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Cosmic ray mass composition measurement in the energy range from 1016.5  eV to 1018.5  eV observed with the TALE hybrid detector

R. U. Abbasi1, T. Abu-Zayyad2,1, M. Allen2, J. W. Belz2, D. R. Bergman2, F. Bradfield3, I. Buckland2, W. Campbell2, B. G. Cheon4 et al. (Telescope Array Collaboration)

B. G. Cheon4, K. Endo3, A. Fedynitch5,6, T. Fujii3,7, K. Fujisue5,6, K. Fujita6,*, M. Fukushima6, G. Furlich2, A. Gálvez Ureña8, Z. Gerber2, N. Globus9, T. Hanaoka10, W. Hanlon2, N. Hayashida11, H. He12,†, K. Hibino11, R. Higuchi12, D. Ikeda11, D. Ivanov2, S. Jeong13, C. C. H. Jui2, K. Kadota14, F. Kakimoto11, O. Kalashev15, K. Kasahara16, Y. Kawachi3, K. Kawata6, I. Kharuk15, E. Kido6, H. B. Kim4, J. H. Kim2, J. H. Kim2,‡, S. W. Kim13,§, R. Kobo3, I. Komae3, K. Komatsu17, K. Komori10, A. Korochkin18, C. Koyama6, M. Kudenko15, M. Kuroiwa17, Y. Kusumori10, M. Kuznetsov15, Y. J. Kwon19, K. H. Lee4, M. J. Lee13, B. Lubsandorzhiev15, J. P. Lundquist20,2, H. Matsushita3, A. Matsuzawa17, J. A. Matthews2, J. N. Matthews2, K. Mizuno17, M. Mori10, S. Nagataki12, K. Nakagawa3, M. Nakahara3, H. Nakamura10, T. Nakamura21, T. Nakayama17, Y. Nakayama10, K. Nakazawa10, T. Nonaka6, S. Ogio6, H. Ohoka6, N. Okazaki6, M. Onishi6, A. Oshima22, H. Oshima6, S. Ozawa23, I. H. Park13, K. Y. Park4, M. Potts2, M. Przybylak24, M. S. Pshirkov15,25, J. Remington2,‖, C. Rott2, G. I. Rubtsov15, D. Ryu26, H. Sagawa6, N. Sakaki6, R. Sakamoto10, T. Sako6, N. Sakurai6, S. Sakurai3, D. Sato17, K. Sekino6, T. Shibata6, J. Shikita3, H. Shimodaira6, H. S. Shin3,7, K. Shinozaki27, J. D. Smith2, P. Sokolsky2, B. T. Stokes2, T. A. Stroman2, H. Tachibana3, K. Takahashi6, M. Takeda6, R. Takeishi6, A. Taketa28, M. Takita6, Y. Tameda10, K. Tanaka29, M. Tanaka30, M. Teramoto10, S. B. Thomas2, G. B. Thomson2, P. Tinyakov18,15, I. Tkachev15, T. Tomida17, S. Troitsky15, Y. Tsunesada3,7, S. Udo11, F. R. Urban8, M. Vrábel27, D. Warren12, K. Yamazaki22, Y. Zhezher6,15, Z. Zundel2, and J. Zvirzdin2 (Telescope Array Collaboration)

