REVIEW
The running coupling from the QCD Schr\"odinger functional -- a one-loop analysis
High Energy Physics - Lattice · Submitted 9 Aug 1995
Pith's verdict: The paper that puts quarks into the Schrödinger functional coupling and hands you the one-loop matching to MSbar plus the boundary improvement coefficient — the numbers hold up.
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Referee report
The paper
The paper claims
A finite-volume definition of the running QCD coupling with quarks is matched to MSbar at one loop, and the boundary improvement coefficient needed for controlled continuum extrapolations is computed.
The reading
What carries the argument
The load-bearing object is the Schroedinger functional effective action $Gamma[B]$ on a space-time cylinder whose spatial boundary values are fixed color-electric gauge fields. The running coupling is defined by differentiating $Gamma$ with respect to a parameter $eta$ that scales the boundary field, normalized by the classical action so that the coupling equals the bare coupling at tree level. Quark fields are periodic up to a phase $theta$; the choice $theta=pi/5$ is made because the massless Dirac operator in the classical background then has a larger lowest eigenvalue, which improves the condition number of the fermion matrix. The quark determinant is evaluated by a first-order recursion in each fixed spatial-momentum and color sector, and the continuum matching is extracted from the asymptotic $1/l$ expansion of the lattice coefficient. The integrated $beta$-function is packaged in the step scaling function $Sigma(s;u,z)$, which is the quantity a non-perturbative simulation would measure.
Extended reading notes
Core claim
The central claim is that the Schroedinger functional gives a non-perturbative definition of a running coupling in QCD with quarks, and that its one-loop relation to the $bar{mathrm{MS}}$ scheme is under control. For massless quarks the quark contribution to the matching coefficient is $c_{1,1}(0)=0.039863(2)$ for $theta=pi/5$ and $0.022504(2)$ for $theta=0$, both small and positive. The quark-mass dependence is parametrized by Chebyshev fits for $z=mL$ below about 10 and by a $ln z$ plus power-correction form for large $z$; the resulting threshold behaviour in the one-loop $beta$-function is broad rather than sharp. The paper also computes the boundary improvement coefficient $c_t^{(1,1)}=0.0191410(1)$ needed to cancel the $O(a)$ artefacts from the time boundaries. For the step scaling function, one massless flavour induces only sub-percent artefacts with the standard action, and these fall roughly like $(a/L)^2$ once the clover term and the boundary improvement are switched on.
Load-bearing premise
The one-loop continuum matching coefficient is extracted from a lattice series assumed to have the asymptotic form $p_{1,1}$ approaching $r_0+s_0ln l+(r_1+s_1ln l)/l$, with $s_0$ fixed to its exact value and only the first $1/l$ corrections retained; if that truncation or the numerical $1/l$ extrapolation is inadequate, the quoted $c_{1,1}$ values shift.
Editorial extensions
If this is right
- Monte Carlo measurements of the finite-volume Schroedinger-functional coupling can be converted to $alpha_{bar{mathrm{MS}}}$ with a one-loop conversion factor.
- Heavy quarks with $z=mL$ above about 2 can be omitted from the one-loop $beta$-function evolution with an error of about $0.003$ in $g^{-2}$, below the precision of the pure-gauge step-scaling result.
- The boundary coefficient $c_t^{(1,1)}$ can be tuned to remove the linear-in-$a$ artefacts from the step scaling function at one loop.
- With the clover term, lattice artefacts in the step scaling function fall roughly like $(a/L)^2$; without it they can look almost constant over the lattice sizes accessible to simulations.
- The construction extends recursive finite-size scaling to QCD with dynamical quarks, which is the step needed for a first-principles determination of the strong coupling.
Reading between the lines
- The $theta=pi/5$ choice is justified by the free Dirac spectrum; a direct Monte Carlo measurement of the low-lying spectrum at realistic coupling would test whether the gain survives gauge fluctuations.
- Because the matching coefficients are small, the Schroedinger-functional coupling may be numerically close to $bar{mathrm{MS}}$ at accessible scales; whether that closeness persists at two loops is an open calculation.
- The broad threshold behaviour and the $1/z$ tail suggest that omitting a heavy flavour is safe at the quoted precision, but a precision extraction near a flavour threshold may need the full mass dependence.
- The improvement coefficient is computed at one loop; non-perturbative tuning at stronger coupling may be needed before continuum extrapolations of dynamical simulations are fully controlled.
Original abstract
Starting from the Schr\"odinger functional, we give a non-perturbative definition of the running coupling constant in QCD. The spatial boundary conditions for the quark fields are chosen such that the massless Dirac operator in the classical background field has a large smallest eigenvalue. At one-loop order of perturbation theory, we determine the matching coefficient to the $\MSbar$-scheme and discuss the quark mass effects in the $\beta$-function. To this order, we also compute the Symanzik improvement coefficient necessary to remove the $\Oa$ lattice artefacts originating from the boundaries. For reasonable lattice resolutions and the standard Wilson action, lattice artefacts are found to be only weakly dependent on the lattice spacing $a$, while they vanish quickly with the improved action of Sheikholeslami and Wohlert.
