1 Proceedings of the XXIX PHYSICS IN COLLISION arXiv:1001.1660v1 [astro-ph.HE] 11 Jan 2010 Observation of Ultra-high Energy Cosmic Rays Radom´ır ˇ Sm´ıda 1 for the Pierre Auger Collaboration (1) Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 182 21, Czech Republic The measurement of ultra-high energy cosmic rays is an unique way to study particle interactions at ener- gies which are well above the capability of current ac- celerators. Significant progress in this field has occurred during last years, particularly due to the measurements made at the Pierre Auger Observatory. The important results which were achieved during last years are de- scribed here. Also future plans for the study of cosmic rays are presented. of the primary particle is distributed among secondary particles in subsequent collisions in the atmosphere. There are two detection techniques used for the ob- servation of extensive air showers. The sampling of sec- ondary particles arriving at the ground is the first one. The surface detector consists of a net of scintillators, wa- ter Cherenkov tanks or muon detectors. An arrival direc- tion and energy of a primary cosmic ray can be recon- structed from timing information of triggered stations and lateral distribution of measured signal, respectively. The main advantages of the surface detector are an auto- matic operation, almost 100% duty cycle and simple cal- culation of its exposure. On the other hand, the energy reconstruction is based on models of hadronic interac- tions extrapolating data from laboratory experiments at lower energies. The uncertainty in hadronic interactions constitute a large contribution to a systematic error of the energy reconstruction. The second technique is based on a measurement of the isotropic fluorescence light emitted by air nitrogen molecules which were excited by secondary particles of the extensive air shower. The longitudinal development of the extensive air shower is measured by a set of fluores- cence telescopes. Unfortunately, the fluorescence detec- tor can be operated only on clear and moonless nights limiting its duty cycle to about 12% of a year. The measured signal, after correction for an attenuation due to Rayleigh and aerosol scattering and for Cherenkov light, is proportional to the number of fluorescence pho- tons emitted in the field of view of each triggered pixel. A Gaisser-Hillas function [3] is used to reconstruct the shower profile which provides a measurement of the en- ergy of the extensive air shower deposited in the atmo- sphere. To derive the primary energy, a correction to missing energy carried into the ground by muons and neutrinos must be made. This correction is however small and its uncertainty is less than 4% of the energy of primary particle. The energy reconstruction is more precise than in a case of the surface detector, because a fluorescence yield for various atmospheric conditions can be measured in a laboratory. However, the un- certainty in the fluorescence yield is still an important source of systematic uncertainty and therefore a large effort is directed into a precise measurement of the fluo- rescence yield by laboratory experiments. Other impor- tant sources of the systematic uncertainty come from the absolute calibration of telescopes, absorption and scat- tering in the atmosphere and reconstruction methods. A significant improvement of the reconstruction can be achieved by using the data measured by both detec- tion techniques for same extensive air showers. A hybrid detector can be well calibrated, has well known aperture, excellent angular resolution (about 0.2 ◦ ) and low energy threshold. The first hybrid detector is the Pierre Auger Observatory and it has measured a clear connection be- tween an energy estimator measured by the surface de- tector and the energy measured by the fluorescence de- tector as is shown in Fig. 1.. In such a way the energy can be attributed to all UHECRs measured only by the surface detector. Cosmic rays with the energy higher than 10 15 eV can- not be measured by satellite or high-altitude balloon ex- periments because of their low flux. Therefore, they are measured indirectly by the observation of extensive air showers from the ground. The extensive air shower is a bundle of secondary particles initiated by a primary particle which arrived into the atmosphere. The energy The methods of UHECRs measurement are described as the first in this article. Then a brief description of current experiments follows with special attention to the Pierre Auger Observatory and the results of its measure- ment. Finally, future plans in cosmic ray measurements are mentioned. Moreover, there is a strict limit on a distance of any $^{source of cosmic rays with the energy above}$∼ $^{4}$×10 19 eV as was shown by [1, 2]. Such energetic cosmic rays can not survive their propagation over the distance longer than ∼ 100 Mpc. This lead to a restriction on the num- ber of possible sources which should be present as a sup- pression in the flux of cosmic rays. This predicted sup- pression in cosmic ray energy spectrum is called Greisen- Zatsepin-Kuzmin (GZK) cut-off. The search for UHECRs sources must take into ac- count the magnetic fields which are measured in in- terstellar and also intergalactic space. Cosmic rays as charged particles are significantly deflected during their propagation because of their electric charge. Therefore their arrival directions do not point back to their sources. Our knowledge of magnetic fields is not sufficient for the calculation of trajectories of measured particles. The main attention is thus focused on the most energetic cos- mic rays which may propagate through magnetic fields along almost linear trajectories. Cosmic rays are observed in a wide range of energy, but their sources remain unknown. The well known prominent source of cosmic rays, but only those with the energy below a few 10 9 eV, is the Sun. We can ex- pect that also other stars emit cosmic ray particles into interplanetary space. There are several indirect measure- ments which indicate that expanding shells of supernova remnants accelerate particles up to very high energies. The maximum energy which can be achieved by parti- cles in these remnants is about 10 15 eV for protons and up to 10 18 eV for iron nuclei. The origin of ultra-high energy cosmic rays (UHECRs) – we will call this name for cosmic rays with the energy above 10 18 eV – is still unknown, but results of the measurements indicate that sources are located outside our Galaxy. Abstract 1. Introduction 2. Cosmic Ray Measurement c ○ 2009 by Universal Academy Press, Inc.