_ Pierre Auger Observatory

Cosmic Rays, Still an Enigma

Analysis of the origin and identity of highest-energy particles moves forward and poses further difficult questions

Carlos Fioravanti n y a miguel boy a

120 _ Special Edition FAPESP 50 YEARS mong the news items circulated by State University of Campinas (Unicamp) and the Reuters international news agen- coordinator of the São Paulo team. Of the nearly cy on November 8, 2007 was an ar- 450 authors of scientific articles referring to data ticle entitled, “Cosmic Rays Believed obtained from the observatory, 30 are Brazilian, to Start in Black Holes.” On the same and 19 of these are from the state of São Paulo. Aday, the British magazine The Economist aban- Through an international collaboration that now doned its customary caution with the headline, includes some 500 physicists from 90 institutions “They Came from Outer Space: A 40-year-old in 19 countries, construction on the observatory Mystery is Solved,” in reference to the possi- began ten years ago at the foot of the Andes on a ble origin of those particles. The findings from semi-desert plain on the outskirts of Malargüe the work of an international team of physicists in northwestern Argentina. The physicists began and engineers at the Pierre Auger Observatory to collect data in January 2004 during the con- in Argentina had enormous repercussion. The struction phase, which was completed in 2008. Auger website chose for publication 65 articles The rate of correlation between cosmic rays from newspapers and magazines from around and active galactic nuclei, which was 69% in 2007, the world to report the news, details of which fell to approximately 40% and stabilized at that appeared in the lead article in Science magazine level in the following years. During that time, the the following day. The mystery, however, has still surface detectors and telescopes detected more not been unraveled. particles, but the rate is still higher than the 21% Four years later, there are still signs that active attributable to pure chance. Unlike particle ac- galactic nuclei, where black holes are formed, celerator physicists, who can produce collisions A plain at the foot of may really be the source of ultrahigh-energy cos- between protons whenever they wish, the physi- the Andes: an ideal 19 place to capture mic rays, above 10 electron volts (eV). “The sig- cists at Pierre Auger have to wait until they ar- particles coming nal is still there, but weaker,” said Carola Dobrig- rive from the sky. Each year, only a few dozen of from space keit Chinellato, a professor of physics at Brazil’s the highest-energy cosmic rays reach the top of

pESQUISA FAPESP _ 121 the Earth’s atmosphere. Each of these two independent methods of observa- (CBPF) in Rio de Janeiro and current cosmic rays traverses the atmosphere tion. Before that time they were used coordinator for Brazil’s participation at and interacts with the nuclei of atoms separately in smaller observatories like the Auger site. Escobar has been work- and molecules of air, producing an at- the Fly’s Eye, which operated in the ing as an invited researcher at Fermilab mospheric shower made up of billions United States from 1981 to 1992 with in Chicago since April 2011. of particles. Some of these particles will 67 telescopes, and the Akeno Giant Air reach the 1,660 surface detectors, the so- Shower Array (AGASA) in Japan, with Rare rays called Cherenkov tanks, each of which 111 surface detectors. “A number of in- French physicist Pierre Victor Auger holds 12,000 liters of highly purified wa- novative details from the original de- identified the showers of particles in ter. The tanks’ sensors detect the bluish sign are now proving valuable,” notes a historic experiment conducted in light emitted in the water when electro- Carlos Escobar, the physics professor at the Alps in 1938. So many years later, magnetic particles traverse the tanks Unicamp who coordinated the Brazilian the composition of the highest-energy (see illustration on the following page). team and handled negotiations with the cosmic rays remains unknown. The construction firms working on the ob- principal reason for this is that they are cattered over 3,300 square kilome- servatory in Argentina starting with the quite rare. The higher the energy, the ters—double the size of the city of initial planning meetings. He was one of rarer the particles. “When the energy SSão Paulo—the surface detectors two Brazilian representatives, along with increases tenfold, the number of cosmic operate in combination with 27 fluo- Ronald Shellard, a researcher from the rays that reach the Earth with energy rescence telescopes, known as Fly’s Brazilian Center for Physics Research above that value is one hundred times Eyes. These telescopes are capable of lower,” Chinellato said. “The recording the faint light emitted by ni- highest-energy cosmic rays trogen molecules in the upper atmos- have energy above 1019 eV, phere when they are excited by particles and only a few of them per from the shower produced by a cosmic Only one highest-energy square kilometer reach the ray that has reached the Earth. Because Earth each year. For parti- they are so sensitive, the fluorescence particle per square cles with energy above 1020 detectors, installed on four buildings, kilometer per century eV, the rate drops to one par- work only in the dark on moonless ni- ticle per square kilometer ghts, while the tanks capture shower reaches the Earth per century.” particles all the time. According to Chinellato, The Pierre Auger facility is the site in the analyses of ultrahigh- of the first experiment to integrate the energy cosmic rays detected at the Auger Observatory, the result of the correlation with active ga- lactic nuclei strengthens the hypothesis Ground detectors that cosmic rays are protons, or hydro- gen nuclei. “The rationale behind this The Pierre Auger Observatory covers 3,300 square kilometers, interpretation is that the cosmic rays double the size of the city of São Paulo and 10 times larger than Paris were likely to have been minimally de- flected by the magnetic fields that they traversed, and that therefore they re- tained angular proximity with their pos- sible sources,” she said. “Had they been nuclei of heavier elements such as iron, for example, they would have undergone greater deflection in the magnetic fields, which would eliminate that correlation.” But things are not quite so simple.

