Search for a new light boson in meson decays

Sammanfattning: The subject of the presented work lies in the field of experimental particle physics. The main topic is the study of e+ e− pairs from η meson decays. The data sample used in this work was collected by the WASA-at-COSY collaboration in proton-proton collisions at 1.4 GeV kinetic beam energy. The experiment took place in 2012 at Forschungzentrum Jülich in Germany at the COSY storage ring. An internal proton beam interacted with a pellet target of frozen hydrogen.We implemented a set of selection criteria in order to extract the η → e+ e− γ event candidates. This is a rare electromagnetic decay of the η meson with branching ratio equal to 6.9·10−3. The resulting set of events served as the basis for three analyses.First, we extracted the η transition form factor that is a function depending on the inner quark and gluon structure of the meson. We implemented a specific method to reduce the contribution of background channels from direct pion production.The second analysis was the search for a narrow structure on the e+ e− invariant mass in the selected sample of η → e+ e− γ candidates. Many theoretical models and some astrophysical and particle physics measurements suggest the existence of a new boson, also called the dark photon, that couples to both dark and to Standard Model particles. This particle would decay to e+ e− pairs of well-defined mass and therefore could be detected by looking for narrow peaks in the e+ e− invariant mass spectra. Since no statistically significant signal was observed, we set an upper limit on the coupling parameter ε2.The third objective of this work was to select a sample of η → e+ e− candidates. This is a very rare decay and therefore sensitive to physics beyond the Standard Model. No signal from η → e+ e− was observed, therefore we were able to set an upper limit on the branching ratio for this decay.