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Precise Measurement of the Cosmic Ray Helium Spectrum above 0.1 PeV

Zhen Cao1,2,3, F. Aharonian3,4,5,6, Y. X. Bai1,3, Y. W. Bao7, D. Bastieri8, X. J. Bi1,2,3, Y. J. Bi1,3, W. Bian7, J. Blunier9 et al. (LHAASO Collaboration)

J. Blunier9, A. V. Bukevich10, C. M. Cai11, Y. Y. Cai7, W. Y. Cao12, Zhe Cao13,4, J. Chang14, J. F. Chang1,3,13, E. S. Chen1,3, G. H. Chen8, H. K. Chen15, L. F. Chen15, Liang Chen16, Long Chen11, M. J. Chen1,3, M. L. Chen1,3,13, Q. H. Chen11, S. Chen17, S. H. Chen1,2,3,§, S. Z. Chen1,3, T. L. Chen18, X. B. Chen19, X. J. Chen11, X. P. Chen14, Y. Chen19, N. Cheng1,3, Q. Y. Cheng1,2,3, Y. D. Cheng1,2,3, M. Y. Cui14, S. W. Cui15, X. H. Cui20, Y. D. Cui21, B. Z. Dai17, H. L. Dai1,3,13, Z. G. Dai4, Danzengluobu18, Y. X. Diao11, A. J. Dong22, X. Q. Dong1,2,3, K. K. Duan14, J. H. Fan8, Y. Z. Fan14, J. Fang17, J. H. Fang23, K. Fang1,3, C. F. Feng24, H. Feng1, L. Feng14, S. H. Feng1,3, X. T. Feng24, Y. Feng23, Y. L. Feng18, S. Gabici9, B. Gao1,3, Q. Gao18, W. Gao1,3, W. K. Gao1,2,3, M. M. Ge17, T. T. Ge21, L. S. Geng1,3, G. Giacinti7, G. H. Gong25, Q. B. Gou1,3, M. H. Gu1,3,13, F. L. Guo16, J. Guo25, K. J. Guo11, X. L. Guo11, Y. Q. Guo1,3, Y. Y. Guo14, R. P. Han1,2,3, O. A. Hannuksela12, M. Hasan1,2,3, H. H. He1,2,3, H. N. He14, J. Y. He14, X. Y. He14, Y. He11, S. Hernández-Cadena7, B. W. Hou1,2,3, C. Hou1,3, X. Hou26, H. B. Hu1,2,3, S. C. Hu1,3,27, C. Huang19, D. H. Huang11, J. J. Huang1,2,3, X. L. Huang22, X. T. Huang24, X. Y. Huang14, Y. Huang1,3,27, Y. Y. Huang19, A. Inventar9, X. L. Ji1,3,13, H. Y. Jia11, K. Jia24, H. B. Jiang1,3, K. Jiang13,4, X. W. Jiang1,3, Z. J. Jiang17, M. Jin11, S. Kaci7, M. M. Kang28, I. Karpikov10, D. Khangulyan1,3, D. Kuleshov10, K. Kurinov10, Cheng Li13,4, Cong Li1,3, D. Li1,2,3, F. Li1,3,13, H. B. Li1,2,3, H. C. Li1,3, Jian Li4, Jie Li1,3,13, K. Li1,3, L. Li29, R. L. Li14, S. D. Li16,2, T. Y. Li7, W. L. Li7, X. R. Li1,3, Y. Li7, Zhe Li1,3, Zhuo Li30, E. W. Liang31, Y. F. Liang31, S. J. Lin21, P. Lipari32, B. Liu14, C. Liu1,3, D. Liu24, D. B. Liu7, H. Liu11, J. Liu1,3, J. L. Liu1,3, J. R. Liu11, M. Y. Liu18, R. Y. Liu19, S. M. Liu11, W. Liu1,3, X. Liu11, Y. Liu8, Y. Liu11, Y. N. Liu25, Y. Q. Lou25, Q. Luo21, Y. Luo7, H. K. Lv1,3, B. Q. Ma30, L. L. Ma1,3,†, X. H. Ma1,3, I. O. Maliy10, J. R. Mao26, Z. Min1,3, W. Mitthumsiri33, Y. Mizuno7, G. B. Mou34, A. Neronov9, K. C. Y. Ng12, M. Y. Ni14, L. Nie11, L. J. Ou8, Z. W. Ou7, P. Pattarakijwanich33, Z. Y. Pei8, D. Y. Peng15, J. C. Qi1,2,3, M. Y. Qi1,3, J. J. Qin4, D. Qu18, A. Raza1,2,3, C. Y. Ren14, D. Ruffolo33, A. Sáiz33, D. Savchenko9, D. Semikoz9, L. Shao15, O. Shchegolev10,35, Y. Z. Shen19, X. D. Sheng1,3, Z. D. Shi4, F. W. Shu29, H. C. Song30, Y. Su14, D. X. Sun4,14, H. Sun24, J. X. Sun19, Q. N. Sun1,3, X. N. Sun31, Z. B. Sun36, N. H. Tabasam24, J. Takata37, P. H. T. Tam21, H. B. Tan19, Q. W. Tang29, R. Tang7, Z. B. Tang13,4, W. W. Tian2,20, C. N. Tong19, L. H. Wan21, C. Wang36, D. H. Wang22, G. W. Wang4, H. G. Wang8, J. C. Wang26, K. Wang30, Kai Wang19, Kai Wang37, L. P. Wang1,2,3,*, L. Y. Wang1,3, L. Y. Wang15, R. Wang24, W. Wang21, X. G. Wang31, X. J. Wang11, X. Y. Wang19, Y. Wang11, Y. D. Wang1,3, Z. H. Wang28, Z. X. Wang17, Zheng Wang1,3,13, D. M. Wei14, J. J. Wei14, Y. J. Wei1,2,3, T. Wen1,3, S. S. Weng34, C. Y. Wu1,3, H. R. Wu1,3, Q. W. Wu37, S. Wu1,3, X. F. Wu14, Y. S. Wu4, S. Q. Xi1,3, J. Xia4,14, J. J. Xia11, G. M. Xiang1,3,27, D. X. Xiao15, G. Xiao1,3, Y. F. Xiao17, Y. L. Xin11, H. D. Xing1,2,3, Y. Xing16, D. R. Xiong26, B. N. Xu1,3, C. Y. Xu23, D. L. Xu7, R. F. Xu1,2,3, R. X. Xu30, S. S. Xu1,3, W. L. Xu28, L. Xue24, D. H. Yan17, T. Yan1,3, C. W. Yang28, C. Y. Yang26, F. F. Yang1,3,13, L. L. Yang21, M. J. Yang1,3, R. Z. Yang4, W. X. Yang8, Z. H. Yang7, Z. G. Yao1,3, X. A. Ye14, L. Q. Yin1,3, N. Yin24, X. H. You1,3, Z. Y. You1,3, Q. Yuan14, H. Yue1,2,3, H. D. Zeng14, T. X. Zeng1,3,13, W. Zeng17, X. T. Zeng21, M. Zha1,3, B. B. Zhang19, B. T. Zhang1,3, C. Zhang19, H. Zhang7, H. M. Zhang31, H. Y. Zhang17, J. L. Zhang20, J. Y. Zhang1,2,3, Li Zhang17, P. F. Zhang17, R. Zhang14, S. R. Zhang15, S. S. Zhang1,3,‡, S. Y. Zhang15, W. Zhang1,3, W. Y. Zhang15, X. Zhang34, X. P. Zhang1,3, Yi Zhang14, Yong Zhang1,3, Z. P. Zhang4, J. Zhao1,3, L. Zhao13,4, L. Z. Zhao15, S. P. Zhao14, X. H. Zhao26, Z. H. Zhao4, F. Zheng36, T. C. Zheng1,3, B. Zhou1,3, H. Zhou7, J. N. Zhou16, M. Zhou29, P. Zhou19, R. Zhou28, X. X. Zhou1,2,3, X. X. Zhou11, B. Y. Zhu4,14, C. G. Zhu24, F. R. Zhu11, H. Zhu20, K. J. Zhu1,2,3,13, Y. C. Zou37, and X. Zuo1,3 (LHAASO Collaboration)

