Search for neutron-hidden neutron interbrane transitions with MURMUR, a low-noise neutron passing-through-walls experiment

Multi-braneworld universe is at the heart of many scenarios of physics beyond the Standard Model and the cosmological model ΛCDM. It is thus an important concern to constrain these scenarios which also allow for experiments below the GeV scale. MURMUR is a new neutron-passing-through-walls experiment designed to constrain neutron-hidden neutron transitions in the context of braneworlds scenarios. As our visible universe could be a 3-brane embedded in a multidimensional bulk, adjacent hidden 3-branes are often expected. Their existence can be constrained as matter exchange between braneworlds must occur with a swapping probability p. A neutron n can convert into a hidden neutron n′ when scattered by a nucleus with cross section σ(n→n′) ∝ σE(n→n) p, where σE is the usual elastic cross-section. Hidden neutrons could therefore be generated in the moderator medium of a nuclear reactor, where high-flux neutrons undergo many elastic collisions. All the theoretical and technological keys of this experiment soon to be installed at the nuclear research reactor BR2 (SCK.CEN, Mol, Belgium) are introduced.