Abstract
We compare the expected effects of so-called gravitationally enhanced depolarization of ultracold neutrons to measurements carried out in a spin-precession chamber exposed to a variety of vertical magnetic-field gradients. In particular, we have investigated the dependence upon these field gradients of spin depolarization rates and also of shifts in the measured neutron Larmor precession frequency. We find excellent qualitative agreement, with gravitationally enhanced depolarization accounting for several previously unexplained features in the data.
Highlights
Ultracold neutrons (UCN) are neutrons of extremely low energy, typically ∼200 neV or less, which can be stored in material bottles and which are routinely used in experiments
The leading systematic error in the neutron electric dipole moment (nEDM) measurement described in [5] arose from shifts in the Larmor precession frequency brought about by the interplay between (a) small magnetic-field gradients within the apparatus and (b) the motional magnetic fields due to the particles moving through the applied electric field [4]
Measurements undertaken at the EDM spectrometer at Paul Scherrer Institute (PSI), showing the dependence upon applied vertical magnetic-field gradients of depolarization rates and of the neutron precession-frequency, have clearly demonstrated features that are characteristic of the anticipated behavior resulting from gravitationally enhanced depolarization and Ramsey wrapping: namely, a sharply peaked rather than parabolic depolarization profile, and significant nonlinearities in the frequency-response curve
Summary
Ultracold neutrons (UCN) are neutrons of extremely low energy, typically ∼200 neV or less, which can be stored in material bottles and which are routinely used in experiments. In the presence of a vertical magnetic-field gradient, the average magnetic field sampled by the neutrons will depend upon the neutron energy The implications of this stratification have been discussed in earlier work [1,2,3], but, in summary, it results in a relative dephasing of the neutrons in different energy bins, which alters the measured Larmor spinprecession frequency. The resulting nonlinearities in frequency response as a function of applied magnetic-field gradients represent potential sources of systematic uncertainty in precision experiments such as nEDM searches [4,5,6,7]. Gravitationally enhanced depolarization can impose a substantial nonlinearity in this relationship: we are unaware of any other mechanism that can do so to the extent required to match our observations
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