It is with great sadness that we learned of the passing of our colleague of many years, Bill Willis, who died on 1 November after a short illness.

Active in physics until his very last days, Bill’s career spanned more than 50 years at the forefront of particle physics.

After his early career in weak interaction studies using bubble chambers in the USA, he was persuaded to join CERN in 1973, where he was introduced by Jack Steinberger as "the cleverest physicist I've ever met".

He was attracted by the physics potential of the CERN ISR to which he brought a vision of a new detector concept – a 4π facility with charged and neutral particle detection capability, which he called the "Impactometer". Its possibility of measuring "missing energy" and the importance of direct lepton detection were prescient concepts of immense discovery potential. The first incarnation of the "Impactometer" was the "Axial Field Spectrometer" at the ISR, which detected high-pt jets in hadronic collisions at the same time as UA1 and UA2. This approach, considered adventurous at hadron colliders at the time, is now the standard detector concept. Subsequently he turned his interest to nuclear matter under extreme conditions of temperature and density: he convinced the CERN management to adapt the SPS to the acceleration of heavy ions, including lead ions, thus opening a new field, which flourished at RHIC and now also at the LHC.

He returned to the United States in 1990, taking up a professorship at Columbia University. In 1993 Bill was part of the first US delegation to visit CERN to explore the US joining the LHC experiments. Some of the ideas from earlier concepts were promoted by Bill and found their way into ATLAS. Bill served as the US ATLAS construction Project Manager until 2005 and was a member of the ATLAS Executive Board for four years.

Most recently he was involved in the MicroBooNE experiment, in which he combined his talent for developing ingenious approaches and his interest in novel detectors to the field of neutrino physics.

Bill was truly a "Renaissance physicist" with remarkably wide-ranging research interests. He was familiar with advanced accelerator physics, having developed a novel plasma-type accelerator scheme in the early 70s.  He is of course well known for the introduction of several novel detector concepts – including Liquid Argon Calorimetry and Transition Radiation Detectors, which would be adopted by many experiments, including ATLAS. In recognition of his many contributions he was awarded the 2003 Panofsky Prize of the APS.

Bill worked with great distinction both here at CERN and in the USA and forged fruitful links with several Russian physicists who became collaborators and friends. He grasped, earlier than many of us, the importance of the international dimension to our field, and showed how we could be both competitive, in a friendly way, and collaborative.

He carried his deep knowledge and experience with the grace of the true scholar: he was hugely respected and admired, and he will be very much missed.

His friends and colleagues at CERN