Towards Solving the Mass-Composition Problem in Ultra High Energy Cosmic Rays
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UCLEI, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
Towards Solving the Mass-Composition Problem in Ultra High Energy Cosmic Rays1 R. Aloisioa,* and V. Berezinskyb,** a
Gran Sasso Science Institute, L’Aquila, 67100 Italy Nazionali Gran Sasso, Assergi (AQ), 67100 Italy *e-mail: [email protected] **e-mail: [email protected]
bINFN—Laboratori
Received April 27, 2018; revised July 5, 2018; accepted July 5, 2018
Abstract—Using the Auger mass-composition analysis of ultra high energy cosmic rays, based on the shape-fitting of Xmax distributions [1], we demonstrate that mass composition and energy spectra measured by Auger, Telescope Array and HiRes can be brought into good agreement. The shape-fitting analysis of Xmax distributions shows that the measured sum of proton and Helium fractions, for some hadronic-interaction models, can saturate the total f lux. Such p + He model, with small admixture of other light nuclei, naturally follows from cosmology with recombination and reheating phases. The most radical assumption of the presented model is the assumed unreliability of the experimental separation of Helium and protons, which allows to consider He/p ratio as a free parameter. The results presented here show that the models with dominant p + He composition explain well the energy spectrum of the dip in the latest (2015–2017) data of Auger and Telescope Array, but have some tension at the highest energies with the expected Greisen–Zatsepin–Kuzmin cutoff. The Auger-Prime upgrade experiment has a great potential to reject or confirm this model. DOI: 10.1134/S1063776118120014
background (CMB). It results in two very specific spectral features: the Greisen–Zatsepin–Kuzmin (GZK) cutoff [2] and the pair-production dip. The former is a sharp cutoff at the end of the spectrum, around E ~ 50 EeV, due to the photo-pion production and the latter is a rather faint feature at E ~ 1–30 EeV first calculated in [3] and studied in detail in [4–6]. The dip is well confirmed in the spectra of all three detectors but its origin as the pair-production dip p + γcmb → p + e– + e+ is now questioned by the Auger mass composition. Before 2011 the data published by HiRes [7] and Auger [8], and later confirmed by TA, showed high energy steepening in good agreement with the predicted GZK cutoff. Nevertheless, the newest data of Auger and TA, released in 2015–2017 seem to be in contradiction with this interpretation, see Fig. 1.
1. INTRODUCTION Mass composition still remains a controversial issue in Ultra High Energy Cosmic Rays (UHECR). The three biggest detectors, Pierre Auger (referred here as “Auger”), Telescope Array (referred as “TA”) and HiRes, have obtained contradictory results concerning mass composition of primary particles in the energy range 3–100 EeV (1 EeV = 1 × 1018 eV). At 1‒3 EeV all three detectors agree with light composition, protons or protons and Helium, but in the range 3–100 EeV the Auger detector, the biggest one, founds a progressively heavier mass composition with increasing energy, while the other two
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