{"id":"b7de5134-be0b-4b38-8641-8808c273fffb","arxiv_id":"2606.18398","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":8.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"First measurement of the K- escape cross section in 12C(K-,p) gives an optical potential W0 ≈ -100 MeV, far more absorptive than single-nucleon models.","lead":"This experiment measured, for the first time, how often a kaon escapes a carbon nucleus after being struck, and used that to extract the absorptive strength of the kaon–nucleus interaction. The extracted imaginary potential W0 ≈ -100 MeV is much stronger than one-nucleon absorption models predict, with consequences for exotic kaonic nuclei and neutron stars.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"W0 extraction is model-dependent: fitted value could be biased by assumed optical-potential form, density profile, or reaction mechanism; 'direct determination' overclaims.","rationale":"The reader's weakest assumption matches exactly the load-bearing issue I see: the extraction is model-dependent. The paper's central claim is that the exclusive measurement yields a direct, decisive constraint on W0, but the chain from measured cross sections to W0 passes through the Green's-function/DWIA framework with a fixed optical-potential form. The exclusive spectrum is a survival probability that is sensitive to W0, but only as interpreted by that model; the inclusive normalization is a free parameter, so the absolute scale of the exclusive spectrum carries the main information, and that scale is subject to the 10.2% efficiency systematic. The quoted W0 = -100 MeV with a one-sided systematic of +0/-16 MeV does not include uncertainty in the reaction model or density profile. This is not an internal inconsistency — the analysis is coherent — but it is a genuine correctness risk: if the model is wrong, the central result is biased. The proposed test — varying the optical-potential form and density profile and refitting — would settle whether the model dependence is significant. If W0 remains around -100 ±20 MeV under such variations, the central claim is robust; if it shifts by 30-50 MeV, the 'significantly stronger than one-nucleon models' conclusion could weaken. Thus I support the conditional verdict and do not recommend a more severe rejection; the measurement is valuable but the interpretation is provisional.","tokens_in":10073,"tokens_out":7542,"duration_ms":78472,"concrete_test":"Take the published spectra (or the authors' templates) and re-run the simultaneous fit with two model variations: (1) a two-parameter Fermi nuclear density with half-density radius and diffuseness varied within the accepted 12C uncertainties (e.g., c=2.19±0.03 fm, a=0.50±0.03 fm) and (2) an energy-dependent imaginary potential of the form W0(E) = W0 * f_phase(E) with f_phase from E05. If the extracted central W0 moves by more than the quoted systematic (16 MeV), the claimed 'direct determination' does not survive model variation. A more definitive check would be to regenerate the escape templates with an independent microscopic calculation (e.g., a coupled-channels or transport code) and see if the same (V0,W0) reproduces the measured ratio.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is that the yield of escaping K- (the exclusive spectrum) and the inclusive spectrum are both described by the same Green's-function DWIA calculation of Ref. [18] with a local optical potential U(r) = (V0 + i W0) ρ(r)/ρ(0). The exclusive K- escape cross section is essentially a survival probability of the recoil K- as it propagates through the nucleus; the conversion from this measured survival probability to W0 is entirely within that model. In particular, (i) the K- p QFES vertex is taken as the free-space amplitude (impulse approximation) — the E05 inclusive analysis relied on the same approximation, and its W0 constraint was weak; (ii) the nuclear density distribution is fixed, so any change in the spatial profile of W0 (e.g., density-dependent W0(ρ) = W0(ρ/ρ0)^alpha) can trade off against the central W0; and (iii) all absorption channels beyond the one-nucleon K-N → Yπ process are encoded as a purely imaginary local potential, with no explicit coupling to the inelastic channels that produce it. If the true in-medium amplitude or the reaction mechanism differs (e.g., two-step processes producing a K- that mimics the escape signal), the fitted W0 will absorb the mismatch. The abstract's phrase 'direct experimental determination' is therefore too strong: the quoted W0 = -100 MeV is a model-dependent estimate, and the claimed discrepancy with ~-40 MeV one-nucleon predictions could shrink or grow under alternative model assumptions. The paper does not include any variation of the optical-potential form or density profile in the systematic error budget; only the escape-spectrum normalization and background fractions are varied.