{"id":"8d42f090-d45a-408d-bd85-31cf4a597e3d","arxiv_id":"2412.03133","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Under assumed upgraded luminosity, the DDVCS asymmetries ALU, ALL, AC_UU (JLab) and ALU, AC_UU, AUT (EIC) are predicted to be measurable and to discriminate among VGG, GK19, KM, and AFKM12 GPD models.","lead":"This paper calculates how a not-yet-measured process, double deeply virtual Compton scattering, could reveal the 3D structure of protons at two future or upgraded electron accelerators. It reports which spin and charge asymmetries are large enough to measure and which GPD models they could tell apart.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The feasibility claims rest on an unvalidated custom off-diagonal extension of valence-quark CFFs, with no independent benchmark or released code to support it.","rationale":"The reader's weakest_assumption is exactly the most load-bearing concern: the custom off-diagonal extension of valence-quark CFFs is the only theory ingredient that is neither standard nor independently validated, and all model predictions shown depend on it. The paper's own text in Section 2 explicitly limits the validation to the diagonal limit, which is a genuine gap rather than an artifact of the writing. I considered whether the assumed 100x luminosity upgrade and 5% efficiency were more central, but those are clearly stated experimental assumptions whose uncertainty is different in kind: they affect the statistical error bars but do not invalidate the model-dependence statements if the theory is correct. The theory input, by contrast, is necessary for every plotted curve, so an error there would be fatal to the central claim. Because this validation gap is already the basis of the reader's CONDITIONAL verdict, my analysis does not shift the verdict; it reinforces it. The recommended next step is either to validate the off-diagonal extension against an independent calculation or to release the code so that the community can perform the check. Until then, the measurability and model-discrimination conclusions should be regarded as conditional on an unverified theory input.","tokens_in":4501,"tokens_out":3793,"duration_ms":36725,"concrete_test":"Independently recompute the valence-quark CFF H_V(xi_prime, xi, t) for the GK19 model at the benchmark JLab point (xB=0.15, t=-0.15 GeV^2, Q^2=2.77 GeV^2, Q'^2=1.0 GeV^2, which fixes xi_prime and xi via Eq. 2) using a standard double-distribution representation or the public code from Deja et al. (Ref. [6]) when available. Compare real and imaginary parts for several xi_prime != xi pairs. If the custom implementation disagrees by more than ~5% at any of these points, the plotted observables and the feasibility conclusions built on them must be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central conclusion that ALU, ALL, AC_UU (JLab) and ALU, AC_UU, AUT (EIC) are measurable and model-discriminating depends entirely on the theoretical predictions plotted in Figs. 2 and 3. For the off-diagonal DDVCS kinematics (xi_prime != xi), the sea-quark contribution uses a known Mellin-Barnes representation, but the valence-quark contribution is implemented through a custom Double Distribution that is only checked to reproduce the diagonal xi_prime = xi result of Ref. [16] (Section 2). No independent off-diagonal benchmark, no analytic cross-check at xi_prime != xi, and no released code is provided. Since the valence CFFs drive the amplitudes and signs of the observables, an error in this custom extension—wrong support, incorrect D-term weighting, or an inconsistent x/xi interpolation—would change the plotted model separations and therefore the stated measurability and model-sensitivity conclusions. The paper states this limitation honestly, but that does not reduce its load: the central claim requires this unvalidated ingredient to be correct, and the current text offers no evidence that it is.