Cosmic Rays I. Cosmic rays continually bombard the Earth. In fact, about cosmic rays pass through a person every hour! Astroparticle Course 1

Cosmic Rays I Cosmic rays continually bombard the Earth. In fact, about 100 000 cosmic rays pass through a person every hour! Astroparticle Course 1

Cosmic Rays I Cosmic rays continually bombard the Earth. In fact, about 100 000 cosmic rays pass through a person every hour! Astroparticle Course 2

Cosmic Rays I Cosmic rays continually bombard the Earth. Where do they come from? In fact, about 100 000 cosmic rays pass through a person every hour! How are they accelerated to such high energies? Astroparticle Course 3

Cosmic Rays I The discovery of cosmic rays Cosmic ray and particle physics CR deflections in magnetic field CR from the Sun Shower theory Astroparticle Course 4

Some essential bibliography Cosmic rays: A dramatic and authoritative account by Bruno Rossi Cosmic Rays and Particle Physics, 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. Astroparticle Course 5

Just before 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 positrons γ rays: turned out to be e.m. radiation Of the known radiation, the one emitted by radioactive substances had the highest energies (MeV). Cosmic ray physics had to involve much greater energies, till 10 20 ev! Astroparticle Course 6

The discovery 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. Astroparticle Course 7

Atmospheric depth 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 absorber 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. Astroparticle Course 10

New particles Colombo, searching for a new route to India, discovered America. In the same way physicists, 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 particle physics were strictly connected. The instrument which made possible these discovers is the cloud (or expansion) chamber, invented by Wilson in 1911. e + Photon conversions γ e + e Photo of α-particles emitted by radioactive source Astroparticle Course 11 e -

Cloud chamber 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 countercontrolled 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). Astroparticle Course 12

An elementary zoo: the positron Anderson, 1932 The positron in the figure is 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. Astroparticle Course 13

An elementary zoo: the positron Ion density in multiples of the density of a minimum-ionizing particle Anderson, 1932 The positron in the figure is 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. Magnetic rigidity Astroparticle Course 14

An elementary zoo: the muon Anderson & Neddermayer,, 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. Astroparticle Course 15

An elementary zoo: the muon Anderson & Neddermayer,, 1937 Photograph by Blackett and Occhialini 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. Astroparticle Course 18

An elementary zoo: the muon Anderson & Neddermayer,, 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, in figure. 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. The circle is the result of the measurement relative to the track Astroparticle Course 19

Muon decay 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. 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. Astroparticle Course 20

Muon decay 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. Kuhlenkampff The proposed µ meson enters a solution the based on cloud the fact chamber that from the above, newly discovered loses cosmic most ray of meson its energy were in unstable, with traversing a decay an time aluminum of the order of µs. plate, In then a 10 decays cm giving layer an of water, equivalent electron to track a (minimum 16000 cm ionizing track) layer of the high atmosphere air, none of the mesons will have the time to decay. Astroparticle Course 21

Nuclear emulsions 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. Grain density in multiple of the one for a minimum ionizing particle 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 range, which depends on its energy and mass. Residual Astroparticle range Course in the emulsion 22

An elementary zoo: the pion Lattes, Occhialini,, & 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. Astroparticle Course 23

An elementary zoo: the kaon Rochester & 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 proton, which appeared to decay in three particles, one of which was a π meson. Astroparticle Course 27

An elementary zoo: more and more 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 Discovery of Ω Astroparticle Course - 28

Magnetic rigidity 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. and inserting unity of measure: 2 mv = R ZeBv BR = p Ze The product BR is called magnetic rigidity. From the definition of ev it follows that: cbr relativistically correct = E( ev ) Z B( gauss) R( cm) = E( ev ) 300 Z Astroparticle Course 29

Magnetic field deflections: latitude effect 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 8 = 0.32 gauss 6.3810 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). Astroparticle Course 31

Magnetic field deflections: E-W E W effect 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 cone for positive particles Störmer cone for negative particles 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. Astroparticle Course 32

Van Allen radiation belt On November 3, 1957, USSR launched Sputnik II, and USA satellites 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. Astroparticle Course 33

Low energy CR from the Sun 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. SOHO images of the flare that occurred on the 15 July 2002 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. Astroparticle Course 34

The solar cycle 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 plasma 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. Astroparticle Course 35

A changing perspective 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. Experimental set-up by Bruno Rossi Astroparticle Course 37

The discovery of extensive air showers Extensive air showers were discovered in the 1930's by the French physicist Pierre Victor Auger. 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 atom 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 Auger Effect. 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. Astroparticle Course 40

Shower development A high-energy primary CR particle (e.g. a proton) collides with a nucleus (O, N, Ar) in the atmosphere producing other particles, mainly pions and kaons. These particles have energies high enough to produce more particles (mainly hadrons). This is called air shower (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 bremsstrahlung radiation. Eventually, π, K and other unstable particles decay into muons and neutrinos (or electrons and neutrinos), whereas low energy electrons lose energy via ionization without generating more photons. Astroparticle Course 41

Branching models Longitudinal shower 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, E c ), but lose energy by ionization: the shower particle number stops increasing and gradually goes to zero. λ=collision length After n branchings the number of particles is Simple branching model of an air shower (Heitler, 1944) N ( X ) = X / λ The energy per particle is E ( X ) = E0 / N( X ) The number of particles at maximum is Then, X max is given by Astroparticle Course 42 X 2 N X ) E / ( max = 0 max ln( E0 / = λ ln 2 Ec E c )

Particles and energy 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): First interaction point N( X ) = N max X X λ and X 0 max X 0 X 0 X max free X λ 0 parameters X exp X λ max Slant depth Slant depth X = X vert sinα The primary energy is given by the track length integral plus the energy carried away by neutrinos: E = α 0 dx N( X ) + E ν 0 α = energy loss per Astroparticle unit length Course per particle 43

Shower characteristics 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 E primary =3 10 20 ev are reduced with respect to a hadronic 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 X max : proton FI: 70 g/cm 2 Fe FI: 15 g/cm 2 at PeV energies X max λ ln[ E 0 )] Astroparticle Course 44 /( AE c

Elongation rate The average of X max is related to the primary energy. For the simple Heitler branching model, for example: ln( E0 / Ec ) X max = λ X ln E0 + a = X ln10 log E + a = 2.3 X log E + ln 2 a The elongation rate is the increase of X max per decade of energy ER dx max = = 2. d log E 3 X The elongation rate is different for different primaries and can be used for obtaining information on the composition of cosmic rays. Astroparticle Course 45

Shower distributions The evolution of a shower is of statistical nature, 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. Three component: e.m., muonic, and hadronic Longitudinal shower distribution Lateral shower distribution Astroparticle Course 46

Fluorescence and Čerenkov light 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. Astroparticle Course 47

Neutrinos as universe messengers 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. Astroparticle Course 48