The HIBEAM/NNBAR program at the European Spallation Source: searching for neutron oscillations

by Valentina Santoro (Lund University and ESS)

Thursday 9 May 2024, 11:00 → 13:00 Europe/Warsaw

The European Spallation Source (ESS), currently in the process of construction in Lund, Sweden, is poised to be the world’s most advanced facility for neutron research. Taking advantage of the unique potential of ESS, the HIBEAM/NNBAR collaboration proposes a two-stage program of experiments [1]. The program aims to conduct high-precision investigations into neutron conversion across various baryon number violation (BNV) channels. The ultimate goal is to achieve a remarkable three-order-of-magnitude increase in sensitivity for n → ¯n compared to the previously established limit at the Institut Laue-Langevin ILL[2]. The observation of neutron into antineutron oscillations that violate conservation of B by two units 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 initial phase of this initiative, HIBEAM (High-Intensity Baryon Extraction and Measurement), will utilize the fundamental physics beamline during the early stages of ESS operation. In this phase, searches for neutron to antineutron oscillations with world-leading sensitivities will be conducted. This represents not only the first search at a spallation source for this process but also serves as a pilot experiment for the second phase (NNBAR). The program also envisions the exploration of neutron conversion to sterile neutrons (n’) associated with a hypothesized ”dark” sector. The construction of the neutron extraction system for this project is currently underway and is expected to be
installed at the ESS next year.

The second stage, NNBAR, will exploit a large beam port, specifically designed in the ESS target station monolith for this experiment. To achieve the proposed sensitivity NNBAR, will use a state-of-the-art annihilation detector, highly efficient magnetic shielding, novel neutron reflectors and a new moderator for the ESS optimized to maximize the intensity of cold neutrons are required. These advancements are crucial for the success of the experiment. The Conceptual Design Report of the experiment has recently been delivered as part of the HighNESS project, financed by the European Framework for Research and Innovation Horizon 2020. During the presentation, I will provide a detailed overview of the recent progress for both the HIBEAM and NNBAR projects.

References
[1] A. Addazi et al., New high-sensitivity searches for neutrons converting into antineutrons and/or sterile neutrons at the HIBEAM/NNBAR experiment at the European Spallation Source, J. Phys. G: Nucl. Part. Phys., 48, 070501 (2021).
[2] M. Baldo-Ceolin et al., A New experimental limit on neutron - anti-neutron oscillations, DFPD-94-EP-13, Z. Phys. C, 63, 409-416 (1994)
[3] V. Santoro, D. Milstead, P. Fierlinger, W. M. Snow, J. Barrow, M. Bartis, P. Bentley, L. Bjork, G. Brooijmans and N. de la Cour, et al. [arXiv:2311.08326 [physics.ins-det]].