Neutron-antineutron oscillations beyond the quasifree limit

Abstract

Prompted by plans to conduct a new neutron oscillation experiment at the European Spallation Source (ESS), we consider issues associated with the magnetic field that must be present, some of which are potentially exacerbated by the significantly larger length l contemplated for the neutron propagation region. To this end, we introduce a stochastic model of the residual magnetic field within the propagation region which draws on features of magnetic profiles measured during the last free neutron oscillation experiment [conducted at the Institut Laue-Langevin (ILL) in the 1990's]. We average over both fluctuations in the magnetic field sampled by neutrons, and representative spectra of neutron speeds. We find that deviations from the quasi-free result for the antineutron probability do not depend quadratically on l (as a naive perturbative estimate would suggest) but increase only linearly with l. As regards the large spikes in the magnetic field which can be expected at, for example, joints in the magnetic shielding of the propagation region (despite compensating currents and magnetic idealization of the shield), we demonstrate that their effect scales, where D is the diameter of the cylindrical magnetic shielding. Our arguments suggest that, provided the dimensions of the propagation region are such that the ratiodoes not exceed the value pertinent to the ILL experiment, and these spikes occur close to either end of the propagation region, they can be neglected. We also establish that any large magnetic field encountered after the propagation region is exited will not diminish the probability for antineutron detection. For the range of values of l of most interest to the ESS experiment, it should suffice to improve on the level of magnetic suppression achieved at the ILL by a factor of two.

Neutron disappearance and regeneration from a mirror state

“…In a disappearance/regeneration experiment, the effects of the local magnetic field non-uniformity and fluctuations can result in some deviation from the quasi-free condition as it was shown in Ref. [35] for neutronantineutron oscillation [36]; however, these effects can be kept under control.…”

Experimental Approach to Search for Free Neutron-Antineutron Oscillations Based on Coherent Neutron and Antineutron Mirror Reflection

“…Recent theoretical studies [50][51][52][53][54] have clarified the subtleties involved in properly understanding the discrete symmetry transformations of a composite strongly interacting bound system like the n. A new ΔB ¼ 2 process, n −n conversion, has been identified and described [52,55], and the possibility of n-mirror n oscillations [56,57], indicated as a possibility in experiments [58][59][60][61] using ultracold neutrons (UCNs) [62,63] is the subject of active studies [64]. Recent studies have investigated in greater depth the limits of the so-called quasifree condition for the evolution of the n=n amplitudes in external magnetic fields [65][66][67]. These developments in theory and experiment show that the approach described here for improving the sensitivity for n −n oscillations is of general interest to the physics community.…”

A new approach to search for free neutron-antineutron oscillations using coherent neutron propagation in gas

“…Signals which can be attributed to n -mirror n oscillations [64,65] seen in experiments [66][67][68][69] using Ultra Cold Neutrons (UCNs) [70,71] motivate several ongoing studies [72]. The limits of the so-called quasifree condition for the evolution of the n/n amplitudes in external magnetic fields [73][74][75] have been investigated in greater depth. All of these developments in theory and experiment show that any idea for improving the sensitivity for n − n oscillation experiments is of general interest to the physics community.…”

“…In practice we expect that the main value of the possibility of operation of the experiment in this mode would be to relax some of the more difficult/expensive experimental conditions, especially the demand for a very small magnetic field. One still must ensure that the magnetic field does not vary significantly in magnitude [73]…”