REVIEW 3 major objections 4 minor 51 references
Charge dependent nucleon-nucleon potentials in covariant chiral effective field theory
T0 review · 3 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read The paper tries to establish that charge-dependent covariant chiral nucleon-nucleon potentials, built from pion mass splitting in one-pion exchange, the static Coulomb interaction, and four proton-proton contact terms, reproduce the np…
desk verdict A first charge-dependent covariant NN potential at NNLO with a known missing NNLO piece; the fit is in-sample, so the agreement is real but not a validation of the specific isospin-breaking mechanism. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the charge-dependent covariant chiral NN potential: the isospin-symmetric NNLO potential of the covariant framework is supplemented by one-pion-exchange potentials with separate $M_{\pi^\pm}$ and $M_{\pi^0}$ propagators (Eqs. 2-4), a covariant static Coulomb potential (Eq. 6), and four pp contact terms (Eq. 5). Phase shifts are produced by solving the Thompson equation, a relativistic two-body scattering equation, with a non-local Gaussian regulator. For pp scattering, the Coulomb interaction is folded into the S-matrix by matching to asymptotic Coulomb wave functions at $R=12$ fm.
What would settle it
Include the isospin-violating pion-nucleon coupling corrections that the paper leaves out (Sect. II B 1) and refit the same low-energy constants to the same PWA93 phase shifts; if the total chi-squared changes significantly or any fitted constant shifts beyond its Bayesian uncertainty, the omitted terms are not negligible and the claimed validation fails.
Extended reading notes
Core claim
The central claim is that the dominant isospin-breaking effects in the covariant chiral two-nucleon force up to NNLO are captured by treating the charged and neutral pion masses as different in one-pion exchange, adding the static Coulomb potential for pp, and introducing four charge-dependent contact terms $C^{pp}_S$, $C^{pp}_V$, $C^{pp}_{AV}$, $C^{pp}_T$. The paper reports that fitting the resulting potential to the np and pp PWA93 phase shifts at laboratory energies 1, 5, 10, 25, 50, 100, and 200 MeV for $J\leq 2$ gives a total unweighted chi-squared of 22.82 at NNLO. The fourth pp contact term is fixed to zero because at most three pp partial waves are available for $J\leq 1$. The paper follows Reference [7] in omitting isospin-violating corrections to the pion-nucleon couplings, on the grounds that their actual size is not well known.
Load-bearing premise
The load-bearing premise is that omitting isospin-violating corrections to the pion-nucleon coupling does not affect the fitted phase-shift data, so the agreement can be credited to the specific charge-dependent potential rather than to flexible constants absorbing missing physics.
Editorial extensions
If this is right
- The fitted NNLO potential reproduces both np and pp phase shifts for $J\leq 2$ with total unweighted chi-squared 22.82, so charge dependence can be included in a covariant chiral framework without going beyond NNLO.
- The four pp contact terms remove most of the low-energy pp $^1S_0$ discrepancy at NNLO, reducing its chi-squared contribution from 17.47 at NLO to 4.06.
- The resulting charge-dependent potential can be used as input for covariant calculations of nuclear structure and reactions involving proton-rich or neutron-deficient systems.
- The work opens a route to studying mirror energy differences, charge-exchange processes, and the symmetry energy of isospin-asymmetric matter, all of which are sensitive to differences among pp, np, and nn forces.
Reading between the lines
- Because the four pp contact terms are fitted only to $J\leq 1$ partial waves and the tensor piece is set to zero, the paper's pp success in higher partial waves is mostly inherited from the isospin-symmetric potential plus the Coulomb term; whether the same contact pattern survives a fit to higher partial waves with other regulators remains open.
- A natural testable extension would be to apply the same charge-dependent construction to neutron-neutron scattering once phase-shift information becomes available, checking whether the same charge-symmetry-breaking contact terms are consistent across all three charge channels.
