γγ→M~+M~-(M=π,K) Processes With Twist-3 Corrections In QCD

In recent years, more and more experimental data have been accumulated about the two-photon exclusive processes. That provides a good experimental reference to understand the mechanism of two-photon hard exclusive processes in QCD on the theoretical. Because of the strong interactions are presented only in final states,two-photon exclusive processes have particular attractions for theoretical and experimental researchers. It is well known that the exclusive processes at large momentum transfer can afford definitely theoretical test for perturbative QCD. The perturbative QCD calculation is based on factorization of the amplitude into a hard scattering amplitudeHT and a soft distribution amplitude f（ x, Q）, in perturbation theory the distribution amplitudes of pseudoscalar mesons are taken as the main nonperturbative input parameters. Although the factorization about hard scattering amplitude is widely used in high energy region, its applicability at currently energy region 1Ge V- 4Ge V is still being questioned. The experimental data indicates that the cross section for two-photon annihilation into pseudoscalar mesons are still noticeably higher than the theoretical prediction. By analyzing the related experimental data and the existing theoretical research, we think that the twist-3 contribution of final mesons cannot be ignored.In this thesis, we recalculate the two-photon annihilation into two pseudoscalar mesons processes（γγ→M⁺M^-,M=Ï€,K） in the perturbative QCD framework. Firstly,we calculate the cross section of Ï€⁺Ï€^- and K⁺K^- production in two-photon collisions by considering the two-particle twist-3 distribution amplitudes of final pseudoscalar mesons. Then the cross section ration σ₀ （K⁺K^-）/σ₀（π⁺Ï€^-） is discussed.Finally, we give a brief analysis of the angular dependence of the cross section10sds/ d| cosq|-. The twist-2 and twist-3 distribution amplitudes of pseudoscalar mesons are taken as the main nonperturbative input parameters. We take into account the meson distribution amplitudes with BHL prescription, where the distribution amplitudes are rewritten with the exponential suppression factors to avoid the end-point effect from twist-3（）pjx term of pseudoscalar mesons.The results show that our twist-2 contribution is lower an order of magnitude than the experiment value, so it is with the previous theoretical prediction, the cross sections with twist-3 corrections are at the same order in magnitude with the data by varying the center-of-mass energy W from 1 Ge V to 6 Ge V. Considering the contribution of twist-3 parts, the correction of cross sections are markedly improved, even than an order of magnitude enhance to compare with the twist-2 contributions. The contributions from twist-3 parts are actually not power suppressed comparing with the leading-twist contributions, because the momentum transfer is not enough large in those processes.We also discuss the cross section ratio σ₀ （K⁺K^-）/（π⁺Ï€^-） and find it close to experimental results 0.89. Numerical analysis for the angular dependence of σ₀^-1dσ/d|cosθ| shows that the ratios are the independence of distribution amplitudes and are a good agreement with the experimental data in the small-angle area, but the ratios with the twist-3 corrections increase faster than the ones from the twist-2 parts in the large-angle area.