{{Multiple issues| {{more citations needed|date=January 2017}} {{technical|date=August 2016}} }}
Produced in [[hadron]]ic collisions, a '''direct photon''' is any real [[photon]] which originates directly from an [[Electromagnetic field|electromagnetic]] vertex in a [[quark]]-quark, quark-[[gluon]] or gluon-gluon scattering subprocess (as opposed to "indirect" photons which arise from the decays of fragmentation products).
Because the [[QCD theory|QCD]] calculations for direct photon production are considerably easier to perform than for other processes studies of direct photons have been used to test predictions made by perturbative [[QCD theory|QCD]].
Direct photons were predicted to exist by C.O. Escobar in 1975<ref>{{Cite journal|last=Escobar|first=C.O.|date=1975|title=Photoproduction of Large Transverse Momentum Mesons and Production of Large pT Photons and Leptons in Proton Proton Collisions|journal=Nuclear Physics B|volume=98|issue=1 |pages=173–188|doi=10.1016/0550-3213(75)90208-4|bibcode=1975NuPhB..98..173E }}</ref> and were first observed by the R412 group at the [[Intersecting Storage Rings]] at [[CERN]] in 1976, and were subsequently studied by various experiments, including E705 and E706 at Fermilab, NA3, NA24, [[WA70 experiment|WA70]] and [[UA6]] at the [[Super Proton Synchrotron|CERN SPS]] as well as [[UA1 experiment|UA1]] and [[UA2 experiment|UA2]] at the [[Super Proton Synchrotron|CERN SPPS collider]].
==References== {{Reflist}}
[[Category:Photons]]
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