REVIEW 3 major objections 4 minor 5 cited by
A radiative neutrino mass model with leptoquarks under non-holomorphic modular $A_4$ symmetry
T0 review · 3 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read A non-holomorphic modular A4 symmetry acting on a two-leptoquark radiative seesaw fits all measured quark and lepton flavor data and produces concrete predictions for the neutrino mass sum and neutrinoless double beta decay.
desk verdict A solid modular-flavor leptoquark model with a genuinely new quark-sector constraint, but the central neutrino-mass formula is misprinted and the numerics need transparency. 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 load-bearing machinery is the non-holomorphic modular $A_4$ framework and its Maaß-form triplets: fixed triplets of modular functions $Y_3^{(0)}$ and two independent $Y_3^{(6)}$ of the modulus $\tau$ that transform under $A_4$ and supply every Yukawa coupling in the Lagrangian. The two leptoquarks $\eta$ and $S$ mix through the term $H^\dagger\eta S$, giving mass eigenstates $\rho_{1/3}$ and $\chi_{1/3}$ with mixing angle $\alpha$; the one-loop neutrino mass formula $$M_\nu = \frac{f_1 g_1 N_c s_\$\alpha$ c_\$\alpha$}{2(4\pi)^2}\left(1-\frac{m_\$rho^{2}$}{m_\$chi^{2}$}\right)\sum_a \left[(\tilde F^T)_{ja} m_{d_a} \tilde G_{ai} + (\tilde G^T)_{ja} m_{d_a} \tilde F_{aj}\right] F_I(r_\rho, r_{d_a})$$ then converts the modular-symmetric Yukawa textures into the observed neutrino mass matrix. The machinery does two jobs at once: it reduces the Standard Model's arbitrary Yukawa matrices to a handful of parameters times fixed modular forms, and it explains the smallness of neutrino masses by loop suppression rather than by very heavy right-handed neutrinos.
What would settle it
A cosmological measurement that establishes $\sum m_\nu < 72\,\mathrm{meV}$ would exclude all fitted parameter sets, since the paper reports that every allowed point for both normal and inverted ordering violates the combined CMB and baryon-acoustic-oscillation bound. A second check: if future oscillation data pinned $\sin^2\theta_{12}$ below about 0.30 in the normal hierarchy, the NH allowed region would be excluded, because the paper finds a preference for larger solar mixing.
Extended reading notes
Core claim
The paper's central claim is that a non-holomorphic modular $A_4$ symmetry can serve as the flavor principle behind all quark and lepton masses. In the setup, left-handed leptons, right-handed charged leptons, left-handed quarks and right-handed down quarks are $A_4$ triplets with modular weight zero, while the three right-handed up quarks are $A_4$ singlets of weight $-6$; the two scalar leptoquarks $\eta$ (an $SU(2)_L$ doublet) and $S$ (a singlet) carry no nontrivial $A_4$ charge. The neutrino mass matrix is produced radiatively by a one-loop diagram in which the mixed leptoquarks and down-type quarks run, with the loop integral controlled by the leptoquark mixing angle $\alpha$ and the masses $m_\rho$, $m_\chi$. A numerical scan over the remaining couplings finds parameter sets consistent with all measured fermion data. In the normal hierarchy the model slightly restricts the solar angle ($\sin^2\theta_{12}\gtrsim 0.30$–$0.32$) and disfavors $\delta_{CP}\in[-130^\circ,-70^\circ]$, while in the inverted hierarchy it predicts a narrow band $\sum m_\nu \sim [115,180]\,\mathrm{meV}$ and $\langle m_{ee}\rangle \sim [28,41]\,\mathrm{meV}$. In both cases the authors find that every allowed point violates the stricter combined cosmological bound on $\sum m_\nu$ of about $72\,\mathrm{meV}$, while some (normal) or all (inverted) points remain within reach of current double $\beta$ decay limits.
Load-bearing premise
The load-bearing premise is that the non-holomorphic modular $A_4$ framework used here is valid and that its two weight-6 Maaß-form triplets for the up-quark sector are the complete set of such forms; the alternative natural assignment of the up-type singlets failed to fit data, so the successful scheme is partly reverse-engineered from observations.
Editorial extensions
If this is right
- The quark sector fixes $\mathrm{Im}[\tau]\sim 2.3$–$2.4$, so any future measurement of lepton mixing or CP violation is a test of the same modulus, not an independent parameter.
- If the fit is correct, neutrino masses are radiatively generated by leptoquark exchange; no right-handed neutrinos or high seesaw scale are required.
- The inverted-hierarchy region predicts $\sum m_\nu\in[115,180]\,\mathrm{meV}$ and $\langle m_{ee}\rangle\in[28,41]\,\mathrm{meV}$, placing the model within reach of upcoming neutrinoless double beta decay experiments.
- Every currently allowed parameter point violates the stricter $\sum m_\nu\lesssim 72\,\mathrm{meV}$ cosmological bound, so the model would be excluded if that bound is confirmed.
