Further search for magnetic-field-induced neutron disappearance in an ultracold neutron beam

Abstract

We report the results of the second iteration of an experiment searching for neutron-hidden-neutron oscillations in a beam of ultracold neutrons, conducted at the PF2 facility of the Institut Laue Langevin (ILL). Oscillations were tested via neutron disappearance as a function of an applied magnetic field, in the context of a phenomenological two-parameter model assuming zero hidden potentials. The magnetic field was varied in a step-wise manner in order to resonantly enhance the oscillation probability at different mass splittings ($δm$) across a 60--1550 peV range. No evidence for neutron disappearance is observed and conservative limits on the neutron-hidden-neutron oscillation period ($τ_{nn'}$) have been set at 95 % confidence level: $τ_{nn'} > 200$ms for $|δm| \in [60, 400]$ peV and $τ_{nn'} > 100$ ms for $|δm| \in [400, 1550]$ peV

Figures (4)

• Figure 1: Schematic of the experimental setup at the PF2 ultracold neutron facility. The red arrow represents a possible UCN trajectory from the beam port to the detector. Note that the stated dimensions are not to scale.
• Figure 2: Pulse-shape analysis histogram ($A$ vs $Q / A$) binned from the data of representative UCN delivery cycles comprising 11357607.0 events. The red contour denotes the boundary of the smallest (Gaussian-smoothed) region containing all non-empty bins, after application of selection cuts 1--7. Approximately 32 % of rejected events (i.e. from cuts 1 and 5--7) are found within this region.
• Figure 3: Numerical calculations of the disappearance probability as a function of coil current for two different $\delta m$ values (red and blue points), the interpolation at those values (red and blue curves), and the interpolation at an intermediate value (green curve), for three distinct $\delta m$ regimes.
• Figure 4: 95% CL limits in the $\delta m$--$\tau_{nn'}$ plane for this work (teal) and previous works: Ban2007Altarev2009Berezhiani2012v2Berezhiani2018Ayres2022 (red), Hostert2023 (purple), Stasser2021 (green), Almazn2022 (blue) , Gonzalez2024 (gray), and saenz (orange).