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REVIEW 3 major objections 4 minor 42 references

Diquark size effects in the quark-diquark approximation for baryons

T0 review · 3 major / 4 minor · reviewed 2026-08-03 · deepseek-v4-flash

Pith's one-line read Baryon masses survive the quark-diquark shortcut when the diquark's internal quark density is folded into the potential.

desk verdict Useful, honest method study: the VDq2 convolution genuinely improves quark-diquark masses, but the acknowledged crude spin-colour prescription weakens the light-quark and compactness claims. read the letter →

arxiv 2512.06016 v4 pith:TZFDJ6MP submitted 2025-12-03 hep-ph

classification hep-ph
keywords quark-diquarkapproximationbaryonmassesdiquarksizeconvolutionpotentialconstituentquarkmodelsemi-relativisticdynamicsthree-bodycompactness
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

A baryon is normally a three-body problem; the quark-diquark approximation reduces it to two two-body problems and is widely used, but its accuracy is rarely tested. This paper tests it against a full three-body calculation with the same semi-relativistic potential and finds that, with a new density-convolved potential, the two-step shortcut reproduces three-body masses for bbb, bbn, nnb, and nnn baryons at the percent level for ground and L=8 states in most cases. The key ingredient is folding the diquark's quark density—not just its squared wave function—into the quark-diquark potential. The paper also concludes that a diquark does not have to be compact for this agreement, and that internal distances (diquark size, quark-diquark separation) are not reliably reproduced by the approximation.

What carries the argument

The central object is the density-convolved quark-diquark potential VDq2(R) = 8∫|ψD(2y)|² Vqq(|y+R|) d³y, obtained by convoluting the quark-antiquark potential Vqq with the one-body density of the two quarks inside the diquark (derived from a particle-density operator). It replaces both the unconvoluted potential and the earlier convolution with |ψD(r)|², and it is what shifts the quark-diquark masses onto the three-body values. The paper evaluates it in a Lagrange-mesh basis, which reduces it to a quadrature sum over diquark expansion coefficients.

What would settle it

Recompute the nnn S=1/2 ground states using the exact spin-colour two-body terms evaluated on the diquark density instead of the SD·s3 prescription; if the quark-diquark masses move from 12–20% to sub-percent agreement with the three-body results, the density convolution is validated and the simplified spin prescription was the main bottleneck—if not, the density treatment itself is at fault.

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Extended reading notes

Core claim

On the paper's own terms, the central claim is that the quark-diquark approximation is quantitatively reliable for baryon masses—computed with the same parameters used in a three-body model—provided the diquark's finite size enters through the spatial density of its two constituent quarks rather than through the diquark wave function alone. With this potential, the quark-diquark masses of bbb, bbn, nnb, and nnn baryons agree with three-body masses to within a few percent for ground states and L=8 excitations, with the best cases at 0.06–1.0% relative difference. In addition, compactness is not the deciding factor: an extended diquark configuration can give closer agreement than a compact one

Load-bearing premise

The spin-colour interaction between the diquark and the third quark is assumed to be proportional to SD·s3, i.e. the two diquark spin operators are simply summed, rather than evaluating s1·s3 + s2·s3 at the actual quark positions; this simplification is flagged in the conclusion and is the assumption most likely to distort the mass comparison, especially for nnn S=1/2 states.

Editorial extensions

If this is right

  • With the density-convolved potential, quark-diquark masses match three-body masses to within a few percent for bbb, bbn, nnb, and nnn ground states, and for L=8 states down to 0.06% (bbb).
  • Diquark compactness is not required: an extended diquark can give closer agreement than a compact one, so compactness alone should not be used as a criterion for trusting quark-diquark mass predictions.
  • The naive |ψ|² convolution overestimates masses and the unconvoluted potential underestimates them; only the quark-density convolution lands near the three-body results.
  • Internal distances are not reproduced (relative differences up to roughly 75%), so geometry-sensitive observables need redefined operators before the quark-diquark wave function is used for them.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because the paper tests only L=0 and L=8, a natural extension is to compute intermediate orbital angular momenta; if the same density convolution holds those percent-level agreements, the shortcut is validated across the whole rotational band.
  • The density operator in the paper assumes two identical quarks; generalising to unequal-mass pairs would open the same treatment to (nb)-n substructures, which the authors note may be energetically preferred—this is our extrapolation, not their result.
  • For multiquark applications (tetraquarks, pentaquarks) that already build on diquark substructures, this density convolution could be adopted as a size correction; the paper mentions such systems as possible beneficiaries, and we infer the percent-level accuracy established here makes that adoption viable.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 4 minor

