Particle physics research

60 years of UChicago particle physics research culminates with experience in Japan

Newswise – Three generations of University of Chicago physicists have spent decades painstakingly cataloging the characteristics of a family of alien particles called kaons, and an upcoming experiment promises to be the most accurate yet.

“Chicago has played a major role in each of the key advancements in kaon physics, each one,” said Yau Wah, professor of physics. Wah is a co-spokesperson for the K0 at Tokai Experiment (KOTO), which is currently researching an extremely rare kaon decay at the Japan Proton Accelerator Research Complex in Tokai, Japan.

Kaons serve as a successful laboratory for physicists because of their tendency to undergo processes that violate charge parity, or CP symmetry. CP symmetry requires that the laws of physics remain unchanged if the universe is reflected in a mirror and each particle is swapped with its antiparticle. This symmetry is maintained in most interactions, and physicists originally believed it to be a fundamental property of nature. In the 1960s, however, this was discovered to be wrong: Kaons could rape CP.

CP violation results in particles and antiparticles that behave differently, which may explain why the phenomenon is a key to the existence of the universe. After the Big Bang, the universe was made up of equal amounts of matter and antimatter. Without the CP violation, matter and antimatter would have annihilated each other until all that was left was radiation. Yet somehow the scales were tilting in favor of matter. How exactly this happened is one of the great unsolved mysteries in physics, and a physicist is eager to explore it.

The tradition of kaon research at UChicago began in the 1950s, with theoretical research by assistant professor and future Nobel laureate Murray Gell-Mann. After James Cronin, SM’53, PhD’55, and Val Fitch discovered CP violation in kaons at Brookhaven National Laboratory in 1964, Cronin joined UChicago faculty. He joined a UChicago department of physicists already interested in kaons, in particular Profs. Valentine Telegdi and Bruce Winstein, who performed several kaon experiments at the Fermi National Accelerator Laboratory in the 1970s. These experiments measured kaon size and a particular property of kaons interacting with matter called regeneration. In 1980, Cronin won the Nobel Prize for his role in uncovering the CP violation.

The CP violation discovered by Cronin was of a type called “indirect” CP violation, as it arises from an asymmetric mixture of kaons and anti-kaons, whereas the “direct” CP violation could only be established by observing a decay violating CP of the particle. Under Winstein’s direction, UCicago Wah physicists Ed Blucher and Roland Winston (now at the University of California, Merced) undertook the task of finding a direct violation of CP through a series of experiments at Fermilab. Finally, with this all-star team, the Kaons at the Tevatron Experiment (KTeV) found compelling evidence of direct CP violation in kaons in 1999.

Put the standard model to the test

Direct CP violation was minimal effect, requiring a very sensitive detector and multiple attempts to find it. Now, the KOTO experiment tries to find an even more elusive effect, measuring a particular kaon decay so rare that it should only occur once every 30 billion decays. The frequency of these decays can be calculated extremely precisely, at 1%, in the Standard Model, the well-tested framework that underpins all particle physics. “The more accurately you can predict something, physicists go wild to measure it,” Wah said.

Measuring this decrease would be a strict test of the Standard Model. Physicists are hoping to find evidence somewhere that the model is collapsing, indicating a hint of previously unknown physics and perhaps solving some of the remaining puzzles in particle physics.

The typical method of discovering new physics is to smash particles together at high energy and sift through the debris, in facilities like the Large Hadron Collider at CERN. The experiments at CERN are built with far-reaching objectives to research a wide variety of processes and, therefore, must be supported by large budgets and huge collaborations of researchers.

But evidence for new particles can also show up in more subtle effects, called “loop effects,” which refer to a loop in a diagram depicting particle interactions, causing disturbance over an estimated amount. Finding effects like these requires very precise and focused experiments. The KOTO experiment uses this tactic and is therefore smaller, cheaper, and designed to take a single measurement.

The legacy of the kaon is more than history. KOTO even reuses physical pieces from the KTeV experiment, cesium iodide crystals, used to measure the energy of particles in the detector.

After the completion of this experiment, if nothing unusual is found, KOTO could be the last in a long line of kaon experiments in Chicago. “Maybe the end is in sight,” Wah said. —Emily Conover


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