Abstract
We investigate different methods to incorporate the effect of photons in hard processes. We compare the two different approaches used for calculating cross sections for the two-photon process. In one of the approaches the photon is treated as a collinear parton in the proton. In the second approach the recently proposed factorization method is used. We discuss how results of the collinear parton model depend on the initial condition for the QCD evolution and discuss an approximate treatment where the photon is excluded from the combined QCD-QED evolution. We demonstrate that it is not necessary to put the photon into the evolution equation as is often done, but it is sufficient to use a simplified approach in which the photon couples to quarks and antiquarks, which by themselves undergo DGLAP evolution equations. We discuss the sensitivity of the results to the choice of structure function parametrization and experimental cuts in the factorization approach. We explicitly display regions of and (arguments of structure functions) relevant for different experiments. We compare the results of our calculations with recent experimental data for dilepton production and find that in most cases the contribution of the photon-photon mechanism is rather small. We discuss how to enhance the photon-photon contribution. We also compare our results to those of recent measurements of exclusive and semiexclusive pair production with certain experimental data by the CMS Collaboration.
11 More- Received 8 January 2016
DOI:https://doi.org/10.1103/PhysRevD.93.074018
© 2016 American Physical Society