Computational Methods for the Fermion Determinant and the Link Between Overlap and Domain Wall Fermions

We define a sparse hermitian lattice Dirac matrix, $H$, coupling $2n+1$ Dirac fermions. When $2n$ fermions are integrated out the induced action for the last fermion is a rational approximation to the hermitian overlap Dirac operator. We provide rigorous bounds on the condition number of $H$ and compare them to bounds for the higher dimensional Dirac operator of domain wall fermions. Our main conclusion is that overlap fermions should be taken seriously as a practical alternative to domain wall fermions in the context of numerical QCD.

We present renormalization constants of overlap quark bilinear operators on 2+1-flavor domain wall fermion configurations.Both overlap and domain wall fermions have chiral symmetry on the lattice.The scale independent renormalization constant for the local axial vector current is computed using a Ward Identity.The renormalization constants for the scalar, pseudoscalar and vector current are calculated in the RI-MOM scheme.Results in the MS scheme are obtained by using perturbative conversion ratios.The analysis uses in total six ensembles with lattice sizes 24 3 × 64 and 32 3 × 64.

The equivalence of domain wall and overlap fermion formulations is demonstrated for lattice gauge theories in 2+1 spacetime dimensions with parity-invariant mass terms. Even though the domain wall approach distinguishes propagation along a third direction with projectors ${1\over2}(1\pm\gamma_3)$, the truncated overlap operator obtained for finite wall separation $L_s$ is invariant under interchange of $\gamma_3$ and $\gamma_5$. In the limit $L_s\to\infty$ the resulting Ginsparg-Wilson relations recover the expected U($2N_f$) global symmetry up to O($a$) corrections. Finally it is shown that finite-$L_s$ corrections to bilinear condensates associated with dynamical mass generation are characterised by whether even powers of the symmetry-breaking mass are present; such terms are absent for antihermitian bilinears such as $i\bar\psi\gamma_3\psi$, markedly improving the approach to the large-$L_s$ limit.

In this paper, we construct the Möbius domain wall fermions (MDWFs) in the Schrödinger functional (SF) scheme for the SU(3) gauge theory by adding a boundary operator at the temporal boundary of the SF scheme setup. Using perturbation theory, we investigate the properties of several constructed MDWFs, including the optimal type domain wall, overlap, truncated domain wall, and truncated overlap fermions. We observe the universality of the spectrum of the effective four-dimensional operator at the tree-level, and fermionic contribution to the universal one-loop beta function is reproduced for MDWFs with a sufficiently large fifth-dimensional extent.

Lattice QCD with domain wall quarks and applications to weak matrix elements

U(1) chiral gauge theory on lattice with gauge-fixed domain wall fermions

We report on an exploratory study of domain wall fermions (DWF) as a lattice regularisation for heavy quarks. Within the framework of quenched QCD with the tree-level improved Symanzik gauge action we identify the DWF parameters which minimise discretisation effects. We find the corresponding effective 4$d$ overlap operator to be exponentially local, independent of the quark mass. We determine a maximum bare heavy quark mass of $am_h\approx 0.4$, below which the approximate chiral symmetry and O(a)-improvement of DWF are sustained. This threshold appears to be largely independent of the lattice spacing. Based on these findings, we carried out a detailed scaling study for the heavy-strange meson dispersion relation and decay constant on four ensembles with lattice spacings in the range $2.0-5.7\,\mathrm{GeV}$. We observe very mild $a^2$ scaling towards the continuum limit. Our findings establish a sound basis for heavy DWF in dynamical simulations of lattice QCD with relevance to Standard Model phenomenology.

We discuss the symmetries of quenched QCD with Wilson fermions, starting from its lagrangian formulation, taking into account the constraints needed for convergence of the ghost-quark functional integral. We construct the corresponding chiral effective lagrangian, including terms linear and quadratic in the lattice spacing. This allows us to study the phase structure of the quenched theory, and compare it to that in the unquenched theory. In particular we study whether there may be an Aoki phase (with parity and flavor spontaneously broken) or a first order transition line (with no symmetry breaking but meson masses proportional to the lattice spacing), which are the two possibilities in the unquenched theory. The presence of such phase structure, and the concomitant long-range correlations, has important implications for numerical studies using both quenched and dynamical overlap and domain-wall fermions. We argue that the phase structure is qualitatively the same as in the unquenched theory, with the choice between the two possibilities depending on the sign of a parameter in the low-energy effective theory.

We briefly review1 the overlap formalism for chiral gauge theories, the overlap Dirac operator for massless fermions and its connection to domain wall fermions. We describe properties of the overlap Dirac operator, and methods to implement it numerically. Finally, we give some examples of quenched calculations of chiral symmetry breaking and topology with overlap fermions.

