Single perturbative splitting diagrams in double parton scattering

Abstract

Abstract We present a detailed study of a specific class of graph that can potentially contribute to the proton-proton double parton scattering (DPS) cross section. These are the ‘2v1’ or ‘single perturbative splitting’ graphs, in which two ‘nonperturbatively generated’ ladders interact with two ladders that have been generated via a perturbative 1 → 2 branching process. Using a detailed calculation, we confirm the result written down originally by Ryskin and Snigirev — namely, that the 2v1 graphs in which the two nonperturbatively generated ladders do not interact with one another do contribute to the leading order proton-proton DPS cross section, albeit with a different geometrical prefactor to the one that applies to the ‘2v2’/‘zero perturbative splitting’ graphs. We then show that 2v1 graphs in which the ‘nonperturbatively generated’ ladders exchange partons with one another also contribute to the leading order proton-proton DPS cross section, provided that this ‘crosstalk’ occurs at a lower scale than the 1 → 2 branching on the other side of the graph. Due to the preference in the 2v1 graphs for the x value at which the branching occurs, and crosstalk ceases, to be very much larger than the x values at the hard scale, the effect of crosstalk interactions is likely to be a decrease in the 2v1 cross section except at exceedingly small x values (≲ 10−6). At moderate x values ≃ 10−3 −10−2, the x value at the splitting is in the region ≃ 10−1 where PDFs do not change much with scale, and the effect of crosstalk interactions is likely to be small. We give an explicit formula for the contribution from the 2v1 graphs to the DPS cross section, and combine this with a suggestion that we made in a previous publication, that the ‘double perturbative splitting’/‘1v1’ graphs should be completely removed from the DPS cross section, to obtain a formula for the DPS cross section. It is pointed out that there are two potentially concerning features in this equation, that might indicate that our prescription for handling the 1v1 graphs is not quite correct.