{"id":"bcca498a-2ed1-4092-a57b-8dd30d44cc02","arxiv_id":"2509.01538","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"RECODE, a reactor experiment with two germanium detectors, could probe axion-photon and axion-electron couplings into the cosmological triangle region at masses around 0.3 to 0.9 MeV.","lead":"The paper calculates the expected sensitivity of a proposed germanium-detector experiment at a 3.4 GW nuclear reactor to light axion-like particles produced in the reactor core. A 10 kg year run could compete with or beat existing beam-dump limits, and a 100 kg year upgrade could cover a previously unexplored 'cosmological triangle' region.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Inverse-Primakoff detection efficiency is ignored, making the projected gaγ limits optimistic; a realistic HPGe photopeak efficiency should be folded into Eq. (6).","rationale":"The reader's weakest-assumption identification is exactly right: Eq. (6) has no detection-efficiency factor for inverse-Primakoff photons or decay products. This is the most load-bearing gap because it directly connects the computed event rate to the advertised sensitivity. Other possible concerns—e.g., the reactor photon flux parameterization in Eq. (5), the use of Z² for uranium, or the omission of systematic uncertainties—are either absorbed into the same sensitivity shift or are standard approximations in this literature. The detector-efficiency effect is concrete, numerically estimable, and easy to test. I agree with the reader's CONDITIONAL verdict: the calculation is otherwise coherent and the projected limits are plausible, but the stated 'full coverage' claim should not be taken at face value until an efficiency correction is applied. My recommendation is UNCHANGED because the reader's verdict already captures this concern appropriately.","tokens_in":12572,"tokens_out":36996,"duration_ms":422410,"concrete_test":"Recompute the 90% C.L. gaγ exclusion contours of Fig. 4 after folding in a realistic detector response. Concretely: (1) use Geant4 (or a published efficiency curve) for a 5 kg HPGe crystal with an internal monoenergetic photon source at energies 0.3–1 MeV to obtain the full-energy-peak efficiency ε(Eγ), and (2) replace the Gaussian-only smearing in Eq. (9) with a full energy-deposit spectrum (photopeak plus Compton continuum). If the resulting gaγ limit at m_a = 0.5 MeV worsens by less than a factor of 1.5, the claim of full cosmological-triangle coverage is preserved; if it worsens by more than that, the claim should be revised to 'partial coverage'.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that RECODE will 'fully cover the cosmological triangle' rests on Eq. (6), which counts every inverse-Primakoff interaction in the germanium detector as a registered signal event. No detection efficiency appears anywhere in the event-rate formula or in the χ² analysis. For the relevant ALP masses (0.3–1 MeV), the dominant detection channel is inverse Primakoff followed by a photon of energy Eγ′ ≈ Ea. In a 5 kg HPGe crystal, the full-energy-peak efficiency for an internal 0.3–1 MeV photon is typically 20–50%, with the rest depositing only part of the energy via Compton scattering before escape. The diphoton-decay channel is negligible at these masses (Γ₂γ ∝ m_a³ is tiny for m_a ≲ 1 MeV), so this efficiency directly gates the sensitivity. Because the signal rate scales as gaγ⁴, a detection efficiency ε shifts the 90% C.L. exclusion limit as ε^{-1/4}: ε = 0.2 raises the limit by ~1.5, ε = 0.1 by ~1.8. The 100 kg·yr curve in Fig. 4 may be only moderately below the cosmological-triangle boundary, so this factor could determine whether the 'full coverage' claim survives. The paper acknowledges that 'systematics depends on realistic detector performances' but does not include any efficiency correction in the projected limits.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a search for sub-MeV axion-like particles at RECODE, a reactor-neutrino experiment with two 5 kg high-purity germanium detectors at 11 m and 22 m from a 3.4 GW reactor. ALPs are assumed to be produced via Primakoff and Compton-like processes in the reactor; detection proceeds through inverse Primakoff scattering, inverse Compton scattering, and two-body decays in the germanium detector. The authors compute event rates and use a binned chi-squared analysis to project 90% C.L. exclusion limits on the axion-photon coupling g_aγ and axion-electron coupling g_ae. They claim that a 10 kg·yr exposure is competitive with or surpasses beam-dump bounds and that a 100 kg·yr upgrade would fully cover the 'cosmological triangle' in the (m_a, g_aγ) plane.","tokens_in":12894,"tokens_out":3656,"duration_ms":43901,"significance":"If the sensitivity projections are reliable, the paper identifies a genuinely promising experimental path: reactor-based searches could probe ALP parameter space that is currently covered only by beam-dump limits and, at higher exposure, the cosmological triangle. The production cross sections and reactor photon flux are standard and properly cited; the analysis is transparent and includes a comparison with existing constraints (beam dump, NEON, stellar, SN1987A). The paper is also candid about the model dependence of the electron-loop-induced decay in the appendix. The main limitation is that the detector response is idealized, which directly affects the quantitative claims.","major_comments":[{"comment":"The event rate in Eq. (6) counts every inverse-Primakoff interaction as a registered event; no detection efficiency factor appears. For the relevant ALP masses (0.3–1 MeV), the signal is a monoenergetic photon with E≈E_a, and the full-energy-peak efficiency of an HPGe detector for internal photons in this energy range is typically 20–50%. Since the sensitivity scales as