{"id":"f607585b-f97c-4b9c-9071-9b1c7ba76394","arxiv_id":"2507.19753","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Absolute production branching ratios after muon capture on 27Al and 28,29,30Si were measured with high precision, showing a clear even-odd proton-number effect in the no-particle-emission channel.","lead":"The authors measured how often muons captured by aluminum and silicon nuclei leave behind each possible daughter nucleus, producing the most precise branching ratios for these reactions to date. The results reveal even-odd patterns in nuclear stability after muon capture and provide new tests for models of highly excited nuclei.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Neutron-induced background on 0n0p channels is a valid but bounded systematic; it should be added to the error budget, but it cannot erase the highly significant even-odd 27Al vs 28Si difference.","rationale":"The reader's weakest assumption—uncorrected neutron-induced background on the 0n0p channels—is the most load-bearing concern because it directly affects the two numbers that define the even-odd conclusion. I agree that this is the right concern to raise, and it justifies a CONDITIONAL verdict. However, the concern does not overturn the central claim: a 3% relative contamination changes 9.90% and 18.92% by only ~0.3 and ~0.6 percentage points, leaving the ~9-point gap at high significance. The RAL–J-PARC agreement for 28Si (18.92 vs 18.91) bounds the external, geometry-dependent background, so the remaining uncertainty is the self-induced neutron component, which is target-dependent but still small. The unresolved 389.7-keV peak for 25Na and the over-parameterized excitation-function fit are secondary issues that do not affect the 0n0p channels or the main even-odd comparison. Thus the appropriate verdict remains CONDITIONAL, with the neutron-background systematic and the 389.7-keV discrepancy to be clarified before the absolute BRs are used as definitive references.","tokens_in":40941,"tokens_out":19073,"duration_ms":212788,"concrete_test":"Re-run the PHITS simulation using the actual experimental geometry and the measured ϵstop values for each target, so that the number of muons stopping in surrounding material is computed rather than assumed equal to the target. Evaluate the resulting neutron-induced production of 27Mg in the 27Al target and 28Al in the 28Si target. If the contamination is below 1% for both plate and powder targets, the quoted 0n0p BRs need no correction; if it reaches or exceeds 3% for the powder targets, the 0n0p BRs and their uncertainties should be revised upward accordingly.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's main physics conclusion is the even-odd proton-number dependence of the 0n0p channel: 9.90(33)% for 27Al versus 18.92(10)(91)% for 28Si. The quoted uncertainties do not include the estimated maximum ~3% contamination of these channels from neutrons emitted in muon capture (Sect. V). This contamination is not corrected. The estimate assumes the number of muons stopping in surrounding material equals that in the target, which is questionable for the powder targets: for 28Si, ϵstop = 0.407(14) at RAL and 0.298(9) at J-PARC, so many more muons stop in the surroundings than in the target, possibly making the 3% an underestimate for the powder samples. However, the even-odd gap is ~9 percentage points, roughly 9σ even if a 3% relative systematic is added, so the central conclusion is robust. The RAL and J-PARC results for 28Si agree to ~0.1% relative, which strongly constrains any geometry-dependent (external) neutron background; the remaining concern is the target self-induced neutron contribution, which would affect both facilities equally and is not independently tested by that agreement. The paper should propagate the estimated contamination as a systematic uncertainty and state whether the 3% applies to plate and powder targets alike.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper reports absolute production branching ratios (BRs) for muon nuclear capture on 27Al and 28,29,30Si, measured with the in-beam activation method at two pulsed muon facilities (RIKEN-RAL and J-PARC). The authors count stopped muons with a plastic scintillator and detect beta-delayed gamma rays with germanium detectors, with a low-intensity countable beam at RAL providing absolute normalization and a high-intensity beam at J-PARC providing high statistics after calibration. The main results are the BRs of residual nuclei after particle emission, presented in Tables II–VI, including the 0n0p channels: 9.90(33)% for 27Al and 18.92(10)(91)% for 28Si. The paper interprets these results as evidence for an even-odd proton-number dependence of the 0n0p channel, a