# Direct photon

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Produced in [hadronic](/source/Hadron) collisions, a **direct photon** is any real [photon](/source/Photon) which originates directly from an [electromagnetic](/source/Electromagnetic_field) vertex in a [quark](/source/Quark)-quark, quark-[gluon](/source/Gluon) or gluon-gluon scattering subprocess (as opposed to "indirect" photons which arise from the decays of fragmentation products).

Because the [QCD](/source/QCD_theory) 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](/source/QCD_theory).

Direct photons were predicted to exist by C.O. Escobar in 1975[1] and were first observed by the R412 group at the [Intersecting Storage Rings](/source/Intersecting_Storage_Rings) at [CERN](/source/CERN) in 1976, and were subsequently studied by various experiments, including E705 and E706 at Fermilab, NA3, NA24, [WA70](/source/WA70_experiment) and [UA6](/source/UA6) at the [CERN SPS](/source/Super_Proton_Synchrotron) as well as [UA1](/source/UA1_experiment) and [UA2](/source/UA2_experiment) at the [CERN SPPS collider](/source/Super_Proton_Synchrotron).

## References

1. **[^](#cite_ref-1)** Escobar, C.O. (1975). "Photoproduction of Large Transverse Momentum Mesons and Production of Large pT Photons and Leptons in Proton Proton Collisions". *Nuclear Physics B*. **98** (1): 173–188. [Bibcode](/source/Bibcode_(identifier)):[1975NuPhB..98..173E](https://ui.adsabs.harvard.edu/abs/1975NuPhB..98..173E). [doi](/source/Doi_(identifier)):[10.1016/0550-3213(75)90208-4](https://doi.org/10.1016%2F0550-3213%2875%2990208-4).

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