Photon structure function
Exceedingly high photon energies may be generated in the future by shining laser light on teraelectronvolt electron beams in a linear collider facility.
The intrinsic quark structure of the target photon beam is revealed by observing characteristic patterns of the scattered electrons in the final state.
Photon structure function 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.
[1] QCD refines the picture [2][3][4] 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 Q, and (approximately) linearly with the momenta x.
The delicate interplay between photon splitting and damped gluon radiation re-normalizes the photon structure function to order unity, leaving the logarithmic behavior in the resolution Q untouched apart from superficially introducing the fundamental QCD scale Λ, but tilting the shape of the structure function fB (x) → f (x) by damping the momentum spectrum at large x.
These characteristics, dramatically different from the proton parton density, are unique features of the photon structure function within QCD.
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.
Whereas the four-momentum transfer squared Q2 can be determined alone from the energy and angle of the scattered electron, x has to be calculated from Q2 and the invariant mass Mh of the hadronic system using x = Q2 / (Q2 + Mh2) .
[6] [7] The first measurement of the photon structure function has been performed using the detector PLUTO at the DESY storage ring PETRA[8] followed subsequently by many investigations at all large electron–positron colliders.
2 at Q2 = 4.3 GeV2 and 39.7 GeV2, is obviously quite different from the behaviour of the proton structure function, which falls with rising x, and it demonstrates nicely the influence of the photon splitting to quark pairs.
In both figures the data are compared to theoretical calculations, the curves representing the analysis of photon structure function data based on the standard higher-order QCD prediction for three light quarks[10] supplemented by the charm quark contribution and a residual hadronic component accounted for by vector meson dominance.
