At the high energy frontier in particle physics there are several key, unanswered questions. As yet undiscovered, more massive particles could explain a number of these mysteries. For example, in the universe today, there is much more ordinary matter than antimatter. However, no mechanism of known particle interactions can explain how the universe evolved from the matter-antimatter balance at the time of the Big Bang to the present, highly unbalanced, situation. By making precision measurements, it is possible to detect the echoes of such massive particles, common at the time of the Big Bang, in quantities like the electric dipole moment of the neutron, which just reflects the separation of the neutron's composite positive and negative charges. In addition to advancing our knowledge in particle physics and cosmology, and generating technological progress, this experiment will involve post-doctoral scholars, graduate and undergraduate students as an essential part, affording the young researchers exceptional opportunities to advance their education and training in a forefront area of nuclear physics.
The neutron electric dipole moment (nEDM) is an explicitly time-reversal-violating observable that has played an important role in descriptions of elementary particle physics; measured upper limits continue to limit extensions of prevailing models. For example, at present, supersymmetry (SUSY) predicts a value of roughly 10^-25 e-cm for a SUSY mass scale of 1 Teraelectronvolt and maximal mixing. It is clear that measurements at the scale of 10^-28 e-cm as proposed here will provide important input at combinations of very high mass scales and small mixing angles. The neutron EDM is also important for understanding the general pattern of T-(CP-) violation and the cause of the observed asymmetry of matter and antimatter in the universe.
This experiment, to be performed at the Fundamental Neutron Physics Beamline of the Spallation Neutron Source at ORNL, is based on a technique that is qualitatively different from the strategies adopted in previous measurements. The basic technique in the present experiment involves formation of a three-component fluid of polarized neutrons and Helium-3 atoms dissolved in a bath of superfluid Helium-4 at a temperature T ~ 0.5 K. The ultracold neutrons in this volume, trapped in a plastic measurement cell, are produced by the collision of 8.9 Ã…ngstrom neutrons with the phonons of the superfluid. The neutron and Helium-3 magnetic dipoles precess in the plane perpendicular to an applied external magnetic field, B0 in a traditional Nuclear Magnetic Resonance arrangement. The nEDM, dn, is determined by measuring the neutron precession frequency in the presence of a strong electric field, E0. Application of the electric field parallel (antiparallel) to B0 changes the Larmor precession frequency, nu, in proportion to dn. With B0 = 30 milliGauss and E0 = 50 kiloVolt/cm, nu = 88 Hertz and the frequency shift is 4.8 nanoHertz for an EDM of 10^-28 e-cm. Operationally, the neutron precession frequency is measured relative to that of the Helium-3 by taking advantage of the strongly spin dependent nuclear capture reaction and detecting the recoiling proton and triton via scintillation produced in the liquid Helium-4. The polarized Helium-3 atoms (in the same volume as the neutrons) also comprise a co-magnetometer (since any EDM of the Helium-3 atoms is suppressed by its atomic electrons); their precession is observed directly using SQUIDS.
This project provides important training opportunities for undergraduates, graduate students and postdocs as they participate in forefront physics research that stretches across areas including low temperature, atomic, nuclear and particle physics. The results of the experiment potentially impact particle physics as well as astrophysics and cosmology. Some technological developments are expected as several SBIR and STTR grants related to the experimental apparatus have been awarded to date.
