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CosmicRaysICosmicRaysI

Cosmic rays continually bombard the Earth.

In fact, about 100 000 cosmic rays pass through a person every hour!

AstroparticleCourse 1 CosmicRaysICosmicRaysI

Cosmic rays continually bombard the Earth.

In fact, about 100 000 cosmic rays pass through a person every hour!

AstroparticleCourse 2 CosmicRaysICosmicRaysI

Cosmic rays continually bombardWhere the Earth. do they come from?

In fact,How about are 100 they 000 accelerated to cosmic rayssuch high pass energies? through a person every hour!

AstroparticleCourse 3 CosmicRaysICosmicRaysI

 ThediscoveryofThediscoveryof cosmicrayscosmicrays  CosmicrayandparticleCosmicrayandparticle physicsphysics  CRdeflectionsinCRdeflectionsin magneticfieldmagneticfield  CRfromtheSunCRfromtheSun  ShowertheoryShowertheory

AstroparticleCourse 4 SomeessentialbibliographySomeessentialbibliography

• Cosmic rays: A dramatic and authoritative account by • Cosmic Rays and Particle , Thomas K. Gaisser • Origin and propagation of Extremely High Energy Cosmic Rays , P. Bhattacharjee & G. Sigl, Phys. Rept. 327 (2000) 109. • Observation and implications of the ultrahigh-energy cosmic rays , M. Nagano & A.A. Watson, Rev. Mod. Phys. 72 (2000) 689.

AstroparticleCourse 5 JustbeforeJustbefore ……

When scientists first started studying radiation in the early 1900s, they found 3 different types of rays: • α rays: turned out to be Helium nuclei • β rays: turned out to be electrons and • γ rays: turned out to be e.m. radiation Of the known radiation, the one emitted by radioactive substances had the highest energies (MeV). physics had to involve much greater energies, till 10 20 eV!

AstroparticleCourse 6 ThediscoveryThediscovery

“At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi

Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 7 ThediscoveryThediscovery

“At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi

Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 8 ThediscoveryThediscovery

“At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…” from Cosmic rays , Bruno Rossi

Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!). “The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912 Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation. AstroparticleCourse 9 AtmosphericdepthAtmosphericdepth

When comparing radiation absorbers of different substances, it becomes necessary to consider the density as well as the thickness of the absorber. Thus, it is customary to define an ab- sorber not by its geometrical thickness, but by the mass of a column of unit cross sectional area. This quantity – the mass per unit area – is usually measured in grams per square centimeters (g/cm 2). For an absorber of constant density, the mass per unit area is just the product of its thickness and its density: so, it’s like a length which takes into account the density . The mass per unit area of the atmosphere above a given level is known as atmospheric depth .

AstroparticleCourse 10 NewparticlesNewparticles

Colombo, searching for a new route to India, discovered America. In the same way , searching for a solution to the cosmic ray puzzle, discovered a zoo of new particles, opening an entirely new field of research: at the beginning, cosmic ray physics and elementary were strictly connected. The instrument which made possible these discovers is the cloud (or expansion ) chamber , invented by Wilson in 1911. conversions γ→e+ e− e+

e- Photoof αparticles emittedbyradioactive source AstroparticleCourse 11 CloudchamberCloudchamber

The cloud (or expansion ) chamber was invented by Wilson in 1911. The expansion of the gas in the chamber causes condensation around the ions present, producing a visible track along the trajectory of a charged particle. However, to be detected, the particle must traverse the chamber at some time during the so-called expansion phase: so the chamber, in its early version, was sensitive for a period of about 0.01 second at each expansion. A major technical achievement was the counter- controlled chamber, which was triggered by Geiger- Müller counters when they were hitted by a CR particle (Blackett & Occhialini, 1932). For a given velocity, the density of ions per unit length increases with increasing charge of the initial particle. For a given charge, it decreases with increasing velocity. The ion trail of smallest possible density is one left by a singly charged particle moving at nearly the velocity of light ( minimum-ionizing particle ). AstroparticleCourse 12 Anelementaryzoo:thepositronAnelementaryzoo:the

 Anderson,1932Anderson,1932

The positron in the figure is identified as the particle that enters the from below and curves sharply to the left after traversing the lead plate. At first Anderson thought the positive particles were . But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one.

