James W. Cronin: Raising the Profile of Physics Worldwide Karl-Heinz Kampert1 1Department of Physics, University of Wuppertal, Germany E-mail: [email protected] (Received October 7, 2016) James W. Cronin started his career in high energy physics, won the Nobel Prize in Physics in 1980 for the discovery of CP-violation, and amongst others, discovered hard parton scattering phenom- ena in nuclear collisions, an observation known as the “Cronin-effect”. In 1986 he decided to move to cosmic ray physics which lead to the construction of the first 1 km2 class large air-shower array CASA-MIA. In 1991, as his masterpiece, he started building a collaboration to construct an instru- ment of unprecedented size for studying cosmic rays with energies up to 1020 eV. Construction of the Pierre Auger Observatory began in 2002, was finished in 2008, is taking data since like a “Swiss Clock”, as Jim used to call it, and has dramatically advanced our understanding of ultra-high energy cosmic rays. KEYWORDS: Pierre Auger Observatory, UHECR, EAS, CP-violation, Cronin-effect 1. Beginnings and training in nuclear physics Jim Cronin was born on September 29, 1931, in Chicago, Illinois, and raised in the Dallas sub- urb of University Park, Texas, where his father was a professor of classical languages at Southern Methodist University (SMU). As an undergraduate at SMU, he published his first scientific paper on research done with electric eels [1] and presented his studies to the Texas Academy of Science. This early experience of scientific research made him to discover his love “to produce and analyze numer- ical data no matter what the source” [2]. Graduate work started in 1951 at the University of Chicago with an impressive list of teachers that included Enrico Fermi, Edward Teller, Maria Goeppert-Mayer, Edward Adams, Gregor Wenzel, Murray Gell-Mann, Val Telegdi, and Subrahmanyan Chandrasekhar. The PhD thesis, done under supervision of Samuel K. Allison at the local 2 MeV Van de Graaff Ac- celerator, was addressed to measuring “Excitation functions and angular distributions of α-particles leading to the ground and first excited states of Be7 in the reaction B10(p,α)Be7”. Jim defended his work in 1955 at an age of only 24 and got it published in Physical Review [4]. The paper received a total of 14 citations up to now, the lastest being from a re-measurement and extension towards lower energies in 2017 [3]! The first postdoc position lead him together with his wife and their baby to Brookhaven National Laboratory to contribute to the physics program at the 3-GeV Cosmotron. While measuring and analysing within the optical model the absorption and diffractive cross sections of pions on nuclei ranging from carbon to lead, Jim found in his character “a strong desire to contribute significantly to any project in which [he] was involved” [2], an attitude that he would rigorously follow for all his career. While developing a detector to measure K+ mesons from pion-proton collisions over a large solid angle, parity violation was discovered in the β-decay of 60Co by Chien-Shiung Wu [5]. The impact of this fundamental discovery is well known nowadays, and it immediately led Jim to conduct his first experiments in which he felt to have had a creative role. To measure parity violation by asymmetries in as many weak decays as possible, including those of Σ+ and Σ− hyperons, was a natural next 1.