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Puppi: promoting new horizons in physics

4 June 2007

Antonino Zichichi provides a personal look at the work of Gianni Puppi.

Résumé

Puppi: pour ouvrir de nouveaux horizons à la physique

Giampietro Puppi est bien connu pour le «triangle de Puppi», par lequel il a fait le premier pas vers l’universalité des forces faibles en 1948. Toujours animé par le désir d’ouvrir de nouveaux horizons à la physique, il a établi le premier groupe de la section de Bologne de l’INFN, puis est devenu une figure importante de la physique de l’espace. Lorsqu’il était directeur de la Recherche au CERN, au début des années 60, à l’heure où l’expérimentation était dominée par la technologie des chambres à bulles, il s’est fait un point d’honneur d’encourager les nouvelles technologies. Dans cet article, Antonino Zichichi nous fait découvrir l’homme, son triangle et sa vision.

In 1948 Giampietro Puppi, Gianni to his friends, published a paper in Il Nuovo Cimento where he distinguished the neutral counterpart of the muon – now known as the muon neutrino, νμ – from the neutral counterpart of the electron, now called the electron neutrino, νe (Puppi 1948). Fourteen years later, what Puppi had proposed in his famous paper was demonstrated experimentally by a team led by Leon Lederman, Mel Schwartz and Jack Steinberger (Danby et al. 1962). Puppi had calculated three weak processes – pion decay, muon capture and muon decay – and was able to prove that these three different processes were described by “approximately” the same fundamental weak coupling. The coupling of the three vertices of the “Puppi triangle” described all weak processes known at the time with the same strength, represented by the sides of his equilateral triangle.

This work was the first step towards the universality of the weak forces and indeed attracted the attention of Enrico Fermi, since it was the first proof that all weak processes could be described by the same coupling. It came just a year after the discovery by Marcello Conversi, Ettore Pancini and Oreste Piccioni that the negative cosmic-ray “mesons” (now known to be the leptons called muons) were disintegrating as if they were not strongly coupled to the nuclear forces (Conversi et al. 1947).

Fermi, together with Edward Teller and Victor Weisskopf, pointed out that the lifetime of this meson was 12 powers of 10 longer than the time needed for the long-sought Yukawa meson to be captured by a nucleus via the nuclear forces (Fermi et al. 1947). The solution of the puzzle was soon found by Cesare Lattes, Giuseppe Occhialini and Cecil Powell, who discovered that the cosmic-ray muon was the decay product of a particle, now known as the π meson or pion, which the authors considered to be the “primary meson” (the origin of the symbol π, for primary; Lattes et al. 1947).

To prove that rates for pion decay, muon decay and muon capture were “approximately” equal as expected by the universality of the Fermi coupling was indeed remarkable. These were great times for the understanding of the weak forces, and at the time the topic of the universality of the weak interaction was a central focus of the physics community (Klein 1948, Lee et al. 1949 and Tiomno and Wheeler 1948). The Puppi triangle played a crucial role in revealing the basic property of the new fundamental force of nature, the strength of which appeared to be so much weaker than that of the electromagnetic and of the nuclear forces.

At this time, cosmic rays provided the only source of high-energy particles, and Puppi made another valuable contribution to the field with his paper on the energy balance of cosmic rays (Puppi 1953). However, the direction of research was soon to change with the advent of particle accelerators and the newly invented bubble_chamber technology. In 1953 Puppi established the first group of the Bologna section of INFN, which led to the existence of a large collaboration in the field of bubble-chamber physics, leading to the observation of parity non-conservation in hyperon decays in 1957.

I have a personal reason for being grateful to Puppi around this time. When he was research director at CERN (1962–63) and later chair of the Experimental Committee (1964–65), he played a crucial role by being a strong supporter of my Non-Bubble Chamber (NBC) project. Physics was at the time dominated by bubble-chamber technology, and Puppi himself had been fully engaged in promoting the National Hydrogen Bubble Chamber in Italy, and in establishing large international collaborations for the analysis of bubble-chamber pictures. It was the need for powerful computing for this analysis that led him to establish the first computing facility in Bologna, the development of which through the subsequent decades produced what is now the largest computing centre in Italy.

Bubble-chamber technology had revealed an enormous number of baryons and mesons and Puppi was interested in what this could mean. The question arose of whether to encourage other technologies, and in particular to do what? In a meeting in his office as research director at CERN, the subject came up of studying the rare-decay modes of mesons – especially the electromagnetic decay modes. This needed NBC technology. As a typical exponent of the classical culture of Venice, Puppi was open to new horizons and made the point that new technologies had to be encouraged; and this is how the NBC project began. He was no longer at CERN when, in 1968, thanks to the NBC set-up, a new decay mode of the X0 meson (now η’) into two photons was discovered, thus establishing that this heavy meson could not be the missing member of the tensor octet. This was the first step in determining directly the correct value of the pseudoscalar meson mixing.

During a meeting on Meson Resonances and Related Electromagnetic Phenomena at the European Physical Society conference in Bologna in 1971, Dick Dalitz pointed out that it was thanks to physics leaders of the calibre and vision of Puppi that new horizons in the physics of mesons has been opened. The problem of the vector and pseudoscalar meson mixings needed NBC technology in order to be investigated experimentally. These were times when no data on vector mesons existed from electron–positron colliders and direct measurements of the pseudoscalar and vector-meson mixings did not exist. As we now know, to understand the mesonic mixings, it was necessary first to discover the theory of quantum chromodynamics and then to discover instantons. No-one could have imagined these developments, rooted in the physics of mesons, when, in the 1960s, CERN’s research director had encouraged the young fellows to propose new ways to go beyond bubble-chamber technology, and the knowledge of the meson-mixings was based only on their masses, which Puppi correctly considered a tautology.

Puppi’s scientific interests extended beyond particle physics to space physics, which is why he became president of the European Space Research Organization (ESRO) and co-founder of the European Space Agency (ESA). Also, in the field of ecology and the protection of the treasures of civilization, he founded the Istituto delle Grandi Masse to study, on a rigorous scientific basis, the sea-water dynamics so vital for the future of his beloved Venice.

The last time I had the pleasure and the privilege to meet my teacher was a few weeks before he departed this life in December 2006 (CERN Courier April 2007 p41). He never stopped pursuing a multitude of interests, including the future of CERN, having been not only a research director but also a member of the CERN Council. He was very concerned when he learned that the Council now does not always express its full support for the laboratory’s activities.

“During my time, the CERN Council was a strong supporter of the decisions taken, always, for the strengthening of the scientific excellence of the results, to be obtained in the most civilized competition mankind can put forward: physics. No one should underestimate the fact that CERN has the remarkable property of being unique in the world.” These were his last words.

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