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports the first measurement of the exclusive K^- escape cross section in the 12C(K^-,p) reaction at 1.8 GeV/c at J-PARC E42, obtained in coincidence with the inclusive 12C(K^-,p) spectrum. Using a simultaneous likelihood fit of Green's function/DWIA spectral templates to both datasets, the authors extract a K^- optical potential of V0 = -72 +3/-5 (stat) +0/-8 (syst) MeV and W0 = -100 +7/-1 (stat) +0/-16 (syst) MeV at nuclear center. The extracted W0 is considerably stronger than the ~-40 MeV expected from one-nucleon absorption models, and the paper interprets this as evidence for multi-nucleon absorption processes. The measured exclusive escape cross section is 436 ± 6 (stat) ± 44 (syst) μb/sr.","tokens_in":10474,"tokens_out":6352,"duration_ms":67417,"significance":"If the extraction is robust, this is a new and potentially important experimental constraint on the antikaon-nucleus optical potential, providing sensitivity to the absorptive part that is complementary to kaonic atom X-ray data and to the previous inclusive E05 measurement. The experiment itself appears carefully performed: the K^- PID is clean, the efficiency is cross-checked with CH2 and diamond targets with a 5.3% agreement, and the 10.2% total systematic on the escape cross section is documented. The main significance, however, resides in the W0 value, and that value is obtained within a specific theoretical model; the paper's claim of a 'direct experimental determination' overstates the model independence of the result.","major_comments":[{"comment":"The phrase 'direct experimental determination of the imaginary part of the K^- optical potential' is too strong. W0 is not measured directly; it is the result of a fit within the Green's function/DWIA framework of Ref. [18], which assumes U_opt(r)=(V0+iW0)ρ(r)/ρ(0), a fixed density profile, and an impulse-approximation vertex for K^-'p' quasi-free scattering. The escape spectrum is essentially a survival probability, so the conversion to W0 is entirely model-dependent. A density-dependent W0(ρ)=W0(ρ/ρ0)^α, a different ρ(r), or two-step contributions could shift the central value beyond the quoted systematic. Please provide a quantitative model-dependence study (varying ρ(r), the density exponent, the energy dependence) or explicitly reframe the result as an extraction under the assumed optical-model form.","section":"Analysis, 'To determine the \\bar K-nucleus optical potential...'; Abstract"},{"comment":"The systematic uncertainties on V0 and W0 are evaluated only by rescaling the escape-spectrum normalization within its ±10.2% uncertainty and repeating the fit. Model uncertainties—the optical-potential functional form, nuclear density, and reaction mechanism—are not propagated. Since the central physics claim is that W0≈-100 MeV is far from the one-nucleon expectation of ≈-40 MeV, a model uncertainty of a few tens of MeV would affect the conclusion. Please report the model-systematic shifts or state clearly that the quoted uncertainties are conditional on the model.","section":"Results and Discussion, systematic uncertainties"},{"comment":"The overall normalization of the inclusive spectrum is treated as a free parameter. This means the absolute normalization of the inclusive cross section does not constrain the fit. While the escape spectrum provides an absolute normalization, the inclusive spectrum's shape alone may not uniquely fix V0 and W0. Please discuss what the absolute inclusive cross section would add, or demonstrate that the fitted parameters are insensitive to normalizing the inclusive model to the data.","section":"Analysis, inclusive fit normalization"}],"minor_comments":[{"comment":"'Direct experimental determination' and 'first direct constraint' should be replaced with wording that reflects the model dependence noted above.","section":"Abstract / Conclusion"},{"comment":"The sentence about the exclusive K^-π^- state being the only significant background with extra charged particles (27.2% relative to the exclusive K^- yield) is confusing if later 'rejecting events with any additional charged tracks strictly selects the exclusive K^- channel.' Clarify whether the 27.2% refers to the sample before the veto or to a different phase-space region.","section":"Analysis, background discussion"},{"comment":"Please state explicitly the -B_K integration range corresponding to the quoted escape differential cross section 436 ± 6 ± 44 μb/sr. The figure shows double-differential spectra, so the range of the integrated number should be specified.","section":"Fig. 3 and quoted cross section"},{"comment":"The text uses both 'likelihood fit' and 'highest posterior density.' Clarify whether the contours in Fig. 4 are Bayesian posterior contours or frequentist likelihood-ratio contours, and state the prior assumptions if Bayesian.","section":"Statistical method"},{"comment":"A careful proofread is needed for minor grammatical issues, e.g., 'contains the reactions such as' should be 'contains reactions such as', and some equation/notation spacing is inconsistent.