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper presents a model-sensitivity and feasibility study of Double Deeply Virtual Compton Scattering (DDVCS) observables at Jefferson Lab CLAS12 upgrade kinematics and at EIC kinematics. The observables considered are the beam spin asymmetry A_LU, target spin asymmetry A_UL, double spin asymmetry A_LL, beam charge asymmetry A^C_UU, and transverse target spin asymmetry A_UT. Predictions are computed with the VGG, GK19, KM10, KM15, and AFKM12 GPD models, and the article claims that A_LU, A_LL, and A^C_UU are measurable within 100 days at an upgraded CLAS12, while A_LU, A^C_UU, and A_UT are measurable within one year at EIC, with enough model sensitivity to set bounds on GPD models. The main technical ingredient is a custom off-diagonal extension of the valence-quark Compton form factors for the xi' != xi case, which is validated only against the diagonal xi' = xi limit of Ref. [16].","tokens_in":4677,"tokens_out":5372,"duration_ms":55273,"significance":"If the custom off-diagonal extension is correct and the assumed luminosity and efficiency values are realistic, the paper would provide a useful quantitative first scan of DDVCS observables as discriminators among existing GPD models. A notable strength is that the paper does not fit GPD parameters to the target observables, so the model comparison is not circular: the VGG, GK19, KM, and AFKM12 models were constrained by other data. The paper is also honest about the main limitation in Section 2, explicitly stating that the valence-quark implementation is a custom Double Distribution checked only in the diagonal limit. However, this self-stated limitation is exactly the load-bearing point of the manuscript: the plotted amplitudes and signs, and therefore the feasibility conclusions, depend on an unvalidated theoretical ingredient. No code release, analytic cross-check at xi' != xi, or independent benchmark is provided. The experimental feasibility claims also rest on a 100x CLAS12 luminosity upgrade and a 5% combined efficiency without a documented statistical-error calculation or a robustness scan over these assumptions.","major_comments":[{"comment":"The central model-sensitivity and measurability conclusions depend on predictions at xi' != xi, yet the valence-quark CFFs are implemented with a 'custom Double Distribution able to reproduce the xi = xi' result on [16]' and no other validation is reported. The current text contains no derivation of this extension, no analytic cross-check at off-diagonal kinematics, no comparison with an independent implementation, and no released code. Since the valence-quark CFFs dominate the amplitudes and signs of A_LU, A_LL, and A^C_UU in the JLab kinematics, an error in the extension's support, D-term weighting, or x/xi interpolation would directly alter the model separations in Figs. 2 and 3 and hence the stated feasibility and model-discrimination claims. The paper should provide a mathematical definition of the extension and at least one independent off-diagonal benchmark, or make the implementation available for scrutiny before the conclusions are accepted.","section":"Section 2, off-diagonal valence-quark extension"},{"comment":"The statements that A_LU, A_LL, and A^C_UU are 'measurable within 100 days' at JLab and that A_LU, A^C_UU, and A_UT are 'measurable within a year' at EIC rest on error bars whose construction is not documented. The text specifies a 100x luminosity upgrade, a combined 5% acceptance and reconstruction efficiency, and a beam time of 100 days or one year, but it does not give the event-rate formula, the statistical-significance criterion, the bin widths, or the treatment of backgrounds. The plotted error bars therefore cannot be reproduced or checked, and the feasibility claim is not robustly tied to the assumptions. The authors should document the statistical error calculation and show how the conclusion changes when the luminosity upgrade factor and efficiency are varied over a plausible range.","section":"Sections 2.1 and 2.2, Figs. 2 and 3"},{"comment":"For the transverse target asymmetry A_UT, the paper compares only GK19 and AFKM12 because the other models do not support the GPD E. The text calls the resulting separation an 'important model sensitivity' and states that A_UT measurements will play a 'crucial role', but it does not quantify whether the plotted difference between GK19 and AFKM12 is statistically resolvable at EIC luminosity, nor does it isolate the E contribution from contributions of other GPDs. With only two model curves and no significance estimate, the claim that A_UT can 'set bounds on models' is stronger than the evidence presented.","section":"Section 2.2, Fig. 3, A_UT"}],"minor_comments":[{"comment":"The abstract says the study is based on VGG and GK19 model predictions, but Section 2 also uses KM10, KM15, and AFKM12; the abstract should reflect the full set of models used.","section":"Abstract and Section 2"},{"comment":"The caption labels 'A_UT GPD dependence (top right)' while also labeling the top-right panel as A^C_UU; the intended panel for the A_UT