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1.
The second knee in the cosmic ray spectrum observed with the surface detector of the Pierre Auger Observatory
Gabriel Brichetto Orquera, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution

Abstract: The determination of the energy spectrum features with low systematic uncertainty is crucial for interpreting the nature of cosmic rays. In this study, we conducted a measurement of the energy spectrum at the Pierre Auger Observatory using a surface detector with a calorimetric energy scale indirectly set by a fluorescence detector. The surface detector consists of an array of water-Cherenkov detectors that extends over 3000 km^2 with 1500m spacing. Additionally, two nested arrays of the same kind with 750m and 433m spacing were utilized to lower the energy threshold of the measurements. This contribution presents, for the first time, the spectrum measured with the 433m array, which reduces the energy threshold down to 63 PeV, nearly half the energy at which we previously published a steepening using the 750m array. Our measurements include a characterization of the spectral features of the flux steepening around 230 PeV, known as the second-knee. The study benefits from a nearly 100% duty cycle and geometrical exposure. Notably, this is the first simultaneous measurement of the second knee energy and spectral indexes before and after the break, using a surface detector with an energy scale predominantly independent of air shower simulations and assumptions regarding hadronic interaction models.
Keywords: ultra-high energy cosmic rays, Pierre Auger Observatory, hadronic interaction model, water-Cherenkov detectors
Published in RUNG: 23.01.2024; Views: 462; Downloads: 4
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2.
Auger@TA : an Auger-like surface detector micro-array embedded within the Telescope Array Project
S. Mayotte, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution

Abstract: The Pierre Auger Observatory (Auger) and the Telescope Array Project (TA) are the two largest ultra-highenergy cosmic ray (UHECR) observatories in the world. One obstacle in pursuing full-sky UHECR physics is the apparent discrepancy in flux measured by the two experiments. This could be due to astrophysical differences as Auger and TA observe the Southern and Northern skies, respectively. However, the scintillation detectors used by TA have very different sensitivity to the various components of extensive air showers than the water-Cherenkov detectors (WCD) used by Auger. The discrepancy could also be due to systematic effects arising from the differing detector designs and reconstruction methods. The primary goal of the Auger@TA working group is to cross-calibrate the approaches of the two observatories using in-situ methods. This is achieved by placing a self-triggering micro-array, which consists of eight Auger surface detector stations, with both WCDs and AugerPrime scintillators, within the TA array. Seven of the WCDs use a 1-PMT prototype configuration and form a hexagon with the Auger spacing of 1.5 km. The eighth station uses a standard 3-PMT Auger WCD, placed with a TA station at the center of the hexagon to form a triplet for high-statistics, low-uncertainty, cross-calibration of instrumentation. Deployment of the micro-array took place between September 2022 and August 2023, with data-taking foreseen by the Fall of 2023. Details on the instrumentation and deployment of the micro-array, as well as its expected performance, trigger efficiencies, and event rate will be presented. First data from individual stations will also be shown.
Keywords: Pierre Auger Observatory, ultra-high energy cosmic rays, Telescope Array, AugerPrime, scintillators, water-Cherenkov detectors
Published in RUNG: 23.01.2024; Views: 450; Downloads: 6
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3.
Search for primary photons at tens of PeV with the Pierre Auger Observatory
Nicolás González, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, 2023, published scientific conference contribution

Abstract: The observation of primary photons with energies around 10[sup]16 eV would be particularly interesting after the discovery of Galactic gamma-ray sources with spectra extending into the PeV range. Since photons are connected to the acceleration of charged particles, searches for photons enhance the multi-messenger understanding of cosmic-ray sources as well as of transient astrophysical phenomena, while offering wealthy connections to neutrino astronomy and dark matter. Additionally, diffuse photon fluxes are expected from cosmic-ray interactions with Galactic matter and background radiation fields. Previously, the energy domain between 1 PeV and 200 PeV was only explored from the Northern Hemisphere. The Pierre Auger Observatory is the largest astroparticle experiment in operation and, thanks to its location, has a sizable exposure to the Southern sky, including the Galactic center region. In this contribution, we present the first search for photons from the Southern hemisphere between 50 and 200 PeV exploiting the Auger data acquired during ∼4 yr of operation. We describe the method to discriminate photons against the dominating hadronic background; it is based on the measurements of air showers taken with the low-energy extension of the Pierre Auger Observatory composed by 19 water-Cherenkov detectors spanning ∼ 2km[sup]2 and an Underground Muon Detector. The search for a diffuse flux of photons is presented and its results are interpreted according to theoretical model predictions. This study extends the range of Auger photon searches to almost four decades in energy.
Keywords: Pierre Auger Observatory, cosmic rays, photons, water-Cherenkov detectors, underground muon detectors
Published in RUNG: 26.09.2023; Views: 814; Downloads: 6
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4.
Search for photons above ▫$10^19$▫ eV with the surface detector of the Pierre Auger Observatory
P. Abreu, Andrej Filipčič, Jon Paul Lundquist, Shima Ujjani Shivashankara, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2023, original scientific article

Abstract: We use the surface detector of the Pierre Auger Observatory to search for air showers initiated by photons with an energy above 10[sup]19 eV. Photons in the zenith angle range from 30 deg. to 60 deg. 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[sup]-2 diffuse flux of ultra-high energy photons above 10[sup]19 eV, 2 × 10[sup]19 eV and 4 × 10[sup]19 eV amounting to 2.11 × 10[sup]-3, 3.12 × 10[sup]-4 and 1.72 × 10[sup]-4 km-2 sr-1 yr-1, respectively. While the sensitivity of the present search around 2 × 10[sup]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.
Keywords: ultra-high-energy cosmic rays, UHE photons, Pierre Auger Observatory, extensive air showers, water Cherenkov detectors
Published in RUNG: 18.08.2023; Views: 797; Downloads: 8
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Measurement of the water-Cherenkov detector response to inclined muons using an RPC hodoscope
Pedro Assis, Andrej Filipčič, Gašper Kukec Mezek, Ahmed Saleh, Samo Stanič, Marta Trini, Darko Veberič, Serguei Vorobiov, Lili Yang, Danilo Zavrtanik, Marko Zavrtanik, 2015, published scientific conference contribution

Abstract: The Pierre Auger Observatory operates a hybrid detector composed of a Fluorescence Detector and a Surface Detector array. Water-Cherenkov detectors (WCD) are the building blocks of the array and as such play a key role in the detection of secondary particles at the ground. A good knowledge of the detector response is of paramount importance to lower systematic uncertainties and thus to increase the capability of the experiment in determining the muon content of the extensive air showers with a higher precision. In this work we report on a detailed study of the detector response to single muons as a function of their trajectories in the WCD. A dedicated Resistive Plate Chambers (RPC) hodoscope was built and installed around one of the detectors. The hodoscope is formed by two stand-alone low gas flux segmented RPC detectors with the test water-Cherenkov detector placed in between. The segmentation of the RPC detectors is of the order of 10 cm. The hodoscope is used to trigger and select single muon events in different geometries. The signal recorded in the water-Cherenkov detector and performance estimators were studied as a function of the trajectories of the muons and compared with a dedicated simulation. An agreement at the percent level was found, showing that the simulation correctly describes the tank response.
Keywords: Pierre Auger Observatory, Water-Cherenkov detectors, detector calibration, inclined cosmic ray muons, Resistive Plate Chambers (RPC) hodoscope
Published in RUNG: 03.03.2016; Views: 4815; Downloads: 198
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