REVIEW
On the Schroedinger functional in QCD
High Energy Physics - Lattice · Submitted 20 Dec 1993
Pith's verdict: The paper that puts Wilson fermions into Lüscher's Schrödinger functional, and the derivation holds up; the soft spots are small and mostly acknowledged.
T0 · AI review
Why Pith says this
Highlight any text to comment on it.
Referee report
The paper
The paper claims
The Schrödinger functional — a QCD state prepared on one time slice and read off on another — can be defined with quarks, provided only half of each quark spinor is pinned at each boundary.
The reading
What carries the argument
The transfer matrix of Wilson lattice QCD, converted into an integral operator on Grassmann coherent states. The coherent-state representation turns each factor of the transfer matrix into an explicit kernel $exp(-S[U,V,U'])$ times a bilinear quark exponential, and iterating it $N$ times with inserted completeness relations produces the path integral. The two-spinor decomposition with projectors $P_pm$ is what makes the boundary conditions visible: the kernel can only depend on the crossed Grassmann variables, so exactly half of each spinor appears at each time slice. The second piece of machinery is the variational argument of section 5, which asks that the classical action admit smooth stationary points and be parity invariant; solving the resulting conditions on the boundary matrices $R_0$ and $R_T$ leaves only terms proportional to $P_- gamma_0$ and $P_+ gamma_0$, matching the action derived from the lattice.
Extended reading notes
Core claim
The paper establishes that the Schrödinger functional of pure gauge theory extends to lattice QCD with Wilson fermions, and that the extension is fixed rather than chosen. Starting from Lüscher's transfer matrix for Wilson fermions and representing it as an integral operator on a Grassmann algebra via coherent states, Sint obtains the kernel of the $N$-th power of the transfer matrix as a path integral over euclidean gauge and quark fields. The kernel depends on the Grassmann variables only through the combinations selected by the projectors $P_+ = (1+gamma_0)/2$ and $P_- = (1-gamma_0)/2$: at $x_0=0$ the plus components of the quark field and the minus components of the antiquark field are fixed, and at $x_0=T$ the opposite halves are fixed. Taking $a to 0$ yields the continuum action with explicit boundary terms, and a variational argument shows that these terms are exactly the ones required for the action to possess smooth stationary points while remaining parity invariant, so they are not lattice remnants. Finally, the free Dirac operator built from this action, viewed as an elliptic two-term boundary value problem on $H_+ oplus H_-$, is essentially self-adjoint with a purely discrete spectrum $p_0^2 + p^2 + m^2$, a lowest massless eigenvalue $(pi/2T)^2$, and no zero modes for any finite mass.
Load-bearing premise
The claim that the continuum boundary terms are physical rather than lattice artifacts rests on requiring the classical action to have smooth stationary points and to be parity invariant, and parity is what excludes the leftover boundary matrices that anticommute with $gamma_0$.
Editorial extensions
If this is right
- The running coupling of QCD can be computed in small volumes with the same boundary-field technique already used successfully in the pure SU(3) theory, with quark boundary values as an additional dial.
- Because the boundaries induce a minimum frequency $(pi/2T)^2$ in the massless case, simulations at light or vanishing quark mass become feasible in small volumes, where the condition number of the lattice Dirac matrix would otherwise blow up.
- Setting the quark boundary fields to zero is consistent with renormalizability, since the two allowed boundary counterterms $\bar\psi\psi$ and $\bar\psi\gamma_0\psi$ then vanish, leaving boundary effects that are suppressed by a power of the lattice spacing.
- The free propagator is well defined as $D^{-1} = D^\dagger (DD^\dagger)^{-1}$ for all masses, so a perturbative expansion of the QCD Schrödinger functional has a sound starting point.
- The construction supplies a criterion for choosing the gauge background: it must make gauge configurations that produce Dirac zero modes extremely unlikely.
Reading between the lines
- The argument that the continuum boundary terms are not artifacts is a uniqueness argument conditional on parity, not a proof that the regulated lattice theory preserves parity at the boundary; an explicit one-loop check, which the author states is in progress, is the natural place for this to be settled.
- If the boundary conditions really do control the low end of the Dirac spectrum, the same mechanism could be used as a spectroscopic tool: the gap $(pi/2T)^2$ is a boundary-induced infrared cutoff whose value is known exactly, so deviations from it in interacting configurations measure how the gauge field distorts the would-be zero-mode sector.
- The requirement that smooth stationary points exist is doing real work; in a theory where the classical boundary value problem fails to have smooth solutions, the same lattice derivation would produce boundary terms that no continuum variational principle selects, which suggests the criterion is worth testing in other fermionic boundary constructions.