AstroparticleCourse 13 Anelementaryzoo:thepositronAnelementaryzoo:thepositron

Iondensityin multiplesofthe Anderson,1932Anderson,1932 densityofa minimumionizing The positron in the figure is particle identified as the particle that enters the cloud chamber from below and curves sharply to the left after traversing the lead plate. At first Anderson thought the positive particles were protons. But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one.

Magneticrigidity AstroparticleCourse 14 Anelementaryzoo:theAnelementaryzoo:the muonmuon

Anderson&Anderson& NeddermayerNeddermayer ,1937,1937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays.

AstroparticleCourse 15 Anelementaryzoo:theAnelementaryzoo:the muonmuon

Anderson&Anderson& NeddermayerNeddermayer ,1937,1937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays.

AstroparticleCourse 16 Anelementaryzoo:theAnelementaryzoo:the muonmuon

Anderson&Anderson& NeddermayerNeddermayer ,1937,1937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays. Electronenergylosses AstroparticleCourse 17 Anelementaryzoo:theAnelementaryzoo:the muonmuon

Anderson&Anderson& NeddermayerNeddermayer ,1937,1937 Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+- e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the PhotographbyBlackett interaction of primary cosmic rays. andOcchialini

AstroparticleCourse 18 Anelementaryzoo:theAnelementaryzoo:the muonmuon

Anderson&Anderson& NeddermayerNeddermayer ,1937,1937 Physicists observed that cosmic rays contained a soft and hard component; the particlesThecircleisthe of the latter could penetrate asresultofthe much as 1 m of lead. They couldmeasurement not be e+- e-, since their estimatedrelativetothetrack energy should have been absurd,infigure. and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays.

AstroparticleCourse 19 MuonMuon decaydecay

In measuring the numbers of CR at various altitudes in the atmosphere, physicists found a very puzzling result: contrary to the earlier findings of Millikan, it looked as if air absorbed CR more effectively than solid or liquid matter. Moreover, the low density air at very high altitudes appeared to be a better absorber then the denser layer in the lower atmosphere. The German H. Kuhlenkampff proposed a solution based on the fact that the newly discovered cosmic ray meson were unstable, with a decay time of the order of µs. In a 10 cm layer of water, equivalent to a 16000 cm layer of the high atmosphere air, none of the mesons will have the time to decay.

AstroparticleCourse 20 MuonMuon decaydecay

In measuring the numbers of CR at various altitudes in the atmosphere, physicists found a very puzzling result: contrary to the earlier findings of Millikan, it looked as if air absorbed CR more effectively than solid or liquid matter. Moreover, the low density air at very high altitudes appeared to be a better absorber then the denser layer in the lower atmosphere. The German physicist H. KuhlenkampffThe proposedµ mesonentersthe a solution based on cloudchamberfromabove, the fact that the newly discovered losesmostofitsenergyincosmic ray meson were unstable, withtraversinganaluminum a decay time of the order of µplate,thendecaysgivingans. In a 10 cm layer of water, equivalentelectrontrack(minimum to a 16000 cm layer of the highionizingtrack) atmosphere air, none of the mesons will have the time to decay.

AstroparticleCourse 21 NuclearemulsionsNuclearemulsions

The cloud chamber has inherent limitations: because of the low density of the gas, very few of the particles entering it collide with nuclei or stop inside the chamber. In the middle 1940s, physicists succeeded in perfecting the nuclear emulsion technique (Powell&Occhialini). Ionizing particle “sensitize” the grains of silver bromide that they encounter along their path. An appropriate “developer” solution will then reduce the sensitized grains to silver, in such a way that, under a microscope, the trajectories of ionizing particles appear as rows of dark grains. The density of the silver grains along the track is proportional to the density of ion pairs that the particle would produce in a gas, and decreases with increasing velocity. If the particle stops in the emulsion, it is possible to measure its Graindensityin range, which depends on its multipleoftheone energy and mass. foraminimum ResidualrangeintheemulsionAstroparticleCourse 22 ionizingparticle Anelementaryzoo:theAnelementaryzoo:the pionpion

