If Aldo Michelini, who has passed away at the age of 89, was a giant of physics, he was a gentle one. Aldo was known as much for his kindness and care for his colleagues, particularly those just embarking on their careers, as for the physics at which he excelled.

First coming to CERN in 1960, Aldo enjoyed a 35-year career at the Laboratory and was the driving force behind some of its most successful endeavours. In the late 60s, he was part of the small team spearheading the development of the Omega spectrometer, a flexible general-purpose device that could be arranged and configured according to the nature of the physics to be studied. Originally installed at the PS, Omega moved to the West Hall of the SPS in 1976 for the start-up of CERN’s new flagship facility and was a backbone of the SPS programme for 20 years. In 1973, Aldo headed a similar project to build a general-purpose spectrometer for the North Area. This became the NA3 experiment, which carried out a string of valuable experiments under Aldo’s guidance until 1981, when he took up the position of spokesperson of the OPAL experiment being planned for LEP. Aldo remained at the helm of OPAL right up to his retirement in 1995.

One of four experiments at LEP, OPAL was built around tried-and-tested technology. Huge for its time, a collaboration of some 300 people, OPAL was nevertheless the smallest of the four. It was a scale that lent itself well to Aldo’s unique style of management – leading through example and consensus. Colleagues remember him smiling and looking very worried, or more often than not, the other way round. This was strangely motivational, with team members striving to make him smile more and worry less. His personality shaped the very unique OPAL team spirit. If difficult decisions needed to be taken, however, Aldo’s gentle nature could be deceptive: he was more than capable of making tough choices and of winning around those who might initially have disagreed with him.

When OPAL detected the first Z particle at LEP on 13 August 1989, Aldo was heard to remark that the young people had taken over. The average age of those in the control room that day was well under 30, and that youthfulness was no accident. Aldo actively supported the young members of the collaboration, making sure that they were visible at collaboration meetings and conferences. In return, they built a strong reputation for OPAL, with many conference conversations including the words “let’s see what OPAL has to say”.

Aldo was a great leader, commanding respect and affection in equal measure. That the collaboration was still able to gather over 100 members in 2019 to celebrate the 30th anniversary of that first Z decay is testimony to the kind of person Aldo was, and to the spirit that he engendered. Although he was unable to attend that gathering, he sent a message, and was loudly cheered. He will be sorely missed.

Rolf Heuer, David Plane and Mette Stuwe on behalf of all his friends and colleagues in OPAL and at CERN