- The paper itself notes that its regulator is 'old-fashioned' and distorts the long-range part of the potential; switching to a semi-local regulator could change the fitted constants and the apparent convergence, so the specific numerical agreement may be somewhat regulator-dependent.
- The omitted isospin-violating pion-nucleon coupling corrections could be estimated in a one-loop chiral calculation; if their effect on the fitted phase shifts is comparable to the reported chi-squared, the 'satisfactory agreement' would not uniquely pin down the proposed charge-dependent potential.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper constructs a charge-dependent nucleon-nucleon potential in covariant chiral effective field theory by adding pion-mass splitting in one-pion exchange, the static Coulomb interaction, and four charge-dependent pp contact terms to the isospin-symmetric covariant NNLO potential. The 19 isospin-conserving and four charge-dependent low-energy constants are fitted simultaneously to the J <= 2 np and pp phase shifts of PWA93 at seven laboratory energies. The authors report that including the charge-dependent terms reduces the total unweighted chi2 from 70.92 at NLO to 22.82 at NNLO and conclude that the resulting phase shifts are in satisfactory agreement with PWA93.
Significance. If the construction is correct, the paper fills a genuine gap: it provides the first covariant chiral NN potential that includes isospin breaking, with explicit operator expressions and a complete LEC table. The substantial chi2 improvement shows that the added pp contact terms are effective. However, the validation is entirely in-sample, the quoted chi2 is unweighted, and an explicitly acknowledged NNLO isospin-violating term is omitted without an estimate; these points limit the strength of the central claim as it currently stands.
major comments (3)
- [II B 1 and Table I] The NNLO entry 'isospin-breaking in OPE' is listed but not included, with the explanation that the size of the isospin-violating pion-nucleon couplings is poorly known. Because the 23 LECs are fitted to the same PWA93 phase shifts used as the benchmark in Figs. 1 and 2, any contribution from this omitted operator set can be absorbed into the fitted LECs. The abstract's claim of a charge-dependent covariant NN potential 'up to NNLO' is therefore not yet justified; the authors should either include or bound this contribution, or explicitly restate the claim as covering only the included mechanisms and discuss the resulting uncertainty.
- [Eq. (5)] The axial-vector charge-dependent contact term is written with a second bilinear gamma5 gamma5, which equals the identity and is not an axial-vector current. As printed, the equation does not define the claimed C^pp_AV operator. Please correct this to the intended gamma_mu gamma_5 form (or the form actually used in the numerical calculation) and verify that Table II and the phase-shift results correspond to the corrected expression.
- [IV A, Table III, Figs. 1 and 2] The numerical evidence for 'satisfactory agreement' is an in-sample fit: the PWA93 phase shifts used to determine the LECs are the same data plotted for comparison, and the chi2 in Table III is an unweighted sum of squared differences with no PWA93 uncertainties. The reduction from 70.92 to 22.82 demonstrates that the added terms improve the fit, but it does not by itself validate the specific charge-dependent operator content. The authors should report a weighted chi2 (or at least the number of data points and degrees of freedom) and, if possible, include a check on a partial wave or energy not used in the fit.
minor comments (4)
- [Table II header] The header 'in units of 104 GeV-2' should read '10^4 GeV^-2' to avoid ambiguity.
- [IV A] The statement that C^pp_T is set to zero because only three J<=1 pp partial waves exist should be phrased as a convention; a short test of the sensitivity of the results to this choice would strengthen the paper.
- [Figs. 1 and 2] The figure labels in the manuscript source appear corrupted (e.g., '/s49/s83/s48'), and the published figures should include proper axis labels and a legend identifying the LO, NLO, and NNLO lines and the PWA93 points.
- [Eq. (1)] The term -V_ITOPE in Eq. (1) is not defined in the text; please define it explicitly or remove it, since the isospin-symmetric OPE is already written as V_OPE.
