Study of theη→e⁺e⁻γDecay Using WASA-at-COSY Detector System

Abstract

Abstract. At the turn of October and November 2008, the WASA-at-COSY collaboration performed an experiment to collect data on the η meson decays in pd → 3 He η reactions. This report presents results of a study of the η → e + e − γ conversion decay. The desired feature of this decay is that the final state e + e − pairs come from the conversion of a virtual γ quantum and, therefore, they constitute a rich source of knowledge about the electromagnetic structure of decaying meson. This is a very important characteristic when it comes to study of a short-lived neutral particle like the η meson, because the classical methods of particle scattering are not applicable here. The analysis performed on one third of the p-d data sample collected using the WASA-at-COSY detector, lead to the extraction of the η transition form factor as a function of the e + e − mass and to the calculation of the slope parameter, related to the charge radius of the η meson.

D term and the structure of pointlike and composed spin-0 particles

“…It is important to stress that the same result for D follows in 3 different ways, from Eqs. (36a, 36b) and (63). At this point it is also worth stressing that we obtain an analytic result for D which is manifestly negative, in agreement with all available theoretical calculations calculations.…”

“…This can be understood in the liquid drop model of the nucleus, where a nucleus with mass number A has approximately the mass ∼ A × 0.93 GeV and the radius ∼ A 1/3 × 1.2 fm which yields δ rel ∼ 1.2 A −8/3 . Although they are not spin-0 particles, we have included the proton, deuteron and 6 Li in Table I [62] except for the radii of η taken from the estimate [63] and ηc taken from the lattice calculation [64]. Nuclear masses are from [65] and nuclear mean charge radii from [66].…”

ηandη′transition form factors from rational approximants

“…(5) can be compared with b η ¼ 0.428ð89Þ from CELLO [32] and b η ¼ 0.501ð38Þ from CLEO [33]. The TFF was also measured in the timelike region with the results b η ¼ 0.57ð12Þ from Lepton-G [46], b η ¼ 0.585ð51Þ from NA60 [47], b η ¼ 0.58ð11Þ from A2 [48], and b η ¼ 0.68ð26Þ from WASA [49]. Recently, the A2 Collaboration reported b η ¼ 0.59ð5Þ [50], the most precise experimental extraction up to date.…”

Scouring meson decays for true muonium

“…An improved value of BR(η/η → γγ) or constructing a different ratio, as we do below, can allow sensitivity to these corrections. Results for the form factors F η/η γ * γ * (Q 2 ) can be broadly classified into three groups: theoretical predictions [53][54][55][56], experimental extractions [57][58][59][61][62][63], and dispersion analyses [64][65][66][67]. The standard parameterization for f (z) is a series expansion in z…”