Summary. The paper tests the quark-diquark approximation for baryons built from up/down (n) and bottom (b) quarks, using a fixed semi-relativistic BCN potential whose parameters come from an underlying three-body model. Three quark-diquark potentials are compared: the unconvoluted potential V_unc_Dq = 2 V_qq, a standard convolution with |ψ_D|^2 (V_Dq1), and an original convolution with a quark density operator (V_Dq2), Eqs. (9), (10), (19). Masses and characteristic distances (diquark size r_qq, quark-diquark distance r_Dq) are computed for ground (L=0) and high-orbital (L=8) states of bbb, bbn, nnb, nnn, and compared with the three-body benchmark. The paper reports that V_Dq2 substantially improves mass agreement over V_Dq1, finds that characteristic distances are not reproduced accurately, and concludes that a diquark need not be compact to obtain good baryon masses.

Significance. If the result holds, the paper provides a practical and physically motivated procedure for constructing quark-diquark potentials from three-body interactions, with potential applications to tetraquarks, pentaquarks, and hexaquarks. The derivation of V_Dq2 and its Lagrange-mesh implementation are transparent and analytic; the comparison with the exact underlying three-body model is a fair and useful test. The paper is also commendably honest about its limitations, explicitly flagging the crude spin-colour prescription and the need for configuration mixing. However, the strength of the central claims is moderated by the large errors in exactly the light-quark spin-sensitive channels and by the hand-picked state selection used for the distance and compactness analyses.

major comments (3)
  1. [§IV.B.1, Eq. (9)] The spin-colour interaction is replaced by S_D·s_3, i.e., the diquark spin is treated as a single spin with the same coupling coefficient as a quark-antiquark pair. For a composite diquark the physical interaction is the sum of two quark-quark spin terms s_1·s_3 and s_2·s_3 evaluated at the actual quark positions; even in the point-like limit this sum has a different coefficient. The paper's own conclusion acknowledges this ('A better way may be to compute the colour-magnetic moment of the diquark'). This is not just a cosmetic issue: Table IV shows the largest V_Dq2 errors precisely in light-diquark, S=1/2 systems (nnn: 12.7% and 20.1%; nnb: 2.4–3.4%), while bbb/bbn are ≤1.5%. The spin term scales as 1/(m_i m_3), so light diquarks are most affected. The central claim of 'good baryon masses' needs to be qualified, and the compactness comparison in §V.C (bbn extended vs nnb compact) is no
  2. [§V.B, Table V] The characteristic-distance comparison and the subsequent compactness discussion use only the states marked with an asterisk in Table IV, which are selected as the states whose V_Dq2 mass is closest to the three-body mass. This selection biases the distance analysis toward the best-case mass agreement and makes the 'extended vs compact diquark' comparison in §V.C rely on two states chosen by this criterion. For example, the nnb L=8 compact-diquark state has 3.4% mass error while the bbn L=8 extended-diquark state has 1.0%; but the reader cannot tell whether this ordering holds for all relevant states or is an artefact of the asterisk selection. Please present results for all dominant states or justify why the asterisked subset is representative.
  3. [§V.C, Table VII] The statement 'good baryon masses' is difficult to reconcile with the nnn S=1/2 L=0 results: the V_Dq2 mass is 1.075 GeV vs 0.954 GeV (12.7%) and 1.146 GeV vs 0.954 GeV (20.1%). While the paper notes that relative errors on binding energies are comparable to bbb, for a light baryon a 0.12–0.19 GeV discrepancy is substantial. The conclusion that 'a diquark must not necessarily be a compact object to obtain good baryon masses' is supported by bbb and by nnn S=3/2, but the strong formulation in the abstract is not warranted by the full table. Please either soften the claim to specify which channels are accurately reproduced, or add a quantitative criterion for 'good'.
minor comments (4)
  1. [§IV.B.1, Eq. (8)] Equation (8) is typeset as Vqq = 2 Vqq, which is confusing; V_{\bar q q} = 2 V_{qq} is meant. Please correct the notation.
  2. [Table II] The heading 'Diquark Masse' is a typo; should be 'Diquark Masses'.
  3. [§IV.B.1, Eq. (26)] The equal-weight averaging over m for the diquark wave function is an assumption that should be justified more explicitly, since for non-zero l it changes the diquark density from the actual eigenstate. The authors mention it restores spherical symmetry, but a short physical argument would help.
  4. [§V.A and Fig. 2] The statement that 'the physical potentials presented at the beginning of this section show similar behaviours' is not quantitatively demonstrated; a plot for the actual BCN potential would strengthen the claim.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the quark-diquark masses are fixed-convolution outputs, not fitted inputs; the comparison against the three-body model is a legitimate approximation test.

