Dr. Melissa Hutcheson, Professor Myron Campbell and Research Team Find Possible Deviation from the Standard Model of Physics
New Michigan Physics Alum Dr. Melissa Hutcheson (W’21), Research Scientist Monica Tecchio, Professor Myron Campbell, and the international research team on the KOTO experiment have produced new results from the recent data collection which may indicate a deviation from the Standard Model of physics.
Dr. Melissa Hutcheson wrote her thesis on the recent results, and thus, explains the KOTO research and findings below.
The KOTO experiment, a high-energy particle physics experiment located in Tokai, Japan, is searching for an exceptionally rare decay of a particle called the kaon. Kaons exhibit an interesting property called charge-parity, or CP violation, which indicates that the laws of physics behave differently for matter and antimatter. In fact, CP violation is one of the explanations for why we live in a matter-dominated universe, though the amount of CP violation currently observed in the Standard Model of physics doesn't account for the amount of matter we see in the universe. By studying these particles, KOTO is searching for new physics processes that may include additional sources of CP violation or undiscovered particles altogether.
The international KOTO experiment consists of 50 to 60 physicists who aim to measure how often the decay of the neutral kaon, KL→π0 νν, occurs, in which a neutral kaon (KL) decays to a neutral pion (π0) and two neutrinos. The Standard Model theory predicts that this decay only occurs once in every 33 billion kaon decays. While this decay is extremely rare, it is one of the best ways to study CP violation and search for new physics. KOTO is currently the only experiment in the world attempting to measure this process and began collecting data in 2013, utilizing the high-intensity proton beam at the J-PARC research facility to produce billions of kaons.
KOTO then collects the decay products from these billions of kaons using a particle detector system and strategically sifts through the data to identify candidates of the unique KL→π0 νν decay among the large number of other kaon decay processes that generate backgrounds.
From data collected in 2015, KOTO set the world's best experimental upper limit on the fraction of the time this decay occurs, or the branching fraction, which was measured to be less than 3.0 x 10-9. KOTO also collected data from 2016 to 2018 and recently published new results in which three KL→π0 νν candidate events were observed and two new sources of background were identified, resulting in a branching fraction upper limit of 4.9 x 10-9. These new results from the 2016-2018 data may indicate a deviation from the Standard Model of physics, though more data collection is needed to understand the true nature of the observed candidate events.
Since 2018, the KOTO experiment has continued to collect more data and will continue developing new background reduction methods and analysis techniques to continue its aim of measuring the decay at the sensitivity of the Standard Model theoretical prediction. Thus, in the coming years, KOTO will be able to tell us whether there is new physics hiding in the decay KL→π0 νν, which can help us understand how our universe came to be.
The paper, Study of the KL → π 0νν Decay at the J-PARC KOTO Experiment, in Physical Review Letters, has been highlighted by the editors as an Editors’ Suggestion. A highlighted Letter has much significance, as only one Letter in seven is highlighted due to its particular importance, innovation, and broad appeal.
In addition to being a lead author on the paper, Dr. Hutcheson encapsulates the results in the scientific image which accompanies the paper and is displayed as the leading image of the PRL cover story.