Here is an attempt at a less jargon-filled summary: Nuclei have energy levels that arise from how the protons and neutrons oscillate against each other. Nuclei

Here is an attempt at a less jargon-filled summary: Nuclei have energy levels that arise from how the protons and neutrons oscillate against each other. Nuclei generally prefer to be at lower energy levels, and the transitions between levels are dictated by rules having to do with properties of the levels like spin and parity. The group used a proton accelerator to populate a particular level in a particular nucleus that is forbidden from decaying by emitting a gamma ray. They observed electrons and their antimatter counterparts coming out, which often suggests that some neutral particle produced them. Gamma rays can do this too through "pair production," but gamma rays are mass-less and carry very little momentum compared to a heavy particle of the same energy. This is where the angular correlations come in, because they are ultimately influenced by the mass of the intermediate particle. They also measured the energy of the pairs and found it to be comparable to the mass energy of the proposed X17 particle. I think this is really interesting, and I'm not a theorist, but I don't think I'm convinced that this is necessarily evidence of a fifth fundamental force. The transition they used in 4He is really convenient from an experimental standpoint, since tritium is commonly used as a proton target to make neutrons for other experiments.