Group of History and Theory of Science César Lattes and 50 years of the pi meson

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

Popular article on César Lattes (by Cássio Leite Vieira and Antonio Augusto Passos Videira, published in the journal Superinteressante)
Breve histórico de César Lattes (Centro Brasileiro de Pesquisas Físicas)
1947 – Ano do méson pi (Alfredo Marques, CBPF)

            Yukawa and 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 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.

    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.

First click the sentence above, then use your navigator's "save as" resource to save the compacted file (lattes.zip). Afterwards, use any software such as pkunzip, winzip, etc., to obtain the files that contain the text and figures of César Lattes' paper.