One century ago Viktor Hess carried out several balloon flights that led him to conclude that the penetrating radiation responsible for the discharge of electroscopes was of extraterrestrial origin. One century from the discovery of this phenomenon seems to be a good time to stop and think about what we have understood about Cosmic Rays. The aim of this review is to illustrate the ideas that have been and are being explored in order to account for the observable quantities related to cosmic rays and to summarize the numerous new pieces of observation that are becoming available. In fact, despite the possible impression that development in this field is somewhat slow, the rate of new discoveries in the last decade or so has been impressive, and mainly driven by beautiful pieces of observation. At the same time scientists in this field have been able to propose new, fascinating ways to investigate particle acceleration inside the sources, making use of multifrequency observations that range from the radio, to the optical, to X-rays and gamma rays. These ideas can now be confronted with data.

The origin of galactic cosmic rays

We present the simulation framework CRPropa version 3 designed for efficient development of astrophysical predictions for ultra-high energy particles. Users can assemble modules of the most relevant propagation effects in galactic and extragalactic space, include their own physics modules with new features, and receive on output primary and secondary cosmic messengers including nuclei, neutrinos and photons. In extension to the propagation physics contained in a previous CRPropa version, the new version facilitates high-performance computing and comprises new physical features such as an interface for galactic propagation using lensing techniques, an improved photonuclear interaction calculation, and propagation in time dependent environments to take into account cosmic evolution effects in anisotropy studies and variable sources. First applications using highlighted features are presented as well.

https://iopscience.iop.org/article/10.1088/1475-7516/2016/05/038/pdf

CRPropa 3-a public astrophysical simulation framework for propagating extraterrestrial ultra-high energy particles

One century ago Viktor Hess carried out several balloon flights that led him to conclude that the penetrating radiation responsible for the discharge of electroscopes was of extraterrestrial origin. One century from the discovery of this phenomenon seems to be a good time to stop and think about what we have understood about Cosmic Rays. The aim of this review is to illustrate the ideas that have been and are being explored in order to account for the observable quantities related to cosmic rays and to summarize the numerous new pieces of observation that are becoming available. In fact, despite the possible impression that development in this field is somewhat slow, the rate of new discoveries in the last decade or so has been impressive, and mainly driven by beautiful pieces of observation. At the same time scientists in this field have been able to propose new, fascinating ways to investigate particle acceleration inside the sources, making use of multifrequency observations that range from the radio, to the optical, to X-rays and gamma rays. These ideas can now be confronted with data. I will mostly focus on supernova remnants as the most plausible sources of Galactic cosmic rays, and I will review the main aspects of the modern theory of diffusive particle acceleration at supernova remnant shocks, with special attention for the dynamical reaction of accelerated particles on the shock and the phenomenon of magnetic field amplification at the shock. Cosmic ray escape from the sources is discussed as a necessary step to determine the spectrum of cosmic rays at the Earth. In the end of this review I will also discuss the phenomenon of cosmic ray acceleration at shocks propagating in partially ionized media and the implications of this phenomenon in terms of width of the Balmer line emission.

The Origin of Galactic Cosmic Rays

We use a kinetic-equation approach to describe the propagation of ultra high energy cosmic ray protons and nuclei and calculate the expected spectra and mass composition at the Earth for different assumptions on the source injection spectra and chemical abundances. When compared with the spectrum, the elongation rate Xmax(E) and dispersion σ(Xmax) as observed with the Pierre Auger Observatory, several important consequences can be drawn: a) the injection spectra of nuclei must be very hard, ~ E-γ with γ~ 1- 1.6; b) the maximum energy of nuclei of charge Z in the sources must be ~ 5Z× 1018 eV, thereby not requiring acceleration to extremely high energies; c) the fit to the Auger spectrum can be obtained only at the price of adding an ad hoc light extragalactic component with a steep injection spectrum ~ E-2.7). In this sense, at the ankle EA≈ 5× 1018 eV) all the components are of extragalactic origin, thereby suggesting that the transition from Galactic to extragalactic cosmic rays occurs below the ankle. Interestingly, the additional light extragalactic component postulated above compares well, in terms of spectrum and normalization, with the one recently measured by KASCADE-Grande.

Ultra high energy cosmic rays: implications of Auger data for source spectra and chemical composition

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of standard model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the [Formula: see text]m scale up to the Big Bang Nucleosynthesis limit of [Formula: see text] m. Neutral LLPs with lifetimes above [Formula: see text]100 m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

/pdf/long-lived-particles-at-the-energy-frontier-the-mathusla-2nigufnbj0.pdf

Long-lived particles at the energy frontier: the MATHUSLA physics case.