Lattes,Lattes, OcchialiniOcchialini ,&Powell,1947,&Powell,1947 In 1935, H. Yukawa had postulated the existence of a subatomic particle associated with the nuclear forces, like the photon was associated to the e.m. ones. Physicists thought that the µ meson was such a particle. Next, Tomonaga and Araki pointed out that positive and negative µ mesons should behave differently after coming at rest in matter. But, the results of an experiment made by Conversi, Pancini and Piccioni, using a magnetic lens , and different materials (lead, carbon, magnesium), showed that in light elements negative mesons could escape nuclear capture. Only later, Lattes, Occhialini and Powell identified the π meson in emulsions. AstroparticleCourse 23 Anelementaryzoo:theAnelementaryzoo:the pionpion

Lattes,Lattes, OcchialiniOcchialini ,&Powell,1947,&Powell,1947 In 1935, H. Yukawa had postulated the existence of a subatomic particle associated with the nuclear forces, like the photon was associated to the e.m. ones. Physicists thought that the µ meson was such a particle. Next, Tomonaga and Araki pointed out that positive and negative µ mesons should behave differently after coming at rest in matter. But, the results of an experiment made by Conversi, Pancini and Piccioni, using a magnetic lens , and different materials (lead, carbon, magnesium), showed that in light elements negative mesons could escape nuclear capture. Only later, Lattes, Occhialini and Powell identified the π meson in emulsions. AstroparticleCourse 24 Anelementaryzoo:theAnelementaryzoo:the pionpion

Lattes,Lattes, OcchialiniOcchialini ,&Powell,1947,&Powell,1947 In 1935, H. Yukawa had postulated the existence of a subatomic particle associated with the nuclear forces, like the photon was associated to the e.m. ones. Physicists thought that the µ meson was such a particle. Next, Tomonaga and Araki pointed out that positive and negative µ mesons should behave differently after coming at rest in matter. But, the results of an experiment made by Conversi, Pancini and Piccioni, using a magnetic lens , and different materials (lead, carbon, magnesium), showed that in light elements negative mesons could escape nuclear capture. Only later, Lattes, Occhialini and Powell identified the π meson in emulsions. AstroparticleCourse 25 Anelementaryzoo:theAnelementaryzoo:the pionpion

Lattes,Lattes, OcchialiniOcchialini ,&Powell,1947,&Powell,1947 In 1935, H. Yukawa had postulated the existence of a subatomic particle associated with the nuclear forces, like the photon was associated to the e.m. ones. Physicists thought that the µ meson was such a particle. Next, Tomonaga and Araki pointed out that positive and negative µ mesons should behave differently after coming at rest in matter. But, the results of an experiment made by Conversi, Pancini and Piccioni, using a magnetic lens , and different materials (lead, carbon, magnesium), showed that in light elements negative mesons could escape nuclear capture. Only later, Lattes, Occhialini and Powell identified the π meson in emulsions. AstroparticleCourse 26 Anelementaryzoo:theAnelementaryzoo:the kaonkaon

Rochester&Butler,1947Rochester&Butler,1947 Just a few months after the discovery of the π meson, Rochester and Butler published two cloud-chamber photographs. Neither the neutral particle invoked to explain the first event, nor the charged particle in the second could possibly be identified as any known particle. Two years later, Powell’s group found in nuclear emulsion a particle, with mass intermediate between that of a π meson and a , which appeared to decay in three particles, one of which was a π meson. AstroparticleCourse 27 Anelementaryzoo:moreandmoreAnelementaryzoo:moreandmore ……

For a while there was a great deal of confusion about the number and properties of the particles required to explain all the experimental data. Then a classification was made in mesons, baryons, and leptons.