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The theoretical framework in Ref. [18] and the comparison analyses in Refs. [17,20] share authorship with the experimental group. This is not a flaw in itself, but it reinforces the need for an independent check of the model dependence or a clear statement of the extracted W0's conditional nature. The measurement appears competently analyzed; the main substantive issue is the load-bearing model dependence of the W0 extraction."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a real experimental first. They measured the K- escape channel in coincidence with the inclusive 12C(K-,p) reaction at 1.8 GeV/c, and the exclusive/inclusive ratio gives a lever arm on the imaginary part of the antikaon optical potential that inclusive spectra never had. The experimental work looks careful: PID in the HypTPC, Geant4 efficiency validation against CH2 target data, 10.2% systematic on the escape cross section, and a clean likelihood minimum. I believe the measurement.\n\nThe fit gives V0 around -72 MeV and W0 around -100 MeV, with a real part consistent with chiral unitary models and an imaginary part far stronger than the ~-40 MeV one-nucleon predictions. That discrepancy is the interesting physics: it points to multi-nucleon absorption playing a major role, consistent with what they note about the K-pp width.\n\nThe soft spot is model dependence. W0 is not measured directly; it is the parameter of a local potential U=(V0+iW0)rho/rho0 embedded in their Green's function/DWIA calculation, with the density fixed and no variation of the potential form in the systematic budget. The inclusive normalization is a free parameter of the fit. So confidence in the central value is tied to the correctness of that model at the 10-20% level. The abstract's phrase 'direct experimental determination' is too strong; a more honest phrasing would be 'first experimental constraint within a specified optical-potential framework.' That said, the claim isn't circular in the input=output sense: the escape/inclusive ratio does respond to W0 in a way inclusive data didn't. The reader's note about same-group theory is fair but not disqualifying; it's the usual situation in this field.\n\nI'd also like data tables or a digitized spectrum, plus explicit model variations (density profile, density-dependent W, energy-dependent imaginary part) to see how robust the -100 MeV is. As is, a skeptical referee can't reproduce the extraction without running their own version of [18], which isn't public.\n\nWho this is for: anyone working on kaonic nuclei, optical potentials, or kaon-condensation onset in neutron-star EoS. It deserves a serious referee; this is the kind of new observable the field needed. My recommendation: engage, don't treat W0 as final, request the model-variation systematics and a softer abstract before publication.","headline":"First K- escape measurement is a genuinely new observable and a real step forward on the shallow-vs-deep question, but the W0 = -100 MeV headline rests on a fixed model framework the systematic budget doesn't yet cover.","tokens_in":11140,"tokens_out":2758,"would_cite":true,"duration_ms":29289,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The first exclusive K- escape measurement in the 12C(K-,p) reaction yields W0 = -100 MeV, far more absorptive than one-nucleon models predict.","keywords":["K- escape","antikaon-nucleus optical potential","imaginary potential","multi-nucleon absorption","(K-,p) reaction","Green's function method","kaonic atoms","neutron stars"],"falsifier":"A direct test would be to measure the K- escape cross section on a nucleus with well-known density (e.g., 4He) at the same beam momentum: a value of W0 close to -40 MeV would contradict the claim, while a value near -100 MeV would confirm the strong absorption.","tokens_in":10018,"feed_emoji":"⚛️","tokens_out":6717,"duration_ms":57644,"temperature":0.7,"pith_summary":"This paper reports the first measurement of the exclusive K- escape process in the 12C(K-,p) reaction. By detecting both the forward proton and the surviving recoil K-, the authors directly constrain the imaginary part of the antikaon-nucleus optical potential, which controls how strongly kaons are absorbed in nuclear matter. A simultaneous fit to the inclusive and escape spectra yields W0 = -100 MeV at the nuclear center, substantially stronger than the -40 MeV expected from one-nucleon absorption alone, implying that multi-nucleon processes play a significant role. This is the first direct experimental determination of W0 and helps resolve the long-standing shallow-vs-deep ambiguity in the antikaon-nucleus potential.","feed_headline":"First kaon escape measurement pins nuclear absorption