Probing neutron-hidden neutron transitions with the MURMUR experiment

TL;DR

The MURMUR experiment searches for neutron-hidden neutron transitions using improved background discrimination and regenerating materials, setting new limits on transition probabilities in the context of braneworld and mirror matter models.

Contribution

This work introduces a novel experimental setup with enhanced background discrimination and regenerating materials, providing the most stringent limit to date on neutron-hidden neutron transition probability.

Findings

New limit on neutron swapping probability p < 4.0 × 10^{-10} at 95% CL

Improved background discrimination techniques

Demonstrated potential of regenerating materials like lead

Abstract

MURMUR is a new passing-through-walls neutron experiment designed to constrain neutron/hidden neutron transitions allowed in the context of braneworld scenarios or mirror matter models. A nuclear reactor can act as a hidden neutron source, such that neutrons travel through a hidden world or sector. Hidden neutrons can propagate out of the nuclear core and far beyond the biological shielding. However, hidden neutrons can weakly interact with usual matter, making possible for their detection in the context of low-noise measurements. In the present work, the novelty rests on a better background discrimination and the use of a mass of a material - here lead - able to enhance regeneration of hidden neutrons into visible ones to improve detection. The input of this new setup is studied using both modelizations and experiments, thanks to tests currently performed with the experiment at the BR2 research nuclear reactor (SCK$\cdot$CEN, Mol, Belgium). A new limit on the neutron swapping probability p has been derived thanks to the measurements taken during the BR2 Cycle 02/2019A: $p < 4.0 \ \times 10^{-10}$ at 95% CL. This constraint is better than the bound from the previous passing-through-wall neutron experiment made at ILL in 2015, despite BR2 is less efficient to generate hidden neutrons by a factor 7.4, thus raising the interest of such experiment using regenerating materials.