Daniel R. Marlow, the Evans Crawford 1911 Professor of Physics, transferred to emeritus status on September 1, 2024. Dan’s fascination with the abstract mathematics that underlie physics has guided him through an outstanding career in particle physics and inspired him to share his passion with forty years’ worth of Princeton students and colleagues. “I enjoy the intellectual enterprise and the thrill of discovery,” he said. “It is a privilege to see firsthand how it all fits together.” 

Dan was born in Ottawa, Canada, in 1954, where his father was an officer in the Royal Canadian Navy. When Dan was ten, his father was assigned to a U.S. Navy facility near Doylestown, Pennsylvania, where he attended junior high and high school. At Carnegie Mellon University in Pittsburgh, he planned to study engineering but found he enjoyed physics, earning his bachelor’s degree in 1976 and Ph.D. in 1981, both in nuclear physics. While still a student, Dan proposed a clever method to measure the lifetime of hypernuclei, which other members of his group pursued after he had graduated. 

As a postdoctoral researcher and assistant professor at Carnegie Mellon University, Dan switched his focus to high-energy physics, working on the Crystal Ball experiment, first at the Stanford Linear Accelerator Center and later at the Deutsches Elektronen Synchrotron Laboratory in Hamburg, Germany. During this time, he became interested in the possibility of discovering an extremely rare decay channel of the subatomic particle K+ into a charged pion, a neutrino, and an antineutrino. Predicted by the Standard Model of particle physics to occur only once or twice per ten billion decays, observation would either give strong confirmation of the Standard Model or, if seen more copiously, indicate new physics. He drew up a “modest proposal,” but soon realized it would take significantly more resources when he learned that scientists at Brookhaven National Laboratory, Princeton University, and TRIUMF laboratory in Vancouver, Canada, were already planning such an experiment. Dan asked the researchers, “Would you mind if I come to your meetings?”

He immediately made important contributions to the design and became an integral member of the collaboration, which later included Japanese institutes. One member of the team, Princeton Professor A.J. Stewart Smith, recognized Dan’s potential and brought him to Princeton as an assistant professor. They and their colleagues quickly produced a suitable detector, which acquired its first data in 1988. Of critical importance, Dan carried out pioneering studies of how efficiently photons could be detected in a practical detector and developed an algorithm to detect the decay of stopped pions into muons. The team’s efforts paid off when they observed the extremely rare kaon decay in 1997 at the level predicted by the Standard Model, a result that was only superseded after more than twenty-five years. 

Once the rare K decay search was well underway, Dan went looking for a new challenge. The United States was eager to construct the world’s largest particle collider, the Superconducting Super Collider, in Texas. Dan led Princeton’s involvement and co-led the design of the electronics that would detect new particles, most importantly the long-sought Higgs Boson, the “holy grail.” Dan and physicists nationwide were severely disappointed when the U.S. government canceled the project in 1993, leaving Dan once again looking for a new challenge in particle physics research.

That new challenge took Dan to Japan as a major contributor to the Belle experiment, designed to search for charge-parity (CP) violation, a physical phenomenon required to explain why the universe is made up only of matter when theory predicts that matter and antimatter were created in equal amounts in the Big Bang. Discovered in 1964 by Princeton scientists Val Fitch and James Cronin, CP violation—“the Princeton effect”—eluded theoretical or experimental explanation until Japanese scientists proposed a model (named CKM after the inventors) requiring six quarks, even though only three quarks were known at the time. This model, which predicted large CP violation in decays of the fifth quark, the b, gained great credence when the b was discovered in 1977. Accordingly, Belle was formed to look for CP violation by measuring and comparing the decays of B mesons and their antiparticles, so named because they contained b quarks. 

Belle was competing with BaBar, a U.S.-based collaboration at the Stanford Linear Accelerator Center, to be the first to observe CP violation, and ironically BaBar was led by Smith, Dan’s mentor and Princeton colleague. Friendly but tough competitors, the BaBar team beat Belle to the finish line in the summer of 2001, followed by Belle just weeks later. Dan’s many contributions to Belle were second to none. As Smith put it, “If they hadn’t had Marlow, we 

would have cleaned their clocks.” Japan realized this too, awarding him the Monbusho Fellowship.

In 2008, Dan turned his attention to the Large Hadron Collider (LHC) in Geneva, Switzerland, working to improve the determination of the LHC’s luminosity—a measure of the number of particle collisions and an important factor in the discovery of the Higgs particle in 2012. He became the principal investigator for the National Science Foundation grant supporting LHC operations, overseeing two $50 million consecutive five-year grants (running from 2012 to 2021), and the deputy operations program manager for the Compact Muon Solenoid (CMS), one of the two main LHC experiments. He continues to be active in research, including publishing a 2024 study using recent LHC data to discover a rare decay mode of the eta meson, observing a clear signal that occurs less than once in 100 million eta decays. 

One of the department’s finest teachers, Dan has taught numerous courses over the years, and he led the modernization of the Advanced Physics Laboratory course required for all junior undergraduates. He has mentored numerous students and postdoctoral researchers, several of whom are now research physicists at major laboratories and universities.

Dan also shared his hobby, ham radio, with students in 2012 when he restored a 1960s-era satellite dish near the New Jersey shore. With senior research physicist Norman Jarosik, he worked with community groups and Princeton students to bring the dish back to functionality, and invited students, amateur radio enthusiasts, and the public to observe radio waves from dying stars. They also beamed radio signals to the moon and intercepted them as they came back to Earth.

Dan advanced through the ranks to become an associate professor in 1990, a professor in 1995, and the Evans Crawford 1911 Professor in 2011. He chaired the department from 2001 through 2008, leading major renovations of Jadwin Hall to establish the Princeton Center for Theoretical Science and a state-of-the-art experimental facility for quantum materials science. He has also been a prominent adviser to U.S. and Canadian funding agencies.

Dan’s science was nationally recognized in 1985, when he was awarded a highly competitive Outstanding Junior Investigator grant by the U.S. Department of Energy, and in 1996 when he was elected to fellowship in the American Physical Society “for important contributions to the physics of rare decays of the K meson, and to the integration of electronics into the design of large detectors.” 

Of special note, in 2015 Dan was the expert witness in an infamous scandal that rocked the National Football League, in which the New England Patriots quarterback was accused of deflating a football just before a championship game to make it easier to grip, an episode that came to be known as “Deflategate.” Dan provided evidence that the deflation indeed had occurred, and the quarterback was suspended for four games. 

Dan plans to retire in Ottawa and has already established connections with the Carleton University physics department, where his fascination with science will likely be contagious.

Written by members of the Department of Physics faculty and the Office of the Dean for Research.