at -100 MeV","feed_subtitle":"Points to multi-nucleon absorption and affects kaon-condensation onset in neutron stars","key_machinery":"The key object is the antikaon-nucleus optical potential U_opt(r) = (V0 + iW0) rho(r)/rho(0), parametrized by a real strength V0 and an imaginary strength W0. The analysis uses the Green's function method with a distorted-wave impulse approximation (DWIA) to compute both the inclusive missing-mass spectrum and the exclusive K- escape spectrum for a grid of (V0, W0). The escape spectrum is uniquely sensitive to W0 because the probability that the quasi-free recoil K- survives final-state absorption depends on the imaginary potential; a simultaneous likelihood fit to both spectra locates a clear minimum in the (V0, W0) plane and yields the quoted values.","core_discovery":"The central discovery is that the K- escape cross section in the 12C(K-,p) reaction at 1.8 GeV/c, measured here for the first time, forces an imaginary optical-potential strength of W0 = -100^{+7}_{-1}(stat)^{+0}_{-16}(syst) MeV at nuclear density, with a real part V0 = -72 MeV. The escape probability of the recoil K- is governed directly by the absorptive strength of the nuclear medium, so the exclusive measurement breaks the degeneracy that made the inclusive measurement alone weakly sensitive to W0. The derived W0 is significantly more absorptive than the predictions of chiral one-nucleon models (around -40 MeV), and it reconciles with the strong absorption deduced from kaonic atom data w","pith_inferences":["If multi-nucleon absorption indeed dominates the imaginary potential, then theoretical approaches that sum only one-nucleon amplitudes (t-rho type) are incomplete; future data on heavier nuclei could trace how W0 scales with density and reveal the microscopic multi-nucleon mechanisms.","The same escape-technique could be applied to other absorbed mesons (e.g., anti-protons or eta mesons) to measure their imaginary potentials, providing a general tool for probing in-medium absorption.","The strong W0 may affect the interpretation of kaonic atom level widths and shifts, suggesting that a re-analysis of global kaonic atom data with this W0 could refine the density dependence of the potential."],"forward_implications":["The absorptive strength W0 = -100 MeV is significantly larger than the -40 MeV predicted by one-nucleon absorption models, indicating that multi-nucleon absorption processes are important in the nuclear medium.","The real potential V0 = -72 MeV is consistent with the shallow chiral-unitary predictions (about -50 to -80 MeV), supporting a shallow rather than deep antikaon-nucleus potential.","The result resolves the long-standing ambiguity from kaonic atom analyses, which could accommodate both shallow and deep potentials; the direct reaction measurement breaks the degeneracy.","A shallow potential with strong absorption raises the threshold density for kaon condensation in neutron stars to about 3-4 times normal nuclear density, consistent with modern neutron-star mass constraints."],"fun_headline_variants":["First K- escape measurement sets nuclear absorption at -100 MeV","Kaon escape cross section reveals absorption stronger than models","First exclusive K- escape pins down kaon optical potential","New kaon data: nuclear absorption double predicted strength","J-PARC kaon escape measurement yields -100 MeV absorption"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The extraction assumes a single optical potential of the form U_opt(r) = (V0 + iW0) rho(r)/rho(0) with a fixed density profile, and that the Green's function/DWIA calculation correctly predicts both the inclusive and escape spectra; if the density shape or the energy dependence of the self-energy differs, the quoted W0 is biased.","fun_headline_variants_meta":{"raw":{"variants":["First K- escape measurement sets nuclear absorption at -100 MeV","Kaon escape cross section reveals absorption stronger than models","First exclusive K- escape pins down kaon optical potential","New kaon data: nuclear absorption double predicted strength","J-PARC kaon escape measurement yields -100 MeV absorption"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000547,"raw_usage":{"total_tokens":2492,"prompt_tokens":826,"completion_tokens":1666,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":570,"completion_tokens_details":{"reasoning_tokens":1585}},"tokens_in":570,"tokens_out":1666,"duration_ms":12680,"temperature":1.0,"reasoning_tokens":1585,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T11:00:11.731747+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A direct test would be to measure the K- escape cross section on a nucleus with well-known density (e.g., 4He) at the same beam momentum: a value of W0 close to -40 MeV would contradict the claim, while a value near -100 MeV would confirm the strong absorption.","supporting_citations":[],"review_version":2}