decreasing 0n1p and 2n2p probability with increasing neutron excess, and a general predominance of neutron emission. The measured BRs are compared with previous activation and prompt-gamma measurements, with PHITS and microscopic evaporation model (MEM) calculations, and with shell-model Gamow-Teller strengths. A method for estimating the excitation energy distribution from the measured BRs is also proposed.","tokens_in":41197,"tokens_out":8570,"duration_ms":100463,"significance":"If the results hold, they provide the most accurate absolute muon-capture production BRs in the aluminum-silicon mass region to date, and the cross-facility consistency between RAL and J-PARC is a notable strength. The even-odd Z dependence of the 0n0p channel, supported by the large 27Al vs 28Si difference, is a valuable constraint on the low-energy isovector response and on proton-neutron pairing effects. The paper also gives a careful uncertainty decomposition that separates relative and absolute components, propagates ENSDF decay-data uncertainties, and treats enriched-target impurities and stopping rates in some detail. The comparisons with PHITS and MEM, and the proposal to extract excitation functions from BRs, are useful contributions even though the model comparisons remain partly qualitative. The manuscript does not include a data-release or code repository, but the methodology is documented in sufficient detail to be reproducible in principle.","major_comments":[{"comment":"The paragraph beginning 'The maximum potential contamination...' states that neutron-induced background from muon stops in surrounding material could contaminate the 0n0p channels by up to approximately 3%, that this is comparable to the measurement uncertainty, and that it was not corrected. Since the even-odd 0n0p comparison (9.90(33)% for 27Al, Table II, vs 18.92(10)(91)% for 28Si, Table IV) is the paper's main physics conclusion, this systematic must be included in the quoted uncertainties. The underlying assumption that the number of muons stopping in the surroundings equals the number stopping in the target is questionable for the enriched powder targets, whose measured stopping rates are only 0.298–0.652 (Table I). Please quantify the contamination separately for plate and powder targets, including the target self-induced neutron contribution, and state explicitly how the quoted BRs change if the estimated contamination is applied as a correction or as a one-sided systematic.","section":"Sect. V"},{"comment":"For 25Na in the J-PARC silicon data, the BR extracted from the 389.7-keV line is significantly lower than those from the 585.0- and 974.7-keV lines, and the 389.7-keV point is excluded from the weighted average without identifying the cause. This unexplained discrepancy raises the possibility of an unaccounted efficiency, self-absorption, or background effect at low energy that could also affect other lines in the same data set. Please either identify the cause or provide a quantitative demonstration that including the 389.7-keV point does not change the compiled 25Na BRs beyond the quoted uncertainties.","section":"Sect. V, Tables III, V, VI"},{"comment":"The stopping rate for the enriched 29Si target has a relative uncertainty of 24–26% at both facilities (0.652(169) at RAL and 0.260(63) at J-PARC), which propagates into roughly 25% scale uncertainties in the 29Si BRs (e.g., 29Al 15.01(48)(373) and 28Al 48.68(18)(1188)). The neutron-excess trend discussion in Section VI B uses these 29Si values, so the trend statements should either be restricted to the well-determined 28Si and 30Si points or explicitly quantify how the 29Si uncertainties affect the conclusions.","section":"Tables I and V; Sect. VI B"}],"minor_comments":[{"comment":"There are several typographical errors that should be corrected: 'irradiating the target. using a plastic scintillator' in the abstract, 'exitat states' in the Introduction, 'T rend' as a section heading, 'SUMMAR Y' in Section VII, 'Comparson' in the conclusions, 'reprocude' in Section VI C, 'Monte-Calro' in Section V, and 'whreas' in Section VI A.","section":"Abstract and headings"},{"comment":"The definition of Pdecay is confusing as written: the text says the numerator is computed 'assuming all the irradiated muon produced the given nucleus (Pcapϵstopb = 1)'. Please clarify that this is a normalization convention used to account for the time structure of the beam, and not a physical assumption that every muon produces every residue.","section":"Sect. IV A, Eq. (5)"},{"comment":"The excitation-function extraction uses H = 20 Gaussian functions with fixed centers and widths, and the hyperparameters were selected to minimize chi-square. Please state explicitly that the resulting excitation functions