Topological susceptibility in two-flavor QCD
We compute the topological susceptibility in QCD with two flavors of dynamical fermions using numerical simulation with overlap fermions.
hep-lat↗
arXiv subjects
Thomas DeGrand
Publications and source records attributed to Thomas DeGrand.
Diquark effects in light baryon correlators from lattice QCD
We study the role of diquarks in light baryons through point to point baryon correlators. We contrast results from quenched simulations with ones with two flavors of dynamical overlap fermions. The scalar, pseudoscalar and axial vector diquarks are combined with light quarks to form color singlets. The quenched simulation shows large zero mode effects in correlators containing the scalar and pseudoscalar diquark. The two scalar diquarks created by gamma_5 and gamma_0gamma_5 lead to different behavior in baryon correlators, showing that the interaction of diquarks with the third light quark matters: we do not see an isolated diquark. In our quark mass range, the scalar diquark created by gamma_5 seems to play a greater role than the others.
hep-ph↗
More about QCD on compact spaces
We present some results about spontaneous breaking of global symmetries for four-flavor, three color QCD on compact spaces with two short directions. When the two short directions have equal length and identical boundary conditions, there is a single transition. When the two short directions have boundary conditions of opposite parity and are of roughly equal extent, the C-breaking and deconfinement transitions separate. When the two short dimensions are of different length, the transitions are modified in qualitative agreement with expectations from dimensional reduction. These features resemble the situation in pure gauge simulations at small and large number of colors.
hep-lat↗
The complete lowest order chiral Lagrangian from a little box
We recently performed a pilot study determining the parameters of the leading order chiral Lagrangian from distributions of the eigenvalues of a quenched Dirac operator coupled to an imaginary isospin chemical potential. We complement a quick survey of our recent preprint arXiv:0708.1731 [hep-lat] by addressing some points raised during discussions at the conference.
hep-lat↗
Parameters of the lowest order chiral Lagrangian from fermion eigenvalues
Recent advances in Random Matrix Theory enable one to determine the pseudoscalar decay constant from the response of eigenmodes of quenched fermions to an imaginary isospin chemical potential. We perform a pilot test of this idea, from simulations with two flavors of dynamical overlap fermions.
hep-lat↗
Quark condensate in one-flavor QCD
We compute the condensate in QCD with a single quark flavor using numerical simulations with the overlap formulation of lattice fermions. The condensate is extracted by fitting the distribution of low lying eigenvalues of the Dirac operator in sectors of fixed topological charge to the predictions of Random Matrix Theory. Our results are in excellent agreement with estimates from the orientifold large-N_c expansion.
hep-th↗
QCD with one compact spatial dimension
The realization of global symmetries can depend on the geometry of the underlying space. In particular, compactification can lead to spontaneous breaking of such symmetries. Four-dimensional QCD with fundamental representation fermions embedded in a space with one compact spatial dimension has a critical length, at which the theory undergoes a phase transition and develops a ground state that is no longer charge conjugation invariant. We show this behavior with simulations of three color, four flavor QCD. We use unrooted staggered fermion at two values of the lattice spacing and several quark masses. We discuss the dependence of the transition on the dynamical fermion mass as well as its connection to the finite temperature and chiral phase transitions.
hep-lat↗
Quark condensate in two-flavor QCD
We compute the condensate in QCD with two flavors of dynamical fermions using numerical simulation. The simulations use overlap fermions, and the condensate is extracted by fitting the distribution of low lying eigenvalues of the Dirac operator in sectors of fixed topological charge to the predictions of Random Matrix Theory.
hep-lat↗
Baryon correlators containing different diquarks from lattice simulations
Point to point vacuum correlators containing diquarks in the color anti-triplet representation are computed both in the quenched approximation and dynamical overlap simulations with two flavors. The scalar, pseudoscalar and axial vector diquarks are combined with light quarks to form color singlets. The scalar ("good") diquark channel shows a stronger attraction than the axial vector ("bad") channel in the quenched data set. The pseudoscalar diquark channel shows a finite volume zero mode artifact: the correlator becomes negative at large distance when the quark mass is small. By separating configurations without zero modes from those with zero modes, we found that the zero modes have an important contribution in both the attraction in the scalar channel and the repulsion in the pseudoscalar channel. In the axial vector diquark channel, we did not find apparent zero mode effects.