Original abstract
In a series of publications [\ref{LNWW},\ref{Schroedinger}], L\"uscher et al. have demonstrated the usefulness of the Schr\"odinger functional in pure SU(2) and SU(3) gauge theory. In this paper, it is shown how their formalism can be extended to include fermions. In the framework of Wilson's lattice QCD, we define the Schr\"odinger functional by making use of the transfer matrix formalism. Boundary conditions for the fermions arise naturally. We then take the naive continuum limit of the action and show that no lattice peculiarities are left over. The corresponding free Dirac operator has a unique self-adjoint extension with purely discrete spectrum and no zero modes.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
In a series of publications [\ref{LNWW},\ref{Schroedinger}], L\"uscher et al. have demonstrated the usefulness of the Schr\"odinger functional in pure SU(2) and SU(3) gauge theory. In this paper, it is shown how their formalism can be extended to include fermions. In the framework of Wilson's lattice QCD, we define the Schr\"odinger functional by making use of the transfer matrix formalism. Boundary conditions for the fermions arise naturally. We then take the naive continuum limit of the action and show that no lattice peculiarities are left over. The corresponding free Dirac operator has a unique self-adjoint extension with purely discrete spectrum and no zero modes.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | In a series of publications [\ref{LNWW},\ref{Schroedinger}], L\"uscher et al. have demonstrated the usefulness of the Schr\"odinger functional in pure SU(2) and SU(3) gauge theory. | background 1.00 | 0.04 | 0.04 | asserted 1.00 |
| s1 | In this paper, it is shown how their formalism can be extended to include fermions. | contribution 0.99 | 0.89 | 0.49 | asserted 1.00 |
| s2 | In the framework of Wilson's lattice QCD, we define the Schr\"odinger functional by making use of the transfer matrix formalism. | method 0.92 | 0.93 | 0.07 | asserted 1.00 |
| s3 | Boundary conditions for the fermions arise naturally. | result 0.53 | 0.70 | 0.05 | asserted 1.00 |
| s4 | We then take the naive continuum limit of the action and show that no lattice peculiarities are left over. | result 0.63 | 0.94 | 0.14 | asserted 1.00 |
| s5 | The corresponding free Dirac operator has a unique self-adjoint extension with purely discrete spectrum and no zero modes. | result 1.00 | 0.86 | 0.16 | asserted 1.00 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
References
How the literature uses it
52 arXiv papers cite this in 71 sentences
Papers citing it · 30 sentences without a confident label
Uses its method or tool 27 papers
-
We evaluate this identity on a lattice with Schrödinger functional boundary conditions (periodic in space, Dirichlet in time) [ref] with vanishing background field.
Non-perturbative improvement and renormalization of the axial current in N_f=3 lattice QCD · Renormalization Condition · 0.99
-
As will be discussed in detail in the present article, this problem can be tackled in a systematic way by means of the Symanzik improvement programme [ref] and defining the theory in the Schrödinger functional (SF) scheme [ref] according to the framework presented in Refs. [ref] ; this allows one to cancel the leading, $O(a)$, discretisation artifacts and to achieve $O(a^2)$ scaling towards the continuum limit [ref] .
Non-perturbative renormalisation and improvement of non-singlet tensor currents in $N_\mathrm{f}=3$ QCD · Introduction · 0.92
-
We have carried out the calculation within the $O(a)$ improved Schrödinger Functional formalism [ref] with $T=2L$ and vanishing background fields.
Quenched lattice calculation of the B --> D l nu decay rate · Lattice Observables · 1.00
Compares with it 1 paper
-
These conditions can be compared to those in ferm-bc and be seen to agree – albeit by construction [ref] .
3d Abelian Dualities with Boundaries · Boundary conditions · 1.00
Cites it as background 9 papers
-
In the following, we assume that the reader is familiar with the SF in QCD [ref] .
A formulation of domain-wall fermions in the Schr\"odinger functional · Formulation · 0.98
-
There are many such schemes for the nonperturbative renormalisation (NPR) of bare lattice quantities, such as RI/MOM [ref] , RI/SMOM [ref] and the Schrödinger functional method [ref] .
Charm quark mass using a massive nonperturbative renormalisation scheme · Introduction · 0.97
-
Most finite-volume couplings that are used in practice are based on the QCD Schrödinger functional (SF) [ref] .
Past, present, and future of precision determinations of the QCD coupling from lattice QCD · Finite-volume couplings rescue us again · 0.84
Discussion
Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.