Single Perturbative Splitting Diagrams in Double Parton Scattering
  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' (NP) ladders interact with two ladders that have been generated via a perturbative 1->2 branching process. Using a detailed calculation, we confirm the result first written down by Ryskin and Snigirev, that 2v1 graphs in which the two NP ladders do not interact with one another do contribute to the LO 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 NP ladders exchange partons with one another also contribute to the LO 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/crosstalk ceases to be much larger than x values at the hard scale, the effect of crosstalk interactions is likely to be a decrease in the 2v1 cross section except at very 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 crosstalk effects are 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 the suggestion that `double perturbative splitting'/`1v1' graphs should be removed from this cross section made in a previous publication, to obtain a formula for the DPS cross section. We point 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. 
Jonathan R. Gaunt
hep-ph