Photon Structure

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Structure of Quasi-Real Photons (Q2 ~ 0)

    The quark-gluon structure of the photon gives information on the creation of hadronic matter. At HERA photons accompany the lepton beam. Such photons can fluctuate into a quark-antiquark state which may or may not develop a hadronic bound state, for example a rho meson. For quasi real photons at HERA the fluctuation time for a quark-antiquark pair corresponds to about 10000 fm in the rest frame of the proton. This quark-antiquark state can be analysed in scattering processes with the partons of the proton which can be observed for example in 2-jet events. 

    Processes in which the proton interacts directly with a parton of the proton are called direct photon-proton processes. Resolved photon-proton scattering involves processes where the photon interacts indirectly via its own partons. 
The figure shows the double differential 2-jet cross section as function of the two observables that are relevant for the parton distributions of the photon: the momentum of the parton from the photon x and the transverse energy of the jets E_T. The latter observable is directly related to the resolution scale of the process. 
    In the jet events, the contributions of quarks and gluons from the photon cannot be distinguished. Therefore an effective parton distribution x f was determined that combines both contribution with a weight of color factors:

    x f_gamma = x ( f_q/gamma + 9/4 f_g/gamma )
     
     
The figure shows measurements of the photon structure function as a function of the virtuality Q2 of the exchanged photon as obtained at electron-positron colliders (left and lower scale). In good agreement with these data is the effective parton distribution of the photon as extracted from the H1 jet measurement. This is shown as a function of the scattered parton squared transverse momentum p_t^2 which was determined from the jet transverse energy (right and upper scale). 
    For the first time the gluon distribution of the photon was measured. Since the quark distributions as extracted from the photon structure function measurements are universal (see figures above), the quark induced jet processes and the direct photoproduction of jets can be subtracted. The remaining 2-jet cross section reflects then the gluon contribution of the photon. 
The figure shows the 2-jet cross section as a function of the parton momentum x from the photon side. The histograms show the contributions of the direct photon-proton processes and the processes that were induced by a quark from the photon side. The difference between the measured jet cross section and these two histograms shows the gluon contribution of the photon. 
The first measurement of the gluon distribution of the photon was determined in leading order QCD using a previous measurement of the 2-jet cross section (open symbols). The same method of extraction was later used for particle cross section measurements (closed symbols). At large x the gluon density in the photon is small. It does not rise steeply towards small values of x. 

Structure of Virtual Photons (Q2 > 1 GeV^2)

    The time of fluctuation of the photon into a quark-antiquark pair decreases with increasing virtuality of the photon as 1/Q2. In order to analyse these fluctuations, two-jet events were used with sufficient jet transverse energy to resolve the partonic structure of virtual photons. 
The figure shows the measurement of the triple differential 2-jet cross section as a function of the photon virtuality Q2 at fixed jet transverse energy E_t =50 GeV^2 in two bins of the momentum x of the parton from the photon. 
    The dashed curve represents a leading order QCD calculations using direct photon-proton processes only. For x = 1 this calculation reproduces approximately the measured cross section. At x = 0. 5 and photon virtualities that are small compared to the jet squared transverse energy, the jet process is able to resolve the partonic structure of the virtual photon.

    For the first time an effective parton distribution of virtual photons with Q2 > 1 GeV^2 was determined using the definition mentioned above for the real photon case. 
The figure shows the effective parton density of the virtual photon as a function of the photon virtuality Q2. As expected, the measured parton density decreases with increasing Q2. 
    The moderate decrease of the parton density of the virtual photon is predicted by perturbative QCD: the parton density of the photon decreases only logarithmically with Q2. This shape can be confronted with the measurement of the rho meson production cross section which falls steeply as a function of Q2. Such comparison implies that the time to develop a hadronic bound state from the quark-antiquark pair diminishes with increasing Q2. Therefore the largest part of the virtual photon structure can be calculated with QCD (curve: QCD inspired model SaS1d). 
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last updated by H1 webmaster on 19/05/98