Compartments with the fluorescence Observations of the development of telescopes particle showers by the fluorescence

Surface telescopes and comparisons with theo- detectors retical predictions indicate that cosmic rays—at least some of them—could be heavier nuclei such as iron, i.e., blocks of 26 protons and 30 neutrons. “That interpretation is fairly dependent on the validity of the theoretical models in the description of the development source: Pierre Auger Observatory of the showers,” Chinellato observes.

122 _ Special Edition FAPESP 50 YEARS Waiting for particles

The Pierre Auger Observatory combines two independent ways of detecting cosmic rays

1 When they reach the Earth, cosmic rays collide with nitrogen in the upper atmosphere to produce a particle shower 2 3

The particles are also recorded when they react with the water in the tanks of the The collision surface detectors between the particles produces a faint blue light, 4 captured by the A central computer gathers the fluorescence data from the telescopes and telescopes surface detectors to identify the possible origin of the cosmic rays

“The theoretical models are based in ger have published details of these and the observatory was being built, it didn’t part on extrapolation from observations other conclusions, and have presented seem so, but at 3,000 square kilometers— in experiments with particle accelera- potential new uses for the equipment, the current size of the observatory—it’s tors at much lower energies.” In prac- such as monitoring weather or the mo- actually too small.” tice, for now we cannot simply choose vements of tectonic plates. Built between 2002 and 2008, Pierre one conclusion over the other. Auger is the largest cosmic ray observa- To arrive at a less uncertain conclu- he total energy levels in the par- tory in operation. The area it covers is sion, physicists will have to adjust, cor- ticle collisions being analyzed are so large that even the most frantic mo- rect or expand these theoretical appro- Tabout 100 to 200 times higher torcyclist zooming between cars in the aches in order to explain the identity of than those produced in the Large Ha- city of São Paulo would be unlikely to cosmic rays. Theoretical physics does dron Collider (LHC), the world’s lar- see in one day all of the 1,660 cylindri- not explain how cosmic rays can acquire gest particle accelerator located at the cal tanks, each measuring 3.7 meters in an energy 100 million times greater than European Organization for Nuclear Re- diameter and 1 meter high, spaced 1.5 that of particles of the same type produ- search (CERN) in Geneva, Switzerland. km apart in the form of a triangular grid. ced in the Tevatron, the world’s most The LHC groups are also experiencing In the past two years, Escobar says, the powerful particle accelerator located at progress, setbacks and digressions in tanks have acquired more electronic de- Fermilab. In other words, the conceptu- attempting to confirm the existence of vices and, in addition to recording the

drüm al tools are limited and the options are the so-called Higgs boson, a hypothe- light produced in the collision with the s n

t io still shaky. “Our data from 2007 further tical particle that could give other par- water in the tanks, they are able to re- t ra weaken the top-down models”, Escobar ticles mass and confirm the formulas cord the arrival of particles formed when us ll i said. Defended by other groups of phy- that physicists have worked with for 50 cosmic rays break apart as they hit the sicists, top-down models assume the years. “Heraclitus said that nature loves Earth’s atmosphere, thus reinforcing existence of particles with even higher to hide,” says Escobar. “In order to iden- the debate around solving the identity cirillo tiago energies than the ones detected, above tify precisely whether cosmic rays are of cosmic rays. ics ph 1020 eV. In eight to ten scientific articles protons or nuclei of iron, we will need The electronic detectors in the tanks n fogra i published each year, the physicists at Au- another 15 years of data,” he said. “When at the Pierre Auger Observatory are al-