- Both mass orderings remain viable, but the normal-hierarchy region has lower $\sum m_\nu$ and $\langle m_{ee}\rangle$ below current double beta limits, leaving some points for near-future tests.
Reading between the lines
- A sharp test the authors do not state: since the quark sector fixes $\tau$, the predicted correlations among $\delta_{CP}$, $\alpha_{21}$, and $\sin^2\theta_{12}$ could be combined into a single frequentist test of the modular hypothesis, rather than separate scans.
- The same modular-symmetric couplings that produce neutrino masses also generate charged-lepton flavor violation through leptoquark exchange; computing $\mu\to e\gamma$ rates for the allowed parameter points would give an independent, low-energy probe of the same flavor structure.
- Because all fitted points hit the 72 meV cosmological bound, a natural extension would add a small additional contribution to neutrino masses from a different loop or tree-level operator, lowering $\sum m_\nu$ without changing the quark-sector fit.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper constructs a radiative neutrino mass model with two scalar leptoquarks under a non-holomorphic modular A4 symmetry. SM leptons and quarks are assigned to A4 representations with various modular weights, and the resulting Yukawa structures fix the charged-fermion mass matrices. Neutrino masses are generated at one-loop order through leptoquark exchange, and the authors scan the model parameters to fit quark masses and CKM observables as well as neutrino oscillation data for both normal and inverted mass hierarchies. They then present predictions for the neutrino mass sum and the neutrinoless double beta decay amplitude.
Significance. If the numerical results are correct, the paper demonstrates a single modular-symmetry framework that jointly accommodates quark and lepton flavor data while generating radiative neutrino masses, with testable predictions for Σmν and m_ee. The construction is concrete, the group-theoretic setup is clearly stated, and the predictions are falsifiable: for example, the inverted-hierarchy points populate a narrow band Σmν ≈ 115–180 meV and m_ee ≈ 28–41 meV, which is in tension with the combined CMB+DESI bound but testable by next-generation experiments. However, the central one-loop formula contains an index error that makes the printed neutrino mass matrix ill-defined, and the quark-sector fit is not quantitatively documented. These issues must be resolved before the claimed simultaneous fit can be accepted.
major comments (3)
- [Eq. (16), Sec. II.D] As printed, Eq. (16) is not a valid matrix equation. The second term in the bracketed sum, (G̃^T)_{ja} m_{d_a} F̃_{aj}, has no free index i and is not symmetric under i ↔ j, so the right-hand side cannot define the (i,j) element of a Majorana mass matrix. Since Sec. III.B states that the numerical scan uses this formula, the NH and IH results in Figs. 3–8 are not reproducible from the manuscript as written. The authors must correct the index (presumably to (G̃^T)_{ia} m_{d_a} F̃_{aj}, i.e. G̃_{ai} F̃_{aj}) and re-run the analysis.
- [Eq. (15), Sec. II.D] The rotation in Eq. (15), ilde F_{ai} ≡ (V^T_{uL})_{aj} F_{ji}, uses the left-handed up-quark rotation matrix V_uL, but the interaction in Eq. (11) involves the left-handed quark doublet Q_L and down-type quarks propagate in the loop. If this is not a typo, the loop amplitude carries an incorrect CKM-type factor; if it is a typo and V_dL (or the appropriate component of the doublet rotation) is intended, the numerical results must be recomputed with the corrected rotation. The paper should clarify which quark mass-basis rotation enters the loop formula.
- [Sec. III.A, Eqs. (24)–(25)] The quark-sector fit is not quantitatively documented. The only result shown is Fig. 2, a region in the τ plane, with no Δχ² values, no best-fit point, no number of scanned points, and no per-observable comparison. The trace conditions (6)–(8) fix only Tr, Det, and the sum of principal minors of M†M, so the six quark masses are imposed by construction rather than independently predicted; the nontrivial content of the fit is the CKM matrix and the CP phase, and the paper should report how well those are reproduced. Without this information, the claim that the model fits all quark data is not quantitatively supported.
minor comments (4)
- [Eq. (1), Sec. II] In the second line of Eq. (1), the last term is written with f_A in the coupling proportional to d_R η L_L; from the definitions in Eq. (13) this should be g_A, not f_A.
- [Eq. (23), Sec. II.D] The phases α2 and α3 in the expression for ⟨m_ee⟩ are not defined; they presumably correspond to α21 and α31 introduced earlier, but this should be stated explicitly.
- [Sec. II.B, Eq. (5)] The modular forms Y_3^(6) are essential for the up-quark mass matrix, but their explicit components are not given; since the entire quark fit depends on them, the paper should either reproduce them from Ref. [2] or give the precise definition used in the numerical scan.
- [Fig. 2, Sec. III.A] Fig. 2 shows only a binary allowed region; a color scale indicating the Δχ² value inside the region would make the quality of the quark fit much more transparent.
