We will discuss the main relevant aspects of the physics of ultra high energy cosmic rays. After a short recap of the experimental evidences, we will review theoretical models aiming at describing ...the sources of these extremely energetic particles opening a window on the highest energies universe. We will discuss the production of secondary particles and the possible tests of new physics that ultra high energy cosmic rays could provide. The present proceedings paper is mainly based on review papers written by the author in the last two years.
Context. Features in the spectra of primary cosmic rays (CRs) provide invaluable information on the propagation of these particles in the Galaxy. In the rigidity region around a few hundred GV, these ...features have been measured in the proton and helium spectra by the PAMELA experiment and later confirmed with a higher significance by AMS-02. We investigate the implications of these data sets for the scenario in which CRs propagate under the action of self-generated waves. Aims. We show that the recent data on the spectrum of protons and helium nuclei as collected with AMS-02 and Voyager are in very good agreement with the predictions of a model in which the transport of Galactic CRs is regulated by self-generated waves. We also study the implications of the scenario for the boron-to-carbon ratio: although a good overall agreement is found, at high energy we find marginal support for a (quasi) energy independent contribution from the grammage, which we argue may come from the sources themselves. Methods. The transport equation for both primary and secondary nuclei is solved together with an equation for the evolution of the self-generated waves and a background of pre-existing waves. The solution for this system of nonlinear equations is found with an iterative method elaborated by the same authors in a previous work on this topic. Results. A break in the spectra of all nuclei is found at a rigidity of a few hundred GV, as a result of a transition from self-generated waves to pre-existing waves with a Kolmogorov power spectrum. Neither the slope of the diffusion coefficient, nor its normalization are free parameters. Moreover, at rigidities below a few GV, CRs are predicted to be advected with the self-generated waves at the local Alfvén speed. This effect, predicted in our previous work, provides an excellent fit to the Voyager data on the proton and helium spectra at low energies, providing additional support to the model.
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 the 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
∼
(
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.
The study of the transition between galactic and extragalactic cosmic rays can shed more light on the end of the galactic cosmic rays spectrum and the beginning of the extragalactic one. Three models ...of transition are discussed: ankle, dip and mixed composition models. All these models describe the transition as an intersection of a steep galactic component with a flat extragalactic one. Severe bounds on these models are provided by the Standard Model of galactic cosmic rays according to which the maximum acceleration energy for Iron nuclei is of the order of EFemax≈1×1017eV. In the ankle model the transition is assumed at the ankle, a flat feature in the all particle spectrum which observationally starts at energy Ea∼(3-4)×1018eV. This model needs a new high energy galactic component with maximum energy about two orders of magnitude above that of the Standard Model. The origin of such component is discussed. As observations are concerned there are two signatures of the transition: change of energy spectra and mass composition. In all models a heavy galactic component is changed at the transition to a lighter or proton component. As a result the ankle model predicts a galactic Iron component at E<5×1018eV, while both HiRes and Auger data show that at (2-5)×1018eV primaries are protons, or at least light nuclei. In the dip model the transition occurs at the second knee observed at energy (4-7)×1017eV and is characterized by a sharp change of mass composition from galactic Iron to extragalactic protons. The ankle in this model appears automatically as a part of the e+e- pair-production dip. The mixed composition model describes transition at E∼3×1018eV with mass composition changing from the galactic Iron to extragalactic mixed composition of different nuclei. In most mixed composition models the spectrum is proton-dominated and it better fits HiRes than Auger data. The latter show a steadily heavier mass composition with increasing energy, and we discuss the models which explain it.
We present a detailed analytical study of the propagation of ultra-high- energy (UHE) particles in extragalactic magnetic fields. The crucial parameter that affects the diffuse spectrum is the ...separation between sources. In the case of a uniform distribution of sources with a separation between them much smaller than all characteristic propagation lengths, the diffuse spectrum of UHE particles has a universal form, independent of the mode of propagation. This statement has the status of theorem. The proof is obtained using the particle number conservation during propagation and also using the kinetic equation for the propagation of UHE particles. This theorem can be also proved with the help of the diffusion equation. In particular, it is shown numerically how the diffuse fluxes converge to this universal spectrum, when the separation between sources diminishes. We study also the analytic solution of the diffusion equation in weak and strong magnetic fields with energy losses taken into account. In the case of strong magnetic fields and for a separation between sources large enough, the GZK cutoff can practically disappear, as it has been found early in numerical simulations. In practice, however, the source luminosities required are too large for this possibility.
Using the Auger mass-composition analysis of ultra high energy cosmic rays, based on the shape-fitting of
X
max
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
X
max
distributions shows that the measured sum of proton and Helium fractions, for some hadronic-interaction models, can saturate the total flux. 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.
PDF