g_{aγ}^4, an efficiency ε=0.2 shifts the 90% C.L. limit by ε^{-1/4}≈1.5 (and ε=0.1 by ≈1.8). The 100 kg·yr curve in Fig. 4 is described as only 'almost' excluding the cosmological triangle, so this factor can determine whether the headline 'full coverage' claim survives. The authors should fold an energy-dependent detection efficiency into Eq. (6) or explicitly justify that all deposited energy is used as signal.","section":"Eq. (6) and 'Experimental sensitivity'"},{"comment":"The sensitivity estimate includes only statistical uncertainty; the text concedes that 'systematics depends on realistic detector performances' but does not implement any systematic term. The background rate B=2 cpkkd is taken as an input assumption, and the planned near–far joint analysis is described only as an anticipated improvement. Given that the central claim is to surpass beam-dump bounds and exclude the cosmological triangle, the projected contours should include representative systematic uncertainties on the background rate and signal shape, or state explicitly the conditions under which they are negligible.","section":"Experimental sensitivity, chi-squared paragraph"},{"comment":"The abstract states that the upgrade 'will fully cover the so-called cosmological triangle region,' while the introduction defines the triangle as 0.3 MeV ≲ m_a ≲ 0.9 MeV; the abstract later refers to '0.3 to 1 MeV.' These numbers should be harmonized, and the projection should be presented as 'almost exclude' (as in the body text) rather than 'fully cover' unless the efficiency issue is resolved.","section":"Abstract and introduction"}],"minor_comments":[{"comment":"Typo: 'suprressed' should be 'suppressed.'","section":"Experimental sensitivity"},{"comment":"The loop function f_p in Eq. (11) is defined, but the subscript notation is opaque; a brief definition in words would improve readability.","section":"Equation (10)"},{"comment":"In the supplied text the figure caption contains corrupted glyphs and undefined symbols; the final version should have a clean caption describing the production and detection processes.","section":"Figure 1"},{"comment":"The reference to the XCOM database [81] should include a URL or version identifier, since the cross-section input is central to the flux calculation.","section":"References"},{"comment":"The sentence 'Due to its high thermal power reactor source and short baseline configuration' is awkward; consider rephrasing.","section":"General"}],"recommendation":"major_revision","confidential_remarks":"The central physics calculation appears sound and the paper is not circular: the projected limits are computed independently of the comparison constraints. The load-bearing issue is the idealized detector response (missing efficiency and systematic uncertainties), which directly affects the claimed coverage of the cosmological triangle. A revision that adds an energy-dependent efficiency and systematic error treatment would make the quantitative claims credible. The paper is otherwise within the scope of a hep-ph journal."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nWhat you should know about this paper: it is a clean, standard sensitivity projection for axions at the RECODE reactor experiment, and the new piece is the specific curves for the Sanmen near-far detector configuration. The production and detection formalism is not new, but the application is useful and the paper is well organized. The main issue is that the event rate in Eq. (6) ignores detection efficiency, which affects the key claim of covering the cosmological triangle.\n\nThe paper does several things well. The flux model and cross sections are standard and properly cited. The comparison of event-rate channels (inverse Primakoff vs. decay) is informative, and the authors honestly note that systematics depend on detector performance. The sensitivity curves are clearly presented, and the appendix treats the loop-induced decay in some detail.\n\nThe soft spot is the missing efficiency. The stress-test note is correct: for a monoenergetic photon from inverse Primakoff in a HPGe detector, the full-energy efficiency is typically 20--50%. The signal rate scales as gaγ^4, so a 20% efficiency worsens the limit by about a factor of 1.5. That could turn the \"fully cover\" claim into \"partially cover.\" This is not a cosmetic issue; it directly affects the advertised sensitivity. The authors should include a detection efficiency factor, even a simple energy-dependent placeholder, and show how the triangle coverage changes.\n\nThe other caveats (statistical-only chi-squared, background as an input) are typical for this genre, but they add to an optimistic overall picture. None of these are fatal to the paper's purpose.\n\nFor a sensitivity paper aimed at a real experiment, this one is solid enough to warrant serious peer review. I would send it out with a request for an efficiency treatment. I would bring it to a reading group if the discussion is about projection methodology.\n\nRecommendation: send to review, expect a revision.","headline":"A solid, standard sensitivity forecast for axions at a reactor experiment, but the missing detection efficiency could soften the key cosmological-triangle coverage claim; worth refereeing with a request for revision.","tokens_in":13389,"tokens_out":3059,"would_cite":true,"duration_ms":35319,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["14.80.Va","29.40.Wk"],"model":"deepseek-v4-flash","headline":"RECODE's two germanium detectors at a reactor would set world-leading limits on MeV-scale axions, and a 100 kg·year upgrade would fully close the cosmological triangle.","keywords":["axion-like particles","sub-MeV axions","Primakoff effect","ALP-photon coupling","ALP-electron coupling","reactor neutrino experiment","germanium detectors