{"id":"aa119e95-24df-4f0b-a39f-82e551ccf10e","arxiv_id":"2607.09897","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Chiral-EFT ab initio computations yield a small 2+ energy in 140Sn under a closed 7/2- subshell assumption, contradicting that shell closure.","lead":"Ab initio calculations find that 140Sn has a low first 2+ excitation energy, so it lacks a neutron shell closure at N=90. This settles a long-running theoretical dispute and sharpens models of neutron-rich nuclei that feed the astrophysical r-process.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"No significant objection identified","rationale":"The Reader correctly isolates the interaction extrapolation as the principal uncertainty and correctly judges that it does not overturn the qualitative result. The paper’s multi-method, multi-truncation evidence (Figs. 2–3) already shows that higher-order correlations and valence-space versus EOM formulations all drive the 2+ energy downward, reinforcing rather than weakening the reductio. Because no stronger technical flaw is present, the ACCEPT verdict stands.","tokens_in":17908,"tokens_out":382,"duration_ms":4648,"concrete_test":"Recompute the EOM-CCSD 2+ energy at Nmax=16 with one additional chiral interaction that also reproduces the 133Sn spectrum (e.g., a Delta-full or N3LO soft potential); if that interaction yields a 2+ above ~2.5 MeV while still describing 133Sn, the interaction-dependence concern would become load-bearing.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is a reductio: under the closed 7/2- neutron subshell assumption the computed 2+ excitation energy is small (and still falling with Nmax) and therefore the assumption is false. Both EOM-CC and VS-IMSRG families, with and without approximate triples, produce the same qualitative result; the 133Sn single-particle spectrum is reproduced at the level expected for this interaction; and the same Hamiltonian has previously described known shell closures. Residual model-space incompleteness and the A=140 extrapolation of 1.8/2.0(EM) are real but already quantified by the authors and do not reverse the conclusion that the excitation energy lies well below the values characteristic of doubly-magic nuclei. No internal inconsistency or hidden assumption that would invalidate the reductio is apparent.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript addresses the disputed existence of an N=90 subshell closure in 140Sn by ab initio calculations with the chiral 1.8/2.0(EM) interaction. After validating the Hamiltonian on the low-lying single-particle spectrum of 133Sn (PA-EOM-CCSD and VS-IMSRG), the authors adopt a spherical Hartree–Fock reference that assumes a filled 1f7/2 neutron subshell and compute the first 2+ excitation energy of 140Sn with EOM-CCSD/T-1, VS-IMSRG(2/3f2) and EOM-IMSRG(2). All methods yield a small excitation energy (below ~1.5 MeV and still decreasing with model-space size), which is inconsistent with a robust shell closure and with the much larger 2+ energies of known doubly-magic nuclei. Ground-state energies are also reported and found consistent with mass-model estimates.","tokens_in":18122,"tokens_out":670,"duration_ms":5289,"significance":"The result is a clean, falsifiable reductio that resolves a long-standing controversy with a single, well-validated Hamiltonian and two independent many-body frameworks. The same interaction has previously reproduced or predicted shell closures in oxygen, calcium, nickel, 78Ni, 100Sn and 208Pb; the 133Sn benchmark further anchors the calculation near the region of interest. Explicit model-space trends and approximate triples corrections are shown, so residual incompleteness is quantified rather than hidden. The conclusion that 140Sn is not doubly magic is therefore robust and of direct interest for r-process modeling and for the interpretation of ongoing spectroscopic campaigns beyond 132Sn.","major_comments":[],"minor_comments":[{"comment":"In Sec. V the statement that EOM-CCSD(T) yields negative excitation energies is useful but terse; a short clause clarifying that this signals reference-state breakdown (rather than a numerical artifact) would help non-specialist readers.","section":null},{"comment":"Fig. 3 caption and surrounding text could note more explicitly that the EOM-CCSDT-1 points are restricted to ẽpqr < 100 MeV; the truncation is mentioned in Sec. III but is easy to miss when reading the figure alone.","section":null},{"comment":"The 13/2+ discrepancy in 133Sn (Sec. IV) is acknowledged; a one-sentence remark on whether continuum effects or Hamiltonian dependence is the more likely culprit would strengthen the discussion without requiring new calculations.","section":null},{"comment":"Minor typographical inconsistencies appear (e.g., “HAMIL TONIAN”, “V ALIDA TION”, occasional missing spaces around math mode); a light copy-edit pass would remove them.","section":null}],"recommendation":"accept","confidential_remarks":"The paper is concise, the central claim is load-bearing and cleanly demonstrated, and the methods are standard for the field. I see no reason to delay publication; the minor presentation points can be handled at the proof stage if the journal prefers."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"This paper settles a multi-year controversy with a clean reductio: assume a closed 7/2- neutron subshell in 140Sn, compute the first 2+ with the well-tested 1.8/2.0(EM) interaction, and find an energy that is already below ~1.5 MeV and still falling with model space. That is incompatible with a shell closure. Both EOM-CC (CCSD and limited CCSDT-1) and VS-IMSRG (2 and 3f2) plus EOM-IMSRG give the same qualitative answer, and the same Hamiltonian correctly places the low-lying single-particle states in 133Sn (except the unbound 13/2+, which everyone struggles with).\n\nWhat is new is the first ab-initio numbers for the 140Sn ground state and 2+ under this interaction; previous shell-model and mean-field work was mixed, and no one had done the closed-shell assumption test from chiral EFT. The authors show the model-space trends explicitly, release the data, and keep the many-body truncations honest. The interaction’s track record on known magic nuclei (including predictions that later matched experiment) makes the extrapolation to A=140 reasonable rather than heroic.