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1. New approaches for gamma-hadron separation at the IceCube Neutrino Observatory

   https://doi.org/10.1051/epjconf/202328004004  

   Bontempo, Federico (January 2023, EPJ Web of Conferences)

   Capone, A.; De Vincenzi, M.; Morselli, A. (Ed.)

   The IceCube Neutrino Observatory located at the geographic South Pole is composed of two detectors. One is the in-ice optical array, which measures high-energy muons from air showers and charged particles produced by the interaction of high-energy neutrinos in the ice. The other is an array of ice-Cherenkov tanks at the surface, called IceTop, which is used both as veto for the in-ice neutrino measurements and for detecting cosmic-ray air showers. In the next decade, the IceCube-Gen2 extension will increase the surface coverage including surface radio antennas and scintillator panels on the footprint of an extended optical array in the ice. The combination of the current surface and in-ice detectors can be exploited for the study of cosmic rays and the search for PeV gamma rays. The in-ice detector measures the high-energy muonic component of air showers, whereas the signal in IceTop is dominated by the electromagnetic component. The relative size of the muonic and electromagnetic components is different for gamma-and hadron-induced air showers. Thus, the gamma-hadron separation of cosmic rays is attempted using machine learning techniques including deep learning. Here, different approaches are presented. Finally, the prospects for the detection of PeV photons with IceCube-Gen2 will be discussed. 

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2. Radio detection of ultrahigh-energy cosmic-ray air showers

   https://doi.org/10.1140/epjs/s11734-025-01503-4  

   Schröder, Frank_G (February 2025, The European Physical Journal Special Topics)

   Abstract Radio antennas have become a standard tool for the detection of cosmic-ray air showers in the energy range above$$10^{16}\,$$ ${10}^{16}$eV. The radio signal of these air showers is generated mostly due to the deflection of electrons and positrons in the geomagnetic field, and contains information about the energy and the depth of the maximum of the air showers. Unlike the traditional air-Cherenkov and air-fluorescence techniques for the electromagnetic shower component, radio detection is not restricted to clear nights, and recent experiments have demonstrated that the measurement accuracy can compete with these traditional techniques. Numerous particle detector arrays for air showers have thus been or will be complemented by radio antennas. In particular when combined with muon detectors, the complementary information provided by the radio antennas can enhance the total accuracy for the arrival direction, energy and mass of the primary cosmic rays. Digitization and computational techniques have been crucial for this recent progress, and radio detection will play an important role in next-generation experiments for ultrahigh-energy cosmic rays. Moreover, stand-alone radio experiments are under development and will search for ultrahigh-energy photons and neutrinos in addition to cosmic rays. This article provides a brief introduction to the physics of the radio emission of air showers, an overview of air-shower observatories using radio antennas, and highlights some of their recent results. 

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3. 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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4. Constraints on the ultra-high energy cosmic ray output of gamma-ray bursts

   https://doi.org/10.1093/mnras/stae873  

   Moore, E.; Gendre, B.; Orange, N. B.; Panther, F. H. (March 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Ultra-high energy cosmic rays are the most extreme energetic particles detected on Earth, however, their acceleration sites are still mysterious. We explore the contribution of low-luminosity gamma-ray bursts to the ultra-high energy cosmic ray flux, since they form the bulk of the nearby population. We analyse a representative sample of these bursts detected by BeppoSAX, INTEGRAL, and Swift between 1998–2016, and found that in order to reconcile our theoretical flux with the observed flux, these bursts should accelerate at most 10−13 M⊙ of ultra-high energy cosmic rays. 

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5. An extremely energetic cosmic ray observed by a surface detector array

   https://doi.org/10.1126/science.abo5095  

   Abbasi, RU; Allen, MG; Arimura, R; Belz, JW; Bergman, DR; Blake, SA; Shin, BK; Buckland, IJ; Cheon, BG; Fujii, T; et al (November 2023, Science)

   Smith, KT (Ed.)

   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 $244\pm 29\left(\text{stat}\text{.}\right){}_{-76}^{+51}\left(\text{syst}\text{.}\right)\text{ exa–electron volts}$(~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. 

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