Abstract
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 collected 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.
Highlights
We present a high-statistics study of the cross section for the process γγ → KS0 KS0, through the measurement of e+e− → (e+e−)KS0 KS0 where neither a scattered electron nor positron is detected, in the W region from close to its threshold to 4.0 GeV and in the angular range | cos θ∗| ≤ 0.8, where W is the total energy of the parent photons and θ∗ is the scattering angle of the KS0 in their center-of-mass (c.m.) reference frame
A candidate event must pass the level-4 software trigger (L4, see Ref. [39]), in which a fast track-finding program reconstructs one or more tracks with transverse momentum pt > 0.3 GeV/c, each satisfying the requirements on the point of closest approach of the track to the z axis of dr < 1 cm and |dz| < 4 cm, where dr and dz are the distances between this point and the interaction point (IP) in the rφ plane and along the z direction, respectively
The Q2max = 1 GeV2 used in the Monte Carlo (MC) is larger than this experimental limit, and in this case the choice of Q2max in the MC does not affect the final γγ-based cross section results; i.e., the Q2max value is included in the definitions of the luminosity function calculated by TREPS, as well as in the efficiency
Summary
We present a high-statistics study of the cross section for the process γγ → KS0 KS0 , through the measurement of e+e− → (e+e−)KS0 KS0 where neither a scattered electron nor positron is detected (zero-tag mode), in the W region from close to its threshold to 4.0 GeV and in the angular range | cos θ∗| ≤ 0.8, where W is the total energy of the parent photons and θ∗ is the scattering angle of the KS0 in their center-of-mass (c.m.) reference frame. Were insufficient to resolve and to study higher mass resonances These experiments operated at higher e+e− c.m. energies, the cross section of each two-photon production process in a specific W range rises only logarithmically with the e+e− c.m. energy. The data are based on an integrated luminosity of 972 fb−1 This significantly extends our previous study [5], where the measurement of this process was reported for. In analyzing the differential cross section, we measure the phase difference between the a2(1320) and f2(1270) as well as the parameters (mass, width and product of the two-photon partial decay width and decay branching fraction to the KK , ΓγγB(KK ) ) of the f2(1525) including the interference.
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