition by the Telescope Array Low-energy Extension (TALE) air fluorescence detector (FD). By making use of the Cherenkov light signal in addition to air fluorescence light from cosmic-ray (CR)-induced extensive air showers, the TALE FD can measure the properties of the cosmic rays with energies as low as ~2 PeV and exceeding 1 EeV. In this paper, we present results on the measurement of $${X}_{\max }$$ distributions of showers observed over this energy range. Data collected over a period of ~4 yr were analyzed for this study. The resulting $${X}_{\max }$$ distributions are compared to the Monte Carlo (MC) simulated data distributions for primary cosmic rays with varying composition and a four-component fit is performed. The comparison and fit are performed for energy bins, of width 0.1 or 0.2 in $${\mathrm{log}}_{10}(E/\mathrm{eV})$$, spanning the full range of the measured energies. We also examine the mean $${X}_{\max }$$ value as a function of energy for cosmic rays with energies greater than 1015.8 eV. Below 1017.3 eV, the slope of the mean $${X}_{\max }$$ as a function of energy (the elongation rate) for the data is significantly smaller than that of all elements in the models, indicating that the composition is becoming heavier with energy in this energy range. This is consistent with a rigidity-dependent cutoff of events from Galactic sources. Finally, an increase in the $${X}_{\max }$$ elongation rate is observed at energies just above 1017 eV, indicating another change in the cosmic-ray composition. 

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3. 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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4. 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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5. Performance of a large area photon detector for rare event search applications

   https://doi.org/10.1063/5.0032372  

   Fink, C_W; Watkins, S_L; Aramaki, T.; Brink, P_L; Camilleri, J.; Defay, X.; Ganjam, S.; Kolomensky, Yu_G; Mahapatra, R.; Mirabolfathi, N.; et al (January 2021, Applied Physics Letters)

   We present the design and characterization of a large-area Cryogenic PhotoDetector designed for active particle identification in rare event searches, such as neutrinoless double beta decay and dark matter experiments. The detector consists of a 45.6 cm2 surface area by a 1-mm-thick 10.6 g Si wafer. It is instrumented with a distributed network of Quasiparticle-trap-assisted Electrothermal feedback Transition-edge sensors with superconducting critical temperature Tc=41.5 mK to measure athermal phonons released from interactions with photons. The detector is characterized and calibrated with a collimated 55Fe x-ray source incident on the center of the detector. The noise equivalent power is measured to be 1×10−17 W/Hz in a bandwidth of 2.7 kHz. The baseline energy resolution is measured to be σE=3.86±0.04 (stat.)−0.00+0.19 (syst.) eV. The detector also has an expected timing resolution of σt=2.3 μs for 5 σE events. 

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