π+ p

Discoveryof Ω AstroparticleCourse 28 MagneticrigidityMagneticrigidity

A moving charged particle in a magnetic field experiences a deflecting force. The radius, R, of the circle described in a uniform field (Larmor radius) is obtained from the condition that the centrifugal force and the Lorentz force must balance. relativistically correct mv 2 p = ZeBv BR = R Ze The product BR is called magnetic rigidity. From the definition of eV it follows that: E(eV ) cBR = Z and inserting unity of measure: E(eV ) B(gauss )R(cm ) = 300 Z

AstroparticleCourse 29 MagneticrigidityMagneticrigidity

A moving charged particle in a magnetic field experiences a deflecting force. The radius, R, of the circle described in a uniform field (Larmor radius) is obtained from the condition that the centrifugal force and the Lorentz force must balance. relativistically correct mv 2 p = ZeBv BR = R Ze The product BR is called magnetic rigidity. From the definition of eV it follows that: E(eV ) cBR = Z and inserting unity of measure: E(eV ) B(gauss )R(cm ) = 300 Z

AstroparticleCourse 30 Magneticfielddeflections:latitudeeffectMagneticfielddeflections:latitudeeffect

In 1930 the notions about the possible effects of the earth’s magnetic field upon cosmic rays were still rather nebulous. Consider a particle that circles the earth at the geomagnetic equator: it has to move from east to west if it is positive and on the contrary if it is negative. The product BR , known as magnetic rigidity of the particle, has to be BR = 32.0 gauss ⋅ 38.6 10 8 cm = 2⋅10 8 gauss cm

which correspond to an energy of about 60 GeV. This means that charged particles with energies of this order or less must be strongly deflected by the earth’s magnetic field at the geomagnetic equator, and CR should somehow be channeled toward the poles ( latitude effect ). AstroparticleCourse 31 Magneticfielddeflections:EMagneticfielddeflections:E WeffectWeffect Then, the Norwegian physicist Carl Störmer computed the trajectories of particles with different magnetic rigidities approaching the earth, and distinguished them in allowed (a) and forbidden (b) ones.

Störmer conefor positiveparticles

Störmer conefor negativeparticles

He found that there existed a special class of trajectories, called bounded ones, with the property of remaining forever in the vicinity of the earth. For each point on the earth, there exists a Störmer cone with the axis pointing to the East (West), which contains the bounded (so forbidden) directions for positive (negative) CR. AstroparticleCourse 32 VanAllenradiationbeltVanAllenradiationbelt

On November 3, 1957, USSR launched Sputnik II, and USA Explorer I and III followed on February 1 and March 26, 1958. At every revolution, Explorer I and III swung from several hundred km to several thousands km. Above 2000 km, the counters, installed aboard by the CR group under J. Van Allen, apparently stopped working and started again at lower altitudes. The only explanation was that they become “jammed” when were exposed to a radiation of excessive strength. It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of albedo neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons that are injected from the geomagnetic tail following geomagnetic storms. AstroparticleCourse 33 LowenergyCRfromtheSunLowenergyCRfromtheSun

When systematic measurements were undertaken at altitudes and latitudes where primary CR particles of lower energy could also be observed, it became apparent that the low-energy portion of the cosmic radiation had to do primarily with events in the sun. SOHOimagesofthe flarethatoccurred onthe15July2002 The CR particles from these events, recorded at earth, have energies of the order of tens of GeV, since the effect is usually much smaller near the geomagnetic equator than at high latitudes. The same conclusion is indicated by the fact that neutron detectors record a much greater increase than µ detectors, since µ leptons are produced abundantly only by protons with greater energies. AstroparticleCourse 34 ThesolarcycleThesolarcycle 1995

The general pattern of solar activity follows an 11-year cycle. When cosmic ray observations began to accumulate, it was found that the flux of cosmic rays also changes systematically during this cycle. 1991

In the plot it is reported the intensity of CR measured at a geomagnetic latitude of 88° N by H.V. Neher of CalTech in 1954 and 1958. At the highest altitude, the intensity doubles. The interpretation of these data is that the emitted by the Sun carries materially away with it the magnetic field, which acts as a partial screen against CR particles entering the solar system from the outside.

