Phenomenological Studies Of Two-photon Annihilation Into Pseudoscalar Meson Pairs In The Framework Of Perturbative QCD

The hadron pairs production in two-photon collisions at high energies has long been one of the most interesting research topics.Studies of exclusive processes about hadronic final states in two-photon collisions provide valuable information concerning the physics of light and heavy-quark resonances,perturbative and nonperturbative QCD,and hadron-production mechanisms.Two-photon production of meson final states is one of the simplest hadronic processes.Because of the pointlike structure of the photon,the initial state is simple and controllable,with strong interactions present only in the final state.The energy dependence,angular dependence and the total cross sections of these processes are nontrivial predictions of QCD.These processes have also been the main subject of experimental research.In recent years,the Belle Collaboration has measured exclusive pseudoscalar meson-pair production in two-photon collisions,e.g.????+?-?K+K-,?0?0,KS0KS0,??0,??.The factorization for exclusive processes at large momentum transfer has been suggested by Brodsky and Lepage and,independently,Efremov and Radyshkin.They point out that the amplitude of two-photon processes can be given by a convolution integral of hard kernel with hadronic wave function.The hard kernel describes subprocess ???qq+qq and it can be calcu-lated systematically in the perturbation theory.The wave function describes the nonperturbative overlap of the two quarks with outgoing meson states and can not be obtained from the first prin-ciples.Besides the perturbative QCD approach,two-photon processes have been investigated by many other methods,such as the Handbag model,the QCD sum rule,the soft collinear effective theory and the chiral perturbation theory at currently accessible energies.However,any kind of these approaches can not fully explain the experimental data.The discrepancy between the theoretical prediction and experimental measurement motivates us to find the explanation of this problem.In the standard perturbative approach,the cross section of two-photon process is usually calculated at leading-twist level,and the existing leading-order and next-to-leading order pre-dictions are far below the experimental data.The end-point divergence usually occurs when the higher twist corrections are considered.If the end-point regions are important,one may not drop the transverse momenta kT.Because of the above reasons,we revisit two-photon processes with twist-3 corrections in the framework of perturbative QCD based on the kT factorization,in which transverse degrees of freedom as well as resummation effects are taken into account.These im-provements also make the perturbative calculation become more self-consistent,especially in a few GeV region.The thesis includes the following contents:In the first chapter,we introduce the research background of two-photon processes.The second chapter shows the basic concepts of per-turbative QCD.The third chapter is the brief descriptions of the kT factorization and the in-volved resummation techniques.The proof of the kT factorization theorem for the process??*?? is presented as an illustration.Chapter four and chapter five are the main works completed during my doctoral period.We revisit the processes ????+?-,K+K-and????0?0,KS0KS0,??0,?? in the framework of perturbative QCD based on the kT fac-torization.The twist-3 corrections and the influence of uncertainties of input parameters on the cross section for each channel are analyzed systematically.We also give the predictions of the cross section ratios,which are concerned both in theoretical and experimental aspects.Our main conclusions are as follows:(1)For the ????+?-,K+K-processes,the intrinsic transverse momentum dependence of hadronic wave function and hard kernel is retained in our calculation.The nonperturbative contributions from the end-point regions can be suppressed effectively by the Sudakov re-summation and the threshold resummation when considering the twist-3 corrections.Our work shows that not only transverse momentum effects but also twist-3 corrections play key roles in these two-photon processes.The different influence of the nonperturbative inputs on the cross section comes mainly from the uncertainty of the chiral enhancement parameter.The twist-3 results of the cross sections ?0(?+?-)and ?0(K+K-)are in good agreement with the experiment data.Especially in a few GeV region,the twist-3 contributions be-come dominant in the cross sections.However,our prediction of the cross section ratio?0(?+?-)/?0(K+K-)is still higher than the experimental measurement,and there also ex-ist gaps between the predicted differential cross sections and the experimental data.Possible reasons for these discrepancies are discussed briefly.(2)For the first time,we systematically study the ????0?0,KS0KS0,??0??? processes with twist-3 corrections in the kT factorization,and give the comparison with the Belle experi-mental measurements.Similar to the ?+?-and K+K-channels,our result shows that the twist-3 correction bring a large enhancement to the cross section of each neutral channel in the intermediate energy region,and the theoretical uncertainties in our calculation are mainly from the involved twist-3 parameters.Including the twist-3 corrections,our predictions agree well with the experimental data.In addition,the analysis of the cross section ratios?0(?0?0)/?O(?+?-)and ?0(KS0KS0)/?0(K+K-),as well as the ratios ?0(??0)/?0(?0?0)and ?0(??)/?0(?0?0)is also presented,and it provides more information for us to under-stand the dynamics mechanism of two-photon processes.