hep-lat↗
The one-flavor quark condensate and related problems
I describe a recent calculation (by me, Hoffmann, Liu, and Schaefer) of the chiral condensate in one-flavor QCD using numerical simulations with overlap fermions. The condensate is extracted by fitting the distribution of low lying eigenmodes of the Dirac operator in sectors of fixed topological charge to the predictions of Random Matrix Theory. Our results are in excellent agreement with estimates from the orientifold large-N expansion. Much interesting physics surrounds this calculation, which I will highlight.
hep-lat↗
Simulating an arbitrary number of flavors of dynamical overlap fermions
We present a set of related Hybrid Monte Carlo methods to simulate an arbitrary number of dynamical overlap fermions. Each fermion is represented by a chiral pseudo-fermion field. The new algorithm reduces critical slowing down in the chiral limit and for sectors of nontrivial topology.
hep-lat↗
Dynamical overlap fermions: techniques and results
We summarize our recent investigations of lattice QCD with dynamical overlap fermions. We sketch algorithmic issues and our approach to solving them. We show our measurement of the topological susceptibility. We describe a computation of the chiral condensate using an analysis of the distribution of eigenmodes of the Dirac operator and Random Matrix Theory.
hep-lat↗
Renormalization of Bilinear Quark Operators for Overlap Fermions
We present non-perturbative renormalization constants of fermionic bilinears on the lattice in the quenched approximation at beta=6.1 using an overlap fermion action with hypercubic(HYP)-blocked links. We consider the effects of the exact zero modes of the Dirac operator and find they are important in calculating the renormalization constants of the scalar and pseudoscalar density. The results are given in the RI' and MS bar schemes and compared to the perturbative calculations.
hep-lat↗
Chiral properties of two-flavor QCD in small volume and at large lattice spacing
We present results from simulations of two flavors of dynamical overlap fermions on 8^4 lattices at three values of the sea quark mass and a lattice spacing of about 0.16 fm. We measure the topological susceptibility and the chiral condensate. A comparison of the low-lying spectrum of the overlap operator with predictions from random matrix theory is made. To demonstrate the effect of the dynamical fermions, we compare meson two-point functions with quenched results. Algorithmic improvements over a previous publication and the performance of the algorithm are discussed.
hep-lat↗
Recent developments from lattice QCD
This year lattice QCD has become very public. A new generation of simulations (including light dynamical quarks) have produced results which are in close agreement with many ``easy'' experimental quantities, and precise predictions for quantities which are tests of the Standard Model. Is QCD over? Reality is somewhat more nuanced. I will try to put the recent results into context: as in any theoretical calculation, there are always hidden assumptions! Hopefully I can give you a feel for some of them.
hep-ph↗
Physics issues in simulations with dynamical overlap fermions
We discuss the impact of various improvements on simulations of dynamical overlap fermions using the Hybrid Monte Carlo algorithm. We focus on the usage of fat links and multiple pseudo-fermion fields.
hep-lat↗
Improving meson two-point functions in lattice QCD
We describe and test a method to compute Euclidean meson two-point functions in lattice QCD. The contribution from the low-lying eigenmodes of the Dirac operator is averaged over all positions of the quark sources. The contribution from the higher modes is estimated in the traditional way with one or a few source points per lattice. In some channels, we observe a significant improvement in the two-point functions for small quark masses.
hep-lat↗
Lattice QCD at the end of 2003
I review recent developments in lattice QCD. I first give an overview of its formalism, and then discuss lattice discretizations of fermions. We then turn to a description of the quenched approximation and why it is disappearing as a vehicle for QCD phenomenology. I describe recent claims for progress in simulations which include dynamical fermions and the interesting theoretical problems they raise. I conclude with brief descriptions of the calculations of matrix elements in heavy flavor systems and for kaons.
hep-ph↗