After studying at the École normale supérieure, Paul Baillon quickly moved on to the laboratory of the École polytechnique and then to CERN, where he became a member of the personnel in 1966.
Paul’s career was notable for the sheer variety of his scientific output.
Firstly, he was a pioneer in bubble-chamber physics. In 1961 and 1962, as a member of the CERN-Collège de France collaboration, he participated in an experiment that recorded 750 000 antiproton annihilations at rest in the liquid hydrogen of the 81 cm Saclay Bubble Chamber. In 1965, he defended his thesis on the study of these annihilations “with production of at least one visible neutral K”, before a prestigious jury composed of Francis Perrin, Jean Meyer and Louis Leprince-Ringuet. The thesis presented a new determination of the mass and width of the K and announced new resonances, in particular the first pseudoscalar meson in the 1400-1500 MeV mass region. Paul remained interested in this subject because this meson could be interpreted as being made up of gluons (a “glueball”).
Twenty years after the data was recorded, Paul even carried out a new analysis, looking for baryonium, a pioneering exercise in data preservation!
From 1974 to 1982, Paul took part in electronic experiments at the PS, which focused on the study of two-body hadronic reactions, and then spent a period at SLAC, where he participated in the DELCO experiment at the e+e− PEP ring, studying in particular the charm quark and the tau lepton.
Throughout his career, in parallel with his work at CERN, Paul managed to continue to collaborate with his French colleagues, often in his spare time. He was passionate about astrophysics and was one of the originators of gamma astronomy in France through his involvement in the Themistocle experiment, carried out from 1988 to 1994. This experiment cleverly reused the infrastructure of the Themis solar power plant, which had been shut down in 1986, to detect cosmic gamma-ray showers by concentrating the Cherenkov light from the showers onto photomultipliers. The studies were particularly focused on the Crab nebula and pulsar. Later, Paul participated in the design of the CAT experiment.
He was also involved in the search for dark matter in the cosmos, aimed at detecting dark objects through the gravitational microlensing effect, an amplification of the luminosity of a star when such an object passes between it and the observer. He contributed to two experiments carried out using telescopes at the Observatoire du Pic du Midi: AGAPE and then the POINT-AGAPE pixel-lensing survey of the Andromeda galaxy.
Upon his return from the United States, Paul joined CERN’s major programmes again, first LEP and the DELPHI experiment, where he became a key member of the team that designed and built the complex and innovative RICH Cherenkov detector. Paul worked on all aspects of the barrel part of the RICH and was responsible for its full simulation and for the particle analysis and identification code. He was also a major player in the production of the detector’s 300 highly-reflective mirrors.
Moving on to the LHC, Paul joined CMS and made essential contributions to the design of the scintillating crystal electromagnetic calorimeter, which played a key part in the discovery of the Higgs boson in 2012. Using the skills he developed at DELPHI, Paul worked with André Braem to develop a reflective protective film to coat the interior of the cells containing the crystals. But most importantly, Paul was one of the designers of the system to stabilise the temperature, to within a few hundredths of a degree, of the crystals, which are located just a few centimetres away from electronics emitting a considerable amount of heat. Thanks to this design, the temperature of the crystals has been kept at exactly 18 degrees since 2007.
With a very solid foundation in classical physics and instrumentation, as well as mathematics, Paul could be as passionate about the construction of a detector as about the most abstract ideas of mathematical physics. Many still remember, for example, his highly informative class on the use of tensor calculus at the Herceg Novi school in 1968. In his retirement, he wrote and published a book entitled: “Differential Manifolds, A Basic Approach for Experimental Physicists”. He was writing a second on the basics of quantum field theory.
When faced with a problem, Paul had the knack of approaching it from an unexpected angle. It was a sign of brilliance, of true originality and even of a certain taste for the paradoxical... but it always produced results.
He was gifted and daring in his intellectual pursuits, but also in sporting ones, as an accomplished skier and mountaineer. Beyond science, Paul was interested in history and religion, and found time to get involved in politics and local affairs.
We will treasure the memories of our discussions with Paul, memories that will constantly remind us of our deep admiration for this exceptional scientist and man.
His colleagues and friends