Wilson Twisted Mass Fermions: Towards Realistic Simulations of Lattice-QCD
Urbach, Carsten

HaupttitelWilson Twisted Mass Fermions: Towards Realistic Simulations of Lattice-QCD
TitelvarianteUntersuchung der Wilson Twisted Mass Formulierung für die Gitter QCD mit leichten Quarks
AutorUrbach, Carsten
Geburtsort: Düsseldorf, Deutschland
GutachterProf. Dr. Volkard Linke
weitere GutachterDr. Karl Jansen
Freie Schlagwörterlattice gauge theory, quantum chromodynamics, phase structure, HMC algorithm12.38.-t, 12.38.Gc, 11.15.Ha, 12.40.Yx, 14.65.Bt, 14.40.-n
DDC530 Physik

In the year 2003 it was realized that with the Wilson twisted mass formulation of lattice QCD at maximal twist O(a) improvement can be obtained without the need of additional improvement coefficients. Of course, such a theoretical proposition needs a check in practice and hence we performed a scaling test of maximally twisted mass QCD (mtmQCD), in the quenched approximation as a start.

The most important findings of this scaling test are the following: thanks to the twisted mass term playing the role of an infrared cut-off for the Dirac operator eigenvalue spectrum, it is possible to simulate pseudo scalar masses as low as 270 MeV without having problems with exceptional configurations. Moreover we could show that physical observables determined with mtmQCD show no O(a) lattice artifacts. We studied the effects of the explicit flavor symmetry breaking in mtmQCD. They turn out to be sizable when the charged/neutral pseudo scalar mass splitting is considered. Nevertheless, the splitting is a lattice artifact and we could show that it vanishes proportional to the squared lattice spacing. While simulations with pseudo scalar mass values below 300 MeV are also possible with overlap fermions, a comparison of computational costs revealed that simulations with twisted mass fermions are a factor of 20 to 70 faster than simulations with overlap fermions.

Aiming at large scale simulations in full lattice QCD we developed - in addition to the investigation of mtmQCD as the potential formulation - a new variant of the Hybrid Monte Carlo algorithm in order to make full QCD simulations with light quark masses affordable. The new variant, as presented in this work, is applicable to a wide variety of lattice Dirac operators and moreover straightforward to implement. Our simulations clearly show that with this new HMC variant (full dynamical) simulations with Wilson type fermions and realistic quark masses are possible with reasonable computational effort.

Taking the aforementioned results together, we have now a sound basis for performing large scale simulations with light quark masses. However, our first investigations in full lattice QCD revealed a surprising result: we could show that close enough to the continuum the phase structure of lattice QCD with Wilson type fermions and the Wilson plaquette gauge action exhibits the expected continuum phase structure distorted by lattice artifacts. Our investigation yielded that in a range of lattice spacings between 0.2 and 0.13 fm there exists a first order phase transition at the chiral point. The phase structure in this range of lattice spacings was unknown so far.

This phenomenon finds its natural interpretation in terms of an effective potential model depicted in lattice chiral perturbation theory, where a first order phase transition is predicted as one of two possible scenarios emerging due to lattice artifacts proportional to the squared lattice spacing.

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Fachbereich/EinrichtungFB Physik
Tag der Disputation12.12.2005
Erstellt am13.01.2006 - 00:00:00
Letzte Änderung19.02.2010 - 13:27:12
Alte Darwin URLhttp://www.diss.fu-berlin.de/2006/16/
Statische URLhttp://www.diss.fu-berlin.de/diss/receive/FUDISS_thesis_000000001941