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1959–2000s
Yukawa's Letter: Fireballs and the Centauro Puzzle
Forty years of the longest scientific collaboration between Brazil, Japan and Bolivia
Milestones 07
In 1959 the man who had predicted the pi meson wrote a letter to the man who had established it, proposing that they work together. Out of that letter came the Brazil–Japan Collaboration: forty years of exposures at Chacaltaya, three excited states of nuclear matter with Guaraní names, a phenomenon called Centauro that occupied physics for thirty years, and a detector at the LHC that still bears its name.
Why Japan had to look for a mountain in the Americas
The story begins with an uncomfortable fact that is worth telling, because it explains everything else.
In the late 1950s Japanese cosmic-ray physics had two problems. The first was money: the country was still deep in post-war reconstruction and, as Yoichi Fujimoto recalled, "the door of access to the international cosmic-ray physics community was very narrow for Japanese physicists" (translated from the Spanish). The second was political: Mitsuo Taketani and Shoichi Sakata — and Cecil Powell too, in Britain — were denied visas to enter the United States throughout the 1950s.
Between 1955 and 1959, at the newly created Institute for Nuclear Study of the University of Tokyo, two groups were debating the same problem: how to do experiments outside Japan. One worked with nuclear emulsions; the other with electronic equipment for detecting extensive air showers. The names in that initial group are the ones that would later fill Chacaltaya: Hasegawa, Koshiba, Nishimura, Niu, Oda, Suga and Fujimoto.
And they had to pay their own way. To reach the International Cosmic Ray Conference in Moscow in 1959, the physicists wrote a short popular-science book under the shelter of Professor Tomonaga's fame; the proceeds paid for exactly two airfares: one for Minoru Oda, who was going to talk about extensive air showers with the American physicists and with Ismael Escobar; the other for Jun Nishimura, who was going to talk about nuclear emulsions with César Lattes.
Out of those two tickets came the mountain's two great projects. Oda's led to BASJE (milestone 06). Nishimura's, to the Brazil–Japan Collaboration.
Editorial note. This passage is probably the best narrative opening in the whole dossier: two plane tickets financed by a popular-science book, and a Bolivian mountain as the way around a Cold War visa ban. It deserves to be told in full and with the dates verified.
The letter
Oda had been a student of Bruno Rossi at MIT, and he had no difficulty obtaining a positive answer from Escobar and from his old teacher for opening a new experiment at Chacaltaya. The emulsion group had no such affiliation with the North American community. Aguirre puts it drily: "in this situation, the natural thing was to seek the support of Lattes and Yukawa".
The groundwork was already in place. Between 1958 and 1959 the Japanese physicist Mitsuo Taketani was a visiting professor at the University of São Paulo and became a friend of Lattes, of Schenberg and of other Brazilian physicists. In that same period Hideki Yukawa visited Brazil, invited by the Japanese community to commemorate the fiftieth anniversary of its migration.
In 1959 Yukawa sent a letter to Lattes proposing a collaboration.
It is the gesture that gives the milestone its name. The physicist who in 1935 had theoretically predicted the particle, and who received the Nobel Prize for it in 1949, writing to the physicist who in 1947 had established it experimentally — in part with plates exposed on this mountain (milestone 02) — to propose that they return to the same subject, in the same place.
The concrete plan was drawn up in 1961, during the International Cosmic Ray Conference in Kyoto, at a small colloquium with Lattes, Occhialini, Koshiba, Fujimoto and Taketani.
In April 1962, Fujimoto and Yokoi arrived in Brazil with the first emulsions and, together with Lattes and Alfredo Marques de Oliveira, set up the pilot experiment at Chacaltaya.
