While fixed-order calculations accurately describe collider observables in regions of phase space where hard, well-separated jets dominate, they are insufficient in the resummation region, where additional particles are emitted at low energies or angles. In these collinear- and soft-enhanced phase-space regions, parton showers provide a reliable and versatile tool to resum the leading logarithms arising from QCD matrix elements to all orders in the perturbative expansion in the strong coupling.
In order to achieve an accurate description over all of phase space, parton showers are typically “matched” or “merged” with fixed-order calculations. These matching and merging strategies, however, often build upon the construction of “shower histories“, accounting for all possible branching sequences to arrive at a given configuration. Constructing these histories at high particle multiplicities mandates fairly complex and resource-intensive algorithms. This issue can be mitigated by so-called sector showers, which divide the branching phase space into distinct “sectors”, each of which receives contributions from a single branching history only.
In this talk, I will give a comprehensive review of parton showers and matching and merging techniques and introduce the concept of sector showers as implemented in the Vincia antenna shower in Pythia 8.3. I will discuss how their bijective nature allows to improve the efficiency of fixed-order corrections in parton showers and present ongoing efforts to include higher-order corrections in Vincia.