GPD dependence should be identified consistently.","section":"Figure 3 caption"},{"comment":"In Eq. (1), the symbol P is not defined and the '±' sign is not explained; the authors should state that P denotes the principal value and specify which sign combination defines the Compton form factor used in the subsequent observable calculations.","section":"Eq. (1)"},{"comment":"There is a typo in 'luminosity is asummed to be', which should read 'assumed', and the stated EIC luminosity of about 10^33-10^34 cm^-2 s^-1 is given without a supporting reference.","section":"Section 2.2"},{"comment":"Reference [17] (JLab LOI12-16-004) appears in the bibliography but is not cited in the text; the authors should either cite it where the CLAS12 muon-detection scenario is described or remove it.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"For the editor: this is a compact proceedings contribution with an interesting and honest model scan, but the conclusions are currently stronger than the documented validation. The most important issue is the unvalidated custom off-diagonal valence-quark extension; a derivation, an analytic or numerical cross-check, or a released code would resolve it. The feasibility claims also need a documented statistical-error procedure and a sensitivity scan over the assumed luminosity upgrade and efficiency. There is no indication of circularity or misconduct, and the topic is within the scope of the journal. I recommend major revision rather than rejection because the central approach is defensible and the missing validation appears to be within the manuscript's scope to add."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read this as a proceedings-style feasibility scan, not a finished phenomenology paper. The genuinely new things: a systematic comparison of DDVCS observables at JLab and EIC kinematics across four GPD models, including the transverse target asymmetry AUT and its sensitivity to GPD E, and an attempt to go off-diagonal with xi' != xi. The authors are straight about what they assume: a 100x luminosity upgrade, 5% combined efficiency, 100 days at JLab or one year at EIC. Given those assumptions, the model spreads in Figs. 2 and 3 do suggest some observables could separate VGG from GK19 and the others. That is useful input for the DDVCS experimental program.\n\nThe soft spot is exactly the one the reader flags. The off-diagonal valence-quark CFFs are the backbone of every plotted curve, and they come from a custom double distribution that is only tested against the diagonal xi' = xi limit. No independent off-diagonal benchmark, no analytic cross-check, no code release. The sea-quark piece follows a known Mellin-Barnes representation, but the valence piece can change the signs and magnitudes of the asymmetries. If that implementation is off, the model-separation conclusions could shift. This is a load-bearing assumption, not a cosmetic one. The authors state it honestly, but stating it doesn't reduce the weight.\n\nThe luminosity and efficiency numbers are aggressive but explicit, and the paper frames them as assumptions, so I'd call that a minor-to-moderate concern rather than a flaw. The lack of theoretical error bars makes the model-discrimination claims qualitative, but for a first scan that is acceptable. The citation pattern looks fine: standard models, standard tools, and the prior DDVCS literature is acknowledged.\n\nWho is this for? People planning DDVCS experiments at JLab or the EIC, and GPD modelers who want to know which observables are worth computing. It deserves a serious referee; the field needs this kind of mapping. The referee should push for off-diagonal validation or a code release before the feasibility claims are taken as quantitative. I would suggest the authors benchmark against an independent off-diagonal calculation, or at least show the valence CFFs over the xi', xi plane against a direct numerical integration.