In this paper the current release of the Monte Carlo event generator Sherpa, version 1.1, is presented. Sherpa is a general-purpose tool for the simulation of particle collisions at high-energy colliders. It contains a very flexible tree-level matrix-element generator for the calculation of hard scattering processes within the Standard Model and various new physics models. The emission of additional QCD partons off the initial and final states is described through a parton-shower model. To consistently combine multi-parton matrix elements with the QCD parton cascades the approach of Catani, Krauss, Kuhn and Webber is employed. A simple model of multiple interactions is used to account for underlying events in hadron-hadron collisions. The fragmentation of partons into primary hadrons is described using a phenomenological cluster-hadronisation model. A comprehensive library for simulating tau-lepton and hadron decays is provided. Where available form-factor models and matrix elements are used, allowing for the inclusion of spin correlations; effects of virtual and real QED corrections are included using the approach of Yennie, Frautschi and Suura.

/pdf/event-generation-with-sherpa-1-1-3ihkxphgza.pdf

Event generation with SHERPA 1.1

In this publication the implementation of a new parton shower model based on the Catani-Seymour dipole factorisation, as first suggested by [1,?2], is discussed. First results obtained with the new algorithm are compared with experimental data.

A Parton shower algorithm based on Catani-Seymour dipole factorisation

Recently Lunin and Maldacena used an SL(3,) transformation of the AdS5 × S5 background to generate a supergravity solution dual to a so-called β-deformation of = 4 super Yang-Mills theory. We use a T-duality-shift-T-duality (TsT) transformation to obtain the β-deformed background for real β ≡ γ, and show that solutions of string theory equations of motion in this background are in one-to-one correspondence with those in AdS5 × S5 with twisted boundary conditions imposed on the U(1) isometry fields. We then apply the TsT transformation to derive a local and periodic Lax pair for the bosonic part of string theory in the γ-deformed background. We also perform a chain of three consecutive TsT transformations to generate a three-parameter deformation of AdS5 × S5. The three-parameter background is dual to a nonsupersymmetric marginal deformation of = 4 SYM. Finally, we combine the TsT transformations with SL(2,) ones to obtain a 6+2 parameter deformation of AdS5 × S5.

Lax pair for strings in Lunin-Maldacena background

We derive an improved prescription for the merging of matrix elements with parton showers, extending the CKKW approach. A flavour-dependent phase space separation criterion is proposed. We show that this new method preserves the logarithmic accuracy of the shower, and that the original proposal can be derived from it. One of the main requirements for the method is a truncated shower algorithm. We outline the corresponding Monte Carlo procedures and apply the new prescription to QCD jet production in e+e- collisions and Drell-Yan lepton pair production. Explicit colour information from matrix elements obtained through colour sampling is incorporated in the merging and the influence of different prescriptions to assign colours in the large N_C limit is studied. We assess the systematic uncertainties of the new method.

/pdf/qcd-matrix-elements-and-truncated-showers-4w7a1rvzs0.pdf

QCD matrix elements and truncated showers

Recent lattice data have reported an infrared suppressed, positivity violating gluon propagator which is nonvanishing at zero momentum and a ghost propagator which is no longer enhanced. This paper discusses how to obtain analytical results which are in qualitative agreement with these lattice data within the Gribov-Zwanziger framework. This framework allows one to take into account effects related to the existence of gauge copies, by restricting the domain of integration in the path integral to the Gribov region. We elaborate to great extent on a previous short paper by presenting additional results, also confirmed by the numerical simulations. A detailed discussion on the soft breaking of the Becchi-Rouet-Stora-Tyutin symmetry arising in the Gribov-Zwanziger approach is provided.

Refinement of the Gribov-Zwanziger approach in the Landau gauge: Infrared propagators in harmony with the lattice results

