Reanalysis of thee+e−→ηγreaction cross section

Abstract

In the experiment with the SND detector at the VEPP-2M e + e − collider measuring the e + e − → ηγ cross section in the energy range 0.6 < √ s < 1.38 GeV the reanalysis of data is performed. The goal is to improve the accuracy of the previous results by analysing ambiguities in the approximation of energy dependence of the e + e − → ηγ cross section, which were not taken into account in our previous study. We report new results on the approximation of the e + e − → ηγ cross section based on Vector Dominance Model under new model parameter assumptions. In our previous work [1] we performed a measurement of the e + e − → ηγ reaction cross section for two η-meson decay modes: η → 3π 0 and η → π + π − π 0 . The measured cross section was approximated within Vector Dominance Model (VDM). In this approximation the fact that with the free magnitudes and phases of all four ρ, ω, φ, and ρ ′ resonances, the minimized likelihood function has several minima and thus, a choice of the right minimization solution is ambiguous, was ignored. Additional physical restrictions must be applied to, at least, the parameters of the ρ ′ resonance, in order to make a right choice of the minimization solution.In this work the ρ ′ -meson parameters were estimated from the experimental data on the e + e − → ηρ cross section [2,3]. The cross section value in the ρ ′ -resonance maximum obtained from the approximation of the e + e − → ηρ cross section can be translated to the e + e − → ρ ′ → ηγ cross section using VDM as in Ref. [4]. The approximation of the data on the e + e − → ηρ cross section was done with three different models describing the energy dependence of the ρ ′ resonance width:Under these assumptions the resulting mass, width, and maximum cross section of the ρ ′ resonance varied within 1440-1520 MeV, 220-400 MeV, and 0.08-0.13 nb, respectively. All three ρ ′ parameter sets were used in the e + e − → ηγ cross section approximation. The difference in the experimental results was included in the systematic uncertainty of the final result. The cross section at the ρ ′ resonance maximum acquired an additional 20% systematic error from the uncertainty of the cross section calculation in the VDM.Natural values for the phase of the ρ ′ amplitude are 0 or 180 degrees. The phase shift from these values can be induced by the ρ and ρ ′ mixing via common decay channels, for example, ρ, ρ ′ → ωπ 0 . In the model considered in Ref. [5] it was shown that mixing is strongly energy dependent. For the approximation of the data on e + e − → ηγ cross section the ϕ ρ ′ value at the φ resonance maximum is important. Since at this energy the ρ − ρ ′ mixing is small, no significant phase shifts from 0 or 180 degrees are expected. Two approximations with the ϕ ρ ′ phase fixed at these values were considered. In order to estimate a corresponding systematic uncertainty of the approximation parameters due to the ρ ′ phase uncertainty, we varied ϕ ρ ′ by ±20 • with respect to its initial values.The fitting results with the ρ ′ parameters fixed to described ...

Measurement of η meson production in γγ interactions and Γ(η → γγ) with the KLOE detector

“…Using the branching fraction for η → π + π − π 0 , we derive σ(e + e − → ηγ) = (856 ± 8 stat ± 12 syst ± 11 BR ) pb This value, obtained from a direct measurement, agrees well with the value (6.2) obtained from the analysis of γγ → η → 3π 0 . The result interpolates well with the measurements of the SND experiment [35] and has a better precision.…”

A global treatment of VMD physics up to the φ: II. τ decay and hadronic contributions to g−2

“…As can be seen in Table 3, this reflects the low contributions to the χ 2 yielded by the old ππ data [49,51] (χ 2 /n 0.6) and by the (recent) e + e − → (π 0 /η)γ data [35][36][37][38][39][40] (χ 2 /n 0.75). Instead, the newly collected ππ data sets [19][20][21] yield χ 2 /n 0.9 and the 3-pion data [19, [41][42][43][44][45]] χ 2 /n 1.1, quite comparable to CLEO data χ 2 /n 1.3.…”

“…For the e + e − → π 0 γ data sets [35][36][37] and the e + e − → ηγ data sets [35,36,[38][39][40], taking into account the magnitude of the systematics, we did not find motivated to split them up into their bin-to-bin correlated and uncorrelated parts. We just add in quadrature the reported systematic and statistical errors.…”

Strong isospin symmetry breaking in light scalar meson production

“…The data from these experiments motivated theoretical studies that yielded the first arguments in favor of the four-quark nature of the f 0 (980) and a 0 (980) states [1,7,18,[41][42][43][44][58][59][60][61][62][63][64][65][66][67]. It was shown in [41,65,66], for example, that light scalar mesons in φ(1020)-meson radiative decays, φ(1020) → γf 0 (980) and φ(1020) → γa 0 0 (980), are generated at small distances, implying that the light scalar mesons are compact states rather than loosely bound molecules. It was also shown in [60][61][62] that radiative decays of the φ(1020) meson into light scalar mesons are four-quark transitions and, in the ap-proach based on a large number of colors N c , correspond to the four-quark nature of light scalars.…”