  Multi-braneworld universe is at the heart of many scenarios of physics beyond
the Standard Model and the cosmological model $\Lambda CDM$. 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 $\sigma (n \to n')$
$\propto$ $\sigma_E (n \to n)\ p$, where $\sigma_E$ 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.