Circularity Check
No significant circularity; the new isospin-breaking potential is derived, and the phase-shift agreement is a disclosed fit with partial predictive content.
full rationale
The paper's central derivation—charge-dependent OPE with Mπ±≠Mπ0 (Eqs. 2-4), static Coulomb (Eq. 6), and the pp contact Lagrangian (Eq. 5)—is constructed from chiral Lagrangians and power counting, with masses taken from PDG and gA/fπ external; it is not defined in terms of the PWA93 phase shifts. The 19 isospin-conserving and 4 pp LECs are then fitted to the PWA93 J≤2 phase shifts simultaneously (Sect. IV A), and the same dataset is used as the benchmark in Figs. 1-2; the abstract's 'satisfactory agreement' is therefore an in-sample fit-quality statement rather than an independent prediction. This is disclosed rather than disguised, and partial predictive content remains: the pp LECs act only in J≤1, so the pp 1D2, 3P2, 3F2, and ε2 waves and the higher-energy region are not directly controlled by the four pp contact LECs. The acknowledged omission of isospin-violating πN coupling corrections to OPE at NNLO (Table I note, Sect. II B 1) is an incompleteness/consistency limitation, not a circular reduction. Self-citations to Ref. [23] for the symmetric potential and Ref. [37] for the regulator supply previously published, independently testable input; they do not import the target result. No equation in the paper reduces by construction to its fit target, so no significant circularity is found.
Assumptions & free parameters
free parameters (7)
- O1-O17 (isospin-symmetric covariant contact LECs) =
NNLO values in 10^4 GeV^-2: -18.15, -8.88, -7.16, 9.72, 1.60, 3.84, -8.84, 4.51, 4.54, 4.77, 3.61, 7.28, -5.80, -9.61…
- D1, D2 (subleading covariant contact LECs) =
-1.94, -1.14 in 10^4 GeV^-2 at NNLO
- C_S^pp (pp charge-symmetry-breaking scalar contact LEC) =
-70.41 in 10^4 GeV^-2 at NNLO
- C_V^pp (pp vector contact LEC) =
-13.94 in 10^4 GeV^-2 at NNLO
- C_AV^pp (pp axial-vector contact LEC) =
13.94 in 10^4 GeV^-2 at NNLO
- C_T^pp (pp tensor contact LEC) =
0.00, fixed rather than fitted
- Regulator cutoff Lambda =
700, 800, 900 MeV for LO, NLO, NNLO respectively
assumptions (6)
- domain assumption The covariant chiral EFT power counting assigns the pion mass splitting in OPE to NLO and treats it as the leading strong isospin-breaking effect.
- domain assumption Isospin-violating corrections to the pion-nucleon couplings are negligible at NNLO and are omitted.
- domain assumption Nucleon mass difference corrections to the covariant amplitudes start at order v=6 and can be neglected.
- domain assumption The Thompson equation with a non-local Gaussian regulator at Lambda = 700, 800, 900 MeV is a valid framework for extracting phase shifts.
- domain assumption The static one-photon Coulomb potential, matched at R = 12 fm, adequately represents electromagnetic effects for the phase shifts studied.
- standard math The Stapp parameterization and Coulomb matching formulas in Eqs. (9) through (16) correctly relate the S-matrix to phase shifts.
Cite this review
Pith. "Pith review of Charge dependent nucleon-nucleon potentials in covariant chiral effective field theory." pith.science (2026). https://pith.science/paper/JBZYSWIA
@misc{pith2026250415598,
author = {Pith},
title = {Pith review of: Charge dependent nucleon-nucleon potentials in covariant chiral effective field theory},
year = {2026},
howpublished = {\url{https://pith.science/paper/JBZYSWIA}},
note = {Machine review of arXiv:2504.15598}
}
abstract
The charge-dependent nucleon-nucleon ($NN$) interaction plays a crucial role in understanding the nuclear structure and reaction problems. In this work, we explore the charge-dependent $NN$ interaction in covariant chiral effective field theory. By incorporating the isospin-breaking contributions, we derive the charge-dependent covariant chiral $NN$ potential up to next-to-next-to leading order (NNLO). The calculated $np$ and $pp$ phase shifts are in satisfactory agreement with the PWA93 partial wave analysis. Our results contribute to a deeper understanding of isospin-breaking effects in nuclear forces and provide a solid foundation for future studies of nuclear structure and reactions within the covariant framework.
