Title:

Double parton scattering in protonproton collisions

Double hard partonparton interactions are expected to occur frequently in protonproton (pp) collisions at the LHC. They can give rise to significant backgrounds to certain rare single scattering (SPS) signals, and are an interesting signal process in their own right. In this thesis, we discuss the theoretical description of the double parton scattering (DPS) cross section in the context of Quantum ChromoDynamics (QCD). After an overview of QCD and an introduction to DPS in Chapter 1, we describe in Chapter 2 a framework for calculating the pp DPS cross section introduced by Snigirev et al., in which this cross section is expressed in terms of double PDFs D_p {ij}(x_1,x_2,Q_A 2,Q_B 2) (dPDFs). We show that the equalscale dPDFs are subject to momentum and number sum rule constraints, and use these in the construction of an explicit set of leading order (LO) equalscale dPDFs (the 'GS09' dPDFs). The leptonic samesign WW DPS signal obtained using GS09 dPDFs is compared with that obtained using simple factorised forms, and the prospects of observing this signal taking into account SPS backgrounds are analysed. We discuss two ways in which the dPDF framework for describing pp DPS is deficient in Chapter 3. We discuss interference and correlated parton effects in flavour, spin, colour, and parton type, which are ignored by the dPDF framework. We then study DPStype graphs in which the parton pairs from both protons have arisen from a perturbative 1>2 branching, derive an expression for the part of such graphs associated with the particles arising from the 1>2 branchings being almost onshell, and use this to demonstrate that the treatment of these graphs by the the dPDF framework is unsatisfactory. In Chapter 4, we study DPStype graphs in which the parton pair from only one proton has arisen from a perturbative 1>2 branching. We discover that such graphs contribute to the LO pp DPS cross section, and that crosstalk between partons in the 'nonperturbatively generated' pair is allowed provided that it occurs at a lower scale than that of the perturbative 1>2 branching in the other proton. The result of this analysis is combined with that of the previous chapter to propose a formula for the LO total DPS cross section, and our proposal is compared with those from other authors. We finish in Chapter 5 with some conclusions and suggestions for further work.