full rationale

The paper's derivation chain is self-contained. The quark-diquark Hamiltonian (6) is solved with the diquark mass M_D and wave function Ψ_D obtained from the two-body Hamiltonian (5), and the potential V_Dq2 is the fixed convolution (19) of the external quark-quark potential (7) with the diquark density (18). No parameter is fitted to the quark-diquark masses that are later compared with the three-body model; the BCN parameters come from [23] and the bottom-quark mass is fitted once to Σ_b in the three-body model and then used unchanged in both approaches. The quark-diquark mass is therefore not equal to the three-body mass by construction: for example, the V_Dq2 results differ by 12.7% and 20.1% for the nnn S=1/2 states and by 3.4% for the nnb L=8 state. The state-matching procedure of Sect. V identifies diquark quantum numbers from the dominant three-body component, but this is an identification rule, not a fit; the paper reports all dominant components and the approximation is free to disagree. The authors explicitly flag the spin-color replacement S_D·s_3 as a possible improvement in the conclusion ('A better way may be to compute the colour-magnetic moment of the diquark'); this is a stated approximation weakness, not a circular step. Self-citations appear only as methodological references (oscillator bases [17], Lagrange functions [27]) and are not load-bearing for the central mass or compactness claims. No uniqueness theorem is imported from the authors' prior work, and the convolution/density formulas are either derived in the paper or attributed to [5,6,9,26]. The fitted m_b to Σ_b does mean one row of Table IV is not an independent experimental prediction, but the paper's aim is to test the quark-diquark approximation against the same Hamiltonian, so this overlap does not make the derivation circular.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

No new particles or forces are introduced. The central calculation rests on the external BCN potential parameter set, the fitted bottom quark mass, the color-factor relation V_qqbar=2V_qq, the density-operator convolution ansatz, the spherical orientation-averaging of diquark states, and the dominant-state selection. All are stated, but several are acknowledged approximations.

