Bruno Touschek remembered (1921-2021)

On 2-3-4 December, a Memorial Symposium to celebrate the 100^th anniversary of the birth of Bruno Touschek, the father of electron-positron physics, was held in three different locations, corresponding to the institutions where Touschek gave major scientific contributions: the Physics Department of Sapienza University of Rome, the Frascati National Laboratories of the National Institute for Nuclear Physics (INFN) and the Accademia Nazionale dei Lincei. The meeting, was the occasion for eminent physicists – including the two Nobel Laureates Carlo Rubbia and Giorgio Parisi – and for Bruno Touschek’s former students, friends and colleagues, to celebrate his memory, his brilliant and unique multifaceted personality, and especially his contribution to theoretical physics and particle colliders, as well as his physics legacy worldwide.

Touschek was born in Vienna on February 3^rd 1921, and like many of his generation he went through a dramatic period of the last century, but also experienced the enthusiasm and excitement of participating in the reconstruction and revival of European physics after the tragedy of World War II. He emerged from the war and early-post war years as one of the first physicists in Europe endowed with a unique expertise about the theory and functioning of accelerators and further evolved his theoretical thought reflecting on the power of symmetry concepts in physics. As a way of realizing in practice the idea of symmetry between matter and antimatter, in February 1960, Touschek suggested exploiting particle-antiparticle annihilation processes as a fundamental tool for studying the basic building blocks of the Universe and their interactions, ushering a new era in high-energy physics and in the exploration of the subnuclear realm. After only one year, the first electrons and positrons were circulating in the storage ring AdA (Anello di Accumulazione), built under his leadership at Frascati National Laboratories, while plans for ADONE, a larger and more powerful electron-positron collider, had already started, following Touschek's proposal of November 1960. In 1964, proof that the two beams of particles and antiparticles had collided was obtained at the Laboratoire de l'Accélérateur Linéaire (LAL) in Orsay, were AdA had been brought to improve particle injection in the ring.

Such results – that were recalled during the conference by Jacques Haïssinski – and the early achievements of the second-generation machines where new types of elementary constituents of matter were detected, further encouraged the development of particle-antiparticle colliders in all the world's major laboratories, notably at CERN, where the Large Electron Positron collider (LEP) has provided during the 1990s precision measurements that have been instrumental in confirming the Standard Model as a theory codifying the properties and interactions of the fundamental constituents of all the visible matter in the Universe. With the detection of the long-sought Higgs boson at the Large Hadron Collider in 2012, the Standard Theory was eventually completed.

The fundamental contribution of AdA as a progenitor of entire generations of colliders was recognized on 5 December 2013, when the world's first particle-antiparticle accelerator– still visible on the grounds of INFN Frascati National Laboratories – was declared an Historic Site by the European Physical Society. This important recognition has definitely marked AdA's role as a milestone in the Italian and European scientific heritage.

As well as establishing a major tradition in the art of accelerators that continues to this day, Touschek set up a theoretical physics group in Frascati National Laboratories. He also mentored a new generation of theorists in Rome, including the late Nicola Cabibbo, one of the first students to graduate with him, who went on to become a leading figure in world physics. As a former student of Cabibbo, Giorgio Parisi, has emphasized his academic descent from Touschek during the last day of the Symposium, and he officially dedicated his 2021 Nobel Prize in Physics to his former teacher and mentor Cabibbo during the Nobel Ceremony held in Rome on 6 December, at the Sapienza University.

The rich background of Touschek's scientific legacy characterized the three-day Conference which covered an extremely broad collection of topics, providing a wide-ranging perspective that spanned the past, present and future of particle physics, accelerator science and its many applications, neutrino physics, through to multimessenger astronomy and, last but not least, also offering a comprehensive overview of future accelerators.

On this special occasion, Touschek's son, Francis, offered some vivid personal memories of his father's life and took part in the ceremony held to name the Visitor Center of the INFN Frascati National Laboratories after Bruno Touschek.

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