  • 1Department of Physics, Loyola University-Chicago, Chicago, Illinois 60660, USA
  • 2High Energy Astrophysics Institute and Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112-0830, USA
  • 3Graduate School of Science, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
  • 4Department of Physics and The Research Institute of Natural Science, Hanyang University, Seongdong-gu, Seoul 426-791, Korea
  • 5Institute of Physics, Academia Sinica, Taipei City 115201, Taiwan
  • 6Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
  • 7Nambu Yoichiro Institute of Theoretical and Experimental Physics, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
  • 8CEICO, Institute of Physics, Czech Academy of Sciences, Prague 182 21, Czech Republic
  • 9Institute of Astronomy, National Autonomous University of Mexico Ensenada Campus, Ensenada, BC 22860, Mexico
  • 10Graduate School of Engineering, Osaka Electro-Communication University, Neyagawa-shi, Osaka 572-8530, Japan
  • 11Faculty of Engineering, Kanagawa University, Yokohama, Kanagawa 221-8686, Japan
  • 12Astrophysical Big Bang Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
  • 13Department of Physics, Sungkyunkwan University, Jang-an-gu, Suwon 16419, Korea
  • 14Department of Physics, Tokyo City University, Setagaya-ku, Tokyo 158-8557, Japan
  • 15Institute for Nuclear Research of the Russian Academy of Sciences, Moscow 117312, Russia
  • 16Faculty of Systems Engineering and Science, Shibaura Institute of Technology, Minumaku, Tokyo 337-8570, Japan
  • 17Academic Assembly School of Science and Technology Institute of Engineering, Shinshu University, Nagano, Nagano 380-8554, Japan
  • 18Service de Physique Théorique, Université Libre de Bruxelles, Brussels 1050, Belgium
  • 19Department of Physics, Yonsei University, Seodaemun-gu, Seoul 120-749, Korea
  • 20Center for Astrophysics and Cosmology, University of Nova Gorica, Nova Gorica 5297, Slovenia
  • 21Faculty of Science, Kochi University, Kochi, Kochi 780-8520, Japan
  • 22College of Science and Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan
  • 23Quantum ICT Advanced Development Center, National Institute for Information and Communications Technology, Koganei, Tokyo 184-8795, Japan
  • 24Doctoral School of Exact and Natural Sciences, University of Lodz, Lodz, Lodz 90-237, Poland
  • 25Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Moscow 119991, Russia
  • 26Department of Physics, School of Natural Sciences, Ulsan National Institute of Science and Technology, UNIST-gil, Ulsan 689-798, Korea
  • 27Astrophysics Division, National Centre for Nuclear Research, Warsaw 02-093, Poland
  • 28Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 277-8582, Japan
  • 29Graduate School of Information Sciences, Hiroshima City University, Hiroshima, Hiroshima 731-3194, Japan
  • 30Institute of Particle and Nuclear Studies, KEK, Tsukuba, Ibaraki 305-0801, Japan

  • *Contact author: kfujita@icrr.u-tokyo.ac.jp
  • †Present address: Purple Mountain Observatory, Nanjing 210023, China.
  • ‡Present address: Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
  • §Present address: Korea Institute of Geoscience and Mineral Resources, Daejeon, 34132, Korea.
  • ‖Present address: NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA.
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Phys. Rev. D 113, 062003 – Published 26 March, 2026

DOI: https://doi.org/10.1103/vrky-dxn7

Abstract

We report on the cosmic ray mass composition measured by the Telescope Array Low-energy Extension (TALE) hybrid detector. The TALE detector consists of a fluorescence detector (FD) station with 10 FD telescopes located at the Telescope Array (TA) Middle Drum FD Station (itself made up of 14 FD telescopes), and a surface detector (SD) array of scintillators. The array consists of 40 SDs with 400 m spacing and 40 SDs with 600 m spacing. In this paper, we present results on the measurement of the depth of shower maxima (Xmax) in the energy range from 1016.5  eV to 1018.5  eV collected over five years of the TALE hybrid detector. The Xmax distributions were analyzed and compared with Monte Carlo simulations of proton, helium, nitrogen, and iron primaries, using the QGSJet II-04 hadronic interaction model. Our results indicate that the elongation rate of the mean Xmax, which is defined as the slope of ⟨Xmax⟩ versus cosmic ray energy, exhibits a break around 1017  eV. Up to this energy, the composition becomes increasingly heavy, characterized by a growing dominance of heavy nuclei and a steadily decreasing fraction of light primaries. Beyond this energy, the proton fraction increases significantly with energy. These findings suggest a transition from Galactic to extra-Galactic cosmic ray sources around the so-called second knee.

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Phys. Rev. D 113, 062003– Published 26 March, 2026
  • Received 8 July 2025
  • Accepted 20 February 2026
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