free parameters (4)
  • BCN potential parameters (V0, C, b=αsΛ, αs) = V0=-0.409 GeV, C=0.121 GeV^2, b=0.532 GeV, αs=0.57
    Taken from Theußl et al. fit to baryon spectra; used in Eq. (7) for both the three-body and quark-diquark calculations.
  • Light quark mass m_n = 0.337 GeV
    From the same prior fit [23]; controls light-diquark and nnn/nnb masses.
  • Bottom quark mass m_b = 5.415 GeV
    Fitted in this paper to the experimental Σb mass using the three-body model; affects all b-containing baryon masses.
  • Diquark-state selection per baryon = e.g. (0,0,0,0,1,0) for bbb ground state
    Chosen by hand as the dominant component of the three-body expansion (Sect. V); this selection is load-bearing for the comparison and is not independently predicted.
assumptions (5)
  • domain assumption Semi-relativistic mass operator H = Σ sqrt(p_i^2+m_i^2) + Σ V_ij is an adequate baryon description.
    Eq. (1), Sect. II; the point-form formalism is assumed without derivation.
  • ad hoc to paper The quark-antiquark potential is twice the quark-quark potential, V_qqbar = 2 V_qq, including for the spin-colour part; and the quark-diquark spin operator is S_D·s_3.
    Eqs. (8)-(9), Sect. IVB; the spin replacement is acknowledged as a simplification in the conclusion.
  • domain assumption Convolution with quark density σ(y)=8|ψ_D(2y)|^2 gives the effective quark-diquark potential.
    Eqs. (16)-(19), Sect. IVB1; assumes identical quarks and that colour density can be identified with quark density.
  • ad hoc to paper Diquark orientations can be averaged with equal weights, so the wave function takes the form (2l+1)^(-1/2) R_nl Σ_m Y_lm.
    Eq. (26), Sect. IVB2; motivated by the absence of m-dependent phenomena in the spectra.
  • domain assumption The dominant component of the three-body oscillator expansion defines the corresponding quark-diquark state.
    Sect. V, procedure steps 1-3; ignores superposition of diquark states, as the paper explicitly notes.

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Pith. "Pith review of Diquark size effects in the quark-diquark approximation for baryons." pith.science (2026). https://pith.science/paper/TZFDJ6MP

@misc{pith2026251206016,
  author       = {Pith},
  title        = {Pith review of: Diquark size effects in the quark-diquark approximation for baryons},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TZFDJ6MP}},
  note         = {Machine review of arXiv:2512.06016}
}
read the original abstract

Baryons can be described within several theoretical frameworks. Among them, the constituent approach is widely used. In this context, we aim to evaluate the accuracy of a particular model of baryons: the quark-diquark approximation. It consists in separating the three-body system into two subsequent two-body ones: a pair of two quarks, the diquark, and a second system consisting of the diquark and the third quark. This approximation is widely used, but its accuracy is rarely evaluated. The goal of this work is to perform this evaluation by comparing the quark-diquark model with a three-body model, both using the same semi-relativistic interaction. The baryon masses and some characteristic distances are computed and analysed within both approaches. Additionally, an original procedure to establish the quark-diquark potential will be presented with the aim to increase the precision of this approximation. It is shown that a diquark must not necessarily be compact to obtain good baryon masses.

Figures

Figures reproduced from arXiv: 2512.06016 by the authors.

Figure 1
Figure 1. Illustration of formula (14). To illustrate the behaviour of the three potentials (9), (10) and (19), and the effect of the quark density on the quark-diquark potential, we consider a generic case with a simple quark-antiquark potential in arbitrary units, similar to the physical one presented at the beginning of this section Vqq(r) = − 1 r + r. (20) [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Comparison of the three quark-diquark potentials for potential (20) and wave function (21). The solid grey curve [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Structures of the ground (left) and L = 8 (right) states of the bbb baryon obtained by computing the mean values of ρ and λ. Both chosen states have a total spin S = 3/2. The ratio defined in (38) takes the values η = 1.152 and η = 1.105 for these states, respectively [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Structures of the ground S = 3/2 (left) and S = 1/2 (right) states of the nnn baryon obtained by computing the mean values of ρ and λ. The ratio defined in (38) takes the values η = 1.153 and η = 1.151 for these states, respectively [PITH_FULL_IMAGE:figures/full_fig_p…
Figure 5
Figure 5. Figure 5: Structures of the ground (left) and L = 8 (right) states of the bbn baryon obtained by computing the mean values of ρ and λ. Both chosen states have a total spin S = 1/2, and a total isospin I = 1/2. The ratio defined in (38) takes the values η = 0.669 and η = 2.056 fo…
Figure 6
Figure 6. Figure 6: Structures of the ground (left) and L = 8 (right) states of the nnb baryon obtained by computing the mean values of ρ and λ. Both chosen states have a total spin S = 1/2, and a total isospin I = 0. The ratio defined in (38) takes the values η = 1.344 and η = 0.342 for …