  • 1State Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, China
  • 2University of Chinese Academy of Sciences, 100049 Beijing, China
  • 3TIANFU Cosmic Ray Research Center, 610000 Chengdu, Sichuan, China
  • 4University of Science and Technology of China, 230026 Hefei, Anhui, China
  • 5Yerevan State University, 1 Alek Manukyan Street, Yerevan 0025, Armenia
  • 6Max-Planck-Institut for Nuclear Physics, P.O. Box 103980, 69029 Heidelberg, Germany
  • 7Tsung-Dao Lee Institute and School of Physics and Astronomy, Shanghai Jiao Tong University, 200240 Shanghai, China
  • 8Center for Astrophysics, Guangzhou University, 510006 Guangzhou, Guangdong, China
  • 9APC, Université Paris Cité, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, 119 75205 Paris, France
  • 10Institute for Nuclear Research of Russian Academy of Sciences, 117312 Moscow, Russia
  • 11School of Physical Science and Technology and School of Information Science and Technology, Southwest Jiaotong University, 610031 Chengdu, Sichuan, China
  • 12Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
  • 13State Key Laboratory of Particle Detection and Electronics, Beijing, China
  • 14Key Laboratory of Dark Matter and Space Astronomy and Key Laboratory of Radio Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, 210023 Nanjing, Jiangsu, China
  • 15Hebei Normal University, 050024 Shijiazhuang, Hebei, China
  • 16Shanghai Astronomical Observatory, Chinese Academy of Sciences, 200030 Shanghai, China
  • 17School of Physics and Astronomy, Yunnan University, 650091 Kunming, Yunnan, China
  • 18Key Laboratory of Cosmic Rays (Tibet University), Ministry of Education, 850000 Lhasa, Tibet, China
  • 19School of Astronomy and Space Science, Nanjing University, 210023 Nanjing, Jiangsu, China
  • 20Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, 100101 Beijing, China
  • 21School of Physics and Astronomy (Zhuhai) and School of Physics (Guangzhou) and Sino-French Institute of Nuclear Engineering and Technology (Zhuhai), Sun Yat-sen University, 519000 Zhuhai and 510275 Guangzhou, Guangdong, China
  • 22School of Physics and Electronic Science, Guizhou Normal University, 550025 Guiyang, China
  • 23Research Center for Astronomical Computing, Zhejiang Laboratory, 311121 Hangzhou, Zhejiang, China
  • 24Institute of Frontier and Interdisciplinary Science, Shandong University, 266237 Qingdao, Shandong, China
  • 25Department of Engineering Physics and Department of Physics and Department of Astronomy, Tsinghua University, 100084 Beijing, China
  • 26Yunnan Observatories, Chinese Academy of Sciences, 650216 Kunming, Yunnan, China
  • 27China Center of Advanced Science and Technology, Beijing 100190, China
  • 28College of Physics, Sichuan University, 610065 Chengdu, Sichuan, China
  • 29Center for Relativistic Astrophysics and High Energy Physics, School of Physics and Materials Science and Institute of Space Science and Technology, Nanchang University, 330031 Nanchang, Jiangxi, China
  • 30School of Physics and Kavli Institute for Astronomy and Astrophysics, Peking University, 100871 Beijing, China
  • 31Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, 530004 Nanning, Guangxi, China
  • 32INFN, Sezione Roma “Sapienza,” Piazzale Aldo Moro 2, 00185 Roma, Italy
  • 33Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
  • 34School of Physics and Technology, Nanjing Normal University, 210023 Nanjing, Jiangsu, China
  • 35Moscow Institute of Physics and Technology, 141700 Moscow, Russia
  • 36National Space Science Center, Chinese Academy of Sciences, 100190 Beijing, China
  • 37School of Physics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China

