Proton structure in transverse space and the effective cross section - art. no. 054023

The observation of double parton collisions by the Collider Detector at Fer milab provided the first direct information on the structure of the proton in transverse space. The actual quantity measured is the "effective cross s ection" sigma(eff), which is related to the transverse size of the region w here hard interactions are localized. The measured value is sizably smaller than naively expected and it is an indication of important correlation eff ects in the many-body parton distribution of the proton. We discuss the pro blem pointing out a possible source of correlations in the proton structure which could have a significant effect on the value of sigma(eff). [S0556-2 821(99)00117-4].