pESQUISA FAPESP _ 123 Brazilian participation ready capable of detecting showers ini- runs out is a never-ending job, and the in the Pierre Auger tiated by neutrinos and photons in the support from FAPESP will enable us to upper atmosphere. Neutrinos are very do the replacements for the next four Observatory low-mass particles and are extremely years,” Chinellato says. Each year an av- abundant, but less abundant than the erage of 600 batteries must be replaced, particles of light known as photons. or an average of two per day. She says Investments “Theoretical models now exist that she is monitoring tests in the United FAPESP, US$2.5 million; Finep/MCT, US$1 predict that neutrinos and photons States on ten experimental tanks with million CNPq, US$300,000; are produced by the same sources as a new format and new electronics, “also FAPERJ: R$ 00,000 cosmic rays or even during the propa- obtained with FAPESP support,” she gation of cosmic rays through space, points out. The new tanks use only one Institutions but so far we have not detected any of light detector instead of the three used Brazilian Center for Physics Research them reaching Earth,” Chinellato notes. in the current tanks. (CBPF); Pontifical Catholic University – “The fact that we have not detected any Rio de Janeiro (PUC-RJ); University of São is also significant.” he present work is relatively sim- Paulo (USP), Physics Institute, São Paulo; As the current coordinator of the ple, given the number of unfore- State University of Campinas (Unicamp); thematic project connected with Au- Tseen events they have already en- State University of Feira de Santana (UEFS); ger (the previous projects were coor- countered. At the beginning, the Che- State University of Southwestern Bahia dinated by Escobar), she is monitoring renkov tanks did not work for the simple (Uesb); Federal University of Bahia; Federal the replacement of the batteries for the reason that grazing cows took an interest University of the ABC (UFABC); Federal Cherenkov tanks. The batteries store in their new neighbors, the tanks, and University of Rio de Janeiro (UFRJ) the energy that is produced by the solar some of them began to chew on the data panels and used by a miniprocessor that transmission wires. Ricardo Perez, the Companies detects the signals from cosmic rays and Argentinian in charge of tank mainte- Alpina Termoplásticos; Rotoplastyc transmits them to the central computer nance, used his knowledge as a former Indústria de Rotomoldados; Equatorial some kilometers away. mining expert to come up with the idea Sistemas; Schwantz Ferramentas “Replacing the two batteries for each of a box to protect the wires, and the co- Diamantadas; and Acumuladores Moura of the 1,600 tanks as their service life ws ceased to confound science.

124 _ Special Edition FAPESP 50 YEARS Science on the ground: Patrick Allison, an American, and Xavier Bertou, a Frenchman, install the electronics on a surface detector. At left, one of the cosmic ray photodetectors

“When this project began,” Chinellato says Marcelo Leigui, a physicist who its to the observatory’s headquarters—a recalls, “it all seemed like an impossible took part in that investigation as a post- beautiful building with huge expanses of dream.” The world’s largest cosmic ray doctoral researcher at Unicamp and is glass instead of walls—on their weekend observatory was first planned in 1992 now a professor at the Federal Univer- strolls. Mendoza, known as Argentina’s by American physicist James Cronin, a sity of the ABC. “Brazilian participation principal wine-producing region, was University of Chicago professor and 1980 would have been lower if one of the oth- also advancing in the field of science. Nobel Prize winner, and Alan Watson, a er two candidates—South Africa or Aus- Then began the arrival of trucks— Scot from England’s University of Leeds. tralia—had been chosen.” Brazil’s par- many trucks—carrying equipment. Be- As the need for inter- ticipation, made of- ginning in early 2001, Alpina, a São Pau- national cooperation ficial on July 17, 2000 lo-based company, manufactured and became clear due to at Unicamp, result- sent the Cherenkov tanks on trips that the proportions that ed in investments of took at least two weeks and were subject the original project around US$4 million to all manner of unforeseen events, from took on, they invited in the form of equip- potholes in narrow roads to highway pa- a few interested col- ment purchased from trolmen who asked to see what was in leagues with experi- The job of Brazilian manufac- the tanks. Rotoplastyc, a company based ence in the field of turers and the fund- in the Brazilian state of Rio Grande do particle physics for replacing the ing of post-graduate Sul, made some of the tanks currently initial talks in June 1,600 batteries research grants and in operation and participated in the de- 1995. One of the par- travel expenses. velopment and production of the new- ticipants in those for the surface Malargüe, a town -format tanks. talks was Escobar, of 23,000 with two Schwantz, a company based in Indaia- then a professor at detectors began stoplights located tuba in the state of São Paulo, made and the University of São 420 kilometers from sent the corrector lenses for the fluores- Paulo (USP). in 2011 Mendoza, Argentina’s cence telescopes. The trapezoid-shaped At a meeting held at closest urban center 25-centimeter-high converging lenses, UNESCO headquar- with regular airline made of German glass, form a corrector ters in Paris in No- connections, began to ring around the edges of the diaphragm vember 1995, Escobar, change as work began that regulates the entrance of light, like Ronald Shellard of the CBPF, Armando in mid-1999. Researchers started arriving the diaphragm of a camera, and increase Turtelli of Unicamp, and Argentine col- from the United States, Italy, Germany, the radius of the lens from 85 to 110 cen- leagues Alberto Etchegoyen and Alberto Poland, Slovenia, and many other foreign timeters without losing image quality. Filevich strenuously supported the idea countries. After they got over the initial Equatorial, a company in São José dos miguel boyayan of the new observatory being built in Ar- strangeness, Malargüenses hastened to Campos in the state of São Paulo, manu- o t os ph gentina. “That was a crucial moment,” learn English and began including vis- factured the prototype for a device 2.5

pESQUISA FAPESP _ 125 Indonesian physicist Richard Randria analyzes particle showers at the data acquisition center