  • *Contact author: wanglp@ihep.ac.cn
  • †Contact author: llma@ihep.ac.cn
  • ‡Contact author: zhangss@ihep.ac.cn
  • §Contact author: chensh@ihep.ac.cn
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Phys. Rev. Lett. 136, 121001 – Published 26 March, 2026

DOI: https://doi.org/10.1103/d838-49gt

Abstract

We report a measurement of the cosmic ray helium energy spectrum in the energy interval 0.16–13 PeV, derived by subtracting the proton spectrum from the light component (proton and helium) spectrum obtained with observations made by the Large High Altitude Air Shower Observatory (LHAASO) under a consistent energy scale. The helium spectrum shows a significant hardening centered at E≃1.1  PeV, followed by a softening at ∼7  PeV, indicating the appearance of a helium “knee.” Comparing the proton and helium spectra in the LHAASO energy range reveals some remarkable facts. In the lower part of this range, in contrast to the behavior at lower energies, the helium spectrum is significantly softer than the proton spectrum. This results in protons overtaking helium nuclei and becoming the largest cosmic ray component at E≃0.7  PeV. A second crossing of the two spectra is observed at E≃5  PeV, above the proton knee, when helium nuclei overtake protons to become the largest cosmic ray component again. These results have important implications for our understanding of the Galactic cosmic ray sources.

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synopsis

Elucidating the Cosmic-Ray Knee

Published 26 March, 2026

New observations of cosmic rays that distinguish between hydrogen and helium find unexpected complexity in a long-observed spectral feature.

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Phys. Rev. Lett. 136, 121001– Published 26 March, 2026
  • Received 3 November 2025
  • Revised 6 January 2026
  • Accepted 6 February 2026
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