X17 - 5th Force Of Nature

The X17 particle is a hypothetical subatomic particle proposed by Attila Krasznahorkay and his colleagues to explain certain anomalous measurement results.[1] The X17 particle could be the force carrier for a postulated fifth force, possibly connected with dark matter,[2] and has been described as a protophobic (ie, ignoring protons)[3] X boson with a mass near 17 MeV.[2] The particle has been proposed to explain wide angles in the trajectory paths consistently observed in the decay products of beryllium-8 atoms, and observed once in stable helium atoms.[2]

History

In 2015, Krasznahorkay and his colleagues at ATOMKI posited the existence of a new, light boson weighing 17 MeV (17 megaelectronvolts and inspiration of the team's X17 naming convention[4]), only 34 times heavier than the electron.[5] In an effort to find a dark photon, the Hungarian team fired protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that then decayed and produced pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the e+ and e− particles, and a combined energy of approximately 17 MeV. This indicated that a small fraction of beryllium-8 might shed its excess energy in the form of a new particle. The result was successfully repeated by the team.[2]

In 2016, Jonathan Feng et al. proposed that a protophobic X-boson, with a mass of 16.7 MeV, suppressed couplings to protons relative to neutrons, and electrons at femtometer range, could explain the data. The force may explain the g-2 muon anomaly and provide a dark matter candidate. As of 2019, several research experiments are underway to attempt to validate or refute these results.[5][6]

In November 2019, it was announced by Krasznahorkay that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium-8, strengthening the case for the X17 particle's existence.[7] This was covered in science journalism, focusing largely on the implications that the existence of the X17 particle and a corresponding fifth force would have in the search for dark matter. 

(Original Post And Source Of Content) - https://en.wikipedia.org/wiki/X17_particle