Evidence for a mixed mass composition at the ‘ankle’ in the cosmic-ray spectrum

Authors

A. Aab, Universität Siegen
P. Abreu, Instituto Superior Técnico
M. Aglietta, Istituto Nazionale di Fisica Nucleare, Sezione di Torino
E. J. Ahn, Fermi National Accelerator Laboratory
I. Al Samarai, Laboratoire de Physique Nucléaire et de Hautes Energies
I. F.M. Albuquerque, Universidade de Sao Paulo - USP
B. Fick, Michigan Technological UniversityFollow
R. Kieckhafer, Michigan Technological UniversityFollow
D. Nitz, Michigan Technological UniversityFollow
et al.

Document Type

Article

Publication Date

11-10-2016

Department

Department of Physics

Abstract

We report a first measurement for ultrahigh energy cosmic rays of the correlation between the depth of shower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneously by the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlation measurement is a unique feature of a hybrid air-shower observatory with sensitivity to both the electromagnetic and muonic components. It allows an accurate determination of the spread of primary masses in the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominated by systematic uncertainties. The present correlation measurement is not affected by systematics in the measurement of the depth of shower maximum or the signal in the water Cherenkov stations. The analysis relies on general characteristics of air showers and is thus robust also with respect to uncertainties in hadronic event generators. The observed correlation in the energy range around the ‘ankle’ at lg⁡(E/eV)=18.5–19.0 differs significantly from expectations for pure primary cosmic-ray compositions. A light composition made up of proton and helium only is equally inconsistent with observations. The data are explained well by a mixed composition including nuclei with mass A> 4. Scenarios such as the proton dip model, with almost pure compositions, are thus disfavored as the sole explanation of the ultrahigh-energy cosmic-ray flux at Earth.

Publisher's Statement

© 2016 The Author(s). Published by Elsevier B.V. Publisher’s version of record: https://doi.org/10.1016/j.physletb.2016.09.039

Publication Title

Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics

Recommended Citation

Aab, A., Abreu, P., Aglietta, M., Ahn, E., Al Samarai, I., Albuquerque, I., Fick, B., Kieckhafer, R., Nitz, D., & et al. (2016). Evidence for a mixed mass composition at the ‘ankle’ in the cosmic-ray spectrum. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 762, 288-295. http://doi.org/10.1016/j.physletb.2016.09.039
Retrieved from: https://digitalcommons.mtu.edu/michigantech-p/6992

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

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Publisher's PDF