⚠ Discrepancy between the two main sources. Vieira and Videira place the first attempt at collaboration in Moscow in 1958 and say that it only prospered after a meeting in Japan in 1962, in which Occhialini also took part; they add that Lattes "ended up not getting on" with the Japanese representative, Jun Nishimura. Aguirre — and the official list of conferences — place Moscow in 1959 and the decisive colloquium in Kyoto, 1961. The 6th International Cosmic Ray Conference was indeed held in Moscow in 1959 and the 7th in Kyoto in 1961, which supports Aguirre's chronology. The site should give Aguirre's sequence and record the Brazilian version in a note, since it adds a human nuance that Aguirre omits.
Why it had to be a mountain, and why this one
Here is the technical reason, and it is the one that turns the site into a condition of the discovery rather than mere scenery.
In those years Lattes was part, from the USP, of the ICEF (International Cooperative Emulsion Flight), the programme of emulsion chambers exposed on balloon flights organised by Marcel Schein from Chicago. The association with the Japanese was not built around that. It was built around a new instrument: a "sandwich" of lead plates, X-ray film and nuclear emulsion.
Aguirre states it bluntly:
"It was clear that such a structure would be impossible to lift with balloons, and hence Chacaltaya was chosen for its exposure."
The instrument was too heavy to fly. That is why the mountain had to be brought to the instrument. For a heritage dossier that sentence is worth more than many arguments: the property was not a convenient place, it was the only possible place.
What, exactly, an emulsion chamber was
It is worth explaining, because it is the physical object of this milestone.
The basic unit was a block: a nuclear emulsion plate of about 0.2 m², covered by an X-ray film and by a lead plate of the same area. Several such blocks stacked together formed an emulsion chamber. The largest reached some 50 m² of surface.
The procedure was of an almost artisanal simplicity, and that is part of its strength:
- The chamber was left exposed for months to cosmic radiation on the mountain.
- It was brought down and developed.
- The X-ray films were examined with the naked eye, to locate the interesting events.
- Only those points were then studied under the microscope, in the emulsions.
Work identifying an interaction of hundreds of teraelectronvolts that began by looking at a plate with the naked eye.
A Bolivian technical contribution. One of Carlos Aguirre's first pieces of work at the Centro Brasileiro de Pesquisas Físicas was to check the fading of the latent image in nuclear emulsion. The result: the emulsions could be exposed under Chacaltaya conditions for more than a year without losing the information. Presented at the XIX and XX Meetings of the Brazilian Society for the Progress of Science (1966 and 1967). Without that result, the long exposures were not defensible.
Who paid: Brazil's first Big Science
The initial division was clean: Brazil supplied the lead; Japan, the emulsions and the X-ray films.
It did not work out that way. Most of the burden ended up falling on the Brazilian side — lead, air fares, civil and electrical works, and the cost of the films as well — because research funding in Japan was still suffering the echoes of the war and because cosmic rays did not enjoy much prestige there. The emulsions and films for the large chambers came to cost some US$100,000, and the project was only viable thanks to funding from FAPESP, the research foundation of the State of São Paulo.
Vieira and Videira describe it without hedging: the Brazil–Japan Collaboration was **"our attempt at Big Science" — one of the longest-lasting agreements in Brazilian physics — and a project on that scale would not occur again in Brazil until the construction of the National Synchrotron Light Laboratory**, in Campinas, in the 1980s.
For the dossier. This paragraph supplies something no other part of the dossier does: a quantification of the material effort three countries sustained for forty years in order to use this site. The heritage value of a scientific site is also measured by what others were prepared to spend in order to reach it.
The fireballs: mirim, açu and guaçu
The main aim of the experiment was to observe the multiple production of pions through the decay of the so-called "fireballs" (H-quantum), an intermediate state of matter. Sun-ichi Hasegawa had postulated the existence of at least three kinds of fireball, of different masses.