The local composite operator ${A}_{\ensuremath{\mu}}^{2}$ is added to the Zwanziger action, which implements the restriction to the Gribov region $\ensuremath{\Omega}$ in Euclidean Yang-Mills theories in the Landau gauge. We prove that Zwanziger's action with the inclusion of the operator ${A}_{\ensuremath{\mu}}^{2}$ is renormalizable to all orders of perturbation theory, obeying the renormalization group equations. This allows us to study the dimension two gluon condensate $⟨{A}_{\ensuremath{\mu}}^{2}⟩$ by the local composite operator formalism when the restriction to the Gribov region $\ensuremath{\Omega}$ is taken into account. The resulting effective action is evaluated at one-loop order in the $\overline{\mathrm{MS}}$ scheme. We obtain explicit values for the Gribov parameter and for the mass parameter due to $⟨{A}_{\ensuremath{\mu}}^{2}⟩$, but the expansion parameter turns out to be rather large. Furthermore, an optimization of the perturbative expansion in order to reduce the dependence on the renormalization scheme is performed. The properties of the vacuum energy, with or without the inclusion of the condensate $⟨{A}_{\ensuremath{\mu}}^{2}⟩$, are investigated. In particular, it is shown that in the original Gribov-Zwanziger formulation, i.e. without the inclusion of the operator ${A}_{\ensuremath{\mu}}^{2}$, the resulting vacuum energy is always positive at one-loop order, independently from the choice of the renormalization scheme and scale. In the presence of $⟨{A}_{\ensuremath{\mu}}^{2}⟩$, we are unable to come to a definite conclusion at the order considered. In the $\overline{\mathrm{MS}}$ scheme, we still find a positive vacuum energy, again with a relatively large expansion parameter, but there are renormalization schemes in which the vacuum energy is negative, albeit the dependence on the scheme itself appears to be strong. Concerning the behavior of the gluon and ghost propagators, we recover the well-known consequences of the restriction to the Gribov region, and this in the presence of $⟨{A}_{\ensuremath{\mu}}^{2}⟩$, i.e. an infrared suppression of the gluon propagator and an enhancement of the ghost propagator. Such a behavior is in qualitative agreement with the results obtained from the studies of the Schwinger-Dyson equations and from lattice simulations.

/pdf/gribov-parameter-and-the-dimension-two-gluon-condensate-in-4wanc0jd9n.pdf

Gribov parameter and the dimension two gluon condensate in Euclidean Yang-Mills theories in the Landau gauge

We point out the existence of a nonperturbative exact nilpotent BRST symmetry for the Gribov-Zwanziger action in the Landau gauge. We then put forward a manifestly BRST invariant resolution of the Gribov gauge fixing ambiguity in the linear covariant gauge.

/pdf/exact-nilpotent-nonperturbative-brst-symmetry-for-the-gribov-2ffi0owpzx.pdf

Exact nilpotent nonperturbative BRST symmetry for the Gribov-Zwanziger action in the linear covariant gauge

We investigate a dynamical mass generation mechanism for the off-diagonal gluons and ghosts in $\mathrm{S}\mathrm{U}(N)$ Yang-Mills theories, quantized in the maximal Abelian gauge. Such a mass can be seen as evidence for the Abelian dominance in that gauge. It originates from the condensation of a mixed gluon-ghost operator of mass dimension two, which lowers the vacuum energy. We construct an effective potential for this operator by a combined use of the local composite operators technique with the algebraic renormalization and we discuss the gauge parameter independence of the results. We also show that it is possible to connect the vacuum energy, due to the mass dimension-two condensate discussed here, with the nontrivial vacuum energy originating from the condensate $⟨{A}_{\ensuremath{\mu}}^{2}⟩$, which has attracted much attention in the Landau gauge.

/pdf/analytic-study-of-the-off-diagonal-mass-generation-for-yang-4z7z2eazvb.pdf

Analytic study of the off-diagonal mass generation for Yang-Mills theories in the maximal Abelian gauge

An elementary introduction to the Gribov ambiguities and their consequences on the infrared behavior of Euclidean Yang-Mills theories is presented.

Introduction to the Gribov Ambiguities In Euclidean Yang-Mills Theories

We present a local setup for the recently introduced BRST-invariant formulation of Yang-Mills theories for linear covariant gauges that takes into account the existence of gauge copies \`a la Gribov and Zwanziger. Through the convenient use of auxiliary fields, including one of the Stueckelberg type, it is shown that both the action and the associated nilpotent BRST operator can be put in local form. Direct consequences of this fully local and BRST-symmetric framework are drawn from its Ward identities: (i) an exact prediction for the longitudinal part of the gluon propagator in linear covariant gauges that is compatible with recent lattice results and (ii) a proof of the gauge-parameter itxndependence of all correlation functions of local BRST-invariant operators.

/pdf/local-and-brst-invariant-yang-mills-theory-within-the-gribov-23fybxudle.pdf

R. F. Sobreiro

Papers

Gribov parameter and the dimension two gluon condensate in Euclidean Yang-Mills theories in the Landau gauge

Exact nilpotent nonperturbative BRST symmetry for the Gribov-Zwanziger action in the linear covariant gauge

Analytic study of the off-diagonal mass generation for Yang-Mills theories in the maximal Abelian gauge

Introduction to the Gribov Ambiguities In Euclidean Yang-Mills Theories

Local and BRST-invariant Yang-Mills theory within the Gribov horizon