Collaborative Research: A Search for the Electric Dipole Moment of the Neutron

California Institute Of Technology, Pasadena CA

Investigators

Abstract

To search for new laws of nature beyond our current understanding, researchers can use very high energy accelerators, such as the Large Hadron Collider at CERN, or alternatively they can make precise experimental measurements of basic particle properties and search for deviations from the known physical laws. For example, the universe now has much more ordinary matter than anti-matter but 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 early particles and interactions, common at the time of the Big Bang, in quantities like the electric dipole moment of the neutron. That characteristic of neutrons reflects the separation of the neutron's internal positive and negative charges. This award supports design and construction of highly specialized instrumentation that is needed to precisely measure the neutron's electric dipole moment. In addition to advancing our knowledge in sub-atomic 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. Some technological developments are expected as several SBIR and STTR grants related to the experimental apparatus have been awarded to date. 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. 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 will be produced by the collision of 8.9 angstrom 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 award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.