We constrain the energy at which the transition from Galactic to extragalactic cosmic rays occurs by computing the anisotropy at Earth of cosmic rays emitted by Galactic sources. Since the diffusion approximation starts to loose its validity for E/Z >~ 10^(16-17) eV, we propagate individual cosmic rays (CRs) using Galactic magnetic field models and taking into account both their regular and turbulent components. The turbulent field is generated on a nested grid which allows spatial resolution down to fractions of a parsec. Assuming sufficiently frequent Galactic CR sources, the dipole amplitude computed for a mostly light or intermediate primary composition exceeds the dipole bounds measured by the Auger collaboration around E ~ 10^18 eV. Therefore, a transition at the ankle or above would require a heavy composition or a rather extreme Galactic magnetic field with strength >~ 10 muG. Moreover, the fast rising proton contribution suggested by KASCADE-Grande data between 10^17 eV and 10^18 eV should be of extragalactic origin. In case heavy nuclei dominate the flux at E >~ 10^18 eV, the transition energy can be close to the ankle, if Galactic CRs are produced by sufficiently frequent transients as e.g. magnetars.

Cosmic Ray Anisotropy as Signature for the Transition from Galactic to Extragalactic Cosmic Rays

Observations are consistent with a significant fraction of heavy nuclei in the cosmic ray flux above a few times 10^19 eV. Such nuclei can be deflected considerably in the Galactic magnetic field, with important implications for the search of their sources. We perform detailed simulations of heavy nuclei propagation within recent Galactic magnetic field models. While such models are not yet sufficiently constrained to predict deflection maps in detail, we find general features of the distribution of (de-) magnified flux from sources. Since in most theoretical models sources of heavy nuclei are located in the local large scale structure of galaxies, we show examples of images of several nearby galaxy clusters and of the supergalactic plane. Such general features may be useful to develop efficient methods for source reconstruction from observed ultrahigh energy cosmic ray arrival directions.

Ultrahigh Energy Nuclei in the Galactic Magnetic Field

We show that the cosmic ray (CR) knee can be entirely explained by energy-dependent CR leakage from the Milky Way, with an excellent fit to all existing data. We test this hypothesis calculating the trajectories of individual CRs in the Galactic magnetic field. We find that the CR escape time ${\ensuremath{\tau}}_{\text{esc}}(E)$ exhibits a knee-like structure around $E/Z=\text{few}\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{eV}$ for small coherence lengths and strengths of the turbulent magnetic field. The resulting intensities for different groups of nuclei are consistent with the ones determined by KASCADE and KASCADE-Grande, using simple power laws as injection spectra. The transition from Galactic to extragalactic CRs is terminated at $\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{eV}$, while extragalactic CRs contribute significantly to the subdominant proton flux already for $\ensuremath{\gtrsim}2\ifmmode\times\else\texttimes\fi{}{10}^{16}\text{ }\text{ }\mathrm{eV}$. The natural source of extragalactic CRs in the intermediate energy region up to the ankle are in this model normal and starburst galaxies. The escape model provides a good fit to $\mathrm{ln}(A)$ data; it predicts that the phase of the CR dipole varies strongly in the energy range between $1\ifmmode\times\else\texttimes\fi{}{10}^{17}$ and $3\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{eV}$, while our estimate for the dipole magnitude is consistent with observations.

Escape model for Galactic cosmic rays and an early extragalactic transition

We calculate the diffusion coefficients of charged cosmic rays (CR) propagating in regular and turbulent magnetic fields. If the magnetic field is dominated by an isotropic turbulent component, we find that CRs reside too long in the Galactic disc. As a result, CRs overproduce secondary nuclei like boron for any reasonable values of the strength and the coherence length of an isotropic turbulent field. We conclude therefore that the propagation of Galactic CRs has to be strongly anisotropic because of a sufficiently strong regular field and/or of an anisotropy in the turbulent field. As a consequence, the number of sources contributing to the local CR flux is reduced by a factor ${\cal O}(100)$ compared to the case of isotropic CR diffusion.

Reconciling cosmic ray diffusion with Galactic magnetic field models

We investigate the possibility that the cosmic ray (CR) knee is entirely explained by the energy-dependent CR leakage from the Milky Way. We test this hypothesis calculating the trajectories of individual CRs with energies between $E/Z={10}^{14}\text{ }\text{ }\mathrm{eV}$ and ${10}^{17}\text{ }\text{ }\mathrm{eV}$ propagating them in the regular and turbulent Galactic magnetic field. We find a knee-like structure of the CR escape time ${\ensuremath{\tau}}_{\text{esc}}(E)$ around $E/Z=\text{few}\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{eV}$ for a coherence length ${l}_{\mathrm{c}}\ensuremath{\simeq}2\text{ }\text{ }\mathrm{pc}$ of the turbulent field, while the decrease of ${\ensuremath{\tau}}_{\text{esc}}(E)$ slows down around $E/Z\ensuremath{\simeq}{10}^{16}\text{ }\text{ }\mathrm{eV}$ in models with a weak turbulent magnetic field. Assuming that the injection spectra of CR nuclei are power laws, the resulting CR intensities in such a turbulence are consistent with the energy spectra of CR nuclei determined by KASCADE and KASCADE-Grande. We calculate the resulting CR dipole anisotropy as well as the source rate in this model.

G. Giacinti

Papers

Cosmic Ray Anisotropy as Signature for the Transition from Galactic to Extragalactic Cosmic Rays

Ultrahigh Energy Nuclei in the Galactic Magnetic Field

Escape model for Galactic cosmic rays and an early extragalactic transition

Reconciling cosmic ray diffusion with Galactic magnetic field models

Explaining the spectra of cosmic ray groups above the knee by escape from the Galaxy