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Works this paper leans on

42 extracted references · 2 linked inside Pith

  1. [1]

    Explore three distinct approaches to describe the quark-diquark approximation, namely (i) a point-like diquark approximation, (ii) a convolution of the potential with the squared modulus of the diquark wave function [5, 6, 9], and (iii) a modified convolution involving a particle density operator

  2. [2]

    More precisely, the distance between the two quarks composing the diquark and the distance between the diquark and the third quark are computed

    Test its capacity to reproduce some characteristic lengths of baryons. More precisely, the distance between the two quarks composing the diquark and the distance between the diquark and the third quark are computed

  3. [3]

    Evaluate its accuracy in predicting various baryon masses. Since our goal is to test the validity of the quark-diquark approximation, we only select baryons consisting of up/down (n) and bottom (b) quarks to highlight the impact of constituent quark masses, without increasing the length of the paper. Both ground and excited states with a total orbital ang...

  4. [4]

    These subsystems must transform under the3represen- tation in order to combine with a third quark into a colour singlet

    Quark-diquark potential Let us first discuss the colour properties of the diquarks. These subsystems must transform under the3represen- tation in order to combine with a third quark into a colour singlet. In terms of colour, a diquark hence transforms in the same way as an antiquark. Moreover, it can be shown that, due to the colour factors, the quark-ant...

  5. [5]

    Some of its characteristics are first introduced, as they will be useful in the following

    Expressions of the convoluted potentials applied to the Lagrange mesh method The expressions of the convoluted potentials can be simplified by considering the method of resolution, namely the Lagrange mesh method. Some of its characteristics are first introduced, as they will be useful in the following. The method relies on expanding the eigenstates|Ψ⟩in ...

  6. [6]

    This provides eigenstates expressed as linear combinations of basis states of the form (4)

    Solve the three-body equations for a baryon with fixed quark content, angular momentumL, spinS, isospinI, and parity. This provides eigenstates expressed as linear combinations of basis states of the form (4)

  7. [7]

    Identify the dominant states in the expansion, i.e., those with the largest coefficients

  8. [8]

    Solve the quark-diquark equations for each of the selected dominant states. Note that, in this procedure, the quark-diquark approximation does not consider a superposition of diquark states, whereas the three-body model considers a linear combination of different states for the two quarks associated with the diquark. Therefore, if the computed three-body ...

Show all 42 references
  1. [9]

    Klempt and J.-M

    E. Klempt and J.-M. Richard, Rev. Mod. Phys.82, 1095 (2010)

  2. [10]

    Anselmino, Rev

    M. Anselmino, Rev. Mod. Phys.65, 1199 (1993)

  3. [11]

    M. Yu. Barabanov, M. A. Bedolla, W. K. Brooks, G. D. Cates, C. Chen, Y. Chen, E. Cisbani, M. Ding, G. Eichmann, R. Ent, et al., Progress in Particle and Nuclear Physics116, 103835 (2021)

  4. [12]

    Gell-Mann, Phys

    M. Gell-Mann, Phys. Lett.8, 214 (1964)

  5. [13]

    M. V. Carlucci, F. Giannuzzi, G. Nardulli, M. Pellicoro, and S. Stramaglia, Eur. Phys. J. C57, 569 (2008)

  6. [14]

    Giannuzzi, Phys

    F. Giannuzzi, Phys. Rev. D79, 094002 (2009)

  7. [15]

    Santopinto and J

    E. Santopinto and J. Ferretti, Phys. Rev. C92, 025202 (2015)

  8. [16]

    Majethiya, K

    A. Majethiya, K. Thakkar, and P. C. Vinodkumar, Chin. J. Phys.54, 495 (2016)

  9. [17]

    Giannuzzi, Phys

    F. Giannuzzi, Phys. Rev. D99, 094006 (2019)

  10. [18]

    L. X. Gutiérrez-Guerrero, A. Bashir, M. A. Bedolla, and E. Santopinto, Phys. Rev. D100, 114032 (2019)

  11. [19]