For thirty years the Collaboration gathered and analysed data that made it possible to describe those processes phenomenologically, and to identify three distinct forms of highly excited states of nuclear matter, named with Guaraní words:
| State | Rest energy |
|---|
| mirim | ~2–3 GeV |
| açu | ~15–30 GeV |
| guaçu | ~100–300 GeV |
The Chacaltaya observations largely confirmed Hasegawa's proposal, and many of the results were later verified and calibrated against the great accelerators of the United States and Europe.
There is also a specific, dated Bolivian contribution. The postgraduate thesis of Carlos Aguirre — supervised by Ana Maria Freire Endler and defended at the CBPF on 13 April 1971 — analysed the gamma-ray families found in several of the Collaboration's chambers, and its results gave "an unequivocal indication of the existence of a second H-quantum", the second fireball predicted by Hasegawa.
A note on the names. Mirim, açu and guaçu are Guaraní words — "small", "big", "very big". That three states of nuclear matter should bear names from an indigenous South American language, given by a Brazilian–Japanese collaboration working on a Bolivian mountain, is exactly the kind of detail a UNESCO dossier should underline. It is not an anecdote: it is the trace of science done from here and not merely on here.
Centauro
And then something turned up that did not fit.
In 1972, in Chacaltaya's two-storey Chamber No. 15, an event was found that seemed impossible: an interaction with dozens of charged hadrons and practically no neutral pions, that is, without the electromagnetic component that should accompany them. It was called Centauro because its brutally asymmetric shape recalled that of the centaur: body on one side, head on the other.
Five "classical" Centauros were identified and analysed in detail in the Chacaltaya chambers, with these characteristics:
- multiplicities of 64 to 90 hadrons;
- practically zero gamma rays produced in the interaction;
- high transverse momentum: ⟨p⊥⟩ = 1.75 ± 0.7 GeV/c;
- mean observed energy of 348 TeV, with a total incident energy estimated at 1,740 TeV.
Aguirre, writing in 1996, summed up the state of the question: "Should this discovery be confirmed, particle physics could take new directions. At present there are some 15 theoretical hypotheses attempting to interpret this experimental result from Chacaltaya."
Counting the Chacaltaya detectors together with those at Pamir, more than forty Centauro-type events have been recorded since the original one.
The turn of 2003, and why the site must tell it
Here the milestone stops being a story of triumph and becomes a story of method. It is more valuable that way.
In 2003, V. Kopenkin, Y. Fujimoto and T. Sinzi published in Physical Review D a paper titled, without hedging, "Solution to the Centauro puzzle". Reanalysing the detector's original data they found angular differences between the cascade blocks of the upper and lower storeys of the chamber: they did not come from a single interaction, but from two distinct ones. With that matching corrected, they conclude that "new physics is no longer required for the explanation".
Note who signs it: Yoichi Fujimoto, one of the founders of the Collaboration and co-author of the canonical 1980 review. The dismantling came from within.
The following year, in 2004, Akinori Ohsawa, Edison H. Shibuya and Masanobu Tamada published in the same journal a re-examination that confirms the matching error pointed out by Kopenkin and his colleagues — there is no upper component that matches the group of cascades in the lower storey — but reaches a different conclusion about the background: the cascades of the lower group probably come from **a bundle of interactions in the target (C-jets), and the event still displays "peculiar characteristics, quite different from those of ordinary cosmic-ray events"**.
Where the discussion stands today. The majority consensus has shifted towards the conventional explanation: Centauro-I is understood as an artefact of the detector's geometry rather than as new physics. But the conclusion is not unanimous, and the re-examiners themselves maintain that the event is not an ordinary one.
Editorial rule applied. The site must not say that Centauro remains unexplained — that was true in 1996 and is no longer — nor that it was simply an error. It must tell the three stages: the finding (1972), the three decades of hypotheses, and the reinterpretation of 2003–2004, still under discussion. A laboratory whose most famous finding was revised and corrected by its own founders does not come off badly: it comes off as a serious laboratory. Self-correction is a credential, not a stain.
