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The beginning and the end of cosmic rays

New studies are expanding our understanding of the possible origins of these subatomic particles, which are accelerated to nearly the speed of light, traverse intergalactic space and break up when they collide with other particles upon reaching the Earth

DRüMDetails about the formation and behavior of cosmic rays—particles that reach the Earth at nearly the speed of light and collide with molecules of nitrogen and oxygen in the Earth’s atmosphere, creating trillions of new particles—have been presented in two recent studies. One of the papers, by researchers from the University of São Paulo (USP) and the United States, showed that cosmic rays could be formed as a result of the encounter and annihilation of magnetic fields of opposite polarity in atmospheres of stars and of compact cosmic objects, such as black holes of star masses or active galactic nuclei. The researchers responsible for this study believe that this mechanism offers an alternative to the most widely accepted model of cosmic ray formation and could explain the extragalactic, though uncertain, origins of those with the highest energy.

The other study—from the Pierre Auger Observatory team with contributing physicists from the universities of São Paulo, Rio de Janeiro and Bahia—analyzes collisions of high-energy cosmic rays with atomic nuclei in the atmosphere and presents the area of interaction between cosmic rays having energies of 1018 to 1018.5 eV (electron volts) and atmospheric atomic nuclei. At these energy levels, the area of interaction—or shock cross section—of these particles is 5.05 x 10-29 square meters (the number zero followed by a decimal point and then 28 zeros before the number 505). “No other experiment had done this measurement of the proton-air shock cross section or of the proton-proton shock cross section at these very high energies,” says Carola Dobrigkeit Chinellato, a researcher at the Physics Institute of the State University of Campinas (Unicamp) and coordinator of the São Paulo team at the Pierre Auger Observatory.

Built between 2000 and 2008 on a semi-desert plain at the foot of the Andes on the outskirts of the town of Malargüe, south of Mendoza, Argentina, the Pierre Auger Observatory is the result of an international collaboration that today numbers about 500 physicists from 18 countries. It is the largest cosmic ray observatory in operation, encompassing 1,660 surface detectors in the form of instrumented cylindrical tanks that measure 3.7 meters in diameter and 1.2 meters in height, spaced 1.5 kilometers apart in a triangular grid. Scattered over an area of 3,300 square kilometers—double the size of the city of São Paulo—the surface detectors operate in combination with 27 fluorescence telescopes—the so-called Fly’s Eyes—that can register the faint light emitted by nitrogen molecules in the upper atmosphere when they are excited by particles from the shower initiated by a cosmic ray that has reached the Earth. Readers of this magazine have followed the progress of the construction of the Pierre Auger Observatory since the August 2000 Pesquisa FAPESP cover story about the behind-the-scenes negotiations.

Cosmic rays were discovered 100 years ago by Austrian physicist and 1936 Nobel Prize winner Victor Hess. The two recent studies have dispelled some of the mystery surrounding the behavior of these particles, although their composition remains uncertain. There are indications that cosmic rays in the energy range up to 1018.5 eV are likely to be protons, while the highest-energy rays may be nuclei of heavy chemical elements such as iron.

Magnetic fields
In the Milky Way, explosions known as supernovas, which signal the end of massive stars, release sufficient energy to explain the formation of low- and high-energy cosmic rays, while the higher-energy rays are believed to be the result of more-distant objects such as active galactic nuclei, explains Elisabete de Gouveia Dal Pino of the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of USP. According to de Gouveia Dal Pino, the protons that form the gas in the interstellar medium could be accelerated to nearly the speed of light and achieve cosmic ray status when they collide with the so-called shock waves that form in supernova explosions and cause abrupt changes in speed, pressure and temperature in nearby regions, like those caused by a passing jet plane or the explosion of an atomic bomb.

020_Raios cosmicos_200-1NasaPhysicists believe that another source of cosmic rays may be the shock waves generated by the impact of the ends of the beams of matter called jets that are emitted by active galactic nuclei into the atmosphere. The problem is that the ends of active galactic nuclei jets may be insufficient to generate particles with energy above 1018 eV. “Cosmic rays have to be able to leave the confinement generated by magnetic fields without losing much energy due to the interaction with the photons in the medium where they were generated,” says de Gouveia Dal Pino. “Another problem, which was encountered in more recent observations of gamma radiation from active galactic nuclei, is that the cosmic rays responsible for that emission are produced in ultracompact regions where shocks are apparently nonexistent.”

De Gouveia Dal Pino and Alexander Lazarian of the University of Wisconsin in the U.S. looked for other mechanisms of formation of highest-energy cosmic rays, and in 2005 they presented a theoretical proposal that expanded their possible sources. Now, through so-called magnetohydrodynamic numerical simulations presented in June of 2012 in the journal Physical Review Letters, Grzegorz Kowal, a Polish astrophysicist who has worked at IAG since 2009, de Gouveia Dal Pino and Lazarian have confirmed the hypotheses of the 2005 article and have shown that cosmic rays could be formed in the magnetized atmospheres, also called coronas, that surround black holes and their accretion disks.

