We report a high-statistics measurement of the differential cross section of the process gamma gamma --> K^0_S K^0_S in the range 1.05 GeV <= W <= 4.00 GeV, where W is the center-of-mass energy of the colliding photons, using 972 fb^{-1} of data coll
ected with the Belle detector at the KEKB asymmetric-energy e^+ e^- collider operated at and near the Upsilon-resonance region. The differential cross section is fitted by parameterized S-, D_0-, D_2-, G_0- and G_2-wave amplitudes. In the D_2 wave, the f_2(1270), a_2(1320) and f_2(1525) are dominant and a resonance, the f_2(2200), is also present. The f_0(1710) and possibly the f_0(2500) are seen in the S wave. The mass, total width and product of the two-photon partial decay width and decay branching fraction to the K bar{K} state Gamma_{gamma gamma}B(K bar{K}) are extracted for the f_2(1525), f_0(1710), f_2(2200) and f_0(2500). The destructive interference between the f_2(1270) and a_2(1320) is confirmed by measuring their relative phase. The parameters of the charmonium states chi_{c0} and chi_{c2} are updated. Possible contributions from the chi_{c0}(2P) and chi_{c2}(2P) states are discussed. A new upper limit for the branching fraction of the P- and CP-violating decay channel eta_c --> K^0_S K^0_S is reported. The detailed behavior of the cross section is updated and compared with QCD-based calculations.
We report a measurement of the process gamma gamma* --> pi0 with a 759 fb^{-1} data sample recorded with the Belle detector at the KEKB asymmetric-energy e+e- collider. The pion transition form factor, F(Q^2), is measured for the kinematical region 4
GeV^2 <~ Q^2 <~ 40 GeV^2, where -Q^2 is the invariant mass squared of a virtual photon. The measured values of Q^2|F(Q^2)| agree well with the previous measurements below Q^2 ~= 9 GeV^2 but do not exhibit the rapid growth in the higher Q^2 region seen in another recent measurement, which exceeds the asymptotic QCD expectation by as much as 50%.
We report the results of a search for a charmonium-like state produced in the process gamma gamma --> omega J/psi in the 3.9-4.2 GeV/c^2 mass region. We observe a significant enhancement, which is well-described by a resonant shape with mass M = (391
5 +/- 3 +/- 2) MeV/c^2 and total width Gamma = (17 +/- 10 +/- 3) MeV. This enhancement may be related to one or more of the three charmonium-like states so far reported in the 3.90-3.95 GeV/c^2 mass region.
We have measured the cross section for pi^0 pi^0 production in two-photon collisions using a data sample with an ntegrated luminosity of 95 fb^-1 collected with the Belle detector. We find at least four resonant structures including a peak from f_0(9
80), and a fitting of the result to partial-wave amplitudes assuming some resonances are applied. We also make a discussion on the higher-energy region data based on the preliminary results from an increased sample corresponding to the integrated luminosity of 223 fb^-1.
We present a high-statistics measurement of differential cross sections and the total cross section for the process gamma gamma -> pi^0 pi^0 in the kinematic range 0.6 GeV <= W <= 4.0 GeV and |cos theta*| <= 0.8, where W and theta* are the energy and
pion scattering angle, respectively, in the gamma gamma center-of-mass system. Differential cross sections are fitted to obtain information on S, D_0, D_2, G_0 and G_2 waves. The G waves are important above W ~= 1.6 GeV. General behavior of partial waves is studied by fitting differential cross sections in a simple parameterization where amplitudes contain resonant contributions and smooth background. The D_2 wave is dominated by the f_2(1270) meson whose parameters are consistent with the with the current world averages. The D_0 wave contains a f_2(1270) component, whose fraction is fitted. For the S wave, the f_0(980) parameters are found to be consistent with the values determined from our recent pi^+ pi^- data. In addition to the f_0(980), the S wave prefers to have another resonance-like contribution whose parameters are obtained.
We perform an Amplitude Analysis of the world published data on $gammagammatopi^+pi^-$ and $pi^0pi^0$. These are dominated in statistics by the recently published results from Belle on the charged pion channel. Nevertheless, having only limited angul
ar information, a range of solutions remain possible. We present two solutions with $Gamma(f_0(980)togammagamma) = 0.42$ and 0.10 keV, and $Gamma(f_2(1270)togammagamma) = 3.14 pm 0.20$ and $3.82 pm 0.30$ keV, respectively: the former being the solution favoured by $chi^2$, the latter at the edge of acceptability. Models of the structure of the $f_0(980)$ predict two photon widths to be between 0.2 and 0.6 keV, depending on its composition as mainly ${bar K}K$, ${bar s}s$ or ${bar{qq}}qq$. Presently available data cannot yet distinguish unambiguously between these predictions. However, we show how forthcoming results on $gammagamma topi^0pi^0$ can not only discriminate between, but also refine, these classes of partial wave solutions.
