Observations of extensive air showers generated by interactions of cosmic rays high in the atmosphere provide our only source of information about hadronic interactions above 1000 TeV. We review the current status of such experiments, discuss their implications for particle physics, and also note the astrophysical implications of the results.

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Particle Physics Books

We place considerable emphasis on a description of the experiments and of the calculations required for their interpretation. We are motivated particularly by the relevance of existing air shower data both to the proposed new generation of accelerators, which will explore the region up to ∼1000 TeV, and to a new generation of air shower experiments, which has the potential to observe longitudinal development of individual showers with energies up to 10 21 eV.DOI: American Physical Society Erratum.

Contents.Etymology The term ray is somewhat of a misnomer due to a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly. In common scientific usage, high-energy particles with intrinsic mass are known as 'cosmic' rays, while, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as or, depending on their.Composition Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the nuclei of well-known atoms (stripped of their electron shells), and about 1% are solitary electrons (similar to ). Serato dj full version mac.

Of the nuclei, about 90% are simple (i.e., hydrogen nuclei); 9% are, identical to helium nuclei; and 1% are the nuclei of heavier elements, called. These fractions vary highly over the energy range of cosmic rays.

A very small fraction are stable particles of, such as. The precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them. Energy Cosmic rays attract great interest practically, due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, and scientifically, because the energies of the most energetic (UHECRs) have been observed to approach 3 × 10 20 eV, about 40 million times the energy of particles accelerated by the. One can show that such enormous energies might be achieved by means of the in.

At 50 J, the highest-energy ultra-high-energy cosmic rays (such as the recorded in 1991) have energies comparable to the kinetic energy of a 90-kilometre-per-hour (56 mph) baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies.

Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks on 0.3 gigaelectronvolts (4.8 ×10 −11 J). History After the discovery of by in 1896, it was generally believed that atmospheric electricity, of the, was caused only by from radioactive elements in the ground or the radioactive gases or isotopes of they produce. Measurements of increasing ionization rates at increasing heights above the ground during the decade from 1900 to 1910 could be explained as due to absorption of the ionizing radiation by the intervening air. Discovery In 1909, developed an, a device to measure the rate of ion production inside a hermetically sealed container, and used it to show higher levels of radiation at the top of the than at its base. However, his paper published in was not widely accepted. In 1911, observed simultaneous variations of the rate of ionization over a lake, over the sea, and at a depth of 3 metres from the surface.

Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth. Pacini makes a measurement in 1910.In 1912, carried three enhanced-accuracy Wulf electrometers to an altitude of 5,300 metres in a flight.

He found the ionization rate increased approximately fourfold over the rate at ground level. Hess ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes.

He concluded that 'The results of the observations seem most likely to be explained by the assumption that radiation of very high penetrating power enters from above into our atmosphere.' In 1913–1914, confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km. Hess lands after his balloon flight in 1912. Identification wrote that:In the late 1920s and early 1930s the technique of self-recording electroscopes carried by balloons into the highest layers of the atmosphere or sunk to great depths under water was brought to an unprecedented degree of perfection by the German physicist and his group. To these scientists we owe some of the most accurate measurements ever made of cosmic-ray ionization as a function of altitude and depth.stated in 1931 that 'thanks to the fine experiments of Professor Millikan and the even more far-reaching experiments of Professor Regener, we have now got for the first time, a curve of absorption of these radiations in water which we may safely rely upon'.In the 1920s, the term cosmic rays was coined by who made measurements of ionization due to cosmic rays from deep under water to high altitudes and around the globe.

Millikan believed that his measurements proved that the primary cosmic rays were gamma rays; i.e., energetic photons. And he proposed a theory that they were produced in interstellar space as by-products of the fusion of hydrogen atoms into the heavier elements, and that secondary were produced in the atmosphere by of gamma rays.

But then, sailing from to the Netherlands in 1927, found evidence, later confirmed in many experiments, that cosmic ray intensity increases from the tropics to mid-latitudes, which indicated that the primary cosmic rays are deflected by the geomagnetic field and must therefore be charged particles, not photons. In 1929, and discovered charged cosmic-ray particles that could penetrate 4.1 cm of gold. Charged particles of such high energy could not possibly be produced by photons from Millikan's proposed interstellar fusion process.

