{"id":"171835e1-58a9-4b1e-abdf-576f2c03af95","arxiv_id":"2411.08337","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In simulated intermediate-energy tin collisions, the free neutron-to-proton yield ratio is more sensitive to the symmetry potential than to the initial neutron-skin thickness in most kinematics.","lead":"This paper simulates tin-on-tin collisions to test whether the ratio of free neutrons to protons can measure the thickness of a nucleus's neutron skin. It finds the ratio mostly reflects the symmetry force in the collision dynamics, with the skin contributing only about a 1% effect in the most favorable case.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Initial neutron-skin configurations are asserted, not shown, to be stable under the ImMDI mean field; if they relax before contact, the reported ~1% skin effect could be an artifact of initialization.","rationale":"The reader's weakest_assumption concerns the chosen brackets for L and effective-mass splitting. My concern is more fundamental: it questions whether the 'initial Δrnp effect' is a genuine ground-state effect at all, because the initial densities come from a different energy-density functional than the one used to propagate the collision. Transport-model studies commonly initialize nuclei with one functional and evolve with another; if the initialized nucleus is not a stationary solution of the evolution Hamiltonian, the skin can relax before meaningful contact. The paper's one-sentence assertion that the distribution is maintained is not backed by quantitative data, and the central negative conclusion (skin effect only ~1%, smaller than the symmetry-potential uncertainty) depends on that assertion. A static stability test is cheap and would settle the issue. If the skin is stable, the paper's conclusions are unchanged; if not, the sensitivity comparison in Table I would need to be redone with self-consistent initial states. I still view the paper as valuable and likely correct, but the missing evidence makes 'conditional acceptance' the appropriate verdict rather than unconditional acceptance at HIGH confidence.","tokens_in":17172,"tokens_out":6480,"duration_ms":75430,"concrete_test":"Run a no-collision IBUU stability test: initialize a single 124Sn nucleus (or boosted 124Sn at large separation) with the same SHF/MSL0 densities for L = 30, 60, 90 and the default ImMDI mean field, and monitor ⟨r_n^2⟩^{1/2}, ⟨r_p^2⟩^{1/2}, and Δrnp as functions of time from t = 0 to t = 50 fm/c, including up to the contact time used in the peripheral and central collision setups. If the initialized Δrnp drifts by more than ~0.01 fm before the collision starts, repeat the Table I and Figs. 8–10 comparisons with initial states that are stationary solutions of the same ImMDI Hamiltonian (e.g., by readjusting the sampled densities or the initialization momentum distribution). If the drift is within statistical uncertainty, the concern is retired and the reported sensitivities stand.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The paper's central quantitative result (Table I and Conclusions) isolates the initial Δrnp effect by initializing 124Sn with SHF/MSL0 densities (L = 30, 60, 90, giving Δrnp = 0.16, 0.19, 0.23 fm) and then evolving all cases with the same ImMDI mean field. This is only a clean separation if the sampled nuclei are quasi-static under the ImMDI Hamiltonian. The text states 'we put the two nuclei close enough so that the sampled density distribution with a desired Δrnp can be maintained before collision' (Sec. III.A), but no quantitative stability check—e.g., time evolution of neutron/proton RMS radii or of Δrnp before contact—is reported. The ImMDI and MSL0 functionals are not identical; their symmetry-energy slopes and surface properties differ. If the skin partially relaxes in the few fm/c before or at contact, the true sensitivity of n/p to the ground-state Δrnp could be larger (or smaller) than the ~1% quoted, and the conclusion that the symmetry potential dominates would be an artifact of the non-equilibrium initialization rather than a robust physics statement. Because the abstract and conclusions generalize to extracting Δrnp from intermediate-energy collisions, this missing check is load-bearing.