The HighNESS Project and Free Neutron Oscillations Searches at the ESS

The European Spallation Source, ESS, currently under construction in Lund, will be the world’s most powerful facility for research using neutrons. The facility’s unique capabilities will exceed and complement those of today’s leading neutron sources.

Taking advantage of the unique potential of the ESS, the NNBAR collaboration proposed a two-stage program of experiments [1] to perform high-precision searches for neutron conversion in a range of baryon number violation (BNV) channels culminating in an ultimate sensitivity increase for n → n ̄ oscillations of three orders of magnitude over the previously attained limit obtained at the Institut Laue-Langevin ILL [2]. The observation of BNV via free neutron oscillations would be of fundamental significance, with implications for many open questions in modern physics which include the origin of the matter-antimatter asymmetry, the possible unification of fundamental forces, and the origin of neutrino mass.

The first stage of this program HIBEAM (High Intensity Baryon Extraction and Measurement) will employ the fundamental physics beamline during the first phase of the ESS operation. This stage focuses principally on searches for neutron conversion to sterile neutrons n’ that would belong to a “dark” sector. The second stage, NNBAR, will exploit a large beam port, specifically designed in the ESS target station monolith for this experiment, to deliver the maximum possible neutron flux and search directly for n → n ̄ oscillations.

Supported by a 3 MEuro Research and Innovation Action within the EU Horizon 2020 program, a design study (HighNESS) will soon deliver the Conceptual Design Report of the ESS second neutron source which has been optimized in order to boost the performance of the NNBAR experiment and the Conceptual Design Report of the NNBAR experiment. This talk will focus on the HighNESS project and the ongoing developments in the NNBAR collaboration.