**1. Introduction**

In 1938, Pierre Auger recorded coincidences by particle detectors separated by large distances at ground level. The source was ultrahigh energy cosmic rays (UHECRs) generating in the atmosphere extensive air showers (EAS). The energy of UHECRs reached up to 1015 eV [1, 2]. In this time energy of particles produced in laboratories was at the level of 107 eV. In 1962, Linsley at Volcano Ranch recorded an air shower from a cosmic ray with giant energy higher than 1020 eV [3]. In 1965, Penzias and Wilson discovered the cosmic microwave background (CMB) radiation. This discovery overshadowed Linsley's experiment, the fantastically huge cosmic ray energy did not receive any attention that it deserved. Just after CMB discovery, **G**reisen, **Z**atsepin, and **K**uzmin (GZK) [4, 5] predicted that photo-pion production by the CMB photons reduces the path length for protons of UHECRs. In the rest frame of proton, the CMB is a beam of very energetic photons. The GZK threshold is the cosmic ray energy at which a Lorentz-boosted CMB photon has energy equal to the pion rest energy. The Planck distribution of CMB photons causes pion photo-production energy loss for protons with energies above approximately 7 × 1019 eV. The effect predicts that the spectrum from distributed homogeneous sources in the universe is suppressed above the GZK threshold at least one order of magnitude compared to the flux without the GZK effect.

If one assumes that the sources accelerate nuclei to a maximum energy above the energy threshold for photo-disintegration on CMB photons, the light elements could then be fragments of heavier nuclei that disintegrated during propagation. Candidate air shower primary particles all suffer severe propagation losses that should produce an effective cut-off at ≤10<sup>20</sup> eV in experiments so far, assuming only that high-energy cosmic rays are normal particles that are produced in sources throughout the universe.

At the moment, the UHECRs remain a puzzle. Reliable conclusions from measurements of the energy spectrum, composition, and anisotropy and the proposed models cannot be obtained without a significant improvement in the observations.

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The statistics of registered events with energy about 1020 eV is definitely insufficient due to extremely low flux estimated less than 0.5 event per km<sup>2</sup> per century per steradian. So that only detectors of a huge size will be able to observe a sufficient number of events, which may be a fundament of a new physics.

Cosmic rays are high-speed particles traveling throughout our Galaxy, including the Solar System. Some of these particles originate from the Sun, but most come from sources outside the Solar System and are known as galactic cosmic rays (GCRs). The origin of the highest energy cosmic rays is expected to be extragalactic. Simple considerations about the confinement of particles in the Galaxy and Galactic halo strongly suggest that most of the highestenergy CR must have an extragalactic origin (unless their charge is unexpectedly large, which is also not favored by the observations). CR particles arriving at the top of the Earth's atmosphere are called primaries; their collisions with atmospheric nuclei give rise to secondaries.

Several experiments have been investigating ultrahigh-energy cosmic rays with energies reported beyond 1020, but their origin is still unknown. For the current physics, astronomy and cosmology, it is a great challenge. In the 1990s, the largest experiments, AGASA and HiRes (both located in the Northern hemisphere), reported a discrepancy in the energy spectrum and clustering of cosmic ray arrival directions near the GZK energy threshold. This fact showed clearly that we require much more accurate and large-scale experiments to investigate this question without any doubts.

At 1015 eV, GCRs consist mostly of protons (nuclei of hydrogen atoms) and alpha particles (helium nuclei). The remainders are electrons and nuclei of heavier atoms. The composition changes with energy. At present, we believe that UHECRs consist mostly of charged nuclei. Gamma rays have been observed with energies as high as ~1012 eV. EAS generated by photons would be almost purely electromagnetic.

Because most cosmic ray primaries are strongly influenced by the solar magnetic field, most of those detected near the Earth have kinetic energies in excess of about 0.1 GeV. The number of particles decreases dramatically with increasing energy, but individual particles with the estimated energies above 1020 eV have also been recorded.

The acceleration mechanism is still not clear and the study requires very careful measurements of the energy spectrum of UHECR to compare to the predictions from different acceleration models. Arrival directions of UHECRs are the second topic requiring a careful attention, and the third is both small- and large-scale anisotropies in their distribution. The fourth: the composition is one of the most difficult measurements because UHECRs cannot be detected directly using conventional particle detectors. Consequently, the composition as well as energy spectrum and arrival directions must be inferred from auxiliary measurements.

**Figure 1.** Observed energy spectrum of primary cosmic rays. The spectrum is expressed by a power law from 1011 to

Introductory Chapter: Ultrahigh-Energy Cosmic Rays http://dx.doi.org/10.5772/intechopen.79535 5

1020 eV with a slight change of slopes around 1015.5 eV (knee), 1017.8 eV (second knee), and 1019 eV (ankle) [6].

The cosmic rays with energies greater than 1014 eV have been investigated by using the Earth's atmosphere itself as part of the detection equipment. The interaction between high-energy

The process begins with the collision of the primary cosmic ray with a nucleus near the top of the atmosphere. This first collision produces typically several tens of secondary particles (depending on initial energy), mainly pions. The charged pions, as relatively long-lived,

cosmic rays and the air produces avalanches of secondary particles.

**2. Extensive air showers**

Due to magnetic fields, primary GCRs that are deflected in the space and arrive at the top of the Earth's atmosphere are nearly uniform from all directions. Thus, identification of UHECR sources based on arrival directions must be excluded. We have to deduce by other ways like, that is, the charge spectrum compared to spectroscopy data of stars and interstellar regions. The abundances of different elements have been well studied for particles with energies from roughly 100 MeV to several hundreds of GeV.

UHECRs are observed in an energy range from 109 eV to above 1020 eV. Over this range, the flux of cosmic rays appears to follow an approximate single power law ~E–2.7, with sharper steepness ~E–3.0 between so-called knee and ankle (see **Figure 1**) corresponding to 1015 eV and 1018 eV, respectively.

Cosmic rays with energies above ~1019 eV, known as ultra-high energy cosmic rays (UHECRs) are microscopic particles with a macroscopic amount of energy about a joule or more. The existence of such energetic particles, the mechanism of the acceleration to such extreme energies, the regions of their creation and the composition remains still a mystery.

**Figure 1.** Observed energy spectrum of primary cosmic rays. The spectrum is expressed by a power law from 1011 to 1020 eV with a slight change of slopes around 1015.5 eV (knee), 1017.8 eV (second knee), and 1019 eV (ankle) [6].

The acceleration mechanism is still not clear and the study requires very careful measurements of the energy spectrum of UHECR to compare to the predictions from different acceleration models. Arrival directions of UHECRs are the second topic requiring a careful attention, and the third is both small- and large-scale anisotropies in their distribution. The fourth: the composition is one of the most difficult measurements because UHECRs cannot be detected directly using conventional particle detectors. Consequently, the composition as well as energy spectrum and arrival directions must be inferred from auxiliary measurements.
