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REVIEW 4 major objections 6 minor 113 references

Candia-v2 solves the DGLAP evolution equations at N3LO accuracy in QCD, directly in x-space.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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2026-08-02 06:15 UTC pith:FDY5KSOJ

load-bearing objection Worth refereeing as a software release, but the N3LO numbers in Table 2 are not backed by any external check. the 4 major comments →

arxiv 2607.14151 v2 pith:FDY5KSOJ submitted 2026-07-14 hep-ph hep-ex

Candia-v2: Logarithmic expansions for DGLAP evolution in x-space

Casey Hampson , Marco Guzzi This is my paper
classification hep-ph hep-ex PACS 12.38.-t12.38.Bx
keywords DGLAP evolutionparton distribution functionsN3LO QCDx-spacelogarithmic expansionsrecursion relationsheavy-quark matchingsubtraction PDFs
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Candia-v2 claims to solve the DGLAP integro-differential equations that govern the scale evolution of proton parton densities directly in Bjorken-x space, at approximate N3LO accuracy in QCD. The method writes the solution as an exponential-type logarithmic power series and determines the unknown coefficient functions through recursion relations, avoiding Mellin-space inversion. The release adds exact four-loop non-singlet splitting functions, state-of-the-art approximations for the four-loop singlet sector, and three-loop operator matrix elements for heavy-quark matching. If the implementation is correct, the package is a production-grade evolution tool for high-precision collider phenomenology and supplies heavy-quark subtraction and residual PDFs for general-mass variable-flavor-number schemes.

Core claim

The paper argues that the DGLAP solution can be built in x-space from the ansatz f(x, mu2) = product over i of [sum_n a_i(x)^n/n! L_i^n] convoluted with the initial PDF, with the logarithmic functions L_i depending only on alpha_s, the starting coupling, and the roots of the four-loop beta-function, and with the coefficient functions fixed by recursion relations. Candia-v2 implements this at N3LO: exact four-loop splitting functions in the non-singlet sector, the currently best available approximations in the singlet sector, and three-loop operator matrix elements for heavy-quark threshold matching. The paper presents evolved gluon and valence densities, heavy-flavor asymmetries, and subtrac

What carries the argument

The central object is the logarithmic-expansion ansatz with the L_i logs: one log of the alpha_s ratio, one log of a quadratic in alpha_s, one log of a linear root term, and an arctangent coming from the roots of the four-loop beta-function. The coefficient functions a_i(x) are built from convolutions and fixed by the four recursion relations, which generate each higher-order coefficient from lower-index convolutions with the splitting-function kernels. Numerically, convolutions are evaluated by Gauss-Legendre quadrature on a three-segment x-grid, with singular plus-distribution and delta-function pieces factored out and precomputed per active flavor number.

Load-bearing premise

The load-bearing premise is that discretizing the convolutions by Gauss-Legendre quadrature on a hand-chosen three-segment grid reproduces the continuum DGLAP solution to the implicit precision, despite there being no grid-refinement or convergence study in the paper and the repository's 'tests' being benchmarking files rather than standard tests.

What would settle it

Run Candia-v2 on the Les Houches toy-model input at mu0^2 = 2 GeV^2 with the same aN3LO splitting functions and operator matrix elements, and compare its evolved xg and xuv at mu = 100 GeV, plus the c(-) and b(-) asymmetries, against an independent evolution code using a different discretization, such as Mellin-space with contour inversion. If the difference at the tabulated x points is much larger than the claimed tolerance, or if doubling the Gauss nodes or grid segments materially changes the output, the numerical claim fails even though the analytic recursion may be correct.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Evolved densities at approximate N3LO with all currently known perturbative ingredients become available in a single open-source package, suitable for LHC and HL-LHC scale predictions.
  • Heavy-flavor asymmetries c(-) and b(-), which first appear at NNLO, now include N3LO evolution and matching corrections, and Fig. 2 shows their size.
  • Subtraction and residual PDFs for charm and bottom can be generated on demand, giving reusable, process-independent pieces for general-mass variable-flavor-number schemes.
  • When exact four-loop singlet splitting functions become available, the recursion structure allows them to be inserted without restructuring the algorithm.
  • LHAPDF input and output let the code be embedded in larger fitting and prediction environments as an x-space alternative to Mellin-space evolution.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Editorial: the paper does not report a grid-refinement or quadrature-order convergence study, so a user should verify that the default 100/50/25 grid and 50 Gauss points are sufficient for the x region of interest before trusting the tabulated digits.
  • Editorial: because the repository's 'tests' are described as benchmarking and plotting files rather than standard tests, an independent comparison of Candia-v2 with a differently discretized evolution code on identical splitting-function inputs is the direct way to check the N3LO numbers.
  • Editorial: the same logarithmic-expansion machinery should carry over to polarized parton densities and to QED corrections, since the recursion relations depend only on the beta-function and splitting-function inputs.
  • Editorial: the exact-ansatz recursion cost scales exponentially with perturbative order, so the truncated ansatz is the practical route at higher orders; the code already exposes both, suggesting a path toward N4LO when the required kernels exist.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The manuscript presents Candia-v2, an open-source C++/Fortran reimplementation of the x-space Candia algorithm for DGLAP evolution. It claims to generalize the previous version to (approximate) N3LO accuracy by implementing four-loop non-singlet splitting functions, state-of-the-art approximations for singlet splitting functions, three-loop operator-matrix-element matching via libome, and optional LHAPDF interfaces. The algorithm is based on logarithmic expansions and recursion relations for coefficients in x-space. Results are shown for the Les Houches toy model evolved from 2 GeV^2 to 10^4 GeV^2 in the VFNS, including scale dependence and heavy-quark subtraction/residual PDFs. The paper also provides a detailed program description, API overview, and a list of bug fixes from Candia-v1.