are model-dependent and not unique, and avoid presenting the quoted average excitation energies (16.4(3) and 15.4(3) MeV) without also stating that the uncertainties do not include the model-choice and evaluation-input uncertainties.","section":"Sect. VI D"},{"comment":"The figure legend contains the typo 'Average Mutiplicity'; also, the use of open triangles for direct and pre-equilibrium emission and open squares for evaporation is described in the text but the symbols in the figure are not labeled directly, which makes the figure difficult to read.","section":"Fig. 12"},{"comment":"The sentence 'This measurement represents the first reliable absolute data...' is stronger than the analysis supports, given the uncorrected neutron background and the unresolved 389.7-keV discrepancy. Please soften the claim to something like 'the first absolute BR measurement with percent-level precision in this mass region' or add the caveats needed to justify 'reliable'.","section":"Sect. VI A"}],"recommendation":"major_revision","confidential_remarks":"The central measurement appears sound, and the even-odd 0n0p effect is likely robust even after adding a few-percent systematic. However, the unpropagated neutron-background systematic is load-bearing for the main physics conclusion and must be addressed. The unexplained 389.7-keV anomaly and the large 29Si stopping-rate uncertainty are additional concerns that should be resolved or explicitly bounded before acceptance. The paper is within scope for a nuclear physics journal and, after these revisions, would be a valuable contribution."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a careful activation measurement of muon-capture production branching ratios for 27Al and 28,29,30Si, with first results for the enriched Si isotopes and improved precision on the others. The main physics claim—an even-odd proton-number dependence in the 0n0p channel (9.90(33)% for 27Al vs 18.92(10)(91)% for 28Si)—looks solid. The two-facility cross-check between RAL and J-PARC is genuinely reassuring.\n\nThe absolute normalization scheme (low-intensity counting at RAL, high-intensity calibration at J-PARC via 27Al) is clever and well explained. The 28Si values agree between facilities to roughly 0.1% relative, which is strong evidence the external geometry-dependent backgrounds are under control. The paper is also honest about its limitations, which counts in its favor.\n\nThe soft spots are real but not fatal. The estimated ~3% neutron-induced contamination of the 0n0p channels is not corrected and not folded into the uncertainties. That should be addressed before these results become reference values; an uncorrected, target-dependent background is not something to leave as an exercise. The stress-test note is right that the even-odd gap is ~9 percentage points, so the main conclusion is robust to a 3% relative shift, but the 28Si self-induced component could affect both facilities equally and is not tested by the cross-facility agreement. Add it as a systematic; don't let it sink the paper.\n\nThe unexplained 389.7-keV exclusion for 25Na in J-PARC silicon data is another thing a referee will want explained. It is a single peak out of many, so it does not undermine the main tables, but 'the cause remains uncertain' is not a satisfying stopping point.\n\nThe excitation-function extraction with twenty Gaussians is a parameterized fit, not a measurement. The average excitation energies (16.4(3) and 15.4(3) MeV) inherit model assumptions and should be labeled as estimates from a fitting procedure rather than direct observables. The comparison with PHITS and MEM is useful, but the model-dependent part of the paper is the weakest.\n\nBottom line: the BR tables are the product here, and they look credible. The paper deserves a serious referee; I would send it to review with the request that the neutron-background systematic be propagated and the 389.7-keV issue either explained or flagged more prominently as an unresolved discrepancy.","headline":"New absolute muon-capture branching ratios for Al and Si with a robust even-odd 0n0p effect; the neutron background needs a systematic and the 389.7-keV exclusion wants an explanation.","tokens_in":41900,"tokens_out":3165,"would_cite":true,"duration_ms":31422,"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":"Absolute muon-capture branching ratios for aluminum and silicon reveal an even-odd proton-number effect.","keywords":["muon nuclear capture","production branching ratio","in-beam activation method","aluminum-27","silicon isotopes","Gamow-Teller strength","proton-neutron pairing","even-odd effect"],"falsifier":"A concrete check: measure the 0n0p branching ratios of 27Al and 28Si with the same beam but with target holders of