    Torcato, A

    A. Torcato, A. Arriaga, G. Eichmann, and M. T. Peña, Few-Body Syst.64, 45 (2023)

  12. [20]

    Farhadi, S

    M. Farhadi, S. M. Moosavi Nejad, and A. Armat, Few-Body Syst.64, 75 (2023)

  13. [21]

    Fleck, B

    S. Fleck, B. Silvestre-Brac, and J. M. Richard, Phys. Rev. D38, 1519 (1988)

  14. [22]

    Santopinto, A

    E. Santopinto, A. Giachino, J. Ferretti, H. García-Tecocoatzi, M. A. Bedolla, R. Bijker, and E. Ortiz-Pacheco, Eur. Phys. J. C79, 1012 (2019)

  15. [23]

    X. Zhu, H. Huang, and J. Ping, Particles8, 83 (2025)

  16. [24]

    Ferretti, A

    J. Ferretti, A. Vassallo, and E. Santopinto, Phys. Rev. C83, 065204 (2011)

  17. [25]

    Silvestre-Brac, R

    B. Silvestre-Brac, R. Bonnaz, C. Semay, and F. Brau (2020), Internal Report ISN-00-66 arXiv:2003.11028

  18. [26]

    Chevalier and S

    C. Chevalier and S. Youcef Khodja, Few-Body Syst.65, 86 (2024)

  19. [27]

    Baye, Phys

    D. Baye, Phys. Rep.565, 1 (2015)

  20. [28]

    R. K. Bhaduri, L. E. Cohler, and Y. Nogami, Nuovo Cimento A65, 376 (1981)

  21. [29]

    Lucha, F

    W. Lucha, F. F. Schöberl, and D. Gromes, Physics Reports200, 127 (1991)

  22. [30]

    Capstick and N

    S. Capstick and N. Isgur, Phys. Rev. D34, 2809 (1986)

  23. [31]

    Theußl, R

    L. Theußl, R. Wagenbrunn, B. Desplanques, and W. Plessas, Eur. Phys. J. A12, 91 (2001)

  24. [32]

    Melde, W

    T. Melde, W. Plessas, and B. Sengl, Phys. Rev. D77, 114002 (2008)

  25. [33]

    Andreev, Phys

    O. Andreev, Phys. Rev. D93, 105014 (2016)

  26. [34]

    V. U. Nazarov, Phys. Rev. B87, 165125 (2013)

  27. [35]

    Semay, D

    C. Semay, D. Baye, M. Hesse, and B. Silvestre-Brac, Phys. Rev. E64, 016703 (2001)

  28. [36]

    Abramowitz and I

    M. Abramowitz and I. A. Stegun,Handbook of Mathematical Functions: With Formulas, Graphs, and Mathematical Tables (Dover Publications Inc, New York, 1964)

  29. [37]

    D. A. Varshalovich, A. N. Moskalev, and V. K. Khersonskii,Quantum Theory Of Angular Momentum(World Scientific Publishing Company, Singapour, 1988)

  30. [38]

    De Sanctis, J

    M. De Sanctis, J. Ferretti, E. Santopinto, and A. Vassallo, Phys. Rev. C84, 055201 (2011)

  31. [39]

    An, S.-Q

    H.-T. An, S.-Q. Luo, and X. Liu, Phys. Rev. D112, 054041 (2025)

  32. [40]

    Cimino, C

    L. Cimino, C. T. Willemyns, and C. Semay, Phys. Rev. D110, 034032 (2024)

  33. [41]

    Galatà and E

    G. Galatà and E. Santopinto, Phys. Rev. C86, 045202 (2012)

  34. [42]

    D. B. Lichtenberg, W. Namgung, J. G. Wills, and E. Predazzi, Z. Phys. C - Particles and Fields19, 19 (1983). 17 Appendix A: Characteristic distances results L S Approach Dominant state ⟨ρ⟩orr qq ⟨λ⟩orr Dq 0 3 2 3-body −0.92(0,0,0,0,1,0) 0.742 0.644 quark-diquark (0,0,0,0,1,0) ...

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