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
Starting from the Schr\"odinger functional, we give a non-perturbative definition of the running coupling constant in QCD. The spatial boundary conditions for the quark fields are chosen such that the massless Dirac operator in the classical background field has a large smallest eigenvalue. At one-loop order of perturbation theory, we determine the matching coefficient to the $\MSbar$-scheme and discuss the quark mass effects in the $\beta$-function. To this order, we also compute the Symanzik improvement coefficient necessary to remove the $\Oa$ lattice artefacts originating from the boundaries. For reasonable lattice resolutions and the standard Wilson action, lattice artefacts are found to be only weakly dependent on the lattice spacing $a$, while they vanish quickly with the improved action of Sheikholeslami and Wohlert.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Starting from the Schr\"odinger functional, we give a non-perturbative definition of the running coupling constant in QCD. | contribution 0.99 | 0.93 | 0.34 | asserted 1.00 |
| s1 | The spatial boundary conditions for the quark fields are chosen such that the massless Dirac operator in the classical background field has a large smallest eigenvalue. | method 0.98 | 0.84 | 0.03 | asserted 0.99 |
| s2 | At one-loop order of perturbation theory, we determine the matching coefficient to the $\MSbar$-scheme and discuss the quark mass effects in the $\beta$-function. | contribution 0.64 | 0.96 | 0.15 | asserted 1.00 |
| s3 | To this order, we also compute the Symanzik improvement coefficient necessary to remove the $\Oa$ lattice artefacts originating from the boundaries. | contribution 0.69 | 0.95 | 0.09 | asserted 1.00 |
| s4 | For reasonable lattice resolutions and the standard Wilson action, lattice artefacts are found to be only weakly dependent on the lattice spacing $a$, while they vanish quickly with the improved action of Sheikholeslami and Wohlert. | result 0.99 | 0.93 | 0.20 | asserted 1.00 |
What was examined, and what was not
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- full text
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- P abstract
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- R cross-paper relations
References
How the literature uses it
44 arXiv papers cite this in 106 sentences
Papers citing it · 59 sentences without a confident label
Builds on it 2 papers
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For this, we calculate the one loop running coupling in our family of SF schemes following closely the discussions in [ref] and adapting them to arbitrary $N$.
Schr\"odinger functional boundary conditions and improvement for N>3 · Introduction · 0.82
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This is a generalisation of the program used earlier to compute the improved action for two fundamental representation fermions in SU(3) gauge theory [ref] .
Nonperturbative improvement of SU(2) lattice gauge theory with adjoint or fundamental flavors · Introduction · 0.98
Uses its method or tool 19 papers
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The renormalized coupling $g^2(2L)$ at the scale $2L$ is corrected perturbatively in order to cancel the deviation of the PCAC mass from zero at the scale $L$ [ref] [equation] The PCAC mass at the scale $L$ has been tuned such that the deviation $Φ(0)u^2mL$ is smaller than the typical statistical error.
Precise determination of the strong coupling constant in Nf=2+1 lattice QCD with the Schr\"odinger functional scheme · Step scaling function · 0.98
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Here $Σ^(k)(u,a/L)$, whose lattice artifacts are perturbatively removed at k-loop, is defined by [equation] where $δ_n$ $(n=1,2…)$ is given by [equation] and $δ_1(a/L)$ for the RG improved action is given in Ref. [ref] .
Universality of the Nf=2 Running Coupling · Result · 0.85
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For the details of the numerical method used to determine coefficient $c_t^(1,1)$, we refer to the original literature where the method was developed for fundamental representation fermions [ref] .
Perturbative Improvement of the Schrodinger Functional for Lattice Strong Dynamics · Introduction · 0.97
Uses its data or values 5 papers
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We translate to the $$ scheme using $Λ_^(2)=2.382035(3)Λ_SF^(2)$ [ref] as well as to physical units Λ_^(2) = 310(20) MeV , where =155 enters.
Non-perturbative computation of the strong coupling constant on the lattice · Scale setting for $=2$ · 0.93
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By combining the previous results from Ref. [ref] , the values of $A_0$ can be obtained, and are shown in the second line (numbers with $∗$) in each $s$ in Table [ref] for $θ=0,π/5$.
Perturbative analysis of the Neuberger-Dirac operator in the Schr\"odinger functional · Coefficients of Symanzik's expansion · 0.92
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[equation] where the right-hand side of eq.([ref]) is taken from [ref] .
Universality of the Nf=2 Running Coupling · Simulation details · 0.81
Compares with it 6 papers
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The first coefficient $A_0$ is generally a function of the parameter $s$, and I get $A_0(s)|_s=0=0.012567(3)$, while by combining the results of Ref. [ref] , $A_0(s)|_s=0=0.012566$ can be deduced.
Universality check of the overlap fermions in the Schroedinger functional · Universality check in perturbation theory · 0.94
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We show numerical results for the one-loop deviation in Tables [ref] and [ref] and the plots in Figure [ref], where we include those of the Wilson type fermions [ref] and overlap fermion [ref] for comparison. $L_s$-dependence of DWFs is small.
A formulation of domain-wall fermions in the Schr\"odinger functional · Lattice artifacts of the step scaling function to one-loop order · 0.98
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We show numerical results of the one-loop deviation in Table [ref] and plots in Figure [ref], where we include those of the Wilson-Dirac and the clover action for comparison [ref] .
Perturbative analysis of the Neuberger-Dirac operator in the Schr\"odinger functional · Lattice artifacts of the step scaling function to one-loop order · 0.87
Cites it as background 6 papers
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The renormalised coupling in the Schrödinger functional (SF) scheme has been defined in [ref] and its scale evolution has been studied in QCD with zero and two quark flavours [ref] .
The SF running coupling with four flavours of staggered quarks · Introduction · 0.87
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In this section we briefly recall the ideas introduced in [ref] and collect the expressions necessary for the subsequent discussion.
Schr\"odinger functional boundary conditions and improvement for N>3 · Schrödinger functional · 0.91
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[ref] for some important examples.]
Past, present, and future of precision determinations of the QCD coupling from lattice QCD · The high-energy regime of $=3$ QCD · 0.94
Discussion
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