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript investigates whether the free neutron-to-proton yield ratio n/p in 124Sn+124Sn collisions at intermediate energies (600A MeV and 2A GeV) can be used to extract the neutron-skin thickness Δrnp. Using the IBUU transport model, the authors initialize 124Sn with Skyrme-Hartree-Fock densities corresponding to Δrnp = 0.16, 0.19, and 0.23 fm, and separately vary the symmetry energy slope L (30–90 MeV) and the neutron-proton effective mass splitting in the collision dynamics. They find that in peripheral collisions and in most central-collision kinematic regions, the n/p yield ratio is more sensitive to the symmetry potential in the dynamics than to the initial Δrnp. The largest skin effect, about 1% at high transverse or longitudinal momenta in central collisions at 2A GeV, is comparable to the effect of varying L but smaller than the effect of the effective mass splitting. The paper concludes that n/p at these energies is not a clean probe of neutron-skin thickness.","tokens_in":17382,"tokens_out":5186,"duration_ms":60455,"significance":"If the conclusions hold, the paper provides a useful negative result for the heavy-ion community: the free n/p yield ratio at intermediate energies is dominated by collision dynamics (symmetry potential and effective mass splitting) rather than by the ground-state neutron-skin thickness, so proposed extractions of Δrnp from such data would be strongly model dependent. The study is a controlled model comparison, with the initial-structure and dynamics effects cleanly separated in the simulation setup. Strengths include the systematic variation of the symmetry energy parameters, the inclusion of a cascade limit that isolates the maximum possible skin effect, and the quantitative summary in Table I with statistical errors. The main limitations are the lack of a reported stability check for the initialized nuclei under the ImMDI mean field and the limited validation of the GEMINI deexcitation treatment, which is responsible for about 80% of free nucleons in peripheral collisions.","major_comments":[{"comment":"The paper asserts (Sec. III.A) that the sampled density distributions with desired Δrnp are maintained before collision, but no quantitative stability check is reported. Since the initial densities are generated with the SHF/MSL0 functional while the dynamics uses ImMDI with different parameterizations, the neutron-skin thickness could partially relax under the ImMDI mean field in the initialization stage or early in the collision. This matters directly for the central claim of Table I, where the effect of varying Δrnp is isolated by keeping the dynamics fixed. Please provide a test, e.g., the time evolution of neutron and proton RMS radii and of Δrnp for an isolated 124Sn nucleus initialized with each L value under the ImMDI Hamiltonian for at least 50–100 fm/c, or an explicit demonstration that the time from initialization to first contact is negligible compared with the relaxation timescale. Without this check, the reported ~1% skin effect could be an artifact of a non-equilibrium initialization, and the conclusion that the symmetry potential dominates would be less robust.","section":"III.A (and Figs. 3, 7)"},{"comment":"In peripheral collisions, about 80% of free nucleons come from deexcitation of residue fragments, and the deexcitation treatment relies on the GEMINI model with the excitation energy computed from Eq. (21) using a simplified SHF-like functional whose parameters are only stated to 'mimic' the ImMDI properties. The only experimental comparison is the total neutron number at forward angles (matching one GSI value). The sensitivity of the peripheral n/p ratios to the deexcitation model (e.g., the parameters a, b, σ, E_pot_sym, γ) is not explored. Since the peripheral-collision conclusions are largely driven by the deexcitation component, a sensitivity study or additional experimental comparisons (e.g., energy spectra or charge distributions of fragments) would materially strengthen the paper.","section":"II.B and Fig. 4"},{"comment":"The quantitative conclusion that the initial Δrnp effect (about 1%) is 'smaller than the uncertainty due to the symmetry potential in the collision dynamics' depends on the chosen ranges of L (30–90 MeV) and of the effective mass splitting, as well as on other model choices (in-medium cross sections, coalescence thresholds, GEMINI parameters). The paper does not propagate systematic uncertainties from these choices. While the chosen ranges are motivated by earlier analyses, a brief discussion of how the ranking of sensitivities would change if, for instance, the symmetry-potential uncertainty were narrower (e.g., L tightly constrained by PREX-II/CREX) would make the claim more precise. As written, the conclusion is tied to the subjective brackets of Fig. 1.","section":"Table I and Conclusions"}],"minor_comments":[{"comment":"There is a typo: 'isovector oberavables' should be 'isovector observables'.","section":"Introduction"},{"comment":"The caption says 'direction production' in the fourth row label; this should read 'direct production'.","section":"Fig. 5 caption"},{"comment":"In the text below Fig. 8, 'freen/p yield ratio' should be 'free n/p yield ratio'.","section":"Sec. III.B"},{"comment":"The subplot labels in Fig. 5 (especially the (j), (k), (l) row) appear misaligned with the description; please check that the labels match the panels as intended.","section":"Figs. 5 and 8–10"},{"comment":"In Eq. (15), the notation C_{τ_i,τ_j} is not explicitly defined; please clarify that it equals C_l when τ_i=τ_j and C_u when τ_i≠τ_j, as used in Eq. (3).","section":"Eq. (15)"},{"comment":"The