The nEDM experiment at Oak Ridge National Laboratory’s Spallation Neutron Source
Our current understanding of the creation of the universe is that it evolved from the Big Bang. Scientists believe this event generated equal populations of particles and anti-particles, which would naturally annihilate each other. However, our universe predominantly contains matter. Why is this the case? The very existence of matter violates our current understanding of the laws of physics. Regardless, the theory that most successfully describes the building blocks of nature and their interactions is the Standard Model of fundamental physics. At its foundation are the intertwined concepts of unification (uniting the various laws of physics into a single theory that explains all the different phenomena) and symmetry (particle properties remain the same after the particle is subjected to transformations).It is well-known, however, that the Standard Model is only an approximate description of the world and is incomplete. For example, it cannot explain the dark sector of the universe – dark matter and dark energy. The mechanism to generate mass for neutrinos is also completely unknown.
The scientific community is taking different approaches to address many remaining fundamental questions. Are there additional particles yet to be discovered? Can symmetries be broken in subtle ways that inform our description of the missing elements of our model? Accelerator laboratories that smash particles at extremely high energies have been a traditional approach to exploring these questions. At the Spallation Neutron Source (SNS), a user facility at the Department of Energy’s Oak Ridge National Laboratory, however, we are seeking to explore these questions from the opposite extreme: using a source of very low energy neutrons, so-called ultra-cold neutrons, to observe the behavior of neutrons influenced by a combination of weak magnetic and strong electric fields. In these extreme conditions, the neutron can expose a violation of a type of symmetry that could help explain the dominance of particles over anti-particles in our universe. The discovery of this violation of symmetry would be a fundamental breakthrough in our understanding of the world.
Although the neutron is an electrically neutral particle overall, deep within it are small positive and negative charges. An electric dipole moment (EDM) would arise if the average positions of the positive and negative charges do not coincide. The neutron also has a spin. Applying the symmetry of charge and parity – that is, flipping all the neutron quantum numbers and observing the neutron in a mirror – the spin would be reversed but not the EDM, giving a different particle that doesn’t exist. Thus, the presence of a permanent neutron electric dipole moment (nEDM) would establish that this symmetry is violated.
How do you measure the neutron EDM? The spin of a neutron will precess around a magnetic field. A neutron EDM manifests itself as a small frequency shift of the precessing spin that is proportional to an applied electric field. While injecting a spin-polarized neutron into a measurement cell with a weak magnetic field, scientists can apply a very high electric field over the measurement cell. Due to the interaction of the neutron EDM with the electric field, the precession frequency will shift ever so slightly – by less than a few parts per billion. Measuring this tiny shift will provide evidence for a neutron EDM.
The implementation of the measurement technique with the nEDM@SNS experiment is unique and ambitious. Spin-polarized cold neutrons are injected into a measurement cell filled with liquid He-4 at a temperature of 0.5 kelvin. Through the interaction with the He-4, the neutrons are cooled to become ultra-cold, allowing their storage and observation for hundreds of seconds. The measurement cell is then filled with polarized He-3 from an atomic beam source. Application of a magnetic pulse will cause the spins of the neutrons and He-3 nuclei to precess about the magnetic field at different rates, with a frequency that depends on their magnetic moments and the magnetic field strength. When the spins of the neutron and the He-3 are opposite, the two will react to form a proton and triton, which creates a flash of scintillation light in the liquid helium that is measured. Since the spins of the neutron and He-3 precess at different rates, the scintillation light signal is modulated. This modulation frequency is measured as a function of the electric field.
The nEDM@SNS experiment aims at measuring the neutron EDM to a precision that is two orders of magnitude better than the state of the art and better than any planned competitor. To achieve this ambitious goal, the experiment uses liquid He-4 also as a dielectric to obtain an electric field of 75 kV/cm generated by a Cavallo voltage multiplier that reaches an absolute voltage of 630kV, the highest electric field projected to be achieved in a neutron EDM experiment. The experiment has two identical measurement cells with the electric field in opposite directions to control systematics. The experiment relies on He-3 to measure the magnetic field in the storage cell to a precision of one-billionth of the Earth’s magnetic field and to correct for variations in the magnetic field that might otherwise be mistaken for a neutron EDM signal. Advanced materials are exploited to shield the experiment from the Earth’s magnetic field and to obtain very high electrical gradients without breakdown. The precision goal is unprecedented: if you can imagine scaling up the neutron to the size of the Earth, the experiment is equivalent to looking for a single positive and negative electric charge separated by less than a micrometer at its center.
Searches for particle electric dipole moments take a prominent place in modern physics because of their bearing on the origin of symmetry violations and the implication these have on understanding the evolution of the universe. The puzzle of what exactly did happen to matter and antimatter after the early days of the universe remains to be solved. This search for the neutron EDM could bring us a step closer to understanding why we are living in a material world. Measurement of the nEDM at the proposed sensitivity constitutes a probe for new physics at a much higher energy scale than can be reached with even the most energetic particle accelerators. Thus, even if the experiment delivers a null result and is only able to set an upper limit on the neutron EDM, it will have profound implications, indicating that the new physics that would complete the Standard Model cannot be probed with the highest energies that can be obtained in the world using particle accelerators.
Summary
Why does matter exist? We don’t know – the very existence of matter violates our current understanding of the laws of physics. But clues may be found in a precise measurement of the neutron’s Electric Dipole Moment (EDM), a tiny displacement between positive and negative electric charge inside the neutron. It manifests itself as a small frequency shift of a precessing neutron spin, proportional to an applied electric field. The experiment with the best planned sensitivity, two orders of magnitude better than the state of the art and better than any planned competitor, is the nEDM@SNS experiment being mounted at Oak Ridge National Laboratory’s Spallation Neutron Source. This experiment takes advantage of an amazing confluence of superfluid helium properties, which increases the number of neutrons, increases the strength of the applied electric field, and suppresses systematic errors. The nEDM@SNS experiment is now being constructed at SNS’s Fundamental Neutron Physics Beam Line.