Projects 1. Study of highest-energy cosmic rays at the Pierre Auger Observatory – nº 2010/07359- 6 (2010-2014) 2. Pierre Auger Observatory – nº 1999/05404-3 (2000-2007) Grant mechanism 1. and 2. Thematic project Coordinators 1. Carola Dobrigkeit meters in diameter that automatically regulates Roberto Leite, a physicist from the University Chinellato (Unicamp) 2. Carlos Ourívio Escobar the telescope lenses and the shutters that expose of São Paulo (USP) and director of the Brazilian Investment the telescope for nighttime observation. Moura, a Physics Society (SBF), who died unexpectedly 1. R$3,064,952.43 Recife company, made the batteries for the solar the following year. 2. R$6,034,341.71 panels on the surface detectors. There were some remarkable aspects about the construction process. First among these is the fact Scientific article ach of the 17 participating countries played that the data were already being collected as the a role in sending equipment and resear- equipment was being installed. Another is that The Pierre Auger Collaboration. Correlation of Echers, so the observatory has some of the the physicists and engineers did not hesitate to the Highest-Energy Cosmic world’s best technology. Argentina supplied in- do any kind of work that was needed. “We have to Rays with Nearby frastructure in the form of machines that purify do whatever is necessary,” I was told in 2003 by Extragalactic Objects. Science. v. 318, n. 5852, water for the tanks, as well as some of the tanks Frenchman Xavier Bertou, coordinator of scien- p. 938-43, 2007. and batteries for the solar panels that supply tific operations. Bertou had left Paris the previous the surface detectors, sharing that undertaking year, settled down in Malargüe, and from then with the Mexicans and the Americans. Austra- never went without his late afternoon gourd of From our archives lia provided the cloud detectors, and France mate. He and other physicists, mostly post-docs More mass for cosmic rays supplied the electronic devices for the surface in their 30s, assembled equipment on the tanks Issue No. 170 – detectors. The Czechs sent the mirrors for the or in the telescope buildings during the day, and April 2010 telescopes, and the Spaniards supplied the so- at night—often until three in the morning—at the Space enigmas lar panels for the tanks. The fluorescent light machine shop in the building in town. Issue No. 167 – detectors for the telescopes used Italian came- “We showed that it is possible to implement January 2010 ras and electronic controls made by the English a large project under budget,” Escobar said in Pierre Auger is inaugurated Issue No. 154 – and the Germans. 2007. Total expenditures came to US$54 million, December 2008 The readers of this magazine followed the which was US$6 million below budget. How Pierre Auger, now ready progress of the construction of Pierre Auger. In did they do it, when most projects break the Issue No. 149 – July 2008 August 2000, the cover story of Pesquisa Fapesp, budget? By negotiating prices with equipment Cosmic rays’ long journey written by Mariluce Moura, told of the behind- suppliers and economizing as much as possible. Issue No. 142 – -the-scenes negotiations and the start of cons- In 2005, when he saw that expenses were piling December 2007

truction. Another story from April 2002 descri- up, Escobar decided that the members of the Shower of particles bes the pace of the work: “At this very moment, Brazilian team would stop taking two flights Issue No. 116 – in a space which sometimes reminds one of the to get to Pierre Auger and would start going by October 2005 refinement of a spaceship or the robust workin- plane only as far as Buenos Aires. From there The eyes of the desert gs of a hydroelectric plant, dozens of operators, they could take a bus and get to Malargüe in 16 Issue No. 90 – August 2003 technicians and researchers are working inten- hours’ travel time. No one objected. One year, sely setting up the cosmic rays measuring equi- Sérgio Carmelo Barroso, a post-doc at Unicamp, The cosmic rays are arriving Issue No. 74 – April 2002 pment.” At that time, José Fernando Perez, then had to go to Malargüe ten times to assemble and scientific director of FAPESP and chairman of test equipment. Now a professor at the State Surround it in the air, capture it on the ground the Auger Observatory’s finance committee, vi- University of Southwestern Bahia (UESB), he Issue No. 56 –

sited the project in Malargüe. With him was José continues to take part in this work. n August 2000 miguel boyayan

126 _ Special Edition FAPESP 50 YEARS