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Proposing Institution

Fakultät für Mathematik und Naturwissenschaften, Theoretische Teilchenphysik, Uni Wuppertal
Project Manager

Prof. Dr. Francesco Knechtli
Gaussstr. 20
42119 Wuppertal
Many simulations of QCD are carried out only with $N_f = 2 + 1$ dynamical light quarks (up, down, strange). This model has so far provided important results and predictions in Particle Physics and can be considered an excellent approximation of the full theory at energies much below the charm quark mass.However, including a dynamical charm quark in Lattice QCD simulations is important for a better understanding of charm physics and removing the systematic errors which are related to the "quenching" of the charm quark in $N_f = 2 + 1$ QCD simulations. To study the impact of charm sea effects on physical observables, our research group has simulated a simplified version of QCD, namely QCD with two degenerate charm quarks. By comparing the results obtained with this model to the ones obtained with pure gauge theory simulations, it is then possible to provide a first reliable estimate of the effects of a dynamical charm quark in QCD. As a lattice discretization scheme, we considered a clover improved doublet of twisted-mass Wilson fermions [1,2,3] and Wilson's plaquette gauge action [4], imposing open boundary conditions in the temporal direction. Expensive ensembles of configurations have already been generated with computer time allocated through a GAUSS large scale project and our first results can be found in [5,6,7].The objective of this application is the computation of the decay constant $f_{\eta_c}$ of the pseudo-scalar meson $\eta_c$. For this observable, the twisted mass formulation of QCD is a particularly convenient setup, because $f_{\eta_c}$ can be extracted from a matrix element of the pseudo-scalar density without the need of any renormalisation factor [8]. We will evaluate $f_{\eta_c}$ from a suitable ratio of two-point correlation functions that involves the computation of boundary-boundary correlators [9]. For heavy quark masses, it is usually hard to determine a boundary-boundary correlator with reasonable accuracy, because the signal decays exponentially with the quark mass. To overcome this problem, we will use the distance preconditioning for the Dirac operator proposed in Refs. [10,11].References[1] B. Sheikholeslami and R. Wohlert, Improved Continuum Limit Lattice Action for QCD with Wilson Fermions, Nucl. Phys. B259 (1985) 572.[2] Alpha Collaboration, R. Frezzotti, P. A. Grassi, S. Sint, and P. Weisz, Lattice QCD with a chirally twisted mass term, JHEP 08 (2001) 058, [hep-lat/0101001].[3] R. Frezzotti and G. C. Rossi, Chirally improving Wilson fermions. 1. O(a) improvement, JHEP 08 (2004) 007, [hep-lat/0306014].[4] K. G. Wilson, Confinement of Quarks, Phys. Rev. D10 (1974) 2445–2459. [,45(1974)].[5] T. Korzec, F. Knechtli, S. Cali, B. Leder, and G. Moir, Impact of dynamical charm quarks, PoS LATTICE2016 (2017) 126, [arXiv:1612.0763].[6] S.Cali, F. Knechtli, T. Korzec, and H. Panagopoulos, Charm quark effects on the strong coupling extracted from the static force, EPJ Web Conf. 175 (2018) 10002, [arXiv:1710.06221].[7] ALPHA Collaboration, F. Knechtli, T. Korzec, B. Leder, and G. Moir, Power corrections from decoupling of the charm quark, Phys. Lett. B774 (2017) 649–655, [arXiv:1706.0498].[8] XLF Collaboration, K. Jansen, A. Shindler, C. Urbach, and I. Wetzorke, Scaling test for Wilson twisted mass QCD, Phys. Lett. B586 (2004) 432–438, [hep-lat/0312013].[9] M. Bruno, T. Korzec, and S. Schaefer, Setting the scale for the CLS 2 + 1 flavor ensembles, Phys. Rev. D95 (2017), no. 7 074504, [arXiv:1608.08900].[10] G.M. de Divitiis, R. Petronzio, and N. Tantalo, Distance preconditioning for lattice Dirac operators , Phys.Lett. B692 (2010) 157-160, [arXiv:1006.4028].[11] S. Collins, K. Eckert, J. Heitger, S. Hofmann, and W. Soeldner, Charmed pseudoscalar decay constants on three-flavour CLS ensembles with open boundaries, PoS LATTICE2016 (2017) 368, [arXiv:1701.05502]

Impressum, Conny Wendler