Figures
Reference graph
Works this paper leans on
-
[1]
This leads to the mass differences of the pi- ons and the nucleons
Isospin-breaking effects from the strong interaction The isospin-breaking effects originating from the strong i n- teraction are caused by the mass differences of the up and down quarks. This leads to the mass differences of the pi- ons and the nucleons. In the chiral effective field theory, th e pion mass terms appear in processes related to the pion ex- ...
-
[2]
γνγ5qν u (−p, s2)] (Ep′ − Ep)2 − (p′ − p)2 − M 2 π0 , (2) VOPE, np, I =1(p, p′) = − g2 A 4f 2π [¯u (p, s1) γµγ5qµu (p′, s′ 1)] [¯u (−p′, s′
-
[3]
γν γ5qνu (−p, s2)] (3) × [ 2 (Ep′ − Ep)2 − (p′ − p)2 − M 2 π± − 1 (Ep′ − Ep)2 − (p′ − p)2 − M 2 π0 ] , VOPE, np, I =0(p, p′) = g2 A 4f 2π [¯u (p, s1) γµγ5qµu (p′, s′ 1)] [¯u (−p′, s′
-
[4]
In principle, the isospin- breaking effects further influence the πN couplings
γνγ5qν u (−p, s2)] (4) × [ 2 (Ep′ − Ep)2 − (p′ − p)2 − M 2 π± + 1 (Ep′ − Ep)2 − (p′ − p)2 − M 2 π0 ] , where I refers to the total isospin. In principle, the isospin- breaking effects further influence the πN couplings. Thus, they would result in the charge-independent and symmetry- breaking corrections to the OPE at NNLO. However, as pointed out in Ref. [...
-
[5]
u (−p, s2)] + Cpp V [¯u (p, s1) γµu (p′, s′ 1)] [¯u (−p′, s′
-
[6]
γµu (−p, s2)] (5) + Cpp AV [¯u (p, s1) γµγ5u (p′, s′ 1)] [ ¯u (−p′, s′
-
[7]
γ5γ5u (−p, s2) ] + Cpp T [¯u (p, s1) σµν u (p′, s′ 1)] [¯u (−p′, s′
-
[8]
σµν u (−p, s2)] , where Cpp S,V,AV,T are the new low-energy constants related to the pp interaction. Notice that in principle the charge-dependent two-nucleon potentials contain not only np and pp interactions, but also nn interactions. However, we do not construct the nn interactions in the present work due to the lack of experimental informa- tion with ...
Show all 51 references
-
[9]
Isospin-breaking from the electromagnetic interaction The electromagnetic interaction also causes isospin- breaking effects because of the different charges of up and down quarks. In the chiral effective field theory, the most si g- nificant contribution from the electromagnetic...
-
[10]
im- proved Coulomb force
γµu (−p, s2)] (E′p − Ep)2 − (p′ − p)2 . (6) In addition to the Coulomb force, the electromagnetic in- teraction also contains other corrections such as the “im- proved Coulomb force” ( VC2), the magnetic moment inter- action ( VMM), and the vacuum polarization potential ( VVP)...