In 1930, predicted a difference between the intensities of cosmic rays arriving from the east and the west that depends upon the charge of the primary particles—the so-called 'east-west effect.' Three independent experiments found that the intensity is, in fact, greater from the west, proving that most primaries are positive. During the years from 1930 to 1945, a wide variety of investigations confirmed that the primary cosmic rays are mostly protons, and the secondary radiation produced in the atmosphere is primarily electrons, photons. In 1948, observations with nuclear emulsions carried by balloons to near the top of the atmosphere showed that approximately 10% of the primaries are helium nuclei and 1% are heavier nuclei of the elements such as carbon, iron, and lead.During a test of his equipment for measuring the east-west effect, Rossi observed that the rate of near-simultaneous discharges of two widely separated was larger than the expected accidental rate.

In his report on the experiment, Rossi wrote '. it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another.' In 1937, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that high-energy primary cosmic-ray particles interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, and photons that reach ground level.Soviet physicist Sergey Vernov was the first to use to perform cosmic ray readings with an instrument carried to high altitude by a balloon. On 1 April 1935, he took measurements at heights up to 13.6 kilometres using a pair of in an anti-coincidence circuit to avoid counting secondary ray showers.derived an expression for the probability of scattering positrons by electrons, a process now known as.

His classic paper, jointly with, published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs. Energy distribution Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of density sampling and fast timing of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the. The experiment employed eleven arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 10 20 eV.

A huge air shower experiment called the is currently operated at a site on the of Argentina by an international consortium of physicists. The project was first led by, winner of the 1980 from the, and of the, and later by other scientists of the international Pierre Auger Collaboration. Their aim is to explore the properties and arrival directions of the very highest-energy primary cosmic rays. The results are expected to have important implications for particle physics and cosmology, due to a theoretical to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 10 20 eV because of interactions with the remnant photons from the origin of the universe. Currently the Pierre Auger Observatory undergoes an upgrade to improve its accuracy and find evidence for the yet unconfirmed origin of the most energetic cosmic rays.High-energy gamma rays (50 MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967. Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles.

Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky. The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.Sources Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and suggesting cosmic rays originated from supernovae. A 1948 proposal by suggested that magnetic variable stars could be a source of cosmic rays. Subsequently, in 1951, Y.

Sekido et al. Identified the as a source of cosmic rays. Since then, a wide variety of potential sources for cosmic rays began to surface, including,. Shock front acceleration (theoretical model for supernovae and active galactic nuclei): Incident proton gets accelerated between two shock fronts up to energies of the high-energy component of cosmic rays.Later experiments have helped to identify the sources of cosmic rays with greater certainty. In 2009, a paper presented at the (ICRC) by scientists at the in Argentina showed (UHECRs) originating from a location in the sky very close to the, although the authors specifically stated that further investigation would be required to confirm Cen A as a source of cosmic rays. However, no correlation was found between the incidence of gamma-ray bursts and cosmic rays, causing the authors to set upper limits as low as 3.4 × 10 −6 cm −2 on the flux of 1 GeV – 1 TeV cosmic rays from gamma-ray bursts.In 2009, supernovae were said to have been 'pinned down' as a source of cosmic rays, a discovery made by a group using data from the. This analysis, however, was disputed in 2011 with data from, which revealed that 'spectral shapes of hydrogen and helium nuclei are different and cannot be described well by a single power law', suggesting a more complex process of cosmic ray formation.