in-medium cross section uses the reduced effective mass (μ*_NN/μ_NN)^2, but the sensitivity of the final n/p ratio to this choice is not discussed; a sentence referencing previous tests would be helpful.","section":"Sec. II.B, Eq. (20)"}],"recommendation":"major_revision","confidential_remarks":"This is a careful model study with a clearly stated negative conclusion. The main technical concern is the lack of a stability check for the initial neutron-skin configurations under the ImMDI mean field; I believe the authors can address this with a straightforward isolated-nucleus test, but it is important enough to request before acceptance. The heavy reliance on GEMINI deexcitation for peripheral collisions is also worth probing, though it may not affect the central-collision high-momentum results in Table I. The self-citation rate (Refs. [24–27]) is high but justified by the authors' previous work on the relativistic analogue."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Xu and collaborators have produced a genuinely useful negative result: at intermediate energies, the free neutron-to-proton yield ratio is not a clean probe of neutron-skin thickness, because the symmetry potential in the collision dynamics dominates in most kinematic regions. The largest skin effect they find—about 1% on n/p at high transverse or longitudinal momentum in central 124Sn+124Sn at 2A GeV—is comparable to the effect from varying L from 30 to 90 MeV and much smaller than the effect from the effective-mass splitting.\n\nWhat is new here is the systematic separation. Previous work proposed n/p as a skin probe; this paper isolates the initial-structure contribution from the dynamics contribution in a single transport framework, adds a GEMINI deexcitation stage, and provides cascade calculations as a control. Table I is a clean summary of the sensitivity hierarchy. The comparison of the forward neutron multiplicity with the GSI datum is a good sanity check.\n\nThe soft spots are real but not fatal. The main one, flagged by our stress test, is that the initial Δrnp values are generated with SHF/MSL0 but evolved with the ImMDI mean field. The paper asserts the sampled density distribution is maintained before contact, but shows no quantitative check—say, the time evolution of the neutron and proton RMS radii or of Δrnp prior to contact. If the skin partially relaxes during the early evolution, the reported 1% sensitivity could be an artifact, and the true sensitivity could be larger. That would weaken the central negative conclusion rather than strengthen it. This is a load-bearing missing check and should be a requested revision, not just a suggestion.\n\nTwo lesser issues: the 80% deexcitation fraction in peripheral collisions comes from GEMINI with an energy functional matched to ImMDI, but it is validated only against one integrated neutron count; and no code or data are released, which limits independent checking. The chosen L and effective-mass brackets are subjective, but the authors acknowledge the ranges and treat them as a sensitivity study, not as a best-fit.\n\nAll in all, the paper is internally consistent, carefully set up, and clearly written. The central claim—that n/p at intermediate energies is more sensitive to the symmetry potential than to the initial skin—holds up for the models considered, modulo the initialization-stability caveat. I would send this to a serious referee, with the stability check as a required addition. It deserves a fair review; the conclusion matters for experimental planning.","headline":"Useful negative result: the free n/p ratio at intermediate energies is dominated by the symmetry potential, not the neutron-skin thickness, but the paper needs a stability check on its initial skin configurations before the ~1% quote is trusted.","tokens_in":18005,"tokens_out":2933,"would_cite":true,"duration_ms":29013,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["25.70.-z","21.10.Gv","24.10.Cn","24.10.Lx"],"model":"deepseek-v4-flash","headline":"Intermediate-energy heavy-ion collisions cannot cleanly extract neutron-skin thickness from the free neutron-to-proton yield ratio, because the symmetry potential in the collision dynamics masks the signal.","keywords":["neutron-skin thickness","free neutron-to-proton yield ratio","IBUU transport model","symmetry energy","effective mass splitting","intermediate-energy heavy-ion collisions","124Sn+124Sn collisions","isospin transport"],"falsifier":"Take a nucleus whose neutron-skin thickness is already known from parity-violating electron scattering (e.g., 208Pb or 48Ca), collide it at 2A GeV with high statistics, and measure the central-collision n/p ratio at pT > 1 GeV/c; if the observed variation with the known skin thickness is comparable to or larger than the variation from the symmetry-potential bracket, the claim that the skin effect is masked would be wrong. A cheaper check is a transport-code intercomparison: if another