Search for a Neutron Dark Decay in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>
M. Le Joubioux, H. Savajols, W. Mittig, X. Fléchard, L. Hayen, Yu. É. Penionzhkevich, D. Ackermann, C. Borcea, L. Cáceres, P. Delahaye, F. Didierjean, S. Franchoo, Anne Grillet, B. Jacquot, M. Lebois, X. Ledoux, N. Lecesne, Emmanuel Liénard, S. M. Lukyanov, O. Naviliat-Cuncic, J. Piot, A. P. Singh, В. В. Смирнов, C. Stödel, D. Testov, D. Thisse, J. C. Thomas, D. Verney
Abstract
Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of $^{6}\mathrm{He}$ into $^{4}\mathrm{He}+n+\ensuremath{\chi}$ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity $^{6}{\mathrm{He}}^{+}$ beam at Grand Acc\'el\'erateur National d'Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of ${\mathrm{Br}}_{\ensuremath{\chi}}\ensuremath{\le}4.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ (95% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of $\mathcal{O}({10}^{\ensuremath{-}5})$, improving over global constraints by one to several orders of magnitude depending on ${m}_{\ensuremath{\chi}}$.