Obtaining the CMS Ridge effect with Multipar- ton Interactions

We show that the “ridge” phenomenon in the two-particle angular correlation function, as observed by the CMS experiment, can be reproduced by implementing an impact parameter dependent azimuthal correlation of the scattering planes of individual partonic interactions. Such an approach is motivated by the observation that even for moderate impact parameters a substantial number of partonic interactions may be produced, while at the same time the protons are sufficiently far apart to create a preferential direction in azimuth. A re-tune of the Pythia6 Z2 tune based on underlying event and minimum bias distributions measured at the LHC shows that a better description of data can be obtained with this approach and that some tension existing between underlying event and minimum bias distributions can be removed. We show that, even though the CMS result on the angular correlation function itself is not used in the re-tune, we can predict the appearance of long-range, near-side angular correlations in proton-proton collisions.