Significance. If the numerical implementation is correct, Candia-v2 would be a useful independent x-space evolution tool at approximate N3LO, with practical features such as subtraction PDFs, LHAPDF I/O, parallelization, and modular design. The paper explicitly uses exact four-loop non-singlet splitting functions, state-of-the-art singlet approximations, and libome for three-loop OMEs, and it openly discusses known v1 bugs. These are strengths that make the package potentially valuable to the community. The main risk is the absence of any quantitative validation of the numerical core, which underlies all reported N3LO results.

major comments (4)
  1. [Eq. (24)] The recursion relations in Eq. (24) contain the undefined symbol gamma in the denominators of Z21, Z22, Z31, Z32, and Z43. Since these relations are the core of the N3LO implementation, the algorithm is not self-contained as written. Please define gamma, presumably gamma = sqrt(4cbar - bbar^2) as suggested by Eq. (21), or explicitly refer to the corresponding definition in Ref. [6].
  2. [Sec. 3, Table 2] No external validation is provided for the central N3LO-accuracy claim. The results are compared only across LO/NLO/NNLO/N3LO within the same code. The paper cites the aN3LO benchmarking paper Ref. [75] but does not compare against it or any other evolution code (e.g., PEGASUS, HOPPET, APFEL, EKO). The repository's tests are explicitly 'not standard tests' (Sec. 5.2), and the disclosed v1 bugs (Sec. 5.5) show that implementation errors are a real risk. A direct numerical comparison with at least one independent evolution code is essential.
  3. [Sec. 5.4.2, Sec. 3] The convolution quadrature and the iteration/truncation of the logarithmic series are not validated. All convolutions use a fixed three-segment grid with 100/50/25 points, 50 Gauss-Legendre nodes, and 4 interpolation points (Sec. 5.4.2). The example uses iterations=10, trunc_idx=10, with no convergence study in these parameters. Since the integrands involve plus-distributions and delta-functions, and the small-x region is highly sensitive, the numerical error could be comparable to the N3LO corrections. Grid-refinement and iteration-convergence tests are needed to show the numerical error is below the target accuracy.
  4. [Sec. 5.4.3, Sec. 5.5] The implementation of heavy-quark threshold matching inside the solver is not described. The AlphaS class handles n_f changes and libome provides OMEs, but the paper does not explain how the OMEs are applied at each threshold within the recursion framework. Given that v1 had a bug in the matching procedure (Sec. 5.5), the N3LO charm and bottom results in Fig. 2 cannot be assessed. A concise description of the matching step and a threshold-crossing check (e.g., PDF continuity) would address this.
minor comments (6)
  1. [Eq. (22)] The text 's=n+m+ell+k' uses an undefined k (and ell is not used in the sums). This appears to be a typographical error.
  2. [Eq. (29)] The notation R(2), R(1), R(0) should be R2, R1, R0 for consistency with Eq. (25).
  3. [Sec. 4] The citation 'GMVFN schemes currently in use [104? ? ?–113]' contains placeholder question marks and is broken.
  4. [Abstract / Sec. 6] The abstract states 'N3LO accuracy in QCD', while the conclusions and Sec. 3 correctly use 'approximate N3LO' (aN3LO). Please make the wording consistent, since the singlet splitting functions are approximations.
  5. [Listing 1] The variables order, iterations, and trunc_idx are declared as const double but represent integer counts; using an unsigned integer type would be clearer.
  6. [Sec. 5.2] The description of the 'tests' as 'not standard tests' is confusing; consider renaming them to 'benchmark and plot-generation programs'.

Circularity Check

0 steps flagged

No significant circularity: the code's outputs are computed from externally specified splitting functions, OMEs, and benchmark input PDFs via recursion relations that are stated in the paper and taken as inputs, not fitted to the outputs.

