Different extensions of the standard model of particle physics, such as braneworld or mirror matter models, predict the existence of a neutron sterile state, possibly as a dark matter candidate. This Letter reports a new experimental constraint on the probability $p$ for neutron conversion into a hidden neutron, set by the STEREO experiment at the high flux reactor of the Institut Laue-Langevin. The limit is $p<3.1\times {10}^{-11}$ at 95% C.L. improving the previous limit by a factor of 13. This result demonstrates that short-baseline neutrino experiments can be used as competitive passing-through-walls neutron experiments to search for hidden neutrons.

• Received 3 November 2021
• Revised 17 December 2021
• Accepted 5 January 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.061801

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Published by the American Physical Society