Editing Photon (section)

=== Hadronic properties ===
{{main|Photon structure function}}
Measurements of the interaction between energetic photons and [[hadron]]s show that the interaction is much more intense than expected by the interaction of merely photons with the hadron's electric charge. Furthermore, the interaction of energetic photons with protons is similar to the interaction of photons with neutrons<ref>{{cite journal |last1=Bauer |first1=T.H. |last2=Spital |first2=R.D. |last3=Yennie |first3=D. R. |last4=Pipkin |first4=F.M. |year=1978 |title=The hadronic properties of the photon in high-energy interactions |journal=[[Reviews of Modern Physics]] |volume=50 |issue=2 |page=261 |bibcode=1978RvMP...50..261B |doi=10.1103/RevModPhys.50.261 }}</ref> in spite of the fact that the electrical charge structures of protons and neutrons are substantially different. A theory called [[Vector Meson Dominance]] (VMD) was developed to explain this effect. According to VMD, the photon is a superposition of the pure electromagnetic photon, which interacts only with electric charges, and vector mesons, which mediate the residual [[nuclear force]].<ref>{{cite journal |last=Sakurai |first=J.J. |year=1960 |title=Theory of strong interactions |journal=Annals of Physics |volume=11 |issue=1 |pages=1–48 |bibcode=1960AnPhy..11....1S |doi=10.1016/0003-4916(60)90126-3 }}</ref> However, if experimentally probed at very short distances, the intrinsic structure of the photon appears to have as components a charge-neutral flux of quarks and gluons, quasi-free according to asymptotic freedom in [[quantum chromodynamics|QCD]]. That flux is described by the [[Photon Structure Function|''photon structure function'']].<ref>{{cite journal |last1=Walsh |first1=T.F. |last2=Zerwas |first2=P. |year=1973 |title=Two-photon processes in the parton model |journal=[[Physics Letters B]] |volume=44 |issue=2 |page=195 |bibcode=1973PhLB...44..195W |doi=10.1016/0370-2693(73)90520-0 }}</ref><ref>{{cite journal |last=Witten |first=E. |year=1977 |title=Anomalous cross section for photon–photon scattering in gauge theories |journal=[[Nuclear Physics B]] |volume=120 |issue=2 |pages=189–202 |bibcode=1977NuPhB.120..189W |doi=10.1016/0550-3213(77)90038-4 }}</ref> A review by {{harvp|Nisius|2000}} presented a comprehensive comparison of data with theoretical predictions.<ref>{{cite journal |last=Nisius |first=R. |year=2000 |title=The photon structure from deep inelastic electron–photon scattering |journal=[[Physics Reports]] |volume=332 |issue=4–6 |pages=165–317 |bibcode=2000PhR...332..165N |arxiv=hep-ex/9912049 |s2cid=119437227 |doi=10.1016/S0370-1573(99)00115-5 }}</ref>