Ingo Sick, Prof. Dr. Dr. h. c. of the University of Basel, passed away on 30 May 2021, two days after his 82nd birthday. During his postdoctoral period at Stanford University with Nobel Prize winner Robert Hofstadter, he experienced the beginnings of electron research with the world's best electron accelerator at the time, SLAC. This domain of research was to remain with him forever.
Ingo Sick then spent several years doing research at CEN Saclay. In 1987, he and Bernard Frois were awarded the Bonn Prize "For their elegant studies of nuclei using high-energy electron scattering. In particular, their precision measurements of nuclear charge and current densities have offered novel perspectives on ground states and valence orbitals. Their studies of few-nucleon systems have demonstrated the need for sub-nucleon degrees of freedom in a complete description of the nucleus. This body of work has provided firm benchmarks against which to test our understanding of the nuclear many-body problems (citation from the laudation)." Indeed, Ingo’s measurements of the electromagnetic structure of light few-body nuclei, including determination of the proton, deuteron, helium-3, and hydrogen-3 elastic form factors, still set a standard in the field today.
In 1983 Ingo Sick became associate professor in Basel and in 1993 full professor of experimental nuclear physics at the Department of Physics of the University of Basel until his retirement in 2004. For many years he was a research councilor and member of the Foundation Council of the Swiss National Science Foundation. In 1988 Ingo Sick was awarded an honorary doctorate from Utrecht University for his services to NIKHEF.
Ingo Sick was active and successful in a broad field of electron scattering exploring the structure of nucleons and light nuclei. He developed an unerring instinct for data analysis and experimental techniques. Early on he recognized the advantages of double polarization experiments due to the amplification of small fractions in the nucleon-nucleon interaction by interference of the wave functions leading to a reduction of systematic errors by measuring asymmetries instead of absolute cross sections. This resulted in several measurements of the electric form factor of the neutron at MAMI, Mainz, and at Jefferson Lab, Virginia, to whose nuclear physics program he contributed significantly, in particular to the Hall C experiments "x > 1" and the investigation of short-range correlations in the nucleon-nucleon interaction using quasi-elastic (e,e’p) scattering experiment off C, Al, Fe and Au nuclei. In the latter experiments the so-called spectral function was extracted, showing that the “missing strength” of the valence proton orbitals is shifted to larger excitation energies and higher proton momenta, significantly increasing towards the heavy nuclei. This still remains a hot topic not only at Jefferson Lab and its interest is reflected in many theoretical works all over the world.
Ingo Sick was also active at the Paul Scherrer Institute, where, together with Jürg Jourdan and several doctoral students and postdocs from Basel, he conducted experiments in the NE-C on the spin-dependent part of the nucleon-nucleon potential exploiting polarised protons from Injector I and neutrons from secondary reactions. He strongly advocated that the Injector I cyclotron should not be shut down, which indeed did not happen until after his retirement. Later, Ingo Sick was very interested in the measurements on muonic hydrogen (https://indico.psi.ch/event/4512), which were performed on a beamline at the meson production target E of PSI’s high intensity proton accelerator HIPA facility, resulting (A. Antognini et al. Science 339, 417-420 (2013)) in 4 % smaller proton radius than from electron scattering. This discrepancy is still not fully understood. Or in Ingo's words, "Many of the ideas that have been stated have all been looked at in more detail. Nobody has come up with a clear result. The idea of fundamental differences between muons and electrons is sort of hard to imagine" (https://doi.org/10.1038/nature.2013.12289).
Ingo was not only active in experimental nuclear research, but also collaborated with numerous theorists. In total he published over 300 papers, many with substantial contribution from him. Ingo Sick was relentless in physics, always searching for the truth. Due to his vast experience with experiments, he immediately recognized when something was not going right and could even give the right advice by remote diagnosis. He was gentle and full of good advice to his doctoral students, postdocs, postdoctoral researchers and staff. We all enjoyed his parties with good wine and meat roasted over the fireplace, the annual asparagus dinners in France and his conversations, peppered with humor and charm as well as a portion of self-irony. We'll miss Ingo.
Daniela Kiselev (ASA/GFA)
Jürg Jourdan, Bernd Krusche, Friedrich-Karl Thielemann, Dirk Trautmann (University of Basel)
Adrian Honegger, representing all the former doctoral students, postdocs and habilitation students of Ingo Sick
[Published: November 2021]