“The idea is simple,” de Gouveia Dal Pino asserts. “As a result of the rapid encounter between magnetic field lines of opposite polarity, the magnetic energy released is able to accelerate low-energy particles to relativistic velocities. The process is very similar to what happens with thermal particles in shock waves. When they are imprisoned between two magnetic field lines of opposite polarity, they collide several times with magnetic fluctuations, gradually gaining energy from these collisions until they reach nearly the speed of light and finally escape from that acceleration region as cosmic rays.”

According to de Gouveia Dal Pino, this proposal was inspired by the intense magnetic activity of the sun. Bent tubes of magnetic field lines called loops, which extend about 10,000 kilometers, frequently emerge on the surface of the sun, forming the so-called solar corona. These loops can have positive or negative polarity, like the magnetic lines of the Earth. When they collide, loops of opposite polarity release energy, produce heat and accelerate any protons nearby, converting them into cosmic rays. De Gouveia Dal Pino says that this process may be the source of many low-energy cosmic rays of up to 1010 eV that reach the Earth.

Gouveia Dal Pino, Lazarian and Kowal concluded that when magnetic fields of opposite polarity are surrounded by intermittent movements called turbulence, they can meet and annihilate one another quickly, which accelerates nearby low-energy protons and turns them into cosmic rays, also in the coronas of magnetized gas close to black holes or stars—or, in general, “in highly magnetized compact regions,” de Gouveia Dal Pino says. In these regions, which can extend hundreds of thousands of kilometers, the protons can gain 10 million times their energy in about a thousand hours (41 days) as they collide with the magnetic fields, according to this study.

020_Raios cosmicos_200-2_novoCERNThe researchers found another possibility that further expands the possible sources of cosmic rays. According to this study, cosmic rays could also be formed, although with less energy gain, in interstellar gas or in the intergalactic medium, which are turbulent and magnetized. According to de Gouveia Dal Pino, the effects of turbulence could cause magnetized regions of gas to meet and annihilate one another, thus transferring energy to nearby particles. The next phase of the study is to combine these findings with physical mechanisms of cosmic ray energy losses and examine observations from telescopes that can confirm or correct these hypotheses.

“We need to see what the predominant mechanism of formation of ultra-high-energy cosmic rays is,” she says. So far the sources of the highest-energy particles have been limited to shock waves in the jets of active galaxies. While shock waves from supernova explosions appear to be the principal mechanism for producing cosmic rays in our galaxy with energies of up to 1016-1017 eV and the sun appears to be one of the main sources of lower energy (109-1010 eV), she says, “the sources of highest-energy cosmic rays remain a mystery, and the mechanism of magnetic reconnection looks like an attractive new possibility.”

Other encounters
The other study also looks at collisions of highest-energy cosmic rays examined at the Pierre Auger Observatory in Argentina. When a highest-energy cosmic ray enters the atmosphere and collides with particles there, new particles are produced. The new particles in turn will continue to propagate in the atmosphere and may also undergo new collisions and generate new particles.

The cascade continues for as long as the particles in the shower have enough energy to produce other particles. “When the particles no longer have sufficient energy, the number of particles in the shower will have reached its maximum, and from then on the number can only diminish,” says Carola Chinellato of Unicamp. The energy from the original cosmic ray will be distributed among that huge number of particles created, she explains. Therefore, if a trillion particles were ultimately produced, the energy of each particle would be approximately one-trillionth of the energy of the original cosmic ray.

Recent measurements at the Pierre Auger Observatory have made it possible for the first time to give a detailed account of the interactions between particles at energies heretofore not achieved in particle accelerators. In a paper published in August in Physical Review Letters, the team at the observatory examined collisions of 11,628 cosmic rays with energy of 1018 to 1018.5 eV with nuclei of nitrogen or oxygen in the atmosphere, recorded between December 2004 and September 2010. According to Chinellato, earlier findings from the Pierre Auger Observatory had indicated that in this energy range, the cosmic particles that reach the Earth must be protons.

In an analysis of the altitudes at which the showers penetrating the farthest into the atmosphere had the greatest number of particles, the researchers determined the inelastic cross section—a fundamental physical number that measures the probability of interaction of one particle with another—in collisions of protons with nuclei of air. For a proton colliding with nuclei of air, the area of interaction is 5.05 x 10-29 square meters. “The larger the cross section, the greater the probability of a collision,” she says. In fact, things are not so simple in the world of particles. “The particles need not be touching one another in order to interact.”

