Willy Haeberli

Willy Haeberli, noted nuclear physicist, teacher, and member of the National Academy of Sciences, died in Madison, Wisconsin, on 4 October 2021 following a period of gradually declining health. He was known internationally for seminal ideas and instrumentation developments that motivated and were crucial for the investigation of spin effects in nuclear processes, fundamental interactions, and symmetry tests.

Haeberli was born in Zürich, Switzerland, on 17 June 1925 and moved soon to Basel, where he was educated. He received his PhD in 1952 from the University of Basel for a study of ionization of gases by alpha particles from radioactive sources. He then emigrated to the US for a postdoctoral position at the University of Wisconsin. In 1954 he married Heidi Speiser from Basel, who had followed him to the US. After a stay at Duke University, he returned to Wisconsin in 1956 to join the physics faculty. Haeberli taught there until his retirement in 2006.

Haeberli’s earliest US experiments included measurements of nucleon spin polarization following low-energy nuclear scattering or reactions. The difficulty of those experiments led him, with students and colleagues, to develop sources of spin-polarized H- and D- ion beams of ever-increasing intensity. Key to these beams’ initial use in the Wisconsin tandem accelerator was demonstration that nuclear depolarization caused by atomic hyperfine coupling was avoided if the ions’ two electrons were added or stripped away more quickly in a thin foil rather than a gas-filled canal. In 1979 Haeberli was a corecipient of the Tom W. Bonner Prize of the American Physical Society, for polarized ion source developments that had spurred rapid growth of nuclear scattering and reaction polarization studies at Wisconsin and similar facilities worldwide.

With a strong preference for studying spin effects in few-nucleon systems, Haeberli began developing gaseous targets of spin-polarized H atoms, improving polarization and avoiding contaminant nuclei present in conventional cryogenic solid polarized targets. Initially, targets used in accelerator storage rings were open-ended, thin-walled tubes to enhance the target thickness in the vicinity of the beam by flow resistance. Walls of these “storage cells” were Teflon coated to prevent depolarization. Later systems provided intense, focused jets of polarized H atoms. Such targets suffer no radiation damage and offer complete freedom in orienting the spin alignment axis. Most of this work was carried out with former student and longtime research colleague Tom Wise.

Beyond his Madison activities, Haeberli contributed extensively, until his retirement, to a number of important international nuclear spin physics measurements, such as the first definitive measurement (with unpolarized target) of the tiny parity violation in proton–proton scattering, arising from weak interaction contributions, carried out in collaboration with Markus Simonius at the Schweizerisches Institut für Nuklearforschung. Later, Haeberli’s lab provided an internal storage-cell target of polarized protons and vector- and tensor-polarized deuterons for the Cooler at the Indiana University Cyclotron Facility. This work led to a complete map of spin and spin-correlation observables in the pp and pd systems at medium energy. A similar target was installed at the HERMES experiment carried out at the HERA electron–proton collider at DESY in Hamburg for a study of quark contributions to the spin of the proton.

The Haeberli lab in Madison also contributed the internal polarized hydrogen jet target for the Relativistic Heavy Ion Collider at Brookhaven and collaborated on the calibration experiment comparing beam-spin to target-spin asymmetries in identical-particle pp scattering in the Coulomb-nuclear interference regime. This allows one to deduce the beam polarization at unprecedented energies of hundreds of GeV from the precisely known target polarization.

Haeberli was a popular classroom teacher too. He delighted in finding new ways to explain physics concepts using simple demonstrations and examples from students’ own experience, especially for a Physics and the Arts course for liberal arts students that he developed with Wisconsin faculty colleagues Ugo Camerini and Pupa Gilbert. He and Gilbert later published a similarly titled textbook for the course that has been adopted worldwide.

Haeberli and his second wife, Gabriele Haberland, were avid art collectors and generous patrons of the arts, particularly for the Madison Museum of Contemporary Art, the University of Wisconsin Chazen Museum, and Tandem Press.

Haeberli was the research adviser to some 40 doctoral students and a dozen research associates, including the three of us. Many remember his penchant for detailed, thorough experimental planning and execution, for careful later data analysis, and for oral exposition or written manuscripts clear enough not to be misunderstood.

His many graduate students and colleagues also remember fondly his love of frequent, relaxed socializing, at a summer picnic after a long hike, or while sharing funny stories or jokes over a delicious meal at his home—often one he had personally prepared. Whether as a revered mentor, respected teacher, professional colleague, or close friend, Willy’s scientific and personal influence for many was large, and will long be remembered.