Abstract
We present the results of the analysis of η → e+e−γ and η → e+e− decays. The experimental data were collected in proton-proton collisions at incident proton kinetic energy 1.4 GeV using the WASA detector and the COSY storage ring. We describe the extraction procedure of the η meson transition form factor, based on a sample of around 108 η mesons, and show an attempt to search for physics beyond the Standard Model that led to the setting of an upper limit on the coupling between photons and hypothetical dark bosons. We also provide an estimate of the branching ratio upper limit for the very rare η → e+e− decay.
Highlights
The data sample used in this paper was collected by the WASA-at-COSY collaboration in proton-proton collisions at 1.4 GeV beam kinetic energy
An internal proton beam interacted with a pellet target of frozen hydrogen
We extracted the η transition form factor that is a function depending on the inner quark and gluon structure of the meson
Summary
The data sample used in this paper was collected by the WASA-at-COSY collaboration in proton-proton collisions at 1.4 GeV beam kinetic energy. We constrained the data sample with 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 of about 6.9 · 10−3 The first observations of η → e+e−γ channel, called the η meson Dalitz decay, were performed in the 70’s (see reference [11]) They were based on limited data samples with high background contribution (mostly from photon conversion in the target and the detector material). We use the properties of the WASA detector such as the negligible pair production in the windowless ∼ 35 μm thick pellet target and its exclusive channel measurement capacity to extract a clean sample of this channel
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