credible BUU or QMD code with the same inputs gives a skin-induced change of several percent rather than about 1%, the conclusion fails.","tokens_in":1845,"feed_emoji":"⚛️","tokens_out":1931,"duration_ms":71405,"temperature":0.7,"pith_summary":"This paper asks whether the free neutron-to-proton yield ratio in intermediate-energy heavy-ion collisions can serve as a clean probe of the neutron-skin thickness of the colliding nuclei. Using the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model for 124Sn+124Sn collisions at 600A MeV and 2A GeV, the authors vary the initial skin thickness independently of the symmetry potential and compare sensitivities. They find that in most kinematic windows the ratio is driven more strongly by the symmetry potential in the collision dynamics than by the initial skin. The clearest skin imprint appears at large transverse or longitudinal momenta in central collisions at a few GeV per nucleon, where a reasonable skin range still changes the ratio by only about 1%, smaller than the symmetry-potential uncertainty. The paper therefore concludes that the n/p yield ratio is not a clean skin probe at these energies.","feed_headline":"Neutron-skin signal swamped by symmetry potential in collisions","feed_subtitle":"Transport simulations show the symmetry potential, not the initial skin, controls the free neutron-to-proton yield ratio.","key_machinery":"The load-bearing machinery is the isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, solved with the improved momentum-dependent interaction (ImMDI) for the mean field and the lattice-Hamiltonian method, plus Skyrme-Hartree-Fock (MSL0) initial density distributions that provide different Δrnp values. These pieces let the authors vary the initial skin and the symmetry potential separately and compare their influence on the same observable—the free n/p yield ratio from direct emission and from GEMINI deexcitation of residues.","core_discovery":"The central claim is that the free neutron-to-proton yield ratio n/p in 124Sn+124Sn collisions at intermediate energies is, in most cases, more sensitive to the symmetry potential Usym in the transport dynamics than to the neutron-skin thickness Δrnp imprinted in the initial density distributions. The authors demonstrate this by comparing three sources of variation: initial skin thickness Δrnp=0.16, 0.19, and 0.23 fm from Skyrme-Hartree-Fock densities; symmetry-energy slope L=30, 60, and 90 MeV; and two opposite neutron-proton effective mass splittings in the mean-field potential. In peripheral collisions the skin effect is very small relative to the symmetry-energy effects on residue N/Z, excitation energy, and deexcitation; in central collisions the largest skin effect appears for nucleons at large transverse or longitudinal momenta at 2A GeV, but Table I shows it is about a 1% change in n/p, comparable to the L effect and much smaller than the effective-mass-splitting effect. The paper's conclusion is that extracting Δrnp from this observable alone is not viable without controlling the symmetry-potential uncertainty.","pith_inferences":["If an independent constraint on the symmetry potential (e.g., from flow or pion ratios in the same collision system) were available, the residual high-momentum n/p signal could be inverted for Δrnp; the paper's Table I provides the sensitivity coefficients needed for such a two-step extraction.","The same comparison could be repeated for neutron-rich isotopes like 208Pb or for isobaric collision pairs, where the skin difference may be larger relative to the potential uncertainty; the paper's method would tell whether any variant escapes the masking.","The 1% level is a transport-model statement; a high-statistics experiment at 2A GeV with event-by-event neutron and proton identification at pT > 1 GeV/c could test whether the predicted ordering with Δrnp appears once the effective mass splitting is fixed.","A natural extension is to use double ratios or ratios between two collision systems with similar symmetry-potential dynamics but different known skin thicknesses, which could cancel the dominant mean-field uncertainty."],"forward_implications":["The free n/p yield ratio at 600A MeV and 2A GeV cannot, by itself, determine Δrnp in 124Sn unless the symmetry potential is independently constrained.","At 2A GeV central collisions, the high-transverse-momentum (pT > 1 GeV/c) and high-longitudinal-momentum (|pz| > 1 GeV/c) n/p ratios carry the largest skin signal; even there the signal is about 1% for Δrnp = 0.16–0.23 fm.","Cascade calculations without mean-field or Coulomb effects set an upper bound on the skin sensitivity of n/p, and that bound remains smaller than the potential effects in full transport.","Forward-rapidity n/p in peripheral collisions is dominated by deexcitation of residues, so its skin sensitivity is diluted by symmetry-energy effects on fragment excitation.","The same comparison strategy can be applied to other observables