-
[11]
Weinberg, Phys
S. Weinberg, Phys. Lett. B 251, 288 (1990)
1990
-
[12]
Weinberg, Nucl
S. Weinberg, Nucl. Phys. B 363, 3 (1991)
1991
- [13]
-
[14]
Ordonez, L
C. Ordonez, L. Ray, and U. van Kolck, Phys. Rev. Lett. 72, 1982 (1994)
1994
-
[15]
van Kolck, Phys
U. van Kolck, Phys. Rev. C 49, 2932 (1994)
1994
-
[16]
D. R. Entem and R. Machleidt, Phys. Rev. C 68, 041001 (2003) , arXiv:nucl-th/0304018
2003 arXiv
-
[17]
Epelbaum, W
E. Epelbaum, W. Glockle, and U.-G. Meissner, Nucl. Phys. A 747, 362 (2005) , arXiv:nucl-th/0405048
2005 arXiv
-
[18]
Epelbaum, H.-W
E. Epelbaum, H.-W. Hammer, and U.-G. Meissner, Rev. Mod. Phys. 81, 1773 (2009) , arXiv:0811.1338 [nucl-th]
2009 arXiv
-
[19]
Machleidt and D
R. Machleidt and D. R. Entem, Phys. Rept. 503, 1 (2011) , arXiv:1105.2919 [nucl-th]
2011 arXiv
-
[20]
Epelbaum, H
E. Epelbaum, H. Krebs, and U. G. Meißner, Phys. Rev. Lett. 115, 122301 (2015) , arXiv:1412.4623 [nucl-th]
2015 arXiv
-
[21]
D. R. Entem, R. Machleidt, and Y . Nosyk, Phys. Rev. C 96, 024004 (2017) , arXiv:1703.05454 [nucl-th]
2017 arXiv
-
[22]
Reinert, H
P . Reinert, H. Krebs, and E. Epelbaum, Eur. Phys. J. A 54, 86 (2018) , arXiv:1711.08821 [nucl-th]
2018 arXiv
-
[23]
H. W. Hammer, S. K¨ onig, and U. van Kolck, Rev. Mod. Phys. 92, 025004 (2020) , arXiv:1906.12122 [nucl-th]
2020 arXiv
-
[24]
Machleidt, Few Body Syst
R. Machleidt, Few Body Syst. 64, 77 (2023) , arXiv:2307.06416 [nucl-th]
2023 arXiv
-
[25]
G. A. Miller, B. M. K. Nefkens, and I. Slaus, Phys. Rept. 194, 1 (1990)
1990
-
[26]
V . G. J. Stoks, R. A. M. Klomp, M. C. M. Rentmeester, and J. J. de Swart, Phys. Rev. C 48, 792 (1993)
1993
-
[27]
D. E. Gonzalez Trotter et al., Phys. Rev. C 73, 034001 (2006)
2006
-
[28]
Chen et al., Phys
Q. Chen et al., Phys. Rev. C 77, 054002 (2008)
2008
- [29]
-
[30]
van Kolck, M
U. van Kolck, M. C. M. Rentmeester, J. L. Friar, J. T. Gold- man, and J. J. de Swart, Phys. Rev. Lett. 80, 4386 (1998) , arXiv:nucl-th/9710067
1998 arXiv
-
[31]
Walzl, U
M. Walzl, U. G. Meissner, and E. Epelbaum, Nucl. Phys. A 693, 663 (2001) , arXiv:nucl-th/0010019
2001 arXiv
-
[32]
J. L. Friar, U. van Kolck, G. L. Payne, and S. A. Coon, Phys. Rev. C 68, 024003 (2003) , arXiv:nucl-th/0303058
2003 arXiv
-
[33]
Lu, C.-X
J.-X. Lu, C.-X. Wang, Y . Xiao, L.-S. Geng, J. Meng, and P . Ring, Phys. Rev. Lett. 128, 142002 (2022) , arXiv:2111.07766 [nucl-th]
2022 arXiv
-
[34]
Ren, K.-W
X.-L. Ren, K.-W. Li, L.-S. Geng, B.-W. Long, P . Ring, and J. Meng, Chin. Phys. C 42, 014103 (2018) , arXiv:1611.08475 [nucl-th]
2018 arXiv
-
[35]