In February 2013, though, research analyzing data from revealed through an observation of neutral pion decay that supernovae were indeed a source of cosmic rays, with each explosion producing roughly 3 × 10 42 – 3 × 10 43 of cosmic rays. However, supernovae do not produce all cosmic rays, and the proportion of cosmic rays that they do produce is a question which cannot be answered without further study. As an explanation of the acceleration in supernovae and active galactic nuclei the model of shock front acceleration is used.In 2017, the published the observation of a weak in the arrival directions of the highest energy cosmic rays. Since the Galactic Center is in the deficit region, this anisotropy can be interpreted as evidence for the extragalactic origin of cosmic rays at the highest energies. This implies that there must be a transition energy from galactic to extragalactic sources, and there may be different types of cosmic-ray sources contributing to different energy ranges.Types Cosmic rays can be divided into two types, galactic cosmic rays ( GCR) and extragalactic cosmic rays, i.e., high-energy particles originating outside the solar system, and, high-energy particles (predominantly protons) emitted by the sun, primarily in. However, the term 'cosmic ray' is often used to refer to only the extrasolar flux. Primary cosmic particle collides with a molecule of atmosphere.Cosmic rays originate as primary cosmic rays, which are those originally produced in various astrophysical processes.

Primary cosmic rays are composed primarily of protons and (99%), with a small amount of heavier nuclei (≈1%) and an extremely minute proportion of. Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include, and, such as,. The latter three of these were first detected in cosmic rays.Primary cosmic rays Primary cosmic rays mostly originate from outside the and sometimes even the. When they interact with Earth's atmosphere, they are converted to secondary particles.

The mass ratio of helium to hydrogen nuclei, 28%, is similar to the primordial ratio of these elements, 24%. The remaining fraction is made up of the other heavier nuclei that are typical nucleosynthesis end products, primarily,. These nuclei appear in cosmic rays in much greater abundance (≈1%) than in the solar atmosphere, where they are only about 10 −11 as abundant as. Cosmic rays made up of charged nuclei heavier than helium are called. Due to the high charge and heavy nature of HZE ions, their contribution to an astronaut's in space is significant even though they are relatively scarce.This abundance difference is a result of the way secondary cosmic rays are formed. Carbon and oxygen nuclei collide with interstellar matter to form, and in a process termed. Spallation is also responsible for the abundances of, and in cosmic rays produced by collisions of iron and nickel nuclei with.At high energies the composition changes and heavier nuclei have larger abundances in some energy ranges.

Current experiments aim at more accurate measurements of the composition at high energies.Primary cosmic ray antimatter. See also:Satellite experiments have found evidence of and a few antiprotons in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays. These do not appear to be the products of large amounts of antimatter from the Big Bang, or indeed complex antimatter in the universe. Rather, they appear to consist of only these two elementary particles, newly made in energetic processes.Preliminary results from the presently operating ( AMS-02) on board the show that positrons in the cosmic rays arrive with no directionality. In September 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters.

A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275±32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500 GeV, but peaks at energies far higher than electron energies, which peak about 10 GeV. These results on interpretation have been suggested to be due to positron production in annihilation events of massive particles.Cosmic ray antiprotons also have a much higher average energy than their normal-matter counterparts (protons). They arrive at Earth with a characteristic energy maximum of 2 GeV, indicating their production in a fundamentally different process from cosmic ray protons, which on average have only one-sixth of the energy.There is no evidence of complex antimatter atomic nuclei, such as nuclei (i.e., anti-alpha particles), in cosmic rays.

These are actively being searched for. A prototype of the AMS-02 designated AMS-01, was flown into space aboard the on in June 1998. By not detecting any at all, the AMS-01 established an upper limit of 1.1 × 10 −6 for the antihelium to helium ratio.

The Moon as seen by the, in gamma rays with energies greater than 20 MeV. These are produced by cosmic ray bombardment on its surface. Secondary cosmic rays When cosmic rays enter the they collide with and, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called secondary radiation that rains down, including, protons,. All of the produced particles stay within about one degree of the primary particle's path.Typical particles produced in such collisions are and charged such as positive or negative. Some of these subsequently decay into and, which are able to reach the surface of the Earth.

Some high-energy muons even penetrate for some distance into shallow mines, and most neutrinos traverse the Earth without further interaction. Others decay into, subsequently producing electromagnetic cascades. Hence, next to photons and usually dominate in air showers. These particles as well as muons can be easily detected by many types of particle detectors, such as, water-Cherenkov or detectors. The observation of a secondary shower of particles in multiple detectors at the same time is an indication that all of the particles came from that event.Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high-energy emissions by gamma-ray telescope.

