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 source of hidden neutrons, 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 (SCKcdot 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<4.0times 10^{-10} ; text {at 95}%text { 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 of 7.4, thus raising the interest of such experiment using regenerating materials.
Highlights
We consider the generic subclass of hidden sector models for which the fermions could exist in a hidden state as well as in the visible state
The present paper describes a new passing-through-wall experiment performed at the BR2 reactor (SCK·CEN, Mol, Belgium)
The MURMUR experiment [27] is installed at 6.18 meters from the centre of the BR2 nuclear reactor of the SCK·CEN at Mol (Belgium) as shown in Fig. 5 where the global layout of
Summary
We consider the generic subclass of hidden sector models for which the fermions could exist in a hidden state as well as in the visible state. The hidden state is sterile as it does not interact via the SM interactions with the visible matter. The hidden state could mix with the visible state through mass mixing (in the theoretical context of mirror matter [1,9,24]), or through kinetic or geometric mixing (in the context of the parallel brane model [19,20,21,25,26,27]). Ev and Eh describe the energy of the visible and hidden states, respectively, and the mixing is described by. Those are in principle 2 × 2 Hermitian matrices acting
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