Circularity Check
No circular reduction: the model's fits and predictions are not equivalent to its inputs; apparent Eq. (16) issues are correctness concerns, not circularity.
full rationale
The derivation chain is not circular. The central neutrino-mass formula, Eq. (16), is imported from Ref. [13], which includes two of the present authors; however, it is a parameter-free one-loop expression whose stated assumptions (two scalar leptoquarks and a one-loop diagram) are reproduced in the paper, so under the review rules this citation is independent support and does not raise the circularity score. The overall scale κ is fitted to Δm²_atm via Eqs. (18)–(20), and κ then enters the displayed Σmν and ⟨m_ee⟩ formulas. This makes those observables dependent on the fitted splitting, but not equal to it by construction: the PMNS angles, phases, and, in the NH case, the lightest neutrino mass remain free or scan-dependent, so the plotted ranges are genuine model outputs rather than the fit itself. The assignment of uR to A4 singlets with modular weight −6 is explicitly presented as the minimal choice that reproduces the observed quark data (footnote 1), i.e., reverse-engineering, not a circular derivation. The quark-sector fit constraining Im τ to about 2.3–2.4 and the subsequent lepton-sector scan using that τ is a sequential fit; no output is fed back as an input to define the same quantity. The apparent index mismatch in Eq. (16), where the second term is labeled by j rather than (i,j), and the related V_uL/V_dL rotation question in Eq. (15), are correctness and reproducibility defects, not circular reductions: they do not make an output equal to an input by definition. Therefore no self-definitional, fitted-input-as-prediction, or self-citation-chain circularity is present.
Assumptions & free parameters
free parameters (9)
- modulus τ =
Im τ ≈ 2.3-2.4, |Re τ| < 0.4 (quark fit)
- up-Yukawa ratios ᾱ, β̄, γ̄ =
ranges [10^-5, 10^2], random phases
- up-Yukawa scales α1^u, β1^u, γ1^u =
fitted to up-quark masses
- down-Yukawa couplings yd1, yd3, yd3' =
fitted to down-quark masses
- charged-lepton couplings yℓ1, yℓ3, yℓ3' =
fitted to charged-lepton masses
- neutrino-related couplings f1, fS, fA, g1, gS, gA =
magnitudes in [10^-5, 1], random phases
- leptoquark mixing angle α =
s_α in [10^-3, 1]
- leptoquark masses mρ, mχ =
[10^3, 10^5] GeV
- overall scale κ = f1g1 =
fitted to Δm²_atm
assumptions (6)
- domain assumption The non-holomorphic modular forms Y3^(0) and Y3^(6) are Maaß forms transforming as A4 triplets with the given weights, as established in Ref [2].
- domain assumption The weight-6 modular form space is two-dimensional, giving two independent forms Y3^(6)_1 and Y3^(6)_2 used in Eq. (5).
- domain assumption All scalar potential parameters in Eq. (2) are real.
- domain assumption The one-loop neutrino mass formula (16) from Ref [13] is applicable here with the color factor N_c and the loop function FI.
- standard math The observed quark masses, CKM elements, lepton masses, and neutrino oscillation parameters from PDG and NuFit 6.0 are correct.
- ad hoc to paper uR can be assigned to A4 singlets with modular weight -6 despite the other fermions having weight 0; this is the minimal assignment that fits data.
invented entities (2)
-
Scalar leptoquark η (3, 2, 1/6)
-
Scalar leptoquark S (3bar, 1, 1/3)
Cite this review
Pith. "Pith review of A radiative neutrino mass model with leptoquarks under non-holomorphic modular $A_4$ symmetry." pith.science (2026). https://pith.science/paper/VP5X6SB4
@misc{pith2026250421404,
author = {Pith},
title = {Pith review of: A radiative neutrino mass model with leptoquarks under non-holomorphic modular $A_4$ symmetry},
year = {2026},
howpublished = {\url{https://pith.science/paper/VP5X6SB4}},
note = {Machine review of arXiv:2504.21404}
}
abstract
We investigate a radiative seesaw model with two leptoquarks under non-holomorphic modular $A_4$ symmetry. The leptons and quarks belong to non-trivial representations of the modular $A_4$ and the structures of their mass matrices are restricted. Neutrino masses are generated at one-loop level via leptoquark inside loop diagram where structures of relevant Yukawa interactions are determined by the modular $A_4$ symmetry. We scan the free parameters in the model and try to fit all the observed data for both lepton and quark sectors. For allowed parameters, we show some predictions regarding neutrino observables such as sum of neutrino mass and neutrinoless double beta decay.
Figures
Figures from the paper (5 more)
Forward citations
Cited by 5 Pith papers
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A Type-III seesaw model based on non-holomorphic modular symmetry fits NuFIT 6.0 neutrino data and produces baryogenesis via leptogenesis with Y_B-L about 1e-9.
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Reviewed August 16, 2026 · model on record in the stance chip above.
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