1. The Radio Detector of the Pierre Auger ObservatoryJ. R. Hörandel, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: To measure the properties of the highest-energy particles in the Universe with unprecedented precision, we have upgraded the Pierre Auger Observatory. A crucial component of this upgrade is the Radio Detector. Radio antennas have been added to all 1660 positions of the surface detector array, covering an area of 3000 sq. km. The antennas detect radio emission, emitted by extensive air showers in the frequency band from 30 to 80 MHz in two polarization directions - one parallel and one perpendicular to the Earth magnetic field. For inclined air showers with zenith angles above 60 degrees, the radio antennas provide a clean measurement of the electromagnetic shower component, while the water-Čerenkov detectors measure the muonic component. Large-scale deployment in the Argentinian Pampa Amarilla started around June 2023 and has been completed in 2024. The deployment is accompanied by extensive calibration efforts both, in the laboratory and in the field. The signal chain is characterized in the laboratory. Galactic radio emission is used as a reference signal and the antenna patterns are verified through in-situ calibrations with a reference antenna. Commissioning of the system is in full progress as well as the analysis of first measured air showers. We present first air showers measured with the largest radio detector for cosmic rays in the world. Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, surface detectors, Water-Cherenkov detectors, Surface Scintillator detectors Published in RUNG: 16.05.2025; Views: 151; Downloads: 0
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2. Machine learning-based analyses using surface detector data of the Pierre Auger ObservatorySteffen Hahn, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The Pierre Auger Observatory is the largest detector for the study of extensive air showers induced
by ultra-high-energy cosmic rays (UHECRs). Its hybrid detector design allows the simultaneous
observation of different parts of the shower evolution using various detection techniques. To
accurately understand the physics behind the origin of UHECRs, it is essential to determine their
mass composition. However, since UHECRs cannot be measured directly, estimating their masses
is highly non-trivial. The most common approach is to analyze mass-sensitive observables, such as
the number of secondary muons and the atmospheric depth of the shower maximum.
An intriguing part of the shower to estimate these observables is its footprint. The shower footprint is detected by ground-based detectors, such as the Water-Cherenkov detectors (WCDs) of the Surface Detector (SD) of the Observatory, which have an uptime of nearly 100%, resulting in a high number of observed events. However, the spatio-temporal information stored in the shower footprints is highly complex, making it very challenging to analyze the footprints using analytical and phenomenological methods. Therefore, the Pierre Auger Collaboration utilizes machine learning-based algorithms to complement classical methods in order to exploit the measured data with unprecedented precision. In this contribution, we highlight these machine learning-based analyses used to determine high-level shower observables that help to infer the mass of the primary particle, with a particular focus on analyses using the shower footprint detected by the WCDs and the Surface Scintillator Detectors (SSD) of the SD. We show that these novel methods show promising results on simulations and offer improved reconstruction performance when applied to measured data. Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, surface detector, Water-Cherenkov detectors (WCDs), Surface Scintillator Detectors (SSDs), UHECR mass composition, air-shower footprint, machine learning Published in RUNG: 16.05.2025; Views: 88; Downloads: 0
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3. Communicating astroparticle physics with the public : the Pierre Auger experienceBeatriz García, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The Pierre Auger Observatory has developed various activities along the last year in the field of education and dissemination of astrophysics and high-energy astroparticle physics. Since 1999 different approaches and improvements of the tools used to explain what cosmic rays are have been implemented, tested, and redesigned based on public feedback in its headquarters in Malargüe city. This presentation highlights the Observatory's engagement with the community, focusing on the new initiatives. The open data policy, master classes for middle-level students and teachers, the design of innovative activities that involve the community, and the presence of the Observatory in special events with the participation of students are also described. Keywords: open data policy, ultra-high-energy cosmic rays, Pierre Auger Observatory, education Published in RUNG: 16.05.2025; Views: 131; Downloads: 0
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4. Calibrating the Surface Scintillator Detector of AugerPrimePaul Filip, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: We present the methods used to calibrate the Surface Scintillator Detector of the Pierre Auger
Observatory. We describe how to obtain an accurate calibration of the scintillator by examining
peak and charge distributions of atmospheric muons. This is done whenever an extensive air
shower event measured by the Surface Detector is reconstructed. We also describe the work
towards an algorithm that uses a rate-based approach to calibrate the Surface Scintillator
Detectors independently. Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, AugerPrime upgrade, detector calibration Published in RUNG: 16.05.2025; Views: 130; Downloads: 0
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5. Investigations of CORSIKA thinning levels suitable for studies of photon-hadron discrimination at ultra-high energiesFiona Ellwanger, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: Cosmic ray detectors like the 3000 sq. km surface array of the Pierre Auger Observatory are
capable of observing high-energy photons in the range of 10[sup]18 to 10[sup]20 eV if the flux
is sufficiently high.