The topological susceptibility is one of the quantities that has a large discretization error, and the error can be sensitive to the choice of fermion action. We report on our results from physical point simulations with 2+1 flavor Möbius domain wall fermion at finite temperature. We also present the chiral condensate and disconnected susceptibility. The temporal lattice size is Nt=12 and 16, and the temperature range is around 140 MeV to 250 MeV for the chiral condensate and susceptibility. A coarse lattice with Nt=10 covers up to 500 MeV to measure the topological susceptibility.

I review the Thirring model in 2+1$d$ dimensions, focussing in particular on possible strongly-interacting UV-stable fixed points of the renormalisation group, corresponding to a continuous phase transition where a U($2N$) global symmetry spontaneously breaks to U($N)\otimes$U($N$). Since there is no small parameter in play, a systematic non-perturbative approach such as numerical simulation of lattice field theory is mandated. I compare and contrast various formulations, paying particular attention to models formulated with either staggered or domain wall lattice fermions. Domain wall fermions, which faithfully capture U($2N$) symmetry in the limit of wall separation $L_s\to\infty$, predict a critical flavor number $1<N_c<2$.

Massive domain wall fermions

We study the vacuum topology of 2+1 flavor QCD above the chiral crossover transition, at $T \lesssim 1.2~T_c$, on lattices of size $32^3\times 8$. Since overlap fermions have exact chiral symmetry and an index theorem even on a finite lattice, we use them to detect the topological content of gauge fields generated using domain wall fermion discretization for quarks. We further use different periodicity phases along the temporal direction for the valence overlap quarks, which allows us to probe different topological structures present in the gauge field ensembles, through its zero modes. This procedure provides strong evidences that fermion zero-modes can be quantitatively understood to arise due to different species of instanton-dyons. We estimate their relative abundances from the Dirac-eigenvalue density and resolve the so called "topological clusters" via multi-parameter fits to their density, providing therefore an understanding of the interactions between instanton-dyons. The typical separation between dyons we obtain, is $\sim 0.3$ fm. Surprisingly, it emerges out from this study that a semi-classical description of the fermionic zero modes in the QCD vacuum is quite accurate just above $T_c$.

We calculate the nucleon electric dipole moment (EDM) from the $\ensuremath{\theta}$ term with overlap fermions on three domain wall lattices with different sea pion masses at lattice spacing 0.11 fm. Due to the chiral symmetry conserved by the overlap fermions, we have well-defined topological charge and chiral limit for the EDM. Thus, the chiral extrapolation can be carried out reliably at nonzero lattice spacings. We use three to four different partially quenched valence pion masses for each sea pion mass and find that the EDM dependence on the valence and sea pion masses behaves oppositely, which can be described by partially quenched chiral perturbation theory. With the help of the cluster decomposition error reduction technique, we determine the neutron and proton EDM at the physical pion mass to be ${d}_{n}=\ensuremath{-}0.00148(14)(31)\overline{\ensuremath{\theta}}\text{ }\text{ }\mathrm{e}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{fm}$ and ${d}_{p}=0.0038(11)(8)\overline{\ensuremath{\theta}}\text{ }\text{ }\mathrm{e}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{fm}$. This work is a clear demonstration of the advantages of using chiral fermions in the nucleon EDM calculation and paves the road to future precise studies of the strong $CP$ violation effects.

We present the vector, scalar, and tensor renormalization constants (RCs) using overlap fermions with either regularization independent momentum subtraction ($\mathrm{RI}/\mathrm{MOM}$) or symmetric momentum subtraction ($\mathrm{RI}/\mathrm{SMOM}$) as the intermediate scheme on the lattice with lattice spacings $a$ from 0.04 fm to 0.12 fm. Our gauge field configurations from the MILC and RBC/UKQCD collaborations include sea quarks using either the domain wall or the HISQ action, respectively. The results show that $\mathrm{RI}/\mathrm{MOM}$ and $\mathrm{RI}/\mathrm{SMOM}$ can provide consistent renormalization constants to the $\overline{\mathrm{MS}}$ scheme, after proper ${a}^{2}{p}^{2}$ extrapolations. But at $p\ensuremath{\sim}2\text{ }\text{ }\mathrm{GeV}$, both $\mathrm{RI}/\mathrm{MOM}$ and $\mathrm{RI}/\mathrm{SMOM}$ suffer from nonperturbative effects which cannot be removed by the perturbative matching. The comparison between the results with different sea actions also suggests that the renormalization constant is discernibly sensitive to the lattice spacing but not to the bare gauge coupling in the gauge action.