⚠ This makes it necessary to correct the timeline and the script of the ten milestones, which still say "still without an accepted explanation". It has already been corrected in the timeline; it remains to be corrected in Diez_hitos_de_Chacaltaya.md.
What the Collaboration founded
The project was not exhausted by its results. It generated international structure, and that is directly arguable before UNESCO.
The International Emulsion Chamber Committee. The Collaboration's effort led to the creation, on the occasion of the International Cosmic Ray Conference held in Paris, of an International Emulsion Chamber Committee within IUPAP's Cosmic Ray Commission — the same union that in 1964 had declared Chacaltaya a laboratory of world interest (milestone 04). Aguirre says its seminars "have become genuine landmarks in the advance of knowledge in this field". Carlos Aguirre was a member of that committee.
⚠ Aguirre does not give the year. The 17th International Cosmic Ray Conference was held in Paris in 1981, which dates the Committee to that year or immediately afterwards. To be confirmed before publishing it as a date.
The Chacaltaya–Pamir Collaboration. The project also led to a joint research effort with nuclear emulsions at three high-altitude stations:
| Station | Country | Altitude |
|---|
| Chacaltaya | Bolivia | 5,200 m |
| Pamir | former USSR (today Tajikistan) | 4,370 m |
| Mount Fuji | Japan | 3,750 m |
Chacaltaya was the highest of the three. At the height of the Cold War, a network linking Bolivia, Brazil, Japan and the Soviet Union around a single experimental method — and in which the Bolivian station was the reference one by altitude — is an argument for international cooperation that is hard to match.
The collaboration's seminars, according to Aguirre: I Nakhodka (1980) · II La Paz and Rio de Janeiro (1982) · III Tokyo (1984) · IV Beijing (1986) · V Łódź (1986 ⚠) · VI Tarbes (1990) · VII Michigan (1992) · VIII Tokyo (1994).
⚠ Two different seminars dated to 1986 is almost certainly a transcription error; the V is probably 1988. To be checked against the printed page.
The Bolivians in Rio
The Collaboration was not only an experiment on the mountain: it was a route for training Bolivian physicists, and that is material for milestone 08 and for the people sheets.
Bolivian students who passed through the Centro Brasileiro de Pesquisas Físicas and took part in its cosmic-ray projects: Magín Zubieta, Mario Bravo, Oscar Troncoso, Gustavo Pérez, Ricardo Anda and Rafael Vidaurre. Aguirre singles out César Lattes as "one of the most determined promoters of this exchange".
Carlos Aguirre himself was in Brazil between 1966 and 1968, invited into the nuclear emulsion group by Dr Neusa Amato, and learned the craft alongside a team of microscopists whose names are worth preserving: Lele Ribeiro, Ermelinda, Regina, Teresinha, Nair Miranda. The mathematician Francisco Mendes de Oliveira Castro was part of the group.
His description of working with Lattes deserves to be quoted in full, because it humanises the man:
"I took classes in the Centre's postgraduate programme, between 8 in the morning and 4 in the afternoon; at that hour my work as an assistant in the Nuclear Emulsion Laboratory began, and at 8 in the evening Lattes would appear, after sleeping for many hours, to start his working 'day', which went on well into the small hours."
Aguirre also accompanied Lattes in setting up the cosmic-ray and geochronology laboratory of the newly created University of Campinas, on "Culto a Ciência" street — a unit that at first had nothing more than a couple of stools.
Chacaltaya's name at the LHC
The phenomenon found on this mountain gave its name to a detector at the world's largest accelerator.
CASTOR — Centauro And STrange Object Research — is a forward-region calorimeter designed to search, in the LHC's heavy-ion collisions, for precisely the phenomena observed in the high-altitude emulsion chambers. Its documentation says so explicitly: "Centauro related phenomena were discovered and analysed in emulsion chamber experiments investigating cosmic ray interactions at the high mountain laboratories of Mt. Chacaltaya (5,200 m above sea level) and the Pamirs." Initially approved as a subsystem of the ALICE experiment, it ended up integrated into CMS.
