Experimental Particle Physics Research Group

Neutron EDM

Why does the Universe contain vast quantities of matter, but almost no antimatter?
In order to address this fundamental question, which has perplexed cosmologists for half a century, we need to look beyond our Standard Model (SM) of particle physics.  Some of the most sensitive probes we have of such “new” physics arise from non-accelerator experiments.  One such is the search for an electric dipole moment (EDM) of the neutron.  EDMs violate both parity and time-reversal symmetries, and they therefore provide some of the tightest constraints upon models beyond the SM that attempt to reconcile the low level of CP violation observed in K and B systems with the large baryon asymmetry of the Universe.  The current world limit of 2.9 x 10-26 e.cm, set in 2006 by the RAL-Sussex-ILL EDM collaboration, already requires considerable fine-tuning of MSSM parameters - the so-called "SUSY CP problem".

The measurement is carried out by using NMR to determine the Larmor precession frequency, in parallel and antiparallel magnetic and electric fields, of ultracold neutrons trapped in a storage cell - a sort of atomic clock, but using neutrons instead of atoms. A change in frequency with applied electric field is the signature of an EDM.  The level of precision is astonishing: the system can detect an energy-level splitting of 10-21 eV, and yet it is sensitive to aspects of physics at energy scales well beyond that achievable at the LHC. 

We are part of a collaboration operating at the Paul Scherrer Institute, near Zurich (http://nedm.web.psi.ch/). The Sussex group has responsibilities that lie at the heart of the experiment: we calculate and model potential systematic errors, we will coordinate the design of an upgraded version of the so-called Ramsey chamber where the measurement actually takes place, and we will be responsible for precision magnetometry using 3He cells.