Secrets of the Neutron: How the Electric Dipole Moment (EDM) Reveals CP Violation and the Origin of the Universe
I have just released the latest explanatory video on Nico Nico Douga! First, please be sure to watch the full video via this link 🎥
In this video, we explore the greatest mystery in physics—CP violation and the asymmetry between matter and antimatter in the universe—through the search for the "neutron electric dipole moment (nEDM)."
If a neutron possesses even the slightest electrical bias (EDM), it signifies a violation of parity (P) symmetry and CP symmetry. This is directly linked to the fundamental question: "Why is the universe filled with matter instead of being an empty world of only light?"
🧩 Content Structure
What is an EDM?: Symmetry breaking seen from spin and charge distribution
The limits of the Standard Model (SM) and new CP violation predicted by BSM (such as supersymmetry)
Ultracold neutrons (UCN) and precision measurement using the Ramsey separated oscillatory fields method
The phantom of "systematic error": The battle against false signals using simultaneous mercury magnetometry
Cutting-edge experimental facilities like PSI (n2EDM) and TRIUMF (TUCAN), and nEDM as a "cosmic seismograph" to unravel the matter dominance of the universe
💡 For those unfamiliar with the technical background! 3 highlights of the video
① The reason the universe didn't end up empty is "a tiny difference in the rules"
Immediately after the Big Bang, matter and antimatter should have been created in equal amounts as pairs. Normally, they would have collided and annihilated, leaving behind an empty world of only energy (light). However, in reality, about one in a billion particles of matter survived to create stars and ourselves. The key to this survival is a tiny difference in the rules for matter and antimatter, known as "CP violation."
② Measuring the "tiny compass" inside the neutron
Investigating whether there is an electrical bias (EDM) inside a neutron is like searching for a "tiny electrical compass needle" along the axis of its spin. By measuring this, we can open the door to new physics that transcends the limits of the current foundation of physics (the Standard Model).
③ "Ultracold neutrons" cooled to the speed of a running person and the battle against phantoms
To capture such incredibly small changes, we use "ultracold neutrons (UCN)" cooled to 7 meters per second (about the speed of a running person). We trap these in a bottle for over 100 seconds and observe them using a mechanism like an ultra-high-precision clock. Furthermore, the greatest enemy in the experiment is the false signal (phantom) known as "systematic error" caused by fluctuations in the magnetic field. The drama of the scientists' painstaking ingenuity and struggle, such as using mercury atoms like spies to monitor the magnetic field, is a must-see.
🩵 Supplement: Why does EDM imply CP violation?
The existence of an EDM means there is an electrical bias in the direction of a particle's spin. When a neutron is reflected in a mirror, the direction of its spin rotation is reversed, but the direction of the EDM bias does not change. This state means that the laws of physics change between the real world and the mirror world, indicating a violation of parity (P) symmetry. Since this P-symmetry violation is directly linked to "CP violation"—which combines it with the symmetry (C) that swaps particles and antiparticles—the existence of an EDM (EDM ≠ 0) serves as a "sign of CP violation" that explains the mystery of why only matter remains in the universe.
Next-generation experiments aiming for an astonishing sensitivity of 10 to the power of minus 27 are approaching the "promised land" where signals of new physics might be found. Please enjoy the latest page of this grand detective story, where all of modern science and technology is poured into listening for the fluctuations of the Big Bang, in the video! 👉
📚 References and Source Materials
The Neutron EDM Experiment — Philip Harris (2007) https://arxiv.org/pdf/0709.3100.pdf
A Revised Experimental Upper Limit on the Electric Dipole Moment of the Neutron — J.M. Pendlebury et al. (2015) https://arxiv.org/abs/1509.04411
The Quest for an Electric Dipole Moment of the Neutron (2016) https://arxiv.org/pdf/1602.01997.pdf
A Strange Contribution to the Neutron EDM — Luca Vecchi (2025) https://arxiv.org/pdf/2506.23402v1.pdf
Apparatus for Measurement of the EDM using a Cohabiting Atomic-Mercury Magnetometer — C.A. Baker et al. (2013) https://arxiv.org/abs/1305.7336
The Design of the n2EDM Experiment — Ayres et al. (2021) https://arxiv.org/abs/2101.08730
The n2EDM Experiment at the Paul Scherrer Institute — Abel et al. (2018) https://arxiv.org/pdf/1811.02340.pdf
The Search for the Electric Dipole Moment of the Neutron with the n2EDM Experiment — EuroPartStrategy2026 report https://indico.cern.ch/event/1439855/.../n2EDM___EuroPartStrategy2026.pdf
A Large Active Magnetic Shield for a High-Precision Experiment — Abel et al. (2023) https://arxiv.org/pdf/2307.07588v1.pdf
Current Status of nEDM Experiments — Martin J. (2022) https://arxiv.org/pdf/2103.01898.pdf
TUCAN EDM — TRIUMF Ultra-Cold Advanced Neutron Project — Martin J. (2023) https://indico.triumf.ca/event/413/.../jmartin-pp-eec-april-2023-final.pdf
TUCAN EDM — Japanese/Canadian Collaboration — Kawasaki S. (2024) https://conference-indico.kek.jp/.../nEDM_kawasaki_FPUA2024.pdf
How Neutron EDM-Experiments Really Work I — Lecture Notes (CERN Indico) https://indico.cern.ch/event/957898/.../nEDM_1.pdf
Electric Dipole Moments of Neutron-Odd Nuclei — Fujita & Oshima (2011) https://arxiv.org/abs/1102.2976