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Opis: 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.
Ključne besede: 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
Objavljeno v RUNG: 16.05.2025; Ogledov: 163; Prenosov: 4
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Opis: 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[sup]−5 Mpc[sup]−3, that is the conservative lower limit for the source number density.
Ključne besede: ultrahigh energy cosmic rays, large-scale structure, extragalactic magnetic fields, UHECR propagation, Telescope Array, UHECR mass composition, UHECR arrival directions
Objavljeno v RUNG: 23.04.2025; Ogledov: 284; Prenosov: 2
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Opis: For the past 20 years, the Pierre Auger Observatory has collected the largest dataset of ultra-high-energy cosmic rays (UHECRs) ever achieved using a hybrid detector. The study of this dataset has led to numerous unexpected discoveries that enhance our understanding of the origins of UHECRs. One of the key points in this study is their mass composition. In this work, we will present the most recent results regarding the mass composition of UHECRs at the Pierre Auger Observatory. In particular, we will focus on the measurement of the depth of the maximum of air-shower profiles, denoted as Xmax . This determination has been achieved through both direct measurements from the Fluorescence Detector data and the application of machine learning for estimating Xmax on an event-by-event basis using the Surface Detector data. The latter has allowed us to extend the measurement to energies up to 100 EeV and indicates a correlation between changes in composition and three features of the energy spectrum (ankle, instep, steepening). Moreover, the results provide evidence of a heavy and nearly pure primary beam for energies greater than 50 EeV that is independent of the hadronic interaction model. The implications of these findings for astrophysics and for modelling hadronic interactions will be discussed.
Ključne besede: ultra-high-energy cosmic rays, Pierre Auger Observatory, extensive air showers, UHECR mass composition
Objavljeno v RUNG: 28.03.2025; Ogledov: 442; Prenosov: 9
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Opis: The Pierre Auger Observatory has measured the spectrum of ultra-high-energy cosmic rays with unprecedented precision, as well as the distribution of the depths of the maximum of the shower development in the atmosphere, which provide a reliable estimator of the mass composition. The measurements above 10[sup]17.8 eV can be interpreted assuming two populations of uniformly distributed sources, one with a soft spectrum dominating the flux below few EeV, and another one with a very hard spectrum dominating above that energy. When considering the presence of intense extragalactic magnetic fields between our Galaxy and the closest sources and a high-energy population with low spatial density, a magnetic horizon appears, suppressing the cosmic ray's flux at low-energies, which could explain the very hard spectrum observed at Earth. The distribution of arrival directions, which at energies above 32 EeV shows indications of a correlation with a population of starburst galaxies or the radio galaxy Centaurus A (Cen A), are also important to constrain the sources. It is shown that adding a fractional contribution from these sources of about 20% on top of an homogeneous background leads to an improvement of the model likelihood.
Ključne besede: ultra-high-energy cosmic rays, UHECR energy spectrum, UHECR mass composition, UHECR anisotropies, UHECR propagation, UHECR data interpretation, extragalactic magnetic fields, starburst galaxies, Centaurus A, Pierre Auger Observatory
Objavljeno v RUNG: 24.03.2025; Ogledov: 421; Prenosov: 7
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Opis: We report an investigation of the mass composition of cosmic rays with energies from 3 to 100 EeV (1 EeV = 10[sup]18 eV) using the distributions of the depth of shower maximum Xmax. The analysis relies on ∼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 Xmax 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 Xmax 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 Xmax 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.
Ključne besede: ultra-high-energy cosmic rays, UHECRs, extensive air showers, Pierre Auger Observatory, UHECR mass composition, depth of shower maximum, fluorescence detector, surface detector, deep learning
Objavljeno v RUNG: 20.01.2025; Ogledov: 790; Prenosov: 8
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