However, no clear candidates for ultra-high-energy photons have been identified yet, so
simulations must be used to study typical trigger patterns and observables for discriminating photons from hadrons, e.g., with neural networks.
Thinning algorithms are applied to keep the computation time and file sizes in a manageable range since the simulation of ultra-high-energy particle showers is computationally expensive.
In CORSIKA, particles with energies below a certain fraction of the primary energy, the thinning level, are exposed to thinning.
In the case of thinning, only one of the particles emerging from an interaction is tracked.
By assigning a corresponding weight, this particle then represents a number of its siblings.
However, the weights of particles that originate from electromagnetic interactions can be 100 times larger than for hadronic interactions.
In contrast to hadronic showers, where a major part of the signal in a surface detector
is produced by muons, photon showers are almost purely electromagnetic.
Using simulations of photon-induced showers with two different thinning levels, the influence
on different observables used for photon-hadron discrimination is investigated.
Effects deriving from both statistical sampling and detector simulations are considered.
Possible influences on station-level as well as event-level observables are probed.
With this study, we are reassured that the optimal thinning parameters determined for
hadron-induced showers are also sufficient for photon-induced showers. Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, CORSIKA air-shower simulator Published in RUNG: 16.05.2025; Views: 131; Downloads: 1
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6. The distribution of ultrahigh-energy cosmic rays along the supergalactic plane measured at the Pierre Auger ObservatoryA. Abdul Halim, P. Abreu, M. Aglietta, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, original scientific article Abstract: Ultrahigh-energy cosmic rays are known to be mainly of extragalactic origin, and their propagation is limited by energy losses, so their arrival directions are expected to correlate with the large-scale structure of the local Universe. In this work, we investigate the possible presence of intermediate-scale excesses in the flux of the most energetic cosmic rays from the direction of the supergalactic plane region using events with energies above 20 EeV recorded with the surface detector array of the Pierre Auger Observatory up to 2022 December 31, with a total exposure of 135,000 sq. km sr yr. The strongest indication for an excess that we find, with a posttrial significance of 3.1σ, is in the Centaurus region, as in our previous reports, and it extends down to lower energies than previously studied. We do not find any strong hints of excesses from any other region of the supergalactic plane at the same angular scale. In particular, our results do not confirm the reports by the Telescope Array Collaboration of excesses from two regions in the Northern Hemisphere at the edge of the field of view of the Pierre Auger Observatory. With a comparable integrated exposure over these regions, our results there are in good agreement with the expectations from an isotropic distribution. Keywords: ultra-high-energy cosmic rays, UHECR propagation, large-scale structure, UHECR energy losses, UHECR deflections, supergalactic plane region, Centaurus region, Pierre Auger Observatory, Auger surface detector array Published in RUNG: 06.05.2025; Views: 300; Downloads: 2
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7. Update on full-sky searches for large- and medium-scale anisotropies in the UHECR flux using the Pierre Auger Observatory and the Telescope ArrayGrigory I. Rubtsov, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The flux of ultra-high-energy cosmic rays (UHECRs) is remarkably uniform across all directions in the sky. The only anisotropy detected with a significance greater than 5σ is a large-scale dipolar modulation in right ascension for energies above 8 EeV. To enhance our sensitivity to potential anisotropies, which may be obscured by significant deflections by magnetic fields, two strategies can be employed: (1) focusing on large-scale anisotropies, such as the dipole and quadrupole moments across various energy intervals, which are anticipated to be more resilient to magnetic deflections; or (2) focusing on the highest energies, where the background from distant sources is more attenuated. The unique aspect of our research is achieving full-sky coverage by combining data for the Pierre Auger Observatory and the Telescope Array, which would not be possible with a single detector array. This comprehensive coverage enables the application of analysis techniques that would otherwise require specific assumptions with partial sky coverage. Accounting for potential systematic effects in energy reconstruction is crucial to avoid spurious north–south anisotropies; the overlapping sky region observed by both arrays allows us to address this in an entirely data-driven manner. In this contribution, we present the latest results using the largest UHECR dataset collected to date, with events detected until December 2022 at the Pierre Auger Observatory and until May 2024 at the Telescope Array. It is shown that the dipolar modulation is the only anisotropy that is significantly (4.6σ) identified in the angular power spectrum. The hypothesis of correlations with the starburst galaxies is supported at the significance of 4.4σ. Keywords: ultra-high-energy cosmic rays, Pierre Auger Observatory, UHECR arrival directions, UHECR large-scale anisotropies, UHECR medium-scale anisotropies, full-sky coverage, Telescope Array Published in RUNG: 05.05.2025; Views: 325; Downloads: 4
