Born in 1943 in San Luis de Potosí, Mexico, Arnulfo Zepeda graduated in nuclear engineering from the University of Prague in 1967. He then joined Cinvestav (the Centro de Investigación y de Estudios Avanzados of the Instituto Politécnico Nacional), where he obtained a PhD in physics in 1970, and then Rockefeller University, where he obtained a PhD in elementary particle physics in 1972.

Arnulfo’s initial interest was in theoretical particle physics and he became known for his studies in chiral symmetry breaking, nucleon form factors and the up-quark mass. He created a young and very active particle-physics group at Cinvestav and became one of the leading figures in theoretical physics in Mexico and Latin America.

In the late 1990s Arnulfo’s attention turned to ultra-high-energy cosmic rays. Leading Mexico’s participation in the construction and exploitation of the Auger Observatory in Argentina, he contributed significantly to important discoveries such as establishing the GZK ultra-high-energy cut-off, the nuclear composition of ultra-high-energy cosmic rays and the observation of possible point-like cosmic-ray sources. His activities in this field continued in Mexico with the promotion of the HAWC gamma-ray observatory and participation in its collaboration board.

At the beginning of the new millennium, Arnulfo was among the initiators of the participation of Mexican scientists in CERN’s experimental programmes, in particular the ALICE experiment. In 2002 he formally joined ALICE, where he was involved in the installation of the ACORDE detector, which has acted as a cosmic-ray trigger. ACORDE not only provided precious calibration data but also, together with other ALICE sub-detectors, precise information on cosmic rays with primary energies in the range 1015–1017 eV.

Arnulfo was an associate (1982–1988) and senior associate (1998–2003) at ICTP, Trieste, and was elected fellow of the American Physical Society in 1993 for his original research in high-energy physics and phenomenology, his leadership in high-energy physics in Mexico, and his initiatives in promoting closer communication among physicists in North America. Contributing greatly to the advancement of physics in Mexico and Latin America, he founded the Escuela Mexicana de Partículas y Campos, chaired the División de Partículas y Campos of the Sociedad Mexicana de Física (SMF) in 1991–1992, and was president of SMF between 1992 and 1994. He also directed more than 30 theses and was considered an enthusiastic and outstanding professor, devoted to his students.

In the years 2005 to 2016 Arnulfo promoted Europe–Latin America scientific exchanges as a member of the executive board of two projects financed by the European Commission: HELEN (High-Energy Physics Latin-American European Network) and EPLANET (European Particle Physics Latin American Network). These projects have mobilised exchanges for more than 3000 person-months, making possible the significant participation of physicists from Latin America in the discoveries made at the LHC. In his later years, Arnulfo was the founding director of MAIS (Mesoamerican Institute for Science) at the Universidad Autónoma de Chiapas – an initiative started by ICTP to establish a network of partner UNESCO institutes around the globe. Arnulfo took on this task with great enthusiasm, moving there from 2011 to 2018, and MAIS was established as a category-two UNESCO institute in 2020.

Arnulfo’s broad and deep scientific interests and his human qualities have been instrumental in the growth of high-energy and astroparticle physics in Mexico in recent decades, and raised the profile of the Mexican scientific community at the international level, both at CERN and in international cosmic-ray experiments. He was an excellent scientist and a precious friend for many of us at CERN.

Paolo Giubellino (GSI-FAIR), Luciano Maiani (Università di Roma) and Luciano Musa (CERN).

This obituary will be published in the May-June issue of the CERN Courier.