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

by Damian Pszczel (Uppsala University)

Thursday 4 Oct 2018, 15:15 → 16:15 Europe/Stockholm

Å80109

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".