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8. Improved calibration methods and reconstruction of the underground muon detector of the Pierre Auger ObservatoryJoaquín De Jesús, A. Abdul Halim, P. Abreu, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: As part of the upgrade of the Pierre Auger Observatory, known as AugerPrime, the Underground
Muon Detector is being deployed in the low-energy extension of the Surface Detector. It comprises
an array of 30 m[sup]2 plastic scintillator muon counters, buried 2.3 meters underground near the water-Cherenkov detectors, allowing for direct measurement of the muonic component of air showers in the energy range of 10[sup]16.5 − 10[sup]19 eV. To achieve an extended dynamic range, the detector operates in two modes: the binary mode, which is optimized for low muon densities, and the ADC mode, designed for high muon densities. In this contribution, we present the latest improvements to the calibration procedure of the ADC mode and to the data reconstruction of the binary mode. We assess their performance with simulations. Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, AugerPrime upgrade, Auger underground muon detector (UMD), muonic air-shower component, detector calibration, data reconstruction Published in RUNG: 30.04.2025; Views: 486; Downloads: 5
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9. Amplifying UHECR arrival direction information using mass estimators at the Pierre Auger ObservatoryLorenzo Apollonio, A. Abdul Halim, P. Abreu, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2025, published scientific conference contribution Abstract: The origin of Ultra-High-Energy Cosmic Rays (UHECRs) is one of the biggest mysteries in modern astrophysics. Since UHECRs are deflected by Galactic and extragalactic magnetic fields, their arrival directions do not point to their sources. Previous analyses conducted on the arrival directions of high-energy events (E ≥ 32 EeV) recorded by the Surface Detector of the Pierre Auger Observatory have not shown significant anisotropies. The largest excess found in the first 19 years of data - at the 4.0 sigma level - is in the region around Centaurus A, and it is also the driving force of a correlation of UHECR arrival directions with a catalog of Starburst Galaxies, which is at the 3.8 sigma level. Since UHECRs are mostly nuclei, the lightest ones (least charged) are also the least deflected. While the mass of the events can be estimated better using the Fluorescence Detector of the Pierre Auger Observatory, the Surface Detector provides the necessary statistics needed for astrophysical studies. The introduction of novel mass-estimation techniques, such as machine learning models and an algorithm based on air-shower universality, will help identify high-rigidity events in the Surface Detector data of the Pierre Auger Observatory. With this work, we present how event-per-event mass estimators can help enhance the sensitivity in the search for anisotropies in the arrival directions of UHECRs at small and intermediate angular scales using simulations. Keywords: ultra-high-energy cosmic rays (UHECRs), extensive air showers, Pierre Auger Observatory, UHECR propagation, UHECR arrival directions, UHECR mass composition, Centaurus A radio galaxy, starburst galaxies, air-shower universality Published in RUNG: 30.04.2025; Views: 370; Downloads: 6
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10. An extremely energetic cosmic ray observed by a surface detector arrayR. U. Abbasi, M. Allen, R. Arimura, J. W. Belz, Douglas R. Bergman, S. A. Blake, K. Shin, I. J. Buckland, B. G. Cheon, Jon Paul Lundquist, 2023, original scientific article Abstract: Cosmic rays are energetic charged particles from extraterrestrial sources, with the highest-energy events thought to come from extragalactic sources. Their arrival is infrequent, so detection requires instruments with large collecting areas. In this work, we report the detection of an extremely energetic particle recorded by the surface detector array of the Telescope Array experiment. We calculate the particle’s energy as 244 +- 29 (stat.) +51,-76 (syst.)
exa–electron volts (~40 joules). Its arrival direction points back to a void in the large-scale structure of the Universe. Possible explanations include a large deflection by the foreground magnetic field, an unidentified source in the local extragalactic neighborhood, or an incomplete knowledge of particle physics. Keywords: ultra-high-energy cosmic rays, telescope array, extremely energetic cosmic-ray event Published in RUNG: 23.04.2025; Views: 259; Downloads: 2
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