A CERN experiment carries in its acronym the name of an event recorded on a Bolivian photographic plate. It is the cleanest proof that what was done on this mountain entered the programme of world physics.
And there is a precedent from the Laboratory itself: the OMEGA proposal of 1989 (milestone 10) had among its declared aims "to clarify the character of the exotic interactions found by the Brazil–Japan Collaboration", comparing the events with those from CERN and Fermilab. The line running from Centauro to CASTOR passes through here.
When it ended
The Collaboration began at the USP. In 1967, following disagreements with colleagues, Lattes moved to the newly created Universidade Estadual de Campinas. Shortly afterwards the CBPF itself joined the Collaboration, and remained in it almost until its definitive end, at the beginning of this century.
⚠ Neither of the two main sources gives an exact closing date. Aguirre writes in 1996, when the project was still active; Vieira and Videira say "the beginning of this century". Establishing the year and the circumstances is an archive task for this milestone (§19).
Chronology of the milestone
| Date | Event |
|---|
| 1950s | Taketani, Sakata and Powell are denied entry visas to the United States. Japanese cosmic-ray physics looks for a way out beyond the North American sphere. |
| 1955 | The Institute for Nuclear Study of the University of Tokyo is created. Hasegawa, Koshiba, Nishimura, Niu, Oda, Suga and Fujimoto debate how to experiment outside Japan. |
| 1958–59 | Mitsuo Taketani, visiting professor at the USP, becomes a friend of Lattes. Yukawa visits Brazil, invited by the Japanese community for the fiftieth anniversary of its migration. |
| 1959 | Lattes's nuclear emulsion laboratory at the CBPF is destroyed by a fire, which also reaches part of the institution's library. |
| 1959 | Yukawa sends Lattes the letter proposing the collaboration. That year, at the Moscow Conference, Nishimura meets Lattes and Oda meets Escobar. ⚠ |
| 1961 | At the International Cosmic Ray Conference in Kyoto, a colloquium with Lattes, Occhialini, Koshiba, Fujimoto and Taketani draws up the concrete plan. |
| April 1962 | Fujimoto and Yokoi arrive in Brazil with the first emulsions. With Lattes and Marques de Oliveira they install the pilot experiment at Chacaltaya. |
| From 1963 | The fireballs begin to appear in the chambers. |
| 1966–1968 | Carlos Aguirre works at the CBPF; he verifies that the emulsions withstand more than a year of exposure under Chacaltaya conditions. |
| 1967 | Lattes moves to the University of Campinas. Shortly afterwards the CBPF joins the Collaboration. |
| 13 Apr. 1971 | Aguirre's thesis at the CBPF: unequivocal indication of the second H-quantum. |
| 1972 | Centauro-I is found in Chacaltaya's Chamber No. 15. |
| 1980 | Lattes, Fujimoto and Hasegawa publish the canonical review in Physics Reports 65, pp. 151–358. First Chacaltaya–Pamir seminar, in Nakhodka. |
| 1981 ⚠ | IUPAP's International Emulsion Chamber Committee is created, on the occasion of the Paris Conference. |
| 1982 | Second Chacaltaya–Pamir seminar, in La Paz and Rio de Janeiro. |
| 1980s–90s | Five classical Centauros are identified at Chacaltaya; together with Pamir, more than forty Centauro-type events are exceeded. Some fifteen theoretical hypotheses circulate. |
| 1989 | The OMEGA proposal sets out, among other aims, to clarify the Collaboration's exotic interactions. |
| Early 2000s | Definitive end of the Collaboration. ⚠ Exact date still to be established. |
| 2003 | Kopenkin, Fujimoto and Sinzi, "Solution to the Centauro puzzle", Phys. Rev. D 68, 052007: the matching between the chamber's storeys was incorrect; no new physics is needed. |
| 2004 | Ohsawa, Shibuya and Tamada, Phys. Rev. D 70, 074028: they confirm the matching error, but maintain that the event still has peculiar characteristics. |
| Since 2009 | CASTOR — Centauro And STrange Object Research — operates in the CMS experiment at the LHC. |
Open points
⚠ Yukawa's letter to Lattes (1959). It is the most important document missing from this milestone, and probably one of the three or four most important in the whole dossier. None of the sources consulted reproduces it. It should be sought in the César Lattes archive — Unicamp and CBPF —, in the Yukawa Hall Archival Library of the Yukawa Institute for Theoretical Physics in Kyoto, and in the CBPF archive. A letter from a Nobel laureate proposing the collaboration that filled this mountain for forty years is an exhibition piece and a dossier piece of the first order.
