Three generations of University of Chicago physicists have spent decades painstakingly cataloging the characteristics of a family of exotic particles called kaons, and an upcoming experiment promises to be the most precise one yet.
“Chicago played a major role in each of the key advancements in kaon physics—every single one,” said Yau Wah, professor in physics. Wah is co-spokesperson of the K0 at Tokai (KOTO) experiment, which is currently searching for an extremely rare decay of kaons at the Japan Proton Accelerator Research Complex in Tokai, Japan.
Kaons serve as a fruitful 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 be unchanged if the universe were reflected in a mirror and each particle swapped with its antiparticle. This symmetry is upheld in most interactions, and physicists originally thought it was a fundamental property of nature. In the 1960s, however, this was discovered to be false—kaons could violate CP.
CP violation results in particles and antiparticles that behave differently, which is why the phenomenon may be a key to the existence of the universe. After the Big Bang, the universe consisted of equal amounts of matter and antimatter. Without CP violation, matter and antimatter would have annihilated until only radiation remained—yet somehow the scales were tipped in favor of matter. Exactly how that happened is one of the great unsolved mysteries of physics, and one physicists are eager to explore.
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 the UChicago faculty. He joined a UChicago department of physicists already interested in kaons, particularly Profs. Valentine Telegdi and Bruce Winstein, who completed multiple kaon experiments at Fermi National Accelerator Laboratory in the 1970s. These experiments measured the size of the kaon and a peculiar property of kaons interacting with matter, called regeneration. In 1980, Cronin won the Nobel Prize for his role in the discovery of CP violation.
The CP violation discovered by Cronin was of a type called “indirect” CP violation, because it comes from an asymmetric mixing between kaons and anti-kaons, whereas “direct” CP violation could be established only by observing a CP-violating decay of the particle. Under Winstein’s leadership, UChicago physicists Wah, Ed Blucher and Roland Winston (now at the University of California, Merced) undertook the task of finding direct CP violation through a series of experiments at Fermilab. Finally, with this all-star team, the Kaons at the Tevatron (KTeV) experiment found unquestionable evidence of direct CP violation in kaons in 1999.
Putting Standard Model to the test
Direct CP violation was a tiny effect, requiring a highly sensitive detector and multiple attempts at finding it. Now, the KOTO experiment is attempting to find an even more elusive effect, measuring a particular decay of kaons so rare that it is expected to occur only once in every 30 billion decays. The frequency of such decays can be calculated extremely precisely, to 1 percent, in the Standard Model, the well-tested framework that underlies all of particle physics. “The more precisely you can predict something—physicists go wild to measure it,” said Wah.
Measuring this decay would be a strict test of the Standard Model. Physicists hope to find evidence somewhere that the model breaks down, indicating a hint of previously unknown physics, and perhaps solving some of the remaining conundrums in particle physics.
The typical method of discovering new physics is to smash particles together at high energies and sift through the debris, at facilities like the Large Hadron Collider at CERN. The experiments at CERN are built with broad-ranging goals to search for a wide variety of processes, and as a result must be supported by large budgets and huge collaborations of researchers.
But evidence of new particles also can appear in subtler effects, called “loop effects,” which refers to a loop in a diagram describing particle interactions, causing a perturbation on an estimated quantity. Finding effects like these requires highly precise, focused experiments. The KOTO experiment takes this tactic, and therefore is smaller, less expensive, and designed to make a single measurement.
The kaon legacy is more than just history. KOTO is even reusing physical pieces of the KTeV experiment—cesium iodide crystals, used to measure the energy of the particles in the detector.
After the completion of this experiment, if nothing unusual is found, KOTO may be the last of the long line of kaon experiments at Chicago. “Perhaps the end is in sight,” Wah said.