Xiao, C.-X
Y . Xiao, C.-X. Wang, J.-X. Lu, and L.-S. Geng, Phys. Rev. C 102, 054001 (2020) , arXiv:2007.13675 [nucl-th]
2020 arXiv
-
[36]
Wang, J.-X
C.-X. Wang, J.-X. Lu, Y . Xiao, and L.-S. Geng, Phys. Rev. C 105, 014003 (2022) , arXiv:2110.05278 [nucl-th]
2022 arXiv
-
[37]
Ren, C.-X
X.-L. Ren, C.-X. Wang, K.-W. Li, L.-S. Geng, and J. Meng, Chin. Phys. Lett. 38, 062101 (2021) , arXiv:1712.10083 [nucl-th]
2021 arXiv
-
[38]
Wang, L.-S
C.-X. Wang, L.-S. Geng, and B. Long, Chin. Phys. C 45, 054101 (2021) , arXiv:2001.08483 [nucl-th]
2021 arXiv
-
[39]
Bai, C.-X
Q.-Q. Bai, C.-X. Wang, Y . Xiao, and L.-S. Geng, Phys. Lett. B, 135745 (2020), arXiv:2007.01638 [nucl-th]
2020 arXiv
-
[40]
Bai, C.-X
Q.-Q. Bai, C.-X. Wang, Y . Xiao, J.-X. Lu, and L.-S. Geng, Phys. Lett. B 833, 137347 (2022) , arXiv:2105.06113 [hep-ph]
2022 arXiv
-
[41]
J.-X. Lu, Y . Xiao, Z.-W. Liu, and L.-S. Geng, (2025), arXiv:2501.17185 [nucl-th]
2025 arXiv
-
[42]
Zou, J.-X
W.-J. Zou, J.-X. Lu, P .-W. Zhao, L.-S. Geng, and J. Meng, Phys. Lett. B 854, 138732 (2024) , arXiv:2312.15672 [nucl-th]
2024 arXiv
-
[43]
Zheng, Z.-W
R.-Y . Zheng, Z.-W. Liu, L.-S. Geng, J.-N. Hu, and S. Wang, Phys. Lett. B 864, 139416 (2025) , arXiv:2501.02826 [nucl-th]
2025 arXiv
-
[44]
Fettes, U.-G
N. Fettes, U.-G. Meissner, and S. Steininger, Phys. Lett. B 451, 233 (1999) , arXiv:hep-ph/9811366. 7 /s49/s83/s48 /s80/s83/s40/s49 /s83/s48 /s41/s32 /s40/s68 /s101 /s103 /s46 /s41 /s51/s80/s48 /s80/s83/s40/s51 /s80/s48 /s41/s32 /s40/s68 /s101 /s103 /s46 /s41 /s49/s80/s49 /s80...
1999 arXiv
-
[45]
Muller and U.-G
G. Muller and U.-G. Meissner, Nucl. Phys. B 556, 265 (1999) , arXiv:hep-ph/9903375
1999 arXiv
-
[46]
Fettes and U.-G
N. Fettes and U.-G. Meissner, Phys. Rev. C 63, 045201 (2001) , arXiv:hep-ph/0008181
2001 arXiv
-
[47]
Xiao, J.-X
Y . Xiao, J.-X. Lu, and L.-S. Geng, Phys. Rev. C 110, 064002 (2024) , arXiv:2406.01292 [nucl-th]
2024 arXiv
-
[48]
H. P . Stapp, T. J. Ypsilantis, and N. Metropolis, Phys. Rev. 105, 302 (1957)
1957
-
[49]
C. M. Vincent and S. C. Phatak, Phys. Rev. C 10, 391 (1974)
1974
-
[50]
Navas et al
S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
2024
-
[51]
Chen, D.-L
Y .-H. Chen, D.-L. Yao, and H. Q. Zheng, Phys. Rev. D 87, 054019 (2013) , arXiv:1212.1893 [hep-ph]. 8 /s80/s83/s40/s49/s83/s48/s41/s32/s40/s68/s101/s103/s46/s41 /s49/s83/s48 /s80/s83/s40/s51/s80/s48/s41/s32/s40/s68/s101/s103/s46/s41 /s51/s80/s48 /s80/s83/s40/s51/s80/s49/s41/s3...
2013 arXiv
Reviewed August 16, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.