or collision systems to search for a probe that isolates Δrnp from Usym."],"supporting_citations":[{"why":"Proposed the free n/p ratio of nucleon emissions as a probe of neutron-skin thickness, the observable this paper tests.","marker":"[28]"},{"why":"Recent quantum-molecular-dynamics proposal to probe the neutron skin of unstable nuclei with heavy-ion collisions, providing the motivation.","marker":"[32]"},{"why":"Showed the n/p ratio is also sensitive to the symmetry energy in collision dynamics, the competing effect this paper isolates.","marker":"[41]"},{"why":"Measured neutron and proton transverse emission ratios and used them to constrain the density dependence of the symmetry energy, underpinning the potential effect.","marker":"[44]"},{"why":"Introduced the ImMDI interaction used to generate the different symmetry potentials.","marker":"[49]"},{"why":"MSL0 Skyrme-Hartree-Fock functional used to generate initial densities with different Δrnp.","marker":"[56]"},{"why":"GSI measurement of projectile-fragmentation neutrons at 600 MeV/nucleon used to validate the deexcitation treatment.","marker":"[36]"},{"why":"Earlier work by the authors using free spectator neutrons in ultracentral relativistic isobaric collisions as a neutron-skin probe, the relativistic counterpart this intermediate-energy study extends.","marker":"[24]"}],"fun_headline_variants":["Neutron-skin signal lost in symmetry potential noise","Symmetry potential trumps neutron-skin in n/p probe","Collision probe fails to isolate neutron-skin thickness","n/p ratio: symmetry potential dominates over skin","Skin effect minute in intermediate-energy collisions"],"cache_read_input_tokens":19968,"weakest_assumption_plain":"The conclusion rests on the assumed range of uncertainty for the symmetry potential in the collision dynamics (slope parameter L from 30 to 90 MeV and the two effective-mass-splitting choices); if the true symmetry-potential uncertainty is smaller than this bracketing range, the roughly 1% neutron-skin signal could become visible, while a larger uncertainty would strengthen the negative conclusion.","fun_headline_variants_meta":{"raw":{"variants":["Neutron-skin signal lost in symmetry potential noise","Symmetry potential trumps neutron-skin in n/p probe","Collision probe fails to isolate neutron-skin thickness","n/p ratio: symmetry potential dominates over skin","Skin effect minute in intermediate-energy collisions"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000417,"raw_usage":{"total_tokens":2161,"prompt_tokens":969,"completion_tokens":1192,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":585,"completion_tokens_details":{"reasoning_tokens":1127}},"tokens_in":585,"tokens_out":1192,"duration_ms":8952,"temperature":1.0,"reasoning_tokens":1127,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T21:42:29.007244+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Take a nucleus whose neutron-skin thickness is already known from parity-violating electron scattering (e.g., 208Pb or 48Ca), collide it at 2A GeV with high statistics, and measure the central-collision n/p ratio at pT > 1 GeV/c; if the observed variation with the known skin thickness is comparable to or larger than the variation from the symmetry-potential bracket, the claim that the skin effect is masked would be wrong. A cheaper check is a transport-code intercomparison: if another credible BUU or QMD code with the same inputs gives a skin-induced change of several percent rather than about 1%, the conclusion fails.","supporting_citations":[{"cited_title":"Neutron/proton ratio of nucleon emissions as a probe of neutron skin","cited_arxiv_id":"0906.5281","evidence_quote":"Proposed the free n/p ratio of nucleon emissions as a probe of neutron-skin thickness, the observable this paper tests."},{"cited_title":"Probing the neutron-skin of unstable nuclei with heavy ion collisions","cited_arxiv_id":"2304.12059","evidence_quote":"Recent quantum-molecular-dynamics proposal to probe the neutron skin of unstable nuclei with heavy-ion collisions, providing the motivation."},{"cited_title":"Neutron and Proton Transverse Emission Ratio Measurements and the Density Dependence of the Asymmetry Term of the Nuclear Equation of State","cited_arxiv_id":"nucl-ex/0607016","evidence_quote":"Measured neutron and proton transverse emission ratios and used them to constrain the density dependence of the symmetry energy, underpinning the potential effect."},{"cited_title":"Thermal properties of asymmetric nuclear matter with an improved isospin- and momentum-dependent interaction","cited_arxiv_id":"1410.1604","evidence_quote":"Introduced the ImMDI interaction used to generate the different symmetry potentials."},{"cited_title":"Neutrons from projectile fragmentation at 600 MeV/nucleon","cited_arxiv_id":"2310.00409","evidence_quote":"GSI measurement of projectile-fragmentation neutrons at 600 MeV/nucleon used to validate the deexcitation treatment."}],"review_version":1}