OCPA Colloquium 2022-1210 Chen-Yu Liu


We are excited to announce the Colloquium by Professor Chen-Yu Liu from University of Illinois at Urbana-Champaign. 

Title: Goldilocks and the three bears: How we made the most precise neutron lifetime measurement and why. 



Neutrons are stable inside atomic nuclei. Outside the confines of the nucleus, they decay into a proton, electron, and antineutrino, with a lifetime of ~880 s. The rate of decay can be precisely calculated, using the theory of electroweak interactions, with an uncertainty on the order of 1e-4. Recent measurements using bottled neutrons have achieved uncertainties below 1 s (0.1 %), but other measurements observing neutron decay in flight disagree by 10 s. Attempts to resolve this discrepancy have motivated much experimental effort as well as exotic theoretical conjectures, thus far without a clear conclusion. In this colloquium, I will discuss the challenges of precision measurement of the neutron lifetime, illustrating the UCNtau experiment. It eliminates the dominant loss mechanisms present in previous bottle experiments by levitating polarized ultracold neutrons using a large magnetic trap. Using this approach, a new result, 877.75 +/- 0.28 (stat) +0.22/-0.16 (sys) s [PRL 127, 162501 (2021)], is the most precise measurement of the lifetime. This result, together with improved measurements of the axial coupling constant, will provide a determination of the CKM matrix element Vud, independent of nuclear decays, and address the recent tension in the test of CKM unitarity.



Chen-Yu Liu is a Full Professor in the Physics Department and the Nuclear Physics Laboratory at University of Illinois at Urbana-Champaign (UIUC). Her research focuses on symmetry tests and fundamental neutron physic to understand matter creation and nucleosynthesis in the early Universe. She specializes in experimental tools using ultra-cold neutrons. Dr. Liu completed her doctoral research in Experimental Nuclear Physics at Princeton University, followed by a postdoctoral fellowship at the Los Alamos Neutron Science Center as a Director’s Funded Post-Doctoral Fellow. She is an alumna of National Taiwan University. She was a Sloan Research Fellow (2007-2008), a Rosen Scholar (2016-2017), and an APS fellow since 2018. She has received the NIST precision measurement grants twice to innovate the technique of neutron lifetime measurements. She taught at Indiana University from 2005-2022 and was named the James H. Rudy Professor of Physics. In 2022, she was recruited to join UIUC through the U of I Distinguished Faculty Recruitment Program. Group web page, https://ultracold.web.illinois.edu.



The colloquium will be held online at 9:00–10:30 PM U.S. Eastern Time on Saturday 12/10/2022. 


Zoom link will be announced through emails to all OCPA members. If you are an existing member, please login to check if your email is accurate. If you are not a member, please sign up for a new membership. A free one-year membership is offered through 12/31/2023.


Colloquium Flyer: