Produced in

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...

ic collisions, a direct photon is any real

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

which originates directly from an

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...

vertex in a

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All common ...

-quark, quark-

gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bi ...

or gluon-gluon scattering subprocess (as opposed to "indirect" photons which arise from the decays of fragmentation products). Because the

QCD In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...

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

. Direct photons were predicted to exist by C.O. Escobar in 1975 and were first observed by the R412 group at the

Intersecting Storage Rings The ISR (standing for "Intersecting Storage Rings") was a particle accelerator at CERN. It was the world's first hadron collider, and ran from 1971 to 1984, with a maximum center of mass energy of 62 GeV. From its initial startup, the collid ...

at CERN in 1976, and were subsequently studied by various experiments, including E705 and E706 at Fermilab, NA3, NA24, WA70 and

UA6 The Underground Area 6 (UA6), also referred to as PHOTONS, experiment was a high-energy physics experiment at the Proton-Antiproton Collider (SpS), a modification of the Super Proton Synchrotron (SPS), at CERN. The experiment ran from 1984 to 199 ...

at the CERN SPS as well as

UA1 The UA1 experiment (an abbreviation of Underground Area 1) was a high-energy physics experiment that ran at CERN's Proton-Antiproton Collider (SpS), a modification of the one-beam Super Proton Synchrotron (SPS). The data was recorded between 1 ...

and UA2 at the CERN SPPS collider.

References

Photons {{particle-stub