full rationale

The derivation chain in the paper is not circular. The DGLAP equation (1) is combined with the exponential power-series ansatz (17), whose L-functions (18)-(21) are obtained from the algebraic Mellin-space solution, and the coefficient recursions (23a)-(23d) and (31) are stated explicitly and implemented. These recursions are attributed to the authors' earlier Ref. [6], a self-citation, but they are re-used inputs rather than the target result: the paper's claims are about the code reproducing DGLAP evolution, and the recursions are the method being implemented. The actual numerical outputs (Table 2, Figs. 1-3) are generated from external theory inputs — the exact four-loop non-singlet splitting functions [11], state-of-the-art singlet approximations [13-20], three-loop OMEs through libome [21], and the Les Houches toy-model initial conditions [101] — with no parameter fitted to the evolved PDFs. The beta-function roots in Table 1 are computed numerically, not fitted. The paper even explicitly states that the truncated recursion relations 'generate truncated solutions at arbitrary order κ that are equivalent those obtained using the U-matrix method [7, 56, 64],' acknowledging rather than concealing the algorithm's relation to known methods. The possible weaknesses — fixed Gauss-Legendre grid defaults (Sec. 5.4.2), no convergence study, 'tests' described as 'not standard tests' (Sec. 5.2), and the disclosed v1 bugs (Sec. 5.5) — are numerical validation and correctness risks, not circular reasoning. A self-citation to the prior derivation of the algorithm is a normal, legitimate reference for a software paper and does not by itself make the derivation circular.

Axiom & Free-Parameter Ledger

2 free parameters · 6 axioms · 0 invented entities

Everything the numerical answers depend on is imported: the DGLAP framework (Eq. 1), the recursion relations from the authors' own Ref. [6], the exact 4-loop non-singlet splitting functions of Ref. [11], the approximate singlet inputs of Refs. [13-20], the 3-loop OMEs of libome (Refs. [21-45]), the 4-loop β-function (Refs. [99,100]), and the Les Houches toy-model boundary conditions (Ref. [101]). No parameters are fitted to data; the package's hand-chosen numerical settings (grid sizes, Gauss order, series truncation) are the only adjustable inputs and their convergence is not quantified.

free parameters (2)
  • series truncation settings (iterations s, trunc_idx κ) = 10 (default in examples/Listing 1)
    Numerical output depends on the number of terms kept in the exact (s) and truncated (κ) ansätze; the choice 10 is not justified by a convergence study in the paper.
  • grid/quadrature defaults = 100/50/25 grid nodes; 50 Gauss-Legendre points; 4 interpolation points
    Hand-chosen defaults in GridFillerArgs/ConvIntArgs (§5.4.2); no grid-refinement or quadrature-error study is presented, so discretization error is uncontrolled.
axioms (6)
  • domain assumption Collinear factorization: PDFs evolve with scale according to the DGLAP equations, Eq. (1).
    Standard QCD factorization assumption; the entire package solves Eq. (1).
  • domain assumption The exponential logarithmic ansatz, Eq. (17), with L0-L3 from the roots of the 4-loop β-function (Table 1), spans the exact DGLAP solution in the NS sector.
    The ansatz and recursion relations (23a-d) are taken from the authors' Ref. [6]; this paper does not re-derive them. If the ansatz or recursions are incomplete at N3LO, the output is wrong.
  • domain assumption Envelope approximations (FHM-RUVV, Refs. [13-20]) of the 4-loop singlet splitting functions represent the true singlet P(3) accurately enough for aN3LO evolution.
    The exact singlet 4-loop calculation is in progress (Refs. [92-98]); the code uses approximate inputs by default (Sec. 1, Sec. 5.4.6).
  • domain assumption 3-loop OMEs from libome [21] provide the correct VFNS matching conditions at N3LO.
    Used for heavy-quark threshold matching (Sec. 3, Sec. 5.4.6); external library input treated as exact.
  • domain assumption Les Houches toy-model initial conditions [101] and pole masses mc=µ0, mb=4.5 GeV, mt=175 GeV define the benchmark.
    Inputs adopted from Ref. [101]; Table 2 results are specific to these inputs.
  • standard math β-function coefficients β0-β3 are known and correctly implemented (Eqs. (5)-(8)).
    From Refs. [99,100]; standard four-loop QCD result.

pith-pipeline@v1.3.0-alltime-deepseek · 26043 in / 23759 out tokens · 239504 ms · 2026-08-02T06:15:12.442095+00:00 · methodology

0 comments
read the original abstract

We present Candia-v2, an open-source software package that generalizes the $x$-space Candia algorithm to next-to-next-to-next-to-leading order (N$^{3}$LO) accuracy in Quantum Chromodynamics (QCD). The code solves the DGLAP evolution equations for unpolarized nucleon parton densities using a highly efficient logarithmic expansion technique that can be systematically extended to all orders in QCD. Candia-v2 supersedes the previous original C implementation of the algorithm with significantly increased efficiency and improved API. The software is publicly available on GitHub under the GPLv3 license.

Figures

Figures reproduced from arXiv: 2607.14151 by Casey Hampson, Marco Guzzi.

Figure 1
Figure 1. Figure 1: Results for the Candia-v2 evolution of the gluon and the uv PDFs. 14 [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Results for the Candia-v2 evolution of the c (−) and b (−) heavy flavor combina￾tions. 15 [PITH_FULL_IMAGE:figures/full_fig_p015_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Subtraction PDFs for the charm and bottom quark evolved at [PITH_FULL_IMAGE:figures/full_fig_p018_3.png] view at source ↗

discussion (0)

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Reference graph

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