**2. Extensive air showers**

The statistics of registered events with energy about 1020 eV is definitely insufficient due to

only detectors of a huge size will be able to observe a sufficient number of events, which may

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 investi-

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

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

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

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

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 ener-

gies, the regions of their creation and the composition remains still a mystery.

per century per steradian. So that

extremely low flux estimated less than 0.5 event per km<sup>2</sup>

be a fundament of a new physics.

4 Cosmic Rays

gate this question without any doubts.

would be almost purely electromagnetic.

roughly 100 MeV to several hundreds of GeV.

1018 eV, respectively.

estimated energies above 1020 eV have also been recorded.

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 cosmic rays and the air produces avalanches of secondary particles.

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, collide with another nucleus. The subsequent collisions are similar in nature to the primary collision. This process then leads to a cascade of particles, known as hadronic shower.

Particles scatter from the region of the shower axis throughout their development. The shower core effectively acts as a moving point source of both fluorescence photons and particles, which make their way to detectors far from the core. The shower front itself is slightly curved, resembling a cone. Particles far from the core will arrive behind the shower plane due to simple geometry. Electromagnetic component diffuses away from the shower axis throughout the shower development. It is wider in comparison to the hadronic one. Thus, far from the core particles are spread in time, with the time spread roughly proportional to the distance from the axis. This time spread helps to distinguish distant large showers from nearby small showers, and is thus useful in triggering the surface array. The time spread becomes greater

Introductory Chapter: Ultrahigh-Energy Cosmic Rays http://dx.doi.org/10.5772/intechopen.79535 7

Fluctuations in shower development distinguish detected signals. One of the most important sources of fluctuations is the depth and characteristics of the first few interactions. Fluctuations in later interactions are averaged over a large number of particles and are not important.

We do not know the composition of the UHECRs. However, the set of stable particles as candidates for the UHECRs, which can trespass cosmological distances saving their energy, is quite limited: heavy or light atomic nuclei, photons and neutrinos. No any standard, electromagnetic mechanism can be responsible for photons and neutrinos (as neutral) acceleration. They can only be a product of the interaction of a still higher energy-charged particle. Therefore, in the framework of conventional astrophysics, we believe that light and heavy

There is experimental evidence that the Universe was created some ~14 billion years ago from some singularity in a giant explosion known as the "Big Bang." Perhaps the most conclusive evidence for the Big Bang is the existence of the isotropic, with Planck distribution T = 2.73 K radiation permeating the entire Universe known as the **c**osmic **m**icrowave **b**ackground (CMB). Shortly after the CMB discovery, Greisen and independently Zatsepin and Kuzmin predicted that at very high energies, the universe should become opaque to light or heavy

<sup>p</sup> <sup>+</sup> <sup>γ</sup>CMB <sup>→</sup> <sup>N</sup> <sup>+</sup> <sup>π</sup> Ep <sup>≥</sup> 1.1 <sup>×</sup> <sup>1020</sup> eV p <sup>+</sup> <sup>γ</sup>CMB <sup>→</sup> <sup>Δ</sup> <sup>→</sup> <sup>N</sup> <sup>+</sup> <sup>π</sup> EN <sup>≥</sup> 2.5 <sup>×</sup> <sup>1020</sup> eV, (1)

The energy budget in the center-mass-frame, for an average CMB energy 6.34 × 10−4 eV and protons with energy above 110 EeV, is sufficient for pion-production, during inelastic colli-

Since in each such inelastic collision, protons leave a large part of their energy (of the order of 13% on average), their energy goes below 10 EeV (EeV = 1018 eV) after a few tens of Mpc.

as the depth of shower maximum increases.

nuclei are probably the best candidates for the UHECR.

where EN is the energy of nucleon being disintegrated.

nuclei due to the following reactions:

sions with CMB photons.

**3. The GZK cut-off**

About 33% of pions, created in collisions, are neutral. They are very short-lived and decay very fast into a pair of photons before a next interaction with nuclei in the atmosphere. Next, photons interacting with the nuclei in the air create electron-positron pairs, which thus produce bremsstrahlung photons. This cascading process forms an electromagnetic avalanche. The hadronic shower itself permanently produces neutral pions and thus is developing secondary electromagnetic cascades along its path.

With an EAS development into the atmosphere, the number of generated particles successively increases (**Figure 2**). However, the process of multiplication is continued until the average energy of the shower particles is insufficient to produce more particles in subsequent collisions. Some part of energy is also leaking to the atmosphere due to ionization processes. Finally, the number of the particles traveling in the shower starts to decrease. This point of the EAS development is known as shower maximum. Beyond the maximum, the shower particles are gradually absorbed with an attenuation length of ~200 g/cm<sup>2</sup> . The depth of shower maximum (*X***max**) is a function of energy. With a value of about 500 g/cm<sup>2</sup> at 1015 eV, the average *Xmax* for showers increases by 60–70 g/cm<sup>2</sup> for every decade of energy [7]. The measured value of *Xmax* can also be used to estimate the composition of the primary cosmic ray. Hadronic interaction length in air for protons is about 70 g/cm<sup>2</sup> , and shorter for heavier nuclei. This means EAS are generated by heavier elements higher in the atmosphere.

More muons and fewer electromagnetic particles are produced by heavy primary particles rather than do lighter primaries, of the same primary energy. Iron and proton showers can be distinguished using surface detector data alone through the ratio of muons to electromagnetic particles, as well as through the arrival time distribution of particles in the shower front.

**Figure 2.** The schematic of the hadronic and electromagnetic components generation in the EAS development.

Particles scatter from the region of the shower axis throughout their development. The shower core effectively acts as a moving point source of both fluorescence photons and particles, which make their way to detectors far from the core. The shower front itself is slightly curved, resembling a cone. Particles far from the core will arrive behind the shower plane due to simple geometry. Electromagnetic component diffuses away from the shower axis throughout the shower development. It is wider in comparison to the hadronic one. Thus, far from the core particles are spread in time, with the time spread roughly proportional to the distance from the axis. This time spread helps to distinguish distant large showers from nearby small showers, and is thus useful in triggering the surface array. The time spread becomes greater as the depth of shower maximum increases.

Fluctuations in shower development distinguish detected signals. One of the most important sources of fluctuations is the depth and characteristics of the first few interactions. Fluctuations in later interactions are averaged over a large number of particles and are not important.
