Search for neutron dark decay: n → χ + e⁺e⁻

¹ W.K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125, USA
² Department of Physics, Indiana University, Bloomington, Indiana 47408, USA
³ Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA
⁴ Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁵ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
⁶ Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA
⁷ Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA
⁸ Department of Physics and Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA
⁹ Institut Laue-Langevin, 38042 Grenoble Cedex 9, France
¹⁰ Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China
¹¹ Department of Physics, University of Winnipeg, Winnipeg, MB R3B 2E9, Canada
¹² Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA
¹³ Department of Physics, Idaho State University, Pocatello, Idaho 83209, USA
¹⁴ Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA

^a e-mail: xsun@caltech.edu

Published online: 13 December 2019

Abstract

In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. One of the possible final states discussed includes a single χ along with an e⁺e⁻ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼ 4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e^- + ν̅_e decays. The summed kinetic energy (E_e⁺e⁻) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel.