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تسجيل مستخدم جديدThe quest for an electric dipole moment of the neutron
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P. Schmidt-Wellenburg
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Until this day no electric dipole moment of the neutron (nEDM) has been observed. Why it is so vanishing small, escaping detection in the last 50 years, is not easy to explain. In general it is considered as the most sensitive probe for the violation of the combined symmetry of charge and parity (CP). A discovery could shed light on the poorly understood matter/anti-matter asymmetry of the universe. As nucleon it might one day help to distinguish different sources of CP-violation in combination with measurements of the electron and diamagnetic EDMs. This proceedings articles presents an overview of the most important concepts in searches for an nEDM and presents a brief overview of the world wide efforts.
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Until now no electric dipole moment of the neutron (nEDM) has been observed. Why it is so vanishingly small, escaping detection for the last 65 years, is not easy to explain. In general it is considered as one of the most sensitive probes for the vio
lation of the combined symmetry of charge and parity (CP). A discovery could shed light on the poorly understood matter/antimatter asymmetry of the Universe. The neutron EDM might one day help to distinguish different sources of CP-violation in combination with measurements of paramagnetic molecules, diamagnetic atoms and other nuclei. This review presents an overview of the most important concepts in searches for an nEDM as well as a brief overview of the worldwide efforts.
A permanent electric dipole moment of fundamental spin-1/2 particles violates both parity (P) and time re- versal (T) symmetry, and hence, also charge-parity (CP) symmetry since there is no sign of CPT-violation. The search for a neutron electric dip
ole moment (nEDM) probes CP violation within and beyond the Stan- dard Model. The experiment, set up at the Paul Scherrer Institute (PSI), an improved, upgraded version of the apparatus which provided the current best experimental limit, dn < 2.9E-26 ecm (90% C.L.), by the RAL/Sussex/ILL collaboration: Baker et al., Phys. Rev. Lett. 97, 131801 (2006). In the next two years we aim to improve the sensitivity of the apparatus to sigma(dn) = 2.6E-27 ecm corresponding to an upper limit of dn < 5E-27 ecm (95% C.L.), in case for a null result. In parallel the collaboration works on the design of a new apparatus to further increase the sensitivity to sigma(dn) = 2.6E-28 ecm.
A new measurement of the neutron EDM, using Ramseys method of separated oscillatory fields, is in preparation at the new high intensity source of ultra-cold neutrons (UCN) at the Paul Scherrer Institute, Villigen, Switzerland (PSI). The existence of
a non-zero nEDM would violate both parity and time reversal symmetry and, given the CPT theorem, might lead to a discovery of new CP violating mechanisms. Already the current upper limit for the nEDM (|d_n|<2.9E-26 e.cm) constrains some extensions of the Standard Model. The new experiment aims at a two orders of magnitude reduction of the experimental uncertainty, to be achieved mainly by (1) the higher UCN flux provided by the new PSI source, (2) better magnetic field control with improved magnetometry and (3) a double chamber configuration with opposite electric field directions. The first stage of the experiment will use an upgrade of the RAL/Sussex/ILL groups apparatus (which has produced the current best result) moved from Institut Laue-Langevin to PSI. The final accuracy will be achieved in a further step with a new spectrometer, presently in the design phase.
Novel experimental techniques are required to make the next big leap in neutron electric dipole moment experimental sensitivity, both in terms of statistics and systematic error control. The nEDM experiment at the Spallation Neutron Source (nEDM@SNS)
will implement the scheme of Golub & Lamoreaux [Phys. Rep., 237, 1 (1994)]. The unique properties of combining polarized ultracold neutrons, polarized $^3$He, and superfluid $^4$He will be exploited to provide a sensitivity to $sim 10^{-28},e{rm ,cdot, cm}$. Our cryogenic apparatus will deploy two small ($3,{rm L}$) measurement cells with a high density of ultracold neutrons produced and spin analyzed in situ. The electric field strength, precession time, magnetic shielding, and detected UCN number will all be enhanced compared to previous room temperature Ramsey measurements. Our $^3$He co-magnetometer offers unique control of systematic effects, in particular the Bloch-Siegert induced false EDM. Furthermore, there will be two distinct measurement modes: free precession and dressed spin. This will provide an important self-check of our results. Following five years of critical component demonstration, our collaboration transitioned to a large scale integration phase in 2018. An overview of our measurement techniques, experimental design, and brief updates are described in these proceedings.
We present the result of an experiment to measure the electric dipole moment (EDM) of the neutron at the Paul Scherrer Institute using Ramseys method of separated oscillating magnetic fields with ultracold neutrons (UCN). Our measurement stands in th
e long history of EDM experiments probing physics violating time reversal invariance. The salient features of this experiment were the use of a Hg-199 co-magnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic field changes. The statistical analysis was performed on blinded datasets by two separate groups while the estimation of systematic effects profited from an unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is $d_{rm n} = (0.0pm1.1_{rm stat}pm0.2_{rm sys})times10^{-26}e,{rm cm}$.
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