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César Lattes and 50 years of the pi meson
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    Professor César Lattes was the founder of the Department of Cosmic Rays and Chronology of the "Gleb Wataghin" Physics Institute, UNICAMP. Lattes' most famous contribution to physics was his important participation in the discovery of the pi meson, in 1947. We present in this page a short, elementary description of this discovery. You can also obtain here an electronic copy of Lattes' first paper on mesons.

    Other Internet pages, on César Lattes and the discovery of the pi meson (in Portuguese), containing photographs:

Fifty years of the pi meson

    It was in 1947 that the existence of the pi meson was established. One of the authors of the research that led to its discovery was the Brazilian physicist César Lattes. Everybody knows that this was an important finding. However, who can tell what exactly are those pi mesons? What changes their discovery brought to physics?
    The discovery of the pi meson was a fundamental step in the understanding of the sub-atomic world. Throughout the 20th century, there was a gradual change and increase in complexity of our ideas on the constitution of matter. Atoms are built of electrons and nuclei. The nucleus contains positive charge particles (protons) and other chargeless particles (neutrons). What binds protons and neutrons together to build the nucleus? They cannot be kept together by electric attraction – on the contrary, protons repeal each other. Gravitational forces, on the other side, are much smaller than the repulsive electrical forces. It was necessary to suppose the existence of a new kind of nuclear forces, stronger than the electric repulsion, to keep the nucleus particles together.

            Yukawa and Lattes
Hideki Yukawa and Cesar Lattes
    In 1935, the Japanese theoretical physicist Hideki Yukawa proposed an explanation of nuclear forces. He suggested the existence of a new particle, with a mass about 200 times larger than that of the electron. The unknown particle was supposed to be emitted and absorbed by protons and neutrons. The exchange of those particles between the constituents of the atomic nucleus would produce a short-range attraction between them that would explain the stability of the nucleus. This particle received the name "meson" (from the Greek "mesos" = intermediate) because its mass was intermediate between those of the electron and of the proton. According to Yukawa's theory, mesons can only exist for a very short time. Outside the atomic nucleus, they were supposed to disintegrate in just one thousandth of a millionth of one second.
    In 1937-38, Carl D. Anderson and Seth H. Neddermeyer found in the cosmic radiation, that continually reaches the ground, signs of something that looked like Yukawa's meson: it has the expected mass and disintegrated as Yukawa's particle was expected to disintegrate. For ten years, it seemed that everything fit in the scheme, and that there was a nice theory on the constitution of matter. In 1947, however, this peace was shaken. It became clear that the meson of Anderson and Neddermeyer did not behave as predicted by Yukawa's theory.
    Mesons should be strongly absorbed by protons and neutrons, if they are to explain nuclear forces. It was predicted, therefore, that they should be easily captured by matter. However, a group of Italian physicists (Marcello Conversi, Ettore Pancini and Oreste Piccioni) observed that those mesons that had been found in cosmic radiation could pass through several hundred atomic nuclei without suffering any interaction. They had a very weak interaction with protons and neutrons – the opposite of what was expected. Something was going wrong.
Two photographs of 
César Lattes, 
in 1947

The Bristol group: Powell is standing, at the center, in a light blazer;
Lattes is in front of him (in the middle row).
Occhialini is the second from right to left, in the front row.

    That is where Lattes' group comes in. In 1946, a research group in Bristol, England, under the leadership of Cecil F. Powell, was studying the tracks produced by nuclear reactions in some special (thick and sensitive) photographic plates called "nuclear emulsions". Studying the tracks produced by protons and other charged particles in such emulsions, it was possible to find their mass and energy. Beppo Occhialini and César Lattes analyzed some emulsions of a new kind, that had been placed at the top of a mountain (the Pic du Midi). When the plates were developed and analyzed, they observed a large number of tracks produced by particles that at first they interpreted as being the known mesons. However, after a few days of detailed study, they found two special tracks of mesons that gradually reduced their speed in the emulsion, and finally stopped. At the end of those tracks, they observed that a new meson appeared.

    The cosmic ray laboratory at Mount Chacaltaya, when it was being built

    What could that be? There were several possible interpretations. Perhaps the meson had reacted with an atomic nucleus inside the emulsion, and after the interaction it could have been expelled with a larger speed. Perhaps the meson could have suffered some sort of transformation into another kind of meson. The two initial cases were insufficient evidence to reach any safe conclusion. In order to obtain a larger number of tracks, Lattes traveled to Bolivia, and put several nuclear plates at the top of Mount Chacaltaya, 5,500 m above the sea level. When the plates were developed, it was possible to find about 30 tracks of double mesons. A detailed study led to the determination of the meson masses, and it was possible to establish that there were indeed two types of mesons, with different masses.
    One of the mesons was about 30% or 40% heavier than the other one. The heavier meson was able to disintegrate and to produce the lighter meson. The second particle was the one that was already known from the studies of Anderson and Neddermeyer. To distinguish it from the other one, it was called "mu meson" (nowadays, it is called "muon"). The primary meson, on the other side, was something new, unknown. It was called "pi meson", and its identification was announced in October 1947. Later tests showed that it strongly interacted with nuclei and that its characteristic properties were those required by Yukawa's theory. The particles that hold the nucleus together had been found.
    The discovery of the pi meson was not merely a confirmation of a theory. It opened a whole new world to investigation. First, because it became clear that there were particles (muons) that had not been predicted; their role in nature was unknown. Secondly, because the study of cosmic rays soon led to the unexpected discovery of several other particles. In that same year, several tracks that did not correspond to anything known were detected.
    Powell's group found evidence for a kind of meson twice as heavy as pions. They were called "tau mesons" at first, and nowadays they are called kappa mesons. In the same year (1947), Clifford Butler and George Rochester observed V shaped tracks. They could be explained assuming the existence of new neutral particles (without electric charge) that produced no track and that disintegrated into one positive and one negative particle. In the following years, physics was flooded by new, unexpected particles. It was difficult to understand their properties, at that time. Robert Oppenheimer introduced the phrase "sub nuclear zoo" to describe this new world of particles. Among the exotic animals of this zoo, there were particles heavier than protons (the so-called "hyperons"), of several different types. The new "animals" were first studied in cosmic rays, but powerful particle accelerators were soon built – each one more powerful than the former – and they allowed the creation and investigation of those particles in the laboratory.
    The discovery of the pi meson was something more than finding one special particle. It heralded the beginning of a deep revision of the physical concepts on the structure of matter. The large variety of particles that were discovered in the following years challenged the very concept of "elementary particle" as something indivisible, simple. It led to the search for a substructure for protons, mesons and the other particles. The theory of quarks would never arise without the stimulus of those empirical discoveries, that began 50 years ago.

César Lattes' first paper on the pi meson

    On the 24th May 1947, the British journal Nature published the paper "Processes involving charged mesons", authored by Cesar Lattes, H. Muirhead, G. P. S. Occhialini and C. F. Powell. You can get an electronic copy of the full paper here (The file lattes.zip is 82.3 kB large; it can take some time to be transferred by Internet).

Attention: The copyright of this article belongs to Nature. Our electronic library can provide a digital copy of the paper, to any individual researcher, without the necessity of copyright payment, exactly as a traditional (paper) library can provide a xerox copy of the article. Anyone who might obtain here a copy of the article must be aware that he/she is only allowed to use the provided copy for individual research. Any other use is forbidden.

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This page was updated on 03/Apr./1998