\n\nSoft spots exist but are proportional. Residual Nmax incompleteness is real—the 2+ is still dropping—but the direction only strengthens the no-gap conclusion. The A=140 reach of LECs fixed on A=3,4 is the usual caveat for this interaction; it does not reverse the result. Ground-state energies sit close to mass-model estimates once triples are estimated, so nothing looks pathological. Citation pattern is clean and the methods are standard.\n\nThis is for people who care about shell evolution past 132Sn or r-process waiting points near A=130. It deserves a serious referee and should be engaged. I would cite the 2+ numbers and the exclusion of the N=90 gap.","headline":"Solid ab-initio reductio that cleanly kills the proposed N=90 shell closure in 140Sn; the result is robust across methods and the soft spots are already quantified.","tokens_in":18703,"tokens_out":498,"would_cite":true,"duration_ms":4867,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.5","headline":"Ab initio calculations show no shell closure at neutron number 90 in 140Sn: the first 2+ excitation energy is small, contradicting the closed-subshell assumption.","keywords":["shell closure","140Sn","ab initio nuclear structure","chiral effective field theory","coupled-cluster theory","IMSRG","neutron-rich tin isotopes","N=90"],"falsifier":"A direct experimental measurement of the first 2+ excitation energy in 140Sn itself; a value above roughly 2–3 MeV would restore the possibility of a shell closure, while a value near or below 1 MeV would confirm the calculation.","tokens_in":18810,"feed_emoji":"⚛️","tokens_out":878,"duration_ms":8705,"temperature":0.7,"pith_summary":"The paper settles a long-running disagreement about whether the neutron-rich tin isotope 140Sn has a shell closure at neutron number 90. Using a chiral nuclear interaction already known to reproduce shell closures in lighter magic nuclei, the authors first check that it correctly describes the low-lying spectrum of 133Sn. They then assume that 140Sn has a closed 7/2- neutron subshell and compute the energy of its first excited 2+ state with two independent many-body methods. The calculated excitation energy is low—below about 1.5 MeV and still falling as the model space is enlarged—which is incompatible with a robust shell gap. The result removes the expectation of a new magic number at N=90 and aligns with earlier shell-model trends and with mass and radius systematics that show no kink at that point.","feed_headline":"No shell closure in 140Sn: first 2+ energy stays low","feed_subtitle":"Ab initio calculations contradict the closed-subshell assumption at neutron number 90","key_machinery":"The equation-of-motion coupled-cluster and valence-space in-medium similarity-renormalization-group methods applied to the 1.8/2.0 (EM) chiral Hamiltonian; both methods start from a closed-shell reference and extract the 2+ excitation energy, whose smallness falsifies the reference assumption.","core_discovery":"If 140Sn is assumed to possess a closed 7/2- neutron subshell beyond 132Sn, first-principles calculations with the 1.8/2.0 (EM) interaction yield a first 2+ excitation energy that is small (under 1.5 MeV and still decreasing with model-space size). That low energy contradicts the closed-subshell premise and therefore rules out a shell closure at N=90 for this interaction.","pith_inferences":["The same interaction and methods can now be used to map the entire tin chain from the proton dripline to the neutron dripline without assuming artificial shell closures.","If continuum effects lower the unbound 13/2+ state in 133Sn, similar continuum corrections may further soften the 2+ energy already found in 140Sn.","A systematic survey of other N=90 isotones with the same force would test whether the absence of a shell gap is specific to tin or generic."],"forward_implications":["No new magic number is expected at N=90 in the tin isotopic chain.","Shell-model calculations that already predicted a soft 2+ energy of 0.5–1 MeV are consistent with the ab initio result.","Astrophysical r-process abundance patterns near A≈130 need not incorporate a pronounced N=90 shell gap.","Future mass and radius measurements of even tin isotopes beyond 138Sn should show smooth trends rather than a kink at N=90."],"fun_headline_variants":["Ab initio calc finds no shell closure in 140Sn","Low 2+ energy contradicts closed-subshell claim for 140Sn","140Sn first 2+ too soft for closed 7/2- neutron subshell","Chiral EFT interaction rules out N=90 shell closure in 140Sn","Assumed closed subshell in 140Sn yields too-low 2+ energy"],"cache_read_input_tokens":128,"weakest_assumption_plain":"That a nuclear force whose constants were fixed only on the lightest nuclei remains accurate enough to decide shell structure in the neutron-rich tin region around mass 140.","fun_headline_variants_meta":{"raw":{"variants":["Ab initio calc finds no shell closure in 140Sn","Low 2+ energy contradicts closed-subshell claim for 140Sn","140Sn first 2+ too soft for closed 7/2- neutron subshell","Chiral EFT interaction rules out N=90 shell closure in 140Sn","Assumed closed subshell in 140Sn yields too-low 2+ energy"]},"model":"grok-4.5","effort":"low","cost_usd":0.006698,"raw_usage":{"total_tokens":1617,"prompt_tokens":655,"num_sources_used":0,"completion_tokens":105,"cost_in_usd_ticks":66980000,"prompt_tokens_details":{"text_tokens":655,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":857,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":655,"tokens_out":105,"duration_ms":5954,"temperature":1.0,"reasoning_tokens":857,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-14T14:44:10.348018+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"A direct experimental measurement of the first 2+ excitation energy in 140Sn itself; a value above roughly 2–3 MeV would restore the possibility of a shell closure, while a value near or below 1 MeV would confirm the calculation.","supporting_citations":[],"review_version":1}