AstroparticleCourse 35 AchangingperspectiveAchangingperspective

During several years physicist belief on cosmic rays changed continuously. At the beginning, Millikan though that they were mainly and resulted from the synthesis of heavy elements like nitrogen, oxygen or silicon. But the coincidences observed by Bothe and Kohlhörster in Geiger-Müller counters were difficult to explain by double Compton effects. The observations seemed to indicate that the primary cosmic radiation consisted of charged particles.

AstroparticleCourse 36 AchangingperspectiveAchangingperspective

During several years physicist belief on cosmic rays changed continuously. At the beginning, Millikan though that they were mainly photons and resulted from the synthesis of heavy elements like nitrogen, oxygen or silicon. But the coincidences observed by Bothe and Kohlhörster in Geiger-Müller counters were difficult to explain by double Compton effects. The observations seemed to indicate that the primary cosmic radiation consisted of charged particles.

Then, other experiments showed that high-energy cosmic rays occasionally produced secondary ionizing particles in the matter of the shields. It soon became clear that this was not inusual, but a characteristic of these particles, which arrived to earth in showers .

Experimentalsetup byBrunoRossi AstroparticleCourse 37 AchangingperspectiveAchangingperspective

During several years physicist belief on cosmic rays changed continuously. At the beginning, Millikan though that they were mainly photons and resulted from the synthesis of heavy elements like nitrogen, oxygen or silicon. But the coincidences observed by Bothe and Kohlhörster in Geiger-Müller counters were difficult to explain by double Compton effects. The observations seemed to indicate that the primary cosmic radiation consisted of charged particles.

Then, other experiments showed that high-energy cosmic rays occasionally produced secondary ionizing particles in the matter of the shields. It soon became clear that this was not inusual, but a characteristic of these particles, which arrived to earth in showers .

Experimentalsetup PhotographbyBlackett byBrunoRossi AstroparticleCourseandOcchialini 38 AchangingperspectiveAchangingperspective

During several years physicist belief on cosmic rays changed continuously. At the beginning, Millikan though that they were mainly photons and resulted from the synthesis of heavy elements like nitrogen, oxygen or silicon. But the coincidences observed by Bothe and Kohlhörster in Geiger-Müller counters were difficult to explain by double Compton effects. The observations seemed to indicate that the primary cosmic radiation consisted of charged particles.

Then, other experiments showed that high-energy cosmic rays occasionally produced secondary ionizing particles in the matter of the shields. It soon became clear that this was not inusual, but a characteristic of these particles, which arrived to earth in showers .

PhotographbytheMIT Experimentalsetup cosmicraygroup byBrunoRossi AstroparticleCourse 39 ThediscoveryofextensiveairshowersThediscoveryofextensiveairshowers

Extensive air showers were discovered in the 1930's by the French physicist . In addition to his contributions to the field of cosmic rays, Pierre Auger was most well known for his discovery in the 1920's of a spontaneous process by which an with a vacancy in the K- shell achieves a more stable state by the emission of an electron instead of an X-ray photon, commonly known as the . After physicists began to experiment with coincidences, it became a common practice to test the operation of the equipment by placing the counters out of line, usually on a horizontal plane. Several experimenters noticed that the number of coincidences recorded was too large to be accounted for entirely by chance. In 1938 Pierre Auger and collaborators undertook a systematic study that established beyond any doubt the occurrence of air showers and provided preliminary information about their properties. AstroparticleCourse 40 ShowerdevelopmentShowerdevelopment

A high-energy primary CR particle (e.g. a proton) collides with a nucleus (O, N, Ar) in the atmosphere producing other particles, mainly and . These particles have energies high enough to produce more particles (mainly hadrons). This is called (or hadronic cascade). At very high energies this is an Extensive Air Shower (EAS).

Neutral pions quickly decay into two photons, which start electromagnetic cascade. Photons produce e+e--pairs, which generate photons in their turn via radiation. Eventually, π, K and other unstable particles decay into and neutrinos (or electrons and neutrinos), whereas low energy electrons lose energy via without generating more photons.