very different surrounding mass, or insert an active neutron detector around the target. If the apparent 9.9% versus 18.9% difference moves by more than a few percent, the even-odd conclusion is contaminated by target-dependent neutron background.","tokens_in":40669,"feed_emoji":"⚛️","tokens_out":6956,"duration_ms":75314,"temperature":0.7,"pith_summary":"This paper measures what happens after a negative muon is captured by an atomic nucleus in aluminum and silicon, counting the residual nuclei left after the excited system emits neutrons, protons, and alpha particles. It reports absolute production branching ratios—the probability of each emission channel—for 27Al and 28,29,30Si with the highest accuracy to date. The key result is that the no-particle-emission channel is 9.90(33)% for 27Al but 18.9(9)% for 28Si, a clear even-odd proton-number dependence. The authors trace this difference to low-energy Gamow-Teller strength below the neutron separation energy, enhanced by proton-neutron pairing in even-Z final nuclei. If the interpretation holds, these branching ratios provide a benchmark for muon-capture models and a way to probe isovector transitions in nuclei.","feed_headline":"Muon capture branching ratios reveal an even-odd nuclear effect","feed_subtitle":"The no-emission channel is 9.9 percent for aluminum versus 18.9 percent for silicon, pointing to proton-neutron pairing.","key_machinery":"The in-beam activation method is the central mechanism: a pulsed muon beam irradiates the target, a plastic scintillator counts the muons, germanium detectors record β-delayed γ-rays from the produced nuclei, and the absolute branching ratio is formed as $b=N_{\\rm prod}/N_{\\rm cap}$. Absolute normalisation comes from a low-intensity beam in which each pulse's muon number can be counted, and the calibrated high-intensity beam then gives high statistics for rare channels; enriched powder targets are normalised through a decomposition of the natural-silicon data. Around this core, the paper builds the interpretation from channel-by-channel threshold energies, shell-model Gamow-Teller strengths, and simulations of particle transport and evaporation.","core_discovery":"On the paper's own terms, the discovery is that muon nuclear capture on 27Al and 28,29,30Si leaves a well-measured distribution of residual nuclei, and that distribution carries nuclear-structure information. The 0n0p channel—capture followed by no neutron or proton emission—is 9.90(33)% for 27Al versus 18.9(9)% for 28Si, and the paper argues this roughly two-fold gap cannot come from separation energies or average excitation energy alone. Instead, shell-model Gamow-Teller strengths summed below the one-neutron separation energy are 1.77 and 3.36 for the two transitions, nearly matching the same ratio as the branching ratios. The paper also establishes that neutron emission without charged particles dominates for all four isotopes, that the 0n1p and 2n2p channels fall with increasing neutron excess, and that production branching ratios can be inverted by a statistical evaporation model to estimate the excitation-energy distribution, yielding average excitation energies of about 16 MeV for 28Si and 15 MeV for 30Si.","pith_inferences":["A direct test of the pairing interpretation would be a coincidence measurement of emitted neutron energy and charged particles for 27Al and 28Si, checking whether the extra low-energy Gamow-Teller strength in 28Si actually removes neutrons from the evaporation cascade.","The same absolute-normalisation technique could be extended to other s-d shell nuclei; if the even-odd 0n0p pattern tracks cumulative B(GT) below the neutron separation energy, the branching ratio would become a cheap isovector probe for nuclei where charge-exchange data are scarce.","The uncorrected neutron background is the main threat: if it varies with target size and holder material, the 9.9% versus 18.9% comparison could shift by up to a few percent, so a dedicated background run with a low-mass target holder would sharpen the physics claim.","The observed 4n4p and 6n4p channels in silicon isotopes, interpreted as multi-alpha emission, suggest that production branching ratios could also constrain cluster structures in highly excited states beyond the Gamow-Teller picture."],"forward_implications":["The 0n0p channel becomes a systematic observable: for even-Z final nuclei, enhanced proton-neutron pairing should suppress neutron emission and raise the no-emission branching ratio, and the current data place the first precise anchor points at 27Al and 28,29,30Si.","Prompt γ-ray measurements of the 0n0p channel cover at least 88% of the absolute branching ratio for 28Si and essentially all of it for 27Al, so direct ground-state populations