TUNL Research Groups

Medium Energy (MEP) Group - Neutron Electric Dipole Moment (EDM) Experiement

Group Members / Progress / Related Papers / LANSCE Neutron EDM Experiment /


Precision measurements of the properties of the neutron present an opportunity to search for violations of fundamental symmetries and to make critical tests of the validity of the standard model (SM) of electro-weak (EW) interactions. Recently, a new experiment was proposed on a search of the neutron electric dipole moment (EDM) with an unprecedented sensitivity, by physicists from U.S., Canada, France, Germany, and the Netherland. The proposed search will have a two orders of magnitude improvement over the current neutron EDM limit in the Particle Data Book.

     A search for a non-zero value of the neutron EDM is a direct search of the time reversal symmetry (T) violation, which is a unique way of search for CP (charge conjugation C and parity P) violation because of CPT invariance. The Standard Model (SM) prediction for the neutron EDM is below the reach of the current limit of EDM measurement by six orders of magnitude. However, many proposed models of electroweak interaction which are extensions beyond the SM predict much larger values of neutron EDM. The proposed experiment has the potential to reduce the acceptable range for predictions by two orders of magnitude and to provide a significant challenge to these extensions to the SM and to search for New Physics Beyond the SM. Furthermore, if new sources of CP violation are present in nature beyond the CKM (after Cabibbo, and Kobayashi, and Maskawa) mechanism in the Standard Model and are relevant to hadronic systems, this experiment offers an intriguing opportunity to measure a non-zero value of neutron EDM. Our understanding of the origins of baryogenesis provides one reason for thinking that other sources of CP violation might exist in nature beyond what's described in the Standard Model and beyond what have been observed so far. To explain the baryon number asymmetry in the universe through the grand unified theory or electroweak baryogenesis, substantial {\bf New Physics} in the CP violation sector is required.

     The proposed neutron EDM experiment is based on the nuclear magnetic resonance technique. The overall experimental strategy is to form a three-component fluid of ultracold neutrons (UCN) and 3 He atoms dissolved in a bath of superfluid 4 He at 300 mK. When placed in an external magnetic field (B 0 =1 mG), both the neutron and 3 He magnetic dipoles will precess in the plane perpendicular to the B 0 field. The measurement of the neutron electric dipole moment comes from a precision measurement of the difference in the precession frequencies of the neutrons as modified when a strong electric field (50 kV/cm) parallel to B 0 is turned on (or reversed). Because the magnitude of the precession frequency shift due to the interaction of the neutron EDM and the electric field is extremely small, it is imperative to measure this overall precession frequency with great precision. The technique adopted in this experiment is to make a comparison measurement in which the UCN precession frequency is compared to the 3 He precession frequency. The technique relies on the spin dependence of the nuclear absorption cross section: n + 3 He -> p + t + 764 keV. The absorption cross section, for the two initial spin states of the reaction, is strongly spin dependent: the absorption process is strongly suppressed when the spins of the neutron and 3 He nucleus are aligned parallel to each other. The neutron absorption rate can be measured by monitoring the scintillation light generated in superfluid 4 He from the reaction products. The polarized 3 He nucleus also acts as a magnetometer in the proposed experiment. Knowledge of the B 0 field environment of the trapped neutrons is a crucial issue in the analysis of the systematic errors in the EDM measurement. Understanding the relaxation mechanism of polarized 3 He nuclei and maintaining their polarizations at the EDM experimental conditions is essential to the success to the entire EDM experiment. Currently, an intense effort between Duke and Caltech focused on the 3 He relaxation study is underway at Duke. A spin-exchange optical pumping technique is adopted for producing polarized 3 He nuclei and the nuclear magnetic resonance (NMR) method is employed for the study of the 3 He relaxation. The apparatus at Duke will allow the study of the 3 He relaxation down to a temperature of 1.5 K under the UCN storage cell surface conditions. Preliminary results at Duke is expected in the summer of 2003, and the final measurements from the Duke EDM apparatus will be completed by February, 2004. The ultimate test at 300 mK is anticipated to take place at University of Illinois, Urbana-Champaign, where a low temperature facility is currently being built.

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