Session T7: Cosmic Ray Composition and Air Shower Studies

Fitting the Cosmic Ray Spectrum and Composition with IceCube

The IceCube Neutrino Observatory at the south pole consists of a square-kilometer surface array and a cubic-kilometer underground array. The combined information from cosmic-ray induced air showers that trigger both arrays can be used to measure the energy and mass of the primary particle. The deep IceCube detector, between 1.5 and 2.5 km in the ice, measures the high energy muons, produced early in the shower development, while the surface array, called IceTop, measures the lower energy particles that make up the extensive air shower. These two measurements are used in a likelihood based reconstruction of both energy and mass of the individual primary particle, and through a statistical approach, fitting distributions of reconstructed parameters with simulation. A method for fitting the spectrum and composition will be shown, as well as an optimized approach for breaking degeneracies in the fit.   

The Composition of Ultra High Energy Cosmic Rays at Telescope Array

Telescope Array (TA) is the largest cosmic ray observatory in the Northern Hemisphere and is sensitive to Ultra High Energy Cosmic Rays (UHECR) with energies above $10^{18}$~eV. UHECR observatories rely on the extensive air shower (EAS) produced when cosmic rays interact in the atmosphere. The point in an EAS that has the largest number of particles is called shower maximum, $X_{max}$, and is sensitive to cosmic ray composition. TA uses surface detector (SD) as well as fluorescence detector (FD) observation methods and may observe $X_{max}$ in monocular mode, using single FD station, stereo mode, using a pair of FD stations, or hybrid mode, using one FD station combined with the SD. Hybrid and stereo observation allow for accurate reconstruction of cosmic ray arrival directions and $X_{max}$. Both hybrid and stereo modes provide resolutions better than $1^{\circ}$ in arrival direction and $30$~g/cm$^{2}$ in $X_{max}$. Both of these observation methods are sufficiently accurate for composition analysis using $X_{max}$. In this talk, the most recent TA composition analysis is presented. The results are compatible with the predictions of QGSJet01/II proton Monte Carlo and incompatible with QGSJet01/II iron for energies between $10^{18.2}$ and $10^{19.3}$~eV.   

The nuclear mass composition of UHECR with the Pierre Auger Observatory

The Fluorescence Detector of the Pierre Auger Observatory measures the atmospheric depth Xmax where the longitudinal profile of the UHECR (Ultra High Energy Cosmic Ray) induced electromagnetic cascade reaches its maximum. This is sensitive to the nuclear mass composition of the cosmic rays. Due to its hybrid design, the Pierre Auger Observatory also provides independent experimental observables for the study of the nuclear mass composition coming from the Surface Detector. We present Xmax distributions and an update of the average and RMS values in different energy bins and compare them to the predictions for different nuclear masses and hadronic models. We also present the results of the composition sensitive parameters derived from the ground level component.   

Determination of hadronic interaction characteristics with the Pierre Auger Observatory

The Pierre Auger Observatory measures extensive air showers (EAS) up to the highest energies. One of the biggest challenges in current data analyses is to interpret these data in terms of the primary mass composition. Due to the insufficient constraint of interactions in EAS this is afflicted with large uncertainties. On the other hand, this high sensitivity of EAS to interaction features can be exploited to determine or constrain properties of interactions up to 450TeV. We demonstrate how specific EAS observations are suited for this task and thus may contribute to limit the uncertainties in the interpretation of air showers. These are the estimation of the muon number at ground level and the study of the hadronic cross-section for particle production via EAS fluctuations.   

Irregular Cosmic Ray Air Showers

Ultra-high energy cosmic rays initiate extensive air showers in the atmosphere. The growth of the number of charged particles with depth in the atmosphere is well described by the semi-empirical Gaisser-Hillas function. Simulations indicate that occasionally showers will deviate significantly from this description and exhibit two distinct shower maxima. Such effects partly are due to a single leading particle from the initial interaction penetrating very deeply before it collides with an atmospheric nucleus. We present studies of this phenomena and discuss methods to extract information about interaction cross sections using measurements of shower development profiles.   

Prospects for Sampling the Cherenkov Lateral Distribution of Air Showers from the Highest Energy Cosmic Rays

The origin and nature of the highest-energy cosmic rays remain a puzzle. A critical concern is the composition of primary cosmic rays at energies above $10^{19}$ eV, where the GZK cutoff should be important. In principle, Cherenkov light generated in extensive air showers can be used to determine the energy and estimate the atomic composition of the primary cosmic rays. We present a study to explore the prospects of using an array of photon detectors to sample the lateral distribution of Cherenkov light arriving on the ground. By measuring the arrival time and photon density of Cherenkov light, we could provide an independent mechanism for reconstructing air shower parameters. We also consider a preliminary design for such an array with initial performance estimates based on Monte Carlo air-shower simulations.   

Searching for Exotic Particles at the Pierre Auger Observatory Using Bayesian Inference

Upper limits on the fractional composition of selected exotic particles in hybrid data from the Pierre Auger Observatory were obtained. Exotic particle types considered are magnetic monopoles, Q-balls, strangelets, micro-black holes and UHECRONs. A Bayesian method was used to set limits on the candidates at energies above 10$^{18}$ eV. By systematically comparing distributions of the parameter Xmax from simulated extensive air showers to a given set of data, this method obtains an upper limit on the composition fraction of the selected exotics. The method and results from the study will be discussed.   

Constraining the galactic magnetic field models

Ultra-high energy cosmic rays (UHECRs) are deflected by the Galactic magnetic field (GMF) on their way to Earth. If UHECR properties were well-understood, it would be straightforward to model the intervening GMF. However uncertainties on the composition and source distribution complicate the issue. An independent method of constraining GMF models is using Faraday rotation measurements (RMs) of Galactic and extra-Galactic radio sources. Here we investigate a new composite method for constraining GMF models using simultaneous fits of UHECR and RM simulations. A simulated universe of UHECRs and Galactic RMs are used to test this method.   

Newly-born Pulsars as Sources of Ultrahigh Energy Cosmic Rays

Newly-born pulsars are good candidate sources of ultrahigh energy cosmic ray (UHECR). Young pulsars can strip heavy nuclei from the star surface, and further accelerate them to ultrahigh energies by unipolar induction in the pulsar wind. Once accelerated, nuclei have to escape from the dense supernova envelope surrounding the pulsar. We examine this escape analytically and numerically. Our results show that, protons with energy above $10^{20}\,$eV can hardly survive from interactions with the young supernova shell. In contrast, as a result of their higher charge, iron-peaked nuclei at the highest observed energies are able to escape newly-born pulsars with millisecond periods and dipole magnetic fields of $\sim10^{12-13}\,$Gauss, embedded in core-collapse supernova. The escaped spectrum presents a transition of composition from light to heavy elements at a few EeV as observed by the Auger Observatory, due to the production of secondary nucleons. The interactions also help to soften the spectral slope, which allows a good fit to the observation. We conclude that the acceleration of heavy nuclei in a reasonably small fraction ($\le 0.01\%$) of extragalactic young pulsars would reproduce consistently the current UHECR data.