\n\nNet: worth engaging, but treat the central feasibility numbers as conditional on that off-diagonal model check.","headline":"Useful feasibility map of DDVCS observables at JLab and EIC, but the central claims rest on an unvalidated off-diagonal model extension that needs a benchmark or code release.","tokens_in":5290,"tokens_out":1653,"would_cite":true,"duration_ms":15557,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Double Deeply Virtual Compton Scattering asymmetries are measurable within 100 days at an upgraded CLAS12 and within a year at the EIC, and they show model sensitivity strong enough to set bounds on GPD models.","keywords":["Generalized parton distributions","Double deeply virtual Compton scattering","Compton form factors","Beam spin asymmetry","Target spin asymmetry","Charge asymmetry","Transverse target asymmetry","GPD model sensitivity"],"falsifier":"An independent numerical evaluation of the off-diagonal Compton form factors—for example by direct integration of a known GPD model over the parton momentum fraction at $x_B = 0.15$, $t = -0.15$ GeV$^2$, $Q^2 = 2.77$ GeV$^2$, $Q'^2 = 1.0$ GeV$^2$—should reproduce the reported $A_{LU}$ sign and magnitude; a mismatch larger than the inter-model spread would falsify the feasibility and discrimination conclusions.","tokens_in":4209,"feed_emoji":"⚛️","tokens_out":12708,"duration_ms":99554,"temperature":0.7,"pith_summary":"The paper argues that Double Deeply Virtual Compton Scattering (DDVCS), which has not yet been measured, is within reach of an upgraded CLAS12 detector at Jefferson Lab and of the Electron-Ion Collider. Using model predictions for the beam spin, target spin, double spin, and beam charge asymmetries, the authors claim that several observables—$A_{LU}$, $A_{LL}$, and $A^{C}_{UU}$ at JLab, and $A_{LU}$, $A^{C}_{UU}$, and $A_{UT}$ at the EIC—are measurable with realistic luminosity and efficiency assumptions. The key point is that these observables show strong sensitivity to different GPD models, so a first measurement could set bounds on the VGG, GK19, KM, and AFKM12 parametrizations. This matters because DDVCS is the only channel that probes GPDs at independent momentum-fraction and skewness values, offering a more complete scan of nucleon structure than DVCS or TCS.","feed_headline":"DDVCS spin asymmetries measurable within 100 days at JLab","feed_subtitle":"Predictions for the double-virtual channel differ enough to pick between VGG, GK19, KM, and AFKM12 GPD models.","key_machinery":"The load-bearing machinery is the off-diagonal extension of Compton Form Factors to kinematics where the momentum-fraction variable $\\xi'$ differs from the skewness $\\xi$, the new freedom DDVCS offers through its two independent photon virtualities. For the sea-quark contribution the authors use the Mellin–Barnes representation, and for the valence-quark contribution they implement a custom double distribution—a parametrization of the GPD as an integral over a profile function—that reproduces the diagonal $\\xi=\\xi'$ limit of Kumerički–Mueller. These off-diagonal CFFs are evaluated on a $(Q^2, Q'^2)$ grid at fixed $x_B$ and $t$, producing the asymmetry predictions whose size and model spread determine the feasibility claims.","core_discovery":"On the paper's own terms, the central claim is that the DDVCS observables $A_{LU}$, $A_{LL}$, and $A^{C}_{UU}$ become measurable within 100 days of beam time at an upgraded CLAS12 detector, and that $A_{LU}$, $A^{C}_{UU}$, and $A_{UT}$ become measurable within one year at EIC kinematics, assuming polarized $e^\\mp$ beams and targets, muon detection, and a combined acceptance and reconstruction efficiency of 5%. The accompanying model scan shows an important sensitivity of these asymmetries to the choice of GPD parametrization—VGG, GK19, KM10/KM15, and AFKM12—so the first DDVCS measurements could set bounds on the models. The authors also report a strong GPD $E$ dependence for the transverse-target observable $A_{UT}$, which is notable because $E$ is poorly constrained by existing data.","pith_inferences":["If the off-diagonal CFF implementation is correct, DDVCS would enable a two-dimensional scan of GPDs in $(\\xi', \\xi)$ rather than only the diagonal line, exposing the $x$-dependence of GPD models far more directly than DVCS or TCS.","The custom double distribution could be checked against upcoming lattice QCD calculations of off-diagonal CFFs or against analyticity relations before committing to a dedicated DDVCS experiment.","A measurement of $A_{UT}$ seems to be the most promising route to the poorly known GPD $E$, but only the GK19 and AFKM12 models provide predictions, so adding other model families would sharpen the discrimination claim."],"forward_implications":["At an upgraded CLAS12 with $10^{35}\\ \\mathrm{cm^{-2}s^{-1}}$ luminosity, $A_{LU}$, $A_{LL}$, and $A^{C}_{UU}$ are measurable within 100 days of beam time.","At EIC kinematics, $A_{LU}$, $A^{C}_{UU}$, and $A_{UT}$ are measurable within one effective year of data taking.","The model sensitivity of these observables is strong enough to distinguish between VGG, GK19, KM, and