These are distinguished from radioactive decay processes by their higher energies above about 10 MeV.Cosmic-ray flux. An overview of the space environment shows the relationship between the solar activity and galactic cosmic rays.The of incoming cosmic rays at the upper atmosphere is dependent on the, the, and the energy of the cosmic rays. At distances of ≈94 from the Sun, the solar wind undergoes a transition, called the, from supersonic to subsonic speeds. The region between the termination shock and the acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤ 1 GeV) by about 90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.In addition, the Earth's magnetic field acts to deflect cosmic rays from its surface, giving rise to the observation that the flux is apparently dependent on, and.The combined effects of all of the factors mentioned contribute to the flux of cosmic rays at Earth's surface.

The following table of participial frequencies reach the planet and are inferred from lower energy radiation reaching the ground. Particle energy (eV)Particle rate (m −2s −1)1 ×10 9 (GeV)1 ×10 41 ×10 12 (TeV)11 ×10 16 (10 PeV)1 ×10 −7 (a few times a year)1 ×10 20 (100 EeV)1 ×10 −15 (once a century)In the past, it was believed that the cosmic ray flux remained fairly constant over time.

However, recent research suggests one-and-a-half- to two-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.The magnitude of the energy of cosmic ray flux in interstellar space is very comparable to that of other deep space energies: cosmic ray energy density averages about one electron-volt per cubic centimetre of interstellar space, or ≈1 eV/cm 3, which is comparable to the energy density of visible starlight at 0.3 eV/cm 3, the energy density (assumed 3 microgauss) which is ≈0.25 eV/cm 3, or the (CMB) radiation energy density at ≈0.25 eV/cm 3. Detection methods.

The array of air Cherenkov telescopes.There are two main classes of detection methods. First, the direct detection of the primary cosmic rays in space or at high altitude by balloon-borne instruments. Second, the indirect detection of secondary particle, i.e., extensive air showers at higher energies. While there have been proposals and prototypes for space and ballon-borne detection of air showers, currently operating experiments for high-energy cosmic rays are ground based. Generally direct detection is more accurate than indirect detection. However the flux of cosmic rays decreases with energy, which hampers direct detection for the energy range above 1 PeV.

Both, direct and indirect detection, is realized by several techniques.Direct detection Direct detection is possible by all kinds of particle detectors at the, on satellites, or high-altitude balloons. However, there are constraints in weight and size limiting the choices of detectors.An example for the direct detection technique is a method developed by Robert Fleischer, and for use in high-altitude balloons. In this method, sheets of clear plastic, like 0.25 polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path.

The resulting plastic sheets are 'etched' or slowly dissolved in warm caustic solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves the plastic at a faster rate along the path of the ionized plastic. The net result is a conical etch pit in the plastic.

The etch pits are measured under a high-power microscope (typically 1600× oil-immersion), and the etch rate is plotted as a function of the depth in the stacked plastic.This technique yields a unique curve for each atomic nucleus from 1 to 92, allowing identification of both the charge and energy of the cosmic ray that traverses the plastic stack. The more extensive the ionization along the path, the higher the charge. In addition to its uses for cosmic-ray detection, the technique is also used to detect nuclei created as products of.Indirect detection There are several ground-based methods of detecting cosmic rays currently in use, which can be divided in two main categories: the detection of secondary particles forming extensive air showers (EAS) by various types of particle detectors, and the detection of electromagnetic radiation emitted by EAS in the atmosphere.Extensive air shower arrays made of particle detectors measure the charged particles which pass through them. EAS arrays can observe a broad area of the sky and can be active more than 90% of the time. However, they are less able to segregate background effects from cosmic rays than can air Cherenkov telescopes. Most state-of-the-art EAS arrays employ plastic.

Also water (liquid or frozen) is used as a detection medium through which particles pass and produce Cherenkov radiation to make them detectable. Therefore, several arrays use water/ice-Cherenkov detectors as alternative or in addition to scintillators.By the combination of several detectors, some EAS arrays have the capability to distinguish muons from lighter secondary particles (photons, electrons, positrons). The fraction of muons among the secondary particles in one traditional way to estimate the mass composition of the primary cosmic rays.A historic method of secondary particle detection still used for demonstration purposes involves the use of to detect the secondary muons created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory.

