Initial-state showering based on colour dipoles connected to incoming parton lines

Abstract

A parton-shower model for hadronic collisions based on the emission properties of QCD dipoles is proposed. This proposal therefore extends the well-known radiation pattern of pure final-state colour dipoles to QCD initial-state radiation, both of which are treated perturbatively. Corresponding dipole splitting functions are derived and the kinematics of all dipole splittings is discussed. The application of the resulting dipoleshower model to hadron production in electron-positron annihilation, to Drell-Yan leptonpair and QCD jet production yields encouraging results.

The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations

“…It is important to bear in mind that this implies that eq. (2.110) is therefore valid not only for those PSMCs based on a 1 → 2 branching picture (such as those just mentioned), but more generally for any PSMC consistent with QCD (in particular, those that adopt a dipole picture [34,[176][177][178][179][180][181][182] …”

Efficient matrix-element matching with sector showers

“…The main distinguishing feature of such showers is that only a single radiation antenna contributes to each phase-space point, as compared to a sum over all radiators in traditional "global" showers [16,18,34,36]. The coefficients of the single poles of gluon antennae are modified to reflect this reorganization.…”

“…Showers based on partons and/or partitioned dipoles (such as Altarelli-Parisi [26][27][28][29] or CataniSeymour [30][31][32][33] ones) produce one term per color charge in the event, i.e., one per quark and two per gluon. Showers based on antennae [18,[34][35][36] are slightly more economic, producing only one term per color-connected pair of color charges. This difference is still comparatively insignificant, however, compared to the proliferation of terms caused by the fact that only strongly ordered paths can contribute (using whatever definition of ordering the particular shower algorithm's authors prefer): to see which paths actually contribute to each n-parton configuration, one must check the ordering condition all the way back to the Born configuration, including the effects of any previous matching steps.…”

“…Dipole-antenna showers [18,[34][35][36] are based on nested 2 → 3 splitting processes, with an on-shell, Lorentz-invariant phase-space factorization taking place at each step [37,38]. Following [17,39], we label the participants in a 2 → 3 dipole-antenna branching by IK → ijk.…”

Controlling inclusive cross sections in parton shower + matrix element merging

“…leading order (LO), have been accompanied by efforts of matching parton shower simulations and perturbative calculations at next-to-leading order (NLO), including both matrix elements for additional hard emission at tree level as well as virtual, one-loop corrections to the hard process of interest, [14][15][16][17][18][19]. On the other hand, parton shower algorithms themselves have evolved from crude approximations of multiple parton emission to more and more refined and precise tools [20][21][22][23][24][25][26], eventually aiding attempts of combining fixed-order calculations with shower resummation in a most consistent way.…”