Search for a new light boson in meson decays [Elektronisk resurs]

• 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 ε 2 . 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. 

Ämnesord

Natural Sciences  (hsv)

Physical Sciences  (hsv)

Astronomy, Astrophysics and Cosmology  (hsv)

Naturvetenskap  (hsv)

Fysik  (hsv)

Astronomi, astrofysik och kosmologi  (hsv)

Subatomic Physics  (hsv)

Subatomär fysik  (hsv)

Fysik med inriktning mot elementarpartikelfysik  (uu)

Physics with specialization in Elementary Particle Physics  (uu)

Genre

Indexterm och SAB-rubrik

dark matter

dark vector boson

dark photon

U boson

meson decays

form factor

Dalitz decay

eta meson