On 27 June 2015, David Bruce Cline, UCLA Distinguished Professor of Physics and Astronomy and an influential experimentalist who sought to understand the world of elementary particles and forces, died from cardiac arrest on the UCLA campus.
Cline was remarkably passionate about teaching and research, and he remained active in both until his death. In addition to teaching standard courses in astronomy and physics, he greatly enjoyed presenting modern physics to students in UCLA’s special freshman seminars and honors courses. His authorship of seven articles in Scientific American testifies to his dedication to sharing his research interests with a wide audience. Cline’s profound curiosity drove his scientific path, in which he branched out from particle physics to related topics, including new methods of accelerating particle beams and the quest to identify the nature of dark matter.
Born in Rosedale, Kansas, on 7 December 1933, Cline attended Kansas State University, where he received a BS in 1959 and an MS in 1961, both in physics. He completed his PhD thesis, “A study of some rare decay modes of the positive kaon,” in 1965 at the University of Wisconsin–Madison under the supervision of William Fry. At the time, scientists hypothesized that the weak force was carried by an electrically charged particle, the W boson, and a crucial question was whether it also had an electrically neutral carrier, the Z boson. With his collaborators, Cline looked at the decays of charged kaons, and their observations seemed to rule out the existence of the Z, assuming that the Z, like the W, could change quark strangeness.
Starting in the late 1960s, Cline, Carlo Rubbia, Alfred Mann, and colleagues used neutrino beams at Fermilab in a series of experiments to study the weak force. Following claims in 1973 by CERN scientists that some neutrino interactions required the Z boson, Cline and his collaborators reached varying conclusions about the existence of such interactions in their own data and ultimately agreed with the CERN results. How that conformed with the older kaon-decay data was meanwhile solved by theorists Sheldon Glashow, John Iliopoulos, and Luciano Maiani by invoking a fourth type of quark, charm.
As a member of the physics department faculty at the University of Wisconsin–Madison, which he joined after receiving his PhD, Cline cofounded the “pheno group,” which consisted of experimenters and theorists working closely together on the design of experiments, the construction of theoretical models, and the comparison of data with model predictions. In 1969 he and Vernon Barger published a book on phenomenological theories of high-energy scattering using Regge poles. In 1973 Cline, Francis Halzen, and John Luthe interpreted the production of large-transverse-momentum particles as a consequence of the parton structure of hadrons.
In 1976 Rubbia, Peter McIntyre, and Cline proposed using existing proton accelerators to make antiprotons and smash them head-on with protons to provide enough energy to produce W and Z bosons. First carried out at CERN, the approach led to the discovery of the W and Z in 1983 by physicists including Cline. Rubbia and Simon van der Meer shared the 1984 Nobel Prize in Physics “for their decisive contributions to the large project.”
Cline moved to UCLA in 1986. He pushed for the physics and astronomy department to follow new research paths, particularly in accelerator physics, a rapidly growing field with applications in many other fields, including x-ray free-electron lasers. He recruited Claudio Pellegrini, the university’s first faculty member dedicated to accelerator physics, who in turn recruited other leaders in the field. Collaborating with scientists at Brookhaven National Laboratory, Cline and his group worked on leading-edge experiments on advanced accelerators. He also was a strong advocate of developing a muon collider as an alternative to electron–positron colliders.
Cline chose in the early 1990s to work on CERN’s Large Hadron Collider rather than on the supercollider under construction in Texas, and he was one of the founders of CERN’s Compact Muon Solenoid (CMS) experiment. After the supercollider was canceled in 1993, other UCLA professors joined the CMS collaboration, and the UCLA group eventually grew to include more faculty, undergraduates, and graduate students and to lead several aspects of the CMS experiment.
Cline’s group helped lay the foundations for using liquefied noble gases as particle detectors. He collaborated in the effort to build the ICARUS 600-ton detector in Italy, the largest to date. Cline was an early proponent of scaling up such detectors to tens of thousands of tons, a concept that others have developed into the current international collaboration to put a huge liquid-argon detector deep underground in South Dakota.
An advocate of the emerging field of astroparticle physics, Cline sought to recruit top faculty to UCLA in, for example, gamma-ray astronomy. With his group, Cline advanced the use of liquid argon and xenon to detect dark matter. Beginning in 1994 he organized a biannual conference on dark matter, which continues to be a popular, well-attended event.
With his inquisitive nature, unquenchable thirst to learn new things, and remarkable memory, Cline was unrelenting in investigating scientific ideas. He was creative and passionate about his work, and his infectious enthusiasm for developing new instruments and experimental approaches kept him always on the cutting edge of physics research.
