什麼是低能單舉過程中的主要動力學機制呢？關於微擾量子色動力學是否適用於低能單舉碰撞過程中的爭議在文獻中常常被提起討論，但一直沒有共識。在這篇論文中，我將藉由研究雙光子的碰撞過程來討論其適用性，我將以γγ→ππ、KK及KSKS為例子，藉由兩種不同的計算方法：微擾量子色動力學及量子色動力學求合規則來計算微分散射振幅的角分布，前者是一種微擾計算法，適用於能量較高的範圍，而後者則是非微擾計算法，適用於能量較低的範圍。我們觀察到這兩種方法將得到完全不同的結果，再與實驗值做比較後我們發現藉由計算微分散射振幅的角分布及不同散射過程的散射振幅比，我們可以判斷出這兩種計算法的適用範圍，我們也發現夸克凝結態的貢獻可能是重要的，此外，我將簡單介紹量子色動力學求合規則。

What is the dominant dynamics in the exclusive processes at moderate energies, soft or hard? The issue of the applicability of PQCD to exclusive processes has been widely discussed in the literature, but a general agreement still has not been available. In this thesis, the dynamics of two-photon collisions will be studied for this purpose. We take the processes γγ→ππ, KK and KSKS as an example, and calculate the angular distributions and partial cross sections of the pion and the kaon in the angular range $|cos heta^*|<0.6$ by using non-perturbative QCD sum rules (QSR), which are reliable at a low center-of-mass energy $W_{gammagamma}$, and the PQCD approach based on $k_T$ factorization theorem, which is reliable at a high $W_{gammagamma}$. It will be shown that QSR and PQCD give dramatically different predictions. Comparing to experimental data it then indicates that the angular distributions and the ratio of partial cross section are suitable quantities for discriminating the soft-dominance and hard-dominance pictures. We find that the result of QSR is in better agreement with the data for $gammagamma o K^0_s K^0_s$ at about $2.4mathrm{GeV}leq W_{gammagamma} leq 4mathrm{GeV}$. It seems to indicate that the contribution of quark condensate is important. We also introduce some concepts of QSR.

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