Cassiopeia, remnant of a supernova: Now, the acceleration of protons is probably the result of the shock wave created by movements in the outer layer of gas

NASACassiopeia, remnant of a supernova: Now, the acceleration of protons is probably the result of the shock wave created by movements in the outer layer of gasNASA

“There is no contact between the particles,” notes Marcio Menon, also a researcher at Unicamp. Physicists believe that the particles are probably components of protons called gluons that jump towards other particles, passing along information on speed and modifying their behavior. Menon used the values obtained by the team at the Pierre Auger Observatory to compare with values measured in other experiments and to propose adjustments in the mathematical formulas that govern cross-sectional variations among elementary particles.

Measurement of the shock cross section of collisions between protons and atmospheric nuclei obtained by telescopes at the Pierre Auger Observatory is also providing input for estimating the behavior of encounters between protons induced in the tunnels of the Large Hadron Collider (LHC) in Geneva, Switzerland. The observatory in Argentina and the LHC were built, each in its own way, to expand our understanding of the properties of elementary particles. The Pierre Auger Observatory team is working with natural collisions of particles with energies one million times greater than the highest energies achieved so far at the LHC, but cosmic rays collide with other particles of air that are virtually still, while in the tunnels of the LHC there are two beams of greatly accelerated protons that meet in head-on collisions. According to Chinellato, in this energy range, the total energy from the collision of a proton from cosmic rays with a nucleus of air is only about eight times greater than the energy produced in a collision between two protons in the LHC.

Collisions between protons
On the basis of the results obtained from measuring inelastic proton-air cross sections, the researchers at the Pierre Auger Observatory calculated the total shock cross section in proton-proton collisions and concluded that the area of interaction between particles continues to increase as the energy increases. According to Chinellato, that increase had already been observed at much lower energies 40 years ago, also at the European Organization for Nuclear Research (CERN), and more indirectly in experiments involving cosmic rays. “Surprisingly,” she says, “the results indicated that the protons became larger and more opaque as their energy increased.”

The LHC currently in operation at CERN presents a new opportunity to continue studying the behavior of the proton-proton shock cross section in experiments conducted using accelerators, now at higher energies of around 7 x 1012 eV, nearly 100 times greater than the energies obtained 40 years ago. The initial results obtained in 2011 with the Totem experiment at CERN, also involving proton-proton collisions, confirmed that the size of the protons continues to increase with increasing energies, and consequently, that the total shock cross section continues to grow. According to Chinellato, the researchers in the Totem experiment measured the shock cross section in elastic proton-proton collisions, and on that basis, they estimated the total proton-proton shock cross section using a theoretical model. The published value is 9.83 x 10-30 square meters for the total energy from a collision at 7 x 1012 eV, which she compares with the value of the total shock cross section in the proton-proton collision obtained by the Pierre Auger Observatory researchers, that is, 1.33 x 10-29 square meters, at even higher energies of 5.7 x 1013 eV. “The protons become increasingly large and more opaque at these energies,” Chinellato says.

“In essence,” she points out, “what we are studying at the LHC and  Auger is very similar to what Rutherford was studying at the beginning of the last century.” In England in 1911, physicist Ernest Rutherford conducted a series of experiments in which he fired positively-charged alpha particles against a gold plate and concluded that the atom was made up of a minuscule nucleus surrounded by a much larger region in which the electrons circulated. “The difference is that the scale of energy is much higher and the experiments are much more interesting and more complicated. And it is fantastic that the Pierre Auger Observatory is able to measure such a fundamental number based on the observation of atmospheric showers.”

Investigation of high-energy phenomena and astrophysical plasmas: Theory, observation and numerical simulations (nº 06/50654-3); Grant mechanism Thematic project; Coordinator Elisabete Maria de Gouveia Dal Pino – IAG/USP; Investmen: R$366,429.60 (FAPESP).
2. Magnetic reconnection and acceleration of particles in astrophysical sources and diffuse media (nº 09/50053-8); Grant mechanism Post-doctoral research grant; Coordinator Elisabete Maria de Gouveia Dal Pino – IAG/USP; Grant recipient Grzegorz Kowal – IAG/USP; Investment R$241,582.45 (FAPESP).
3. Study of highest-energy cosmic rays at the Pierre Auger Observatory (nº 10/07359-6); Grant mechanism Thematic project; Coordinator Carola Dobrigkeit Chinellato – IFGW/Unicamp; Investment R$ 3,182,417.76 (FAPESP).

Scientific articles
DE GOUVEIA DAL PINO, E.M. and LAZARIAN, A. Production of the large scale superluminal ejections of the microquasar GRS 1915+105 by violent magnetic reconnection. Astronomy & Astrophysics. v. 441, p. 845-53. 2005.
KOWAL, G. et al. Particle acceleration in turbulence and weakly stochastic reconnection. Physical Review Letters. v. 108, n. 24, p. 241.102. 2012.
ABREU, P. et al.  Measurement of the Proton-Air Cross Section at √s = 57 TeV with the Pierre Auger Observatory. Physical Review Letters. v. 109, n. 6, p. 062002. 2012.