The HighNESS Project and Future Free Neutron Oscillations Searches at the European Spallation Source

Abstract

The European Spallation Source, presently under construction in Lund, Sweden, is a multi-disciplinary international laboratory. It will operate the world's most powerful pulsed neutron source. Taking advantage of the unique potential of the ESS, the HIBEAM/NNBAR collaboration has proposed a two-stage program of experiments to perform high precision searches for neutron conversion in a range of baryon number violation (BNV) channels culminating in an ultimate sensitivity increase $ n\to \bar{n}$ oscillations of three orders of magnitude over the previously attained limit obtained at the Institut Laue-Langevin ILL. The first stage of this program HIBEAM (High Intensity Baryon Extraction and Measurement) will employ a 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\to \bar{n}$ oscillations. Supported by a Research and Innovation Action within the EU Horizon 2020 program, a design study (HighNESS) is now underway for the design of the ESS second neutron source which will be also optimized in order to boost the performance of the NNBAR experiment.