Date and time: Wednesday June 25th at 4:00 PM (CEST)
Speaker: Fernando Arias-Aragón (Laboratori Nazionali di Frascati)
Title: The X17 Journey: From Nuclear Experiments to Atomic Electron Motion.
Astract: The ATOMKI collaboration measured excesses in electron-positron production from nuclear transitions of beryllium, helium and carbon nuclei. All these excesses are compatible with the emission of a new boson with mass around 17 MeV, denoted X17. An independent search for this particle, harnessing a particle physics experiment, was performed at the Positron Annihilation into Dark Matter Experiment (PADME), where a positron beam collided against a diamond target expecting to produce resonantly the X17 particle. Given the presumably narrow width of this particle, a precise theoretical computation of the signal shape was required to account for electron motion within the target. After including the theoretical modelling of this effect in their analysis, the PADME collaboration has reported an excess in electron-positron events centered around 17 MeV. This excess was later independently shown to largely reduce the uncertainty on the expected X17 mass when combined with nuclear observables. As a byproduct of these studies, electron motion in large Z materials has been shown to enhance centre-of-mass energy in fixed-target experiments and allow for a new energy scan mechanism relevant for the measurement of the hadronic vacuum polarization contribution to the muon anomalous magnetic moment
