Colour Reconnection - Models and Tests

Recent progress on colour reconnection within the Pythia framework is presented. A new model is introduced, based on the SU(3) structure of QCD and a minimization of the potential string energy. The inclusion of the epsilon structure of SU(3) gives a new baryon production mechanism and makes it possible simultaneously to describe hyperon production at both $e^+e^-$ and pp colliders. Finally, predictions for $e^+e^-$ colliders, both past and potential future ones, are presented.Comment: 6 pages, 5 figures. To appear in the proceedings of the The European Physical Society Conference on High Energy Physics, 22-29 July 2015 in Vienna, Austri

Instanton effects and chiral symmetry breaking in QCD2

We discuss the spontaneous breakdown of chiral symmetry in Quantum Chromodynamics by considering gluonic instanton configurations in the partition function. It is shown that in order to obtain nontrivial fermionic correlators in a two dimensional gauge theory for the strong interactions among quarks, a regular instanton background has to be taken into account. We work over massless quarks in the -fundamental- representation of SU(N_c). For large N_c, massive quarks are also considered.Comment: 4 pages, uses espcrc2.sty (attached), updated reference

String Formation Beyond Leading Colour

We present a new model for the hadronisation of multi-parton systems, in which colour correlations beyond leading $N_C$ are allowed to influence the formation of confining potentials (strings). The multiplet structure of $SU(3)$ is combined with a minimisation of the string potential energy, to decide between which partons strings should form, allowing also for "baryonic" configurations (e.g., two colours can combine coherently to form an anticolour). In $e^+e^-$collisions, modifications to the leading-colour picture are small, suppressed by both colour and kinematics factors. But in $pp$ collisions, multi-parton interactions increase the number of possible subleading connections, counteracting their naive $1/N_C^2$ suppression. Moreover, those that reduce the overall string lengths are kinematically favoured. The model, which we have implemented in the PYTHIA 8 generator, is capable of reaching agreement not only with the important \left(n_\mathrm{charged}) distribution but also with measured rates (and ratios) of kaons and hyperons, in both $ee$ and $pp$ collisions. Nonetheless, the shape of their $p_\perp$ spectra remains challenging to explain.Comment: 48 pages, 22 figure

Structural Analysis and Matrix Interpetive System /SAMIS/ program Technical report, Feb. - Aug. 1966

Development of characteristic equations and error analysis for computer programs contained in structural analysis and matrix interpretive syste

Recent Pythia 8 developments: Hard diffraction, Colour reconnection and γγ\gamma\gamma collisions

An overview of recent developments in \pythia~8 is given. First the new hard diffraction model, which is implemented as a part of the multiparton interactions (MPI) framework, is discussed. Then the new colour reconnection model, which includes beyond leading colour effects that can become important when MPI are present, is briefly reviewed. As a last topic an introduction is given to our implementation of photon-photon collisions. In particular photon PDFs, required modifications for the initial state radiation algorithm and beam remnant handling with photon beams is discussed.Comment: Contribution to proceedings of 7th International Workshop on Multiple Partonic Interactions at the LHC, 23 - 27 November 2015, Miramare, Trieste, Ital