Confirmation of the existence of the X17 particle

¹ Institute for Nuclear Research (Atomki) P.O. Box 51, H-4001 Debrecen, Hungary
² University of Debrecen, 4010 Debrecen, PO Box 105, Hungary
³ CERN, Geneva, Switzerland and Institute for Nuclear Research (Atomki), P.O. Box 51, H-4001 Debrecen, Hungary

^* e-mail: kraszna@atomki.hu

Published online: 6 April 2020

Abstract

In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1+ state on 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass m_Xc² = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (m_Xc2) and its branching ratio relative to the γ-decay of the 8Be excited state (B_X), to m_Xc² = 17.01(16) MeV and B_X = 6(1) 10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ = 0⁻ 0⁺ transition in 4He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass m_Xc² = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.