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تسجيل مستخدم جديدUltra-High Energy Cosmic Rays
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نشر من قبل
Scientific Information Service CERN
تاريخ النشر
2016
مجال البحث
فيزياء
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English
تأليف
M.T. Dova
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The origin of the ultra high energy cosmic rays (UHECR) with energies above E > 1017eV, is still unknown. The discovery of their sources will reveal the engines of the most energetic astrophysical accelerators in the universe. This is a written version of a series of lectures devoted to UHECR at the 2013 CERN-Latin-American School of High-Energy Physics. We present an introduction to acceleration mechanisms of charged particles to the highest energies in astrophysical objects, their propagation from the sources to Earth, and the experimental techniques for their detection. We also discuss some of the relevant observational results from Telescope Array and Pierre Auger Observatory. These experiments deal with particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.
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We present the main results on the energy spectrum and composition of the highest energy cosmic rays of energy exceeding 10$^{18}$ eV obtained by the High Resolution Flys Eye and the Southern Auger Observatory. The current results are somewhat contra
dictory and raise interesting questions about the origin and character of these particles.
A fundamental question that can be answered in the next decade is: WHAT IS THE ORIGIN OF THE HIGHEST ENERGY COSMIC PARTICLES? The discovery of the sources of the highest energy cosmic rays will reveal the workings of the most energetic astrophysical
environments in the recent universe. Candidate sources range from the birth of compact objects to explosions related to gamma-ray bursts or generated around supermassive black holes in active galactic nuclei. In addition to beginning a new era of high-energy astrophysics, the study of ultra-high energy cosmic rays will constrain the structure of the Galactic and extragalactic magnetic fields. The propagation of these particles from source to Earth also probes the cosmic background radiation and gives insight into particle interactions at orders of magnitude higher energy than can be achieved in terrestrial laboratories. Next generation observatories designed to study the highest energy cosmic rays will have unprecedented sensitivity to ultra-high energy photons and neutrinos, which will further illuminate the workings of the universe at the most extreme energies. For this challenge to be met during the 2010-2020 decade, a significant increase in the integrated exposure to cosmic rays above 6 1019 eV will be necessary. The technical capabilities for answering this open question are at hand and the time is ripe for exploring Charged Particle Astronomy.
We develop a model for explaining the data of Pierre Auger Observatory (Auger) for Ultra High Energy Cosmic Rays (UHECR), in particular, the mass composition being steadily heavier with increasing energy from 3 EeV to 35 EeV. The model is based on th
e proton-dominated composition in the energy range (1 - 3) EeV observed in both Auger and HiRes experiments. Assuming extragalactic origin of this component, we argue that it must disappear at higher energies due to a low maximum energy of acceleration, E_p^{max} sim (4 - 10) EeV. Under an assumption of rigidity acceleration mechanism, the maximum acceleration energy for a nucleus with the charge number Z is ZE_p^{max}, and the highest energy in the spectrum, reached by Iron, does not exceed (100 - 200) EeV. The growth of atomic weight with energy, observed in Auger, is provided by the rigidity mechanism of acceleration, since at each energy E=ZE_p^{max} the contribution of nuclei with Z < Z vanishes. The described model has disappointing consequences for future observations in UHECR: Since average energies per nucleon for all nuclei are less than (2 - 4) EeV, (i) pion photo-production on CMB photons in extragalactic space is absent; (ii) GZK cutoff in the spectrum does not exist; (iii) cosmogenic neutrinos produced on CMBR are absent; (iv) fluxes of cosmogenic neutrinos produced on infrared - optical background radiation are too low for registration by existing detectors and projects. Due to nuclei deflection in galactic magnetic fields, the correlation with nearby sources is absent even at highest energies.
More than 100 years after the discovery of cosmic rays and various experimental efforts, the origin of ultra-high energy cosmic rays (E > 100 PeV) remains unclear. The understanding of production and propagation effects of these highest energetic par
ticles in the universe is one of the most intense research fields of high-energy astrophysics. With the advent of advanced simulation engines developed during the last couple of years, and the increase of experimental data, we are now in a unique position to model source and propagation parameters in an unprecedented precision and compare it to measured data from large scale observatories. In this paper we revisit the most important propagation effects of cosmic rays through photon backgrounds and magnetic fields and introduce recent developments of propagation codes. Finally, by comparing the results to experimental data, possible implications on astrophysical parameters are given.
While there is some level of consensus on a Galactic origin of cosmic rays up to the knee ($E_{k}sim 3times 10^{15}$ eV) and on an extragalactic origin of cosmic rays with energy above $sim 10^{19}$ eV, the debate on the genesis of cosmic rays in the
intermediate energy region has received much less attention, mainly because of the ambiguity intrinsic in defining such a region. The energy range between $10^{17}$ eV and $sim 10^{19}$ eV is likely to be the place where the transition from Galactic to extragalactic cosmic rays takes place. Hence the origin of these particles, though being of the highest importance from the physics point of view, it is also one of the most difficult aspects to investigate. Here I will illustrate some ideas concerning the sites of acceleration of these particles and the questions that their investigation may help answer, including the origin of underline{ultra} high energy cosmic rays.
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