after muon capture are small.","Current transport-code simulations overestimate neutron multiplicities, particularly for 27Al and 30Si, because they underestimate direct and pre-equilibrium emission; reproducing the new absolute branching ratios requires fixing that component.","Microscopic evaporation models can reproduce the overall ordering of channels but underestimate charged-particle emission, so their treatment of the high-energy excitation tail needs revision.","Production branching ratios alone can constrain the low-energy part of the muon-capture excitation function, below roughly 40 MeV, where statistical evaporation dominates."],"supporting_citations":[{"why":"Establishes the in-beam activation method that the present measurement extends to absolute branching ratios.","marker":"[1]"},{"why":"Supplies the muonic-atom lifetimes and capture probabilities used to convert muon counts into capture counts.","marker":"[7]"},{"why":"Provides the prompt-γ comparison that quantifies how much of the 0n0p channel is covered by direct gamma measurement.","marker":"[41]"},{"why":"Gives earlier activation results and the Coulomb-barrier systematics used to compare the new charged-particle channels.","marker":"[35]"},{"why":"Supplies the phenomenological Singer excitation function used in the transport-model predictions of branching ratios.","marker":"[49]"},{"why":"Develops the microscopic-plus-evaporation model whose predicted branching ratios are compared with the measured ones.","marker":"[6]"},{"why":"Provides the shell-model code used to compute the Gamow-Teller strengths that explain the even-odd 0n0p effect.","marker":"[70]"},{"why":"The particle-transport simulation code used both for background estimates and for the theoretical branching-ratio comparison.","marker":"[76]"},{"why":"Gives the earlier neutron-multiplicity data used to estimate average multiplicities and check the simulation's evaporation component.","marker":"[48]"}],"fun_headline_variants":["Muon capture exposes even-odd nuclear pairing","Al vs Si: muon capture shows pairing effect","Nuclear pairing revealed by muon capture ratios","Even-odd gap in muon capture branching ratios","No-emission channel: Al vs Si two-fold difference"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that neutrons from muon captures in material around the target contribute negligibly to the measured 0n0p channels; the paper puts the maximum contamination at about 3%, comparable to its uncertainties, and does not correct for it.","fun_headline_variants_meta":{"raw":{"variants":["Muon capture exposes even-odd nuclear pairing","Al vs Si: muon capture shows pairing effect","Nuclear pairing revealed by muon capture ratios","Even-odd gap in muon capture branching ratios","No-emission channel: Al vs Si two-fold difference"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000277,"raw_usage":{"total_tokens":1728,"prompt_tokens":1099,"completion_tokens":629,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":715,"completion_tokens_details":{"reasoning_tokens":555}},"tokens_in":715,"tokens_out":629,"duration_ms":6603,"temperature":1.0,"reasoning_tokens":555,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T14:03:19.656587+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A concrete check: measure the 0n0p branching ratios of 27Al and 28Si with the same beam but with target holders of very different surrounding mass, or insert an active neutron detector around the target. If the apparent 9.9% versus 18.9% difference moves by more than a few percent, the even-odd conclusion is contaminated by target-dependent neutron background.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the prompt-γ comparison that quantifies how much of the 0n0p channel is covered by direct gamma measurement."},{"cited_title":"Vil’gel’mona and V","cited_arxiv_id":null,"evidence_quote":"Gives earlier activation results and the Coulomb-barrier systematics used to compare the new charged-particle channels."},{"cited_title":"Kozlowski, W","cited_arxiv_id":null,"evidence_quote":"Supplies the phenomenological Singer excitation function used in the transport-model predictions of branching ratios."},{"cited_title":"Matsubara and A","cited_arxiv_id":null,"evidence_quote":"Provides the shell-model code used to compute the Gamow-Teller strengths that explain the even-odd 0n0p effect."},{"cited_title":"Winsberg, Phys","cited_arxiv_id":null,"evidence_quote":"The particle-transport simulation code used both for background estimates and for the theoretical branching-ratio comparison."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the earlier neutron-multiplicity data used to estimate average multiplicities and check the simulation's evaporation component."}],"review_version":1}