Visualisation Room (201)
Lloyd Building
Trinity College Dublin
Time: 2:00 pm
| Date | Speaker | Institute | Title |
| 21 Oct 2008 | Pietro Giudice | Trinity College Dublin | Percolation as a gauge theory: introduction and last results |
| 04 Nov 2008 | Joyce Myers | Swansea University | Phase diagrams of SU(N) gauge theories with fermions in various representations
View abstract We minimize the one-loop effective potential for SU(N) gauge theories with fermions in various representations and compare with lattice simulations of a related model. We consider fundamental (F), adjoint (Adj), symmetric (S), and antisymmetric (AS) representation fermions with finite mass m. We calculate the phase diagram as a function of the length of the compact time dimension beta and the fermion mass for various N and Nf. We also consider the quantum effects of periodic boundary conditions [PBC(+)] on fermions as well as the finite temperature effects of antiperiodic boundary conditions [ABC(-)]. |
| 18 Nov 2008 | Dirk Brömmel | University of Southampton | Meson Distribution Amplitudes and RI/MOM Renormalisation with Momentum Sources
View abstract We present results for the first two moments of the light-cone distribution amplitudes of the pion and kaon pseudo-scalar mesons and of the rho, K* and phi vector mesons. The calculations are performed on the RBC/UKQCD collaborations' ensembles generated with the Iwasaki gauge action and with 2+1 flavours of domain wall fermions. In addition we also provide some results on the necessary non-perturbative renormalisation which we perform using the Rome-Southampton method. We discuss the benefits of the momentum source approach such as much smaller statistical errors and the possibility to see effects of the discretisation. |
| 02 Dec 2008 | Francesco Knechtli | Bergische Universität Wuppertal | SU(2) gauge theories in five dimensions: spontaneous symmetry breaking and dimensional reduction
View abstract We study five-dimensional gauge theories on orbifolds formulated on a Euclidean lattice. In these theories the five-dimensional component of the gauge field can play the role of a four-dimensional Higgs field. We present and compare analytic computations and data from Monte Carlo simulations. |
| 27 Jan 2009 | Luigi Del Debbio | The University of Edinburgh | Minimal Technicolor on the lattice
View abstract I present preliminary results from simulations with dynamical fermions in higher representations. The relevance of these results for models of Dynamical Electroweak Symmetry Breaking are discussed. |
| 03 Feb 2009 | Urs Wenger | Universität Bern | Wilson fermions, vertex models and the solution of a sign problem
View abstract We have developed an efficient simulation algorithm for strongly interacting relativistic fermions in two-dimensional field theories based on a formulation as a loop gas. The loop models describing the dynamics of the fermions can be mapped to statistical vertex models and our proposal is in fact an efficient simulation algorithm for generic vertex models in arbitrary dimensions. The algorithm essentially eliminates critical slowing down by sampling two-point correlation functions and it allows simulations directly in the massless limit. Moreover, it generates loop configurations with fluctuating topological boundary conditions enabling to simulate fermions with arbitrary periodic or anti-periodic boundary conditions. As illustrative examples, the algorithm is applied to the Gross-Neveu model and to the Schwinger model in the strong coupling limit. Finally, we show how the fermionic sign problem is circumvented in the latter. |
| 24 Feb 2009 | Stefan Schäfer | Humboldt Universität Berlin | With Wilson fermions towards the epsilon regime
View abstract The epsilon regime of QCD provides an interesting alternative setting for the computation of low-energy constants on the lattice. Contrary to the more standard p-regime, the pion wave length is taken to be much larger than the size of the box in which we perform the calculation. By definition it is a finite volume regime. This makes it particularly well suited for numerical calculations. However, the light pions, small quark masses required in the epsilon regime made the use of chiral fermions seem necessary and the calculations very costly. In this talk we discuss a variant of Wilson fermions, constructed from HYP smeared links, and algorithmic advances which make it possible to reach the epsilon regime with moderate computer resources. |
| 07 April 2009 | Oliver Bär | Humboldt Universität Berlin | With Wilson quarks towards the epsilon regime (II): Lattice spacing corrections in ChPT
View abstract Recent numerical simulations have shown that the epsilon regime of QCD can be reached with Wilson fermions. Moreover, the lattice data is very well described by the NLO results of continuum chiral perturbation theory (ChPT). This is quite surprising since chiral symmetry is explicitly broken with Wilson fermions. I will report the results of an analysis in Wilson ChPT that explicitly takes into account the lattice spacing in the chiral expansion. It turns out that the lattice spacing corrections are in general highly suppressed and show up at higher order in the epsilon expansion. This offers an explanation for the surprise mentioned before. Taking into account the lattice spacing corrections in a reanalyis of numerical data confirms that the corrections are small. |
| 21 April 2009 | Anton Ilderton | Trinity College Dublin | Modelling the ground state in gauge theories
View abstract I will describe several trial states modelling the heavy quark--antiquark ground state in SU(2) Yang--Mills theory. States describing the confining flux tube as a thin string of glue will be shown to be give poor descriptions of the ground state in the continuum limit (even with smearing). Contrastingly, a state which surrounds the quarks with non--abelian Coulomb fields is found to have a good overlap with the ground state, for all charge separations, which increases as the lattice regulator is removed. The connections between these results and the construction of gauge invariant, physical (colour and electromagnetic) charges will also be discussed. |
| 28 April 2009 | Christof Gattringer | Karl-Franzens-Universität Graz | Canonical partition functions in lattice QCD - new techniques and properties
View abstract Canonical partition functions provide an alternative approach to the notoriously difficult problem of lattice QCD at high density. Based on a domain decomposition we derive a dimensional reduction formula for the fermion determinant which allows to very efficiently evaluate determinants with a fixed quark number. We analyze the properties of the canonical determinants under center transformations and discuss the implications for the QCD phase transition. |