⚠ Moscow: 1958 or 1959? And whether the decisive meeting was Kyoto 1961 (Aguirre) or a meeting in Japan in 1962 (Vieira and Videira). The official calendar of the conferences supports Aguirre. See §2.
⚠ The year of the International Emulsion Chamber Committee. Aguirre associates it with the Paris Conference without giving a year; Paris was 1981. To be confirmed.
⚠ The V Chacaltaya–Pamir seminar, dated 1986 like the IV. Probable transcription error. See §9.
⚠ The closing date of the Collaboration. See §12.
⚠ How many chambers were exposed in total, and over how many years. There is no figure in the sources consulted. It would be a strong data point for the dossier: the material magnitude of the programme.
⚠ Lattes's health. Vieira and Videira document that Lattes suffered a psychiatric episode after the Difini scandal of 1954 and that he went through a difficult period, with episodes of depression, and they suggest that his personal difficulties had a bearing on the start of the collaboration with the Japanese. This is a fact published in an academic source, not a rumour. The decision whether or not to include it on the public site is not mine: it belongs to the editorial director and, if possible, should be discussed with the family and with the CBPF. My recommendation: do not conceal it in the technical dossier — where documentary honesty is obligatory — and treat it on the public page soberly or not at all, but never turn it into an anecdote.
The material element of this milestone
Unlike milestone 06, whose object is a structure of concrete and galena still standing on the mountain, the object of this milestone is fragile, small and probably scattered: the plates.
What the dossier needs to find out:
- Is any emulsion chamber preserved, complete or in part? At Chacaltaya, at the IIF, at the CBPF, at Waseda, at the University of Tokyo?
- Are any developed plates preserved — emulsions or X-ray films — from the Chacaltaya exposures? In particular, where is the material from Chamber No. 15, the Centauro-I chamber?
- Does the exposure infrastructure remain at the Laboratory — the mounts, the developing room, the microscopes?
- Conservation conditions: nuclear emulsion is an unstable material. If anything survives, conservation is urgent.
Observation. An emulsion plate bearing the trace of Centauro-I, conserved and exhibited, would be the most eloquent object on the site: small, authenticatable, and directly linked to a scientific debate that reached the LHC. Locating it should be a high priority for the project.
The case for the UNESCO nomination
Short form (for the page):
In 1959 Hideki Yukawa wrote to César Lattes proposing a collaboration. Out of that letter came forty years of physics at Chacaltaya, three states of nuclear matter with Guaraní names, and a phenomenon — Centauro — that gave its name to a detector at the LHC.
Developed form (for the dossier):
- The site was a condition of the experiment, not its setting. The instrument — the sandwich of lead, film and emulsion — was impossible to lift by balloon. Chacaltaya's altitude was not an advantage: it was the requirement. It is the most direct argument for exceptionality that the site offers.
- Forty years of sustained South–South–North cooperation. Brazil, Japan and Bolivia maintained the same programme for four decades, one of the longest-lasting agreements in Brazilian physics, funded for the most part from the South. The Collaboration later extended the network to the Soviet Union through Chacaltaya–Pamir, at the height of the Cold War and with the Bolivian station as the highest of the three.
