A new release of the Monte Carlo event generator Herwig (version 7.2) is now available. This version introduces a number of improvements over the major version 7.0, notably: multi-jet merging with the dipole shower at LO and NLO QCD; spin correlations in both the dipole and angular-ordered parton showers; an improved choice of evolution variable in the angular-ordered parton shower; improvements to mass effects and top decays in the dipole shower, improvements to the simulation of multiple-parton interactions, including diffractive processes; a new model for baryonic colour reconnection; improvements to strangeness production; as well as a new tune of the hadronisation parameters and support for generic Lorentz structures in BSM models. This article illustrates new features of versions 7.1 and 7.2.

No abstract

We attempt to describe the interplay of confinement and chiral symmetry breaking in QCD by using the string model. We argue that in the quasi-abelian picture of confinement based on the condensation of magnetic monopoles and the dual Meissner effect, the worldsheet dynamics of the confining string can be effectively described by the 1+1 dimensional massless electrodynamics, which is exactly soluble. The transverse plane distribution of the chromoelectric field stretched between the quark and antiquark sources can then be attributed to the fluctuations in the position of the string. The dependence of the chiral condensate in the string on the (chromo-)electric field can be evaluated analytically, and is determined by the chiral anomaly and the θ-vacuum structure. Therefore, our picture allows to predict the distribution of the chiral condensate in the plane transverse to the axis connecting the quark and antiquark. This prediction is compared to the lattice QCD results; a good agreement is found.

We present a new model for soft interactions in the event-generator Herwig. The model consists of two components. One to model diffractive final states on the basis of the cluster hadronization model and a second component that addresses soft multiple interactions as multiple particle production in multiperipheral kinematics. We present much improved results for minimum-bias measurements at various LHC energies.

The propagation of a high energy quark disturbs the confining QCD vacuum inducing the currents in Dirac sea. Since quarks possess electric charge, these induced vacuum quark currents act as a source of soft photon radiation. This can lead to the enhancement of the soft photon production above the expectations based on the charged hadron yields and the Low theorem. We illustrate the phenomenon by using the exactly soluble 1+1 dimensional massless Abelian gauge model that shares with QCD all of the ingredients involved in this mechanism: confinement, chiral symmetry breaking, axial anomaly, and the periodic $\theta$-vacuum. We show that the propagating quark throughout the process of hadronization induces in the vacuum charged transition currents that lead to a strong resonant enhancement of the soft photon yield; the Low theorem however remains accurate in the limit of very soft momenta. We then construct on the basis of our result a simple phenomenological model and apply it to the soft photon production in the fragmentation of jets produced in $Z^0$ decays. We find a qualitative agreement with the recent result from the DELPHI Collaboration.Comment: 14 pages, 2 figure