AstroparticleCourse 41 Longitudinal shower BranchingmodelsBranchingmodels distribution As a result, at first the particles increase in number while their energy decreases. Eventually, as the original energy is shared among more and more particles, individual particles have so little energy that they no longer produce new particles

(they arrive to the so called critical energy , Ec), but lose energy by ionization: the shower particle number stops increasing and gradually goes to zero. λ=collisionlength After n branchings the N(X ) = 2 X / λ number of particles is

The energy per particle is E(X ) = E0 / N(X ) The number of particles at maximum is N(X max ) = E0 / Ec ln( E / E ) Simplebranchingmodel 0 c Then, Xmax is given by X max = λ ofanairshower ln 2 (Heitler,1944) AstroparticleCourse 42 ParticlesandenergyParticlesandenergy

The growth and decline of the number of charged particles of a shower can be defined using various mathematical models. One of these is the Gaisser- Hillas profile (1977): Slantdepth First interaction λ and X 0 free parameters point X − X max 0 X vert  X − X  λ  X − X  X =  0  max sin α N(X ) = Nmax   exp    X max − X 0   λ 

The primary energy is given by the ∞ track length integral plus the energy E0 = α ∫ dX N(X ) + Eν carried away by neutrinos: 0

α =energylossperunitlengthperparticleAstroparticleCourse 43 ShowercharacteristicsShowercharacteristics

Proton induced showers have larger fluctuations than iron or photon induced ones, and the average depth of the shower maximum is intermediate between them. The first thing is due to the fact that a heavy primary like an iron nucleus is viewed as a collection of independent nucleons (superposition model ) and the result of collision is similar to an average on its constituents. On the other side, a photon primary produces an e.m. shower, where the fluctuations 20 are reduced with respect to a hadronic Eprimary =310 eV shower. The second feature depends on the fact that the interactions probabilities of the nucleons in the superposition model add, leading to a faster development of the shower and a

somehow different formula for Xmax : X ∝ λ ln[ E /( AE )] protonFI:70g/cm 2 max 0 c 2 FeFI:15g/cm AstroparticleCourse 44 atPeV energies ElongationrateElongationrate

The average of Xmax is related to the primary energy. For the simple Heitler branching model, for example: ln( E / E ) X = λ 0 c ≡ X ln E + a = X ln 10 log E + a = 3.2 X log E + a max ln 2 0

The elongation rate is the increa- se of Xmax per decade of energy dX ER = max = 3.2 X d log E

The elongation rate is different for different primaries and can be used for obtaining information on the composition of cosmic rays. AstroparticleCourse 45 Threecomponent:e.m.,muonic,andhadronic ShowerdistributionsShowerdistributions

The evolution of a shower is of statistical , since the exact point where a given photon materializes or a given electron radiates, or how the energy is shared between the two particles produced in a single event, is a matter of chance. One may, however, inquire into the average behavior of showers.

Longitudinalshower distribution

Lateralshower distribution AstroparticleCourse 46 FluorescenceandFluorescenceand ČČerenkoverenkov lightlight

A possible source of radiation, practically isotropic, from an air shower is the excitation of air nitrogen by the charged particles, mainly electrons (more correctly, it is scintillation light). First used by the experiment Fly’s Eye.

Moreover, as Blackett first realized in 1948, charged particles that travel faster than light in the atmosphere emit detectable Čerenkov radiation on a narrow cone around the direction of the particle. The opening angle is a function of the density of the air and, thus, of the height of emission. AstroparticleCourse 47 NeutrinosasuniversemessengersNeutrinosasuniversemessengers

High energy neutrino astronomy is one of the most promising research line in astroparticle physics. Similarly to photons and unlike charged cosmic rays, they keep directional information which can be used to perform astronomy. Differently from gamma rays, they are emitted only in hadronic processes and travel unimpeded to the Earth.

Vertical neutrino induced showers cannot be distinguished from ordinary CR showers. But in very inclined showers it is possible to identify different features for the different primaries.

AstroparticleCourse 48