Gravitationally enhanced depolarization of ultracold neutrons in magnetic-field gradients

Trapped ultracold neutrons (UCNs) have for many years been the mainstay of experiments to search for the electric dipole moment (EDM) of the neutron, a critical parameter in constraining scenarios of new physics beyond the Standard Model. Because their energies are so low, UCNs preferentially populate the lower region of their physical enclosure and do not sample uniformly the ambient magnetic field throughout the storage volume. This leads to a substantial increase in the rate of depolarization, as well as to shifts in the measured frequency of the stored neutrons. Consequences for EDM measurements are discussed.

Figure 1

UCN density distribution as a function of height within a storage trap of height 12 cm and a Fermi potential of 93 cm height equivalent.

Figure 2

$T_2$ multiplied by the square of the magnetic-field gradient as a function of UCN energy. See text for details.

Figure 3

Distribution of phase differences for a population of UCNs in a $B$-field gradient of $5\mathrm{nT}/\mathrm{m}$ after 130 s of Larmor precession. See text for details.

Figure 4

Shift in measured frequency as a function of applied $B$-field gradient. The straight line represents the value expected simply from the reduced height of the center of mass of the neutrons. See text for details.

Figure 5

Shift in measured frequency as a function of time, for a $B$-field gradient of $10\mathrm{nT}/\mathrm{m}$.

Figure 6

Polarization as a function of applied magnetic field gradient. See text for details.

Figure 7

Polarization $\alpha$ as a function of time, for a $B$-field gradient of $10\mathrm{nT}/\mathrm{m}$.