Search for new interaction in the sub-micron range with small-angle neutron scattering*

Unknown gravitylike interactions, i.e. new interactions coupling to mass, with sub-millimeter ranges are recently attracting considerable interests, since it is suggested by the large-extra-dimension (LED) models which provide a possible solution for so-called hierarchy problem in the elementally particle physics[1]. So far, such new interactions have been searched with Cavendish-type experiments using torsion pendulum and so on. The sensitivities of those experiments, however, become smaller below a few micrometers due to serious background caused by inter-molecular forces whose strength is proportional to the electric polarizabilities of the test objects. The small-angle neutron scattering (SANS) is considered useful for probing new gravity-like interactions in sub-micron range thanks to an extremely small electric polarizability of the neutron. Previously, SANS experiments with noble gas targets were performed[2,3]. The coupling constant suggested by the theories is, however, far below the sensitivities of those experiments. To improve the experimental sensitivity, we are promoting the SANS experiment using the targets made of nanoparticles[4] in the series of the experiments 2019A0225, 2019B0339, 2020B0416, 2022A0253, and 2022B0333 at Materials and Life-science experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC). In the case of neutron scattering with nanoparticle target, since the coherent scattering intensity is enhanced by a factor of ~10⁶ which corresponds to the number of atoms contained in a particle, a drastic improvement of the sensitivity to new interactions is expected. The key to the present method is the suppression of the background caused by the nuclear coherent scattering which is also enhanced. Therefore, we are developing the nanoparticles made of the elements or isotopes with opposite signs of coherent scattering length. This paper will report the present status of the target development and the SANS experiment.