AFKM12 GPD parametrizations, with $A_{LU}$ discriminating VGG and $A^{C}_{UU}$ discriminating GK19 at EIC.","$A_{LL}$ exhibits a complex GPD dependence, making it a multi-observable probe of nucleon structure.","$A_{UT}$ shows a strong dependence on the poorly known GPD $E$, offering a path to constrain it."],"supporting_citations":[{"why":"Defines the DDVCS kinematics with independent space-like and time-like photon virtualities that make the xi-prime scan possible.","marker":"[6]"},{"why":"Establishes the muon-pair final state and luminosity requirements that define the experimental feasibility scenarios.","marker":"[7]"},{"why":"Letter of intent for a DDVCS program at Jefferson Lab that underlies the upgraded-CLAS12 scenario.","marker":"[8]"},{"why":"Reviews the VGG model, one of the five GPD parametrizations whose predictions are compared.","marker":"[9]"},{"why":"Supplies the Mellin-Barnes representation used to extend the sea-quark Compton form factors to off-diagonal kinematics.","marker":"[12]"},{"why":"Source of the AFKM12 model predictions, one of the models used for the transverse-target observable.","marker":"[13]"},{"why":"Provides the GK19 model predictions through PARTONS.","marker":"[14]"},{"why":"Provides the KM model predictions through Gepard.","marker":"[15]"},{"why":"The diagonal xi=xi' result that the custom double distribution reproduces, the only off-diagonal validation benchmark.","marker":"[16]"}],"fun_headline_variants":["DDVCS spin asymmetries in 100 days at JLab","DDVCS observables can pick between GPD models","DDVCS probes GPD E via transverse spin asymmetry","First DDVCS measurements could set bounds on GPD models","DDVCS at JLab and EIC: 100-day and one-year windows"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The conclusions rest on the assumption that the custom double distribution used to compute the off-diagonal valence-quark Compton form factors is correct, since it is checked only against the diagonal $\\xi=\\xi'$ result and not against any independent off-diagonal benchmark or experimental data.","fun_headline_variants_meta":{"raw":{"variants":["DDVCS spin asymmetries in 100 days at JLab","DDVCS observables can pick between GPD models","DDVCS probes GPD E via transverse spin asymmetry","First DDVCS measurements could set bounds on GPD models","DDVCS at JLab and EIC: 100-day and one-year windows"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001015,"raw_usage":{"total_tokens":4297,"prompt_tokens":970,"completion_tokens":3327,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":586,"completion_tokens_details":{"reasoning_tokens":3237}},"tokens_in":586,"tokens_out":3327,"duration_ms":21658,"temperature":1.0,"reasoning_tokens":3237,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T22:42:56.724765+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"An independent numerical evaluation of the off-diagonal Compton form factors—for example by direct integration of a known GPD model over the parton momentum fraction at $x_B = 0.15$, $t = -0.15$ GeV$^2$, $Q^2 = 2.77$ GeV$^2$, $Q'^2 = 1.0$ GeV$^2$—should reproduce the reported $A_{LU}$ sign and magnitude; a mismatch larger than the inter-model spread would falsify the feasibility and discrimination conclusions.","supporting_citations":[{"cited_title":"Deja et al., Phys","cited_arxiv_id":null,"evidence_quote":"Defines the DDVCS kinematics with independent space-like and time-like photon virtualities that make the xi-prime scan possible."},{"cited_title":"Zhao et al","cited_arxiv_id":null,"evidence_quote":"Establishes the muon-pair final state and luminosity requirements that define the experimental feasibility scenarios."},{"cited_title":"Letter of Intent to PAC 43","cited_arxiv_id":null,"evidence_quote":"Letter of intent for a DDVCS program at Jefferson Lab that underlies the upgraded-CLAS12 scenario."},{"cited_title":"Guidal, H","cited_arxiv_id":null,"evidence_quote":"Reviews the VGG model, one of the five GPD parametrizations whose predictions are compared."},{"cited_title":"Aschenauer et al., JHEP 2013.9 (2013) 1-59","cited_arxiv_id":null,"evidence_quote":"Source of the AFKM12 model predictions, one of the models used for the transverse-target observable."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the KM model predictions through Gepard."},{"cited_title":"Kumerički and D","cited_arxiv_id":null,"evidence_quote":"The diagonal xi=xi' result that the custom double distribution reproduces, the only off-diagonal validation benchmark."}],"review_version":1}