- Results that entered the canon of the discipline. The fireballs confirmed Hasegawa's proposal and were later calibrated against the accelerators of the United States and Europe. The review by Lattes, Fujimoto and Hasegawa in Physics Reports (1980) runs to two hundred pages of one of the most cited review journals in physics.
- A phenomenon with a name of its own, and a thirty-year debate. Centauro occupied the international community for three decades, generated some fifteen theoretical hypotheses, appeared at Pamir as well, and ended up giving its name to CASTOR, a detector at the LHC. That its interpretation was later corrected — and corrected in part by its own discoverers — adds value to the dossier rather than subtracting from it: it documents a complete scientific cycle, from finding to self-correction.
- The site as a school. The Collaboration trained Bolivian physicists at the CBPF — Zubieta, Bravo, Troncoso, Pérez, Anda, Vidaurre, Aguirre — whose names reappear later at the head of the Laboratory and in the founding of Bolivian scientific institutions (milestone 08).
- And a sentence that sums up the milestone better than any argument. Vieira and Videira close their essay like this: "It is the mountain of the South that dared to face the machines of the North."
Sources
- Carlos Aguirre Bastos, Medio siglo de ciencia en Bolivia. El Laboratorio de Física Cósmica de Chacaltaya de la UMSA, Fundación Universitaria Simón I. Patiño, La Paz, 1996 — ch. X, "La Colaboración Brasil–Japón", and the chapters on the 1960s and 1970s.
- Cássio Leite Vieira (CBPF) and Antonio Augusto Passos Videira (UERJ), "Chacaltaya: um laboratório nas nuvens". Version published in Scientific American Brasil, January 2015; reproduced in História da Física (2019), pp. 129–141.
- Y. Fujimoto, "Brasil Japan Collaboration Experiment at Chacaltaya", in Topics in Cosmic Ray Physics. Cited by Aguirre.
- C. M. G. Lattes, Y. Fujimoto and S. Hasegawa, "Hadronic interactions of high energy cosmic-ray observed by emulsion chambers", Physics Reports 65, no. 3 (1980), pp. 151–358. — The canonical review of the experiment.
- V. Kopenkin, Y. Fujimoto and T. Sinzi, "Solution to the Centauro puzzle", Physical Review D 68, 052007 (30 September 2003).
- A. Ohsawa, E. H. Shibuya and M. Tamada, "Exotic characteristics of Centauro-I: Reexamination of the Centauro event", Physical Review D 70, 074028 (22 October 2004).
- E. Gładysz-Dziaduś, "Are Centauros exotic signals of the QGP?", hep-ph/0111163 — quantitative characteristics of the five classical Centauros.
- A. L. S. Angelis et al., "CASTOR: Centauro And Strange Object Research in nucleus-nucleus collisions at LHC", hep-ex/0209008.
- "Boost for Andean cosmic ray laboratory", CERN Courier, 30 October 2000.
- C. Aguirre, "Observação e análise de famílias de cascatas eletromagnéticas detectadas em câmaras de chumbo e fotoemulsões da Colaboração Brasil–Japão, expostas no Monte Chacaltaya", Thesis no. 03-71, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, 13 April 1971.
- A. Marques (ed.), Cesar Lattes 70 Anos: a Nova Física Brasileira, CBPF, Rio de Janeiro, December 1994.
- Official list of the International Cosmic Ray Conferences (Moscow 1959 · Kyoto 1961 · Plovdiv 1977 · Paris 1981), for cross-checking dates.
How to cite this page
«1959–2000s · Yukawa's Letter: Fireballs and the Centauro Puzzle». Chacaltaya Cosmic Ray Laboratory, Instituto de Investigaciones Físicas, UMSA. https://chacaltaya.umsa.bo/en/
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