A New Search for the Electric Dipole Moment of the Neutron and ``what next''?

Speaker: Dr. Prajwal Mohan Murthy
Affiliation: MIT
Date and time: April 24^th, 4 pm
Abstract:

CP violation and consequently T violation endows the neutron with a non-zero electric dipole moment. Thus far, the known sources of CP violation in the Standard Model have been insufficient to explain the observed baryon asymmetry of the universe. Measuring a statistically significant nEDM is one way to gain a handle on the sources of CP violation in the SM and beyond. Therefore, a search for a permanent nEDM has been an ongoing effort for well over half a century. The Paul Scherrer Institute Neutron Electric Dipole Moment (PSI nEDM) experiment is a room temperature experiment using the Ramsey technique of separated oscillating fields to search for a permanent electric dipole moment in neutrons. The PSI nEDM experiment uses ultracold neutrons (UCNs) and a cohabiting ¹⁹⁹Hg magnetometer. But, the key upgrades of the PSI nEDM over previous generations of the experiments come in the form of a simultaneous spin analyzer and an array of ¹³³Cs magnetometers. These upgrades provided PSI nEDM with an unprecedented control over systematics, thereby improving the systematics budget by a factor of 6. The PSI UCN source also is the most intense source of UCNs in the world, and this gave the experiment the best statistical sensitivity. The PSI nEDM experiment collected data from 2015 to 2017, and represents the most precise measurement of the EDM of the neutron. We will describe the background of nEDM in light of a plethora of other EDM searches, the experimental apparatus itself, and finally the result.

We will also discuss modern approaches being investigated to search for CP violating moments in deformed nuclei and atoms, where the moments are vastly enhanced. From Wigner-Eckhart theorem, electric 2^2N+1-pole moments and magnetic 2^2N-pole moments are T and P odd, and consequently also violate CP and CT. Nuclear magnetic quadrupole moment (MQM), the central topic of this work, is CP, P, and T violating. Nucleons and nuclei have a non-zero MQM from sources within the SM. But the nuclear MQM is dramatically enhanced if the nuclei are structurally quadrupole deformed. Multiple sources contribute to an atomic EDM, viz. nuclear EDM, CP violating interactions between the electrons and the nuclei, and finally the nuclear MQM also contributes to the atomic EDM. Unlike nuclear EDM which are shielded by the Schiff effect, nuclear MQMs are not shielded, making the search for them an attractive avenue to probe CP violation.