Pre-defence seminar: Search for a new light boson in meson decays

Seminar in preparation of a PhD defence, about hadron physics.

Abstract:

The subject of this talk lays 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 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. The resulting sample of events served as the basis for three analyses that will be shortly described.

We will begin by discussing the implementation of a set of criteria chosen in order to select 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 .

We will also present a method of extraction of the η meson transition form factor that is a function depending on the inner quark and gluon structure of the meson. We applied a dedicated fitting procedure to reduce the contribution of background channels from direct pion production.

Then, we will describe 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.

The third topic of this talk will relate to the selection of a sample of η → e+e− candidates. This is an extremely rare decay in the Standard Model and therefore constitutes a very sensitive probe to look for "new physics".