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Search for the electric dipole moment (EDM) of the neutron

Kolloquium der Abteilung 8

The discovery of an electric dipole moment (EDM) of the neutron would manifest the invariance time reversal and violate the combined symmetry of charge and parity (CP). Quite naturally many extensions to the standard model of particle physics (SM) provide new CP-violating interactions which could give rise to measurable EDMs while at the same time might explain the dominance of matter of anti-matter, known as baryon asymmetry of the Universe (BAU) [1,2,3]. Although, the complex phase of the CKM in the SM is close to maximal, it is not sufficient to explain the observed BAU, strongly motivating searches for and models of new sources of CP-violation.

I will present a new limit on the neutron EDM, |dn |<1.8×10-26 ecm [4], from an experiment performed at the Paul Scherrer Institute by an international collaboration deploying Ramsey’s method of separated oscillating magnetic fields of stored ultracold neutrons. The salient features of this experiment were the use of a 199Hg co-magnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic field changes. Two separate groups performed the statistical analysis on blinded datasets while the estimation of systematic effects profited from unprecedented knowledge of the magnetic field.

In a second part I will present the design of the new instrument, n2EDM, currently mounted at PSI, which will further increase the sensitivity to about 1×10-27 ecm [5].


[1]         A.D. Sakharov. JETP Lett. 5, 24(1967)


[2]         C.L. Bennett, D. Larson, J. L. Weiland, et al., ApJS 208, 20 (2013)


[3]         D.E. Morrissey and M.J. Ramsey-Musolf, New J. Phys. 14, 125003 (2012)

[4]         C. Abel et al., PRL124, 081803 (2020)


[5]         N.J. Ayres et al., EPJC81, 512 (2021)