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The quark content of the photon is described in quantum field theory by the ''photon structure function'' defined by the process hadrons. It is uniquely characterized by the linear increase in the logarithm of the electronic momentum transfer log and by the approximately linear rise in , the fraction of the quark momenta within the photon.These characteristics are borne out by the experimental analyses of the photon structure function. == Theoretical basis == High energy photons can transform in quantum mechanics to lepton and quark pairs, the latter fragmented subsequently to jets of hadrons, i.e. protons, pions etc. At high energies the lifetime of such quantum fluctuations of mass becomes nearly macroscopic: ; this amounts to flight lengths as large as one micrometer for electron pairs in a 100 GeV photon beam, and still 10 fermi, i.e. the tenfold radius of a proton, for light hadrons. High energy photon beams have been generated by photon radiation off electron beams in circular beam facilities such as PETRA at DESY in Hamburg and LEP at CERN in Geneva. Exceedingly high photon energies may be generated in the future by shining laser light on TeV electron beams in a linear collider facility. The classical technique for analyzing the virtual particle content of photons is provided by scattering electrons off the photons. In high-energy, large-angle scattering the experimental facility can be viewed as an electron microscope of very high resolution , corresponding to the momentum transfer in the scattering process according to Heisenberg's uncertainty principle. The intrinsic quark structure of the target photon beam is revealed by observing characteristic patterns of the scattered electrons in the final state. The incoming target photon splits into a nearly collinear quark-antiquark pair. The impinging electron is scattered off the quark to large angles, the scatter pattern revealing the internal quark structure of the photon. Quark and antiquark finally transform to hadrons. Most exciting is the theoretical analysis of the quark content of the photon, termed "photon structure function". The analysis can be described quantitatively in quantum chromodynamics (QCD), the theory of quarks as constituents of the strongly interacting elementary particles, which are bound together by gluonic forces. The primary splitting of photons to quark pairs, cf. Fig.1, regulates the essential characteristica of the photon structure function, the number and the energy spectrum of the quark constituents within the photon.〔T. F. Walsh and P. M. Zerwas, "Two-photon processes in the parton model", ''Physics Letters'' B44 (1973) 195.〕 QCD refines the picture 〔E. Witten, "Anomalous cross-section for photon-photon scattering in gauge theories", ''Nuclear Physics'' B120 (1977) 189.〕〔W. A. Bardeen and A. J. Buras, "Higher order asymptotic freedom corrections to photon-photon scattering", ''Physical Review'' D20 (1979) 166 (D21 (1980) 2041 ).〕 by modifying the shape of the spectrum, to order unity unlike the small modifications naively expected as a result of asymptotic freedom. Quantum mechanics predicts the number of quark pairs in the photon splitting process to increase logarithmically with the resolution , and (approximately) linearly with the momenta . The characteristic behavior : with : to order unity, leaving the logarithmic behavior in the resolution untouched apart from superficially introducing the fundamental QCD scale , but tilting the shape of the structure function by damping the momentum spectrum at large . These characteristica, dramatically different from the proton parton density, are unique features of the photon structure function within QCD. They are the origin of the excitement associated with the photon structure function.〔A. J. Buras, "Photon structure functions: 1978 and 2005", ''Acta Physica Polonica'' B37 (2006) 609, arXiv:hep-ph/0512238v2.〕 While electron scattering off photons maps out the quark spectra, the electrically neutral gluon content of the photons can best be analyzed by jet pair production in photon-proton scattering. Gluons as components of the photon may scatter off gluons residing in the proton, and generate two hadron jets in the final state. The complexity of these scattering processes, due to the superposition of many subprocesses, renders the analysis of the gluon content of the photon quite complicated. The quantitative representation of the photon structure function introduced above is strictly valid only for asymptotically high resolution , i.e. the logarithm of being much larger than the logarithm of the quark masses. However, the asymptotic behavior is approached steadily with increasing for away from zero as demonstrated next. In this asymptotic regime the photon structure function is predicted uniquely in QCD to logarithmic accuracy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photon structure function」の詳細全文を読む スポンサード リンク
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