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Register a new userComposition determination of cosmic rays from the muon content of the showers
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The origin and nature of ultra high energy cosmic rays remains being a mystery. However, great progress has been made in recent years due to the observations performed by the Pierre Auger Observatory and Telescope Array. In particular, it is believed that the composition information of the cosmic rays as a function of the energy can play a fundamental role for the understanding of their origin. The best indicators for primary mass composition are the muon content of extensive air shower and the atmospheric depth of the shower maximum. In this work we consider a maximum likelihood method to perform mass composition analyses based on the number of muons measured by underground muon detectors. The analyses are based on numerical simulations of the showers. The effects introduced by the detectors and the methods used to reconstruct the experimental data are also taken into account through a dedicated simulation that uses as input the information of the simulated showers. In order to illustrate the use of the method, we consider AMIGA (Auger Muons and Infill for the Ground Array), the low energy extension of the Pierre Auger Observatory that directly measures the muonic content of extensive air showers. We also study in detail the impact of the use of different high energy hadronic interaction models in the composition analyses performed. It is found that differences of a few percent between the predicted number of muons have a significant impact on composition determination.
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The composition of Galactic Cosmic Rays (GCR) presents strong similarities to the standard (cosmic) composition, but also noticeable differences, the most important being the high isotopic ratio of Ne22/Ne20 which is about 5 times higher in GCR than in the Sun. This ratio provides key information on the GCR origin. We investigate the idea that GCR are accelerated by the forward shocks of supernova explosions, as they run through the presupernova winds of the massive stars and through the interstellar medium. We use detailed wind and core yields of rotating and non-rotating models of massive stars with mass loss, as well as simple models for the properties of the forward shock and of the circumstellar medium. We find that the observed GCR Ne22/Ne20 ratio can be explained if GCR are accelerated only during the early Sedov phase, for shock velocities >1500-1900 km/s. The acceleration efficiency is found to be of the order of 1.e-6 - 1.e-5, i.e. a few particles out of a million encountered by the shock escape the SN at GCR energies. We also show quantitatively that the widely publicized idea that GCR are accelerated in superbubbles fails to account for the high Ne22/Ne20 ratio in GCR
ALICE, a general purpose experiment designed to investigate nucleus-nucleus collisions at the CERN Large Hadron Collider (LHC), has also been used to detect atmospheric muons produced by cosmic-ray interactions in the atmosphere. In this contribution the analysis of the multiplicity distribution of the atmospheric muons detected by ALICE between 2010 and 2013 is presented, along with a comparison with Monte Carlo simulations. Special emphasis is given to the study of high-multiplicity events, i.e. those containing more than 100 reconstructed muons. Such high-multiplicity events demand primary cosmic rays with energy above $10^{16}$ eV. The frequency of these events can be successfully described by assuming a heavy mass composition of primary cosmic rays in this energy range, using the most recent interaction models to describe the development of the air shower resulting from the primary interaction.
Galactic cosmic-rays (GCRs) are thought to be accelerated in strong shocks induced by massive star winds and supernova explosions sweeping across the interstellar medium. But the phase of the interstellar medium from which the CRs are extracted has remained elusive until now. Here, we study in detail the GCR source composition deduced from recent measurements by the AMS-02, Voyager 1 and SuperTIGER experiments to obtain information on the composition, ionisation state and dust content of the GCR source reservoirs. We show that the volatile elements of the CR material are mainly accelerated from a plasma of temperature higher than $sim 2$ MK, which is typical of the hot medium found in galactic superbubbles energised by the activity of massive star winds and supernova explosions. Another GCR component, which is responsible for the overabundance of $^{22}$Ne, most likely arises from acceleration of massive star winds in their termination shocks. From the CR-related $gamma$-ray luminosity of the Milky Way, we estimate that the ion acceleration efficiency in both supernova shocks and wind termination shocks is of the order of $10^{-5}$. The GCR source composition also shows evidence for a preferential acceleration of refractory elements contained in interstellar dust. We suggest that the GCR refractories are also produced in superbubbles, from shock acceleration and subsequent sputtering of dust grains continuously incorporated into the hot plasma through thermal evaporation of embedded molecular clouds. Our model explains well the measured abundances of all primary and mostly primary CRs from H to Zr, including the overabundance of $^{22}$Ne.
The origin and properties of the cosmic radiation are one of the most intriguing question in modern astrophysics. The precise measurement of the chemical composition and energy spectra of the cosmic rays provides fundamental insight into these subjects. In this paper we will review the existing experimental data. Specifically, we will analyse results collected by space-born experiments discussing the experimental uncertainties and challenges with a focus on the PAMELA experiment.
We explore the feasibility of estimating primary cosmic ray composition at high energies from the study of two parameters of Extensive Air Showers (EAS) at ground and underground level with Monte Carlo simulations using the new EPOS and QGSJETII hadronic models tuned with LHC data. Namely, the slope and density at a given distance of the muon lateral distribution function are analysed in this work. The power to discriminate primary masses is quantified in terms of merit factor for each parameter. The analysis considers three different primary particles (proton, iron and gamma), four different zenith angles (0$^{circ}$, 15$^{circ}$, 30$^{circ}$ and 45$^{circ}$) and primary energies of $10^{17.25}$ eV, $10^{17.50}$ eV and $10^{17.75}$ eV.
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