We study the process e þ e − → e þ e − η 0 in the double-tag mode and measure for the first time the γ ⋆ γ ⋆ → η 0 transition form factor F η 0 ðQ 2 1 ; Q 2 2 Þ in the momentum-transfer range 2 < Q 2 1 , Q 2 2 < 60 GeV 2. The analysis is based on a data sample corresponding to an integrated luminosity of around 469 fb −1 collected at the PEP-II e þ e − collider with the BABAR detector at center-of-mass energies near 10.6 GeV.

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The free energy analyzed in the framework of the quantum field theory in conjunction with the statistical model for a [Formula: see text] meson is found to undergo an expansion in the condensate wave function. The superconducting and fractal properties of the meson are found to originate from the branch-cut type of singularity in the wave function of the model in which the gauge symmetry breaking is manifest.

The Fermi momentum Pf for the Bc meson has been estimated in the framework of the statistical model. The matrix element |Vcb| has been extracted from the lepton energy spectrum using the estimated value of the Fermi momentum, in the framework of the ACCMM model. The value of Pf for Bc meson has been estimated as Pf=1.14 GeV which is somewhat large compared to the existing estimates.