• Building a consistent parton shower

      Forshaw, Jeffrey R.; Holguin, Jack; orcid: 0000-0001-5183-2673; email: jack.holguin@manchester.ac.uk; Plätzer, Simon (Springer Berlin Heidelberg, 2020-09-01)
      Abstract: Modern parton showers are built using one of two models: dipole showers or angular ordered showers. Both have distinct strengths and weaknesses. Dipole showers correctly account for wide-angle, soft gluon emissions and track the leading flows in QCD colour charge but they are known to mishandle partonic recoil. Angular ordered showers keep better track of partonic recoil and correctly include large amounts of wide-angle, soft physics but azimuthal averaging means they are known to mishandle some correlations. In this paper, we derive both approaches from the same starting point; linking our under- standing of the two showers. This insight allows us to construct a new dipole shower that has all the strengths of a standard dipole shower together with the collinear evolution of an angular-ordered shower. We show that this new approach corrects the next-to-leading- log errors previously observed in parton showers and improves their sub-leading-colour accuracy.
    • Improvements on dipole shower colour

      Holguin, Jack; orcid: 0000-0001-5183-2673; email: jack.holguin@manchester.ac.uk; Forshaw, Jeffrey R.; Plätzer, Simom (Springer Berlin Heidelberg, 2021-04-27)
      Abstract: The dipole formalism provides a powerful framework from which parton showers can be constructed. In a recent paper (Forshaw et al. 2020), we proposed a dipole shower with improved colour accuracy and in this paper we show how it can be further improved. After an explicit check at O(αs2) we confirm that our original shower performs as it was designed to, i.e. inheriting its handling of angular-ordered radiation from a coherent branching algorithm. We also show how other dipole shower algorithms fail to achieve this. Nevertheless, there is an O(αs2) topology where it differs at sub-leading Nc from a coherent branching algorithm. This erroneous topology can contribute a leading logarithm to some observables and corresponds to emissions that are ordered in kt but not angle. We propose a simple, computationally efficient way to correct this and assign colour factors in accordance with the coherence properties of QCD to all orders in αs.