A next generation measurement of the electric dipole moment of the neutron at the FRM II



The electric dipole moment of the neutron

An electric dipole moment of a quantum system would violate time-reversal symmetry (T) breaking effects at low energies. Assuming the conservation of CPT, violation of T also implies CP violation. Currently, such effects have only been observed in the decays particles like the K meson. In the Standard Model of particle physics (SM) this is accommodated for by a complex phase in the CKM matrix. However, this is by far too small to explain the observed Baryon asymmetry in the universe (BAU). As pointed out by Sakharov already in 1967, the explanation of this problem requires new sources of CP violation, baryon number non-conservation and processes out of thermal equilibrium. In addition, there is the unexplained question why the strong interaction does not violate CP, as it would naturally be expected by the CP violating product of the gluon operator and its dual within the QCD Lagrangian, weighted by a strongly restricted parameter theta.
Basically all popular models for physics beyond the SM, in particular Supersymmetry, naturally suggest EDMs close to the current experimental upper limits. Our goal is to lower the experimental bound by > 100 within the next four years, increasing the sensitivity to 5·10-28ecm (3σ).


As a measurement method we will use the commonly known method of separated oscillatory fields by Ramsey. This is an interferometric nuclear magnetic resonance method applied to polarized and trapped ultra-cold neutrons. Spin polarized UCN precess in a highly homogeneous and constant magnetic field of ˜1&mycro;T. In an electric field applied along the magnetic field, a non-zero EDM would cause an additional level splitting, thus changing the Larmor frequency.



Ramsey’s method of separated oscillatory fields: starting with an ensemble of polarized ultra-cold neutrons, the polarization is flipped into a precession plane normal to the constant field B0. During a long period of free precession, an additional electric field E is applied parallel or anti-parallel to B0, causing a small phase in the angle after precession. As the spin is flipped back along B0, the deviation is analyzed.