A fifth method, involving, can be used to detect cosmic ray particles.More recently, the devices in pervasive cameras have been proposed as a practical distributed network to detect air showers from ultra-high-energy cosmic rays (UHECRs). The first, to exploit this proposition was the CRAYFIS (Cosmic RAYs Found in Smartphones) experiment. Then, in 2017, the CREDO (Cosmic Ray Extremely Distributed Observatory) Collaboration released the first version of its completely open source app for Android devices. Since then the collaboration has attracted the interest and support of many scientific institutions, educational institutions and members of the public around the world. Future research has to show in what aspects this new technique can compete with dedicated EAS arrays.The first detection method in the second category is called the air Cherenkov telescope, designed to detect low-energy (. See also:Cosmic rays have sufficient energy to alter the states of circuit components in, causing transient errors to occur (such as corrupted data in or incorrect performance of ) often referred to as '.'

This has been a problem in at extremely high-altitude, such as in, but with becoming smaller and smaller, this is becoming an increasing concern in ground-level electronics as well. Studies by in the 1990s suggest that computers typically experience about one cosmic-ray-induced error per 256 megabytes of per month. To alleviate this problem, the has proposed a cosmic ray detector that could be integrated into future high-density, allowing the processor to repeat the last command following a cosmic-ray event.In 2008, data corruption in a flight control system caused an airliner to twice, resulting in injuries to multiple passengers and crew members. Cosmic rays were investigated among other possible causes of the data corruption, but were ultimately ruled out as being very unlikely.A high-profile recall in 2009–2010 involving vehicles with throttles that became stuck in the open position may have been caused by cosmic rays.

The connection was discussed on the 'Bit Flip' episode of the podcast Significance to aerospace travel. Main article:Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. Cosmic rays also pose a threat to electronics placed aboard outgoing probes. In 2010, a malfunction aboard the space probe was credited to a single flipped bit, probably caused by a cosmic ray. Strategies such as physical or magnetic shielding for spacecraft have been considered in order to minimize the damage to electronics and human beings caused by cosmic rays.

Comparison of radiation doses, including the amount detected on the trip from Earth to Mars by the on the (2011–2013).Flying 12 kilometres (39,000 ft) high, passengers and crews of are exposed to at least 10 times the cosmic ray dose that people at receive. Aircraft flying near the are at particular risk. Role in lightning Cosmic rays have been implicated in the triggering of electrical breakdown in. It has been proposed that essentially all lightning is triggered through a relativistic process, or ', seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through 'conventional breakdown' mechanisms. Postulated role in climate change A role for cosmic rays in climate was suggested by in 1959 and by in 1975.

It has been postulated that cosmic rays may have been responsible for major climatic change and mass-extinction in the past. According to Adrian Mellott and Mikhail Medvedev, 62-million-year cycles in biological marine populations correlate with the motion of the Earth relative to the galactic plane and increases in exposure to cosmic rays. The researchers suggest that this and bombardments deriving from local could have affected and, and might be linked to decisive alterations in the Earth's climate, and to the of the.Danish physicist has controversially argued that because modulates the cosmic ray flux on Earth, they would consequently affect the rate of cloud formation and hence be an indirect cause of. Svensmark is one of several scientists outspokenly opposed to the mainstream scientific assessment of global warming, leading to concerns that the proposition that cosmic rays are connected to global warming could be ideologically biased rather than scientifically based. Other scientists have vigorously criticized Svensmark for sloppy and inconsistent work: one example is adjustment of cloud data that understates error in lower cloud data, but not in high cloud data; another example is 'incorrect handling of the physical data' resulting in graphs that do not show the correlations they claim to show. Despite Svensmark's assertions, galactic cosmic rays have shown no statistically significant influence on changes in cloud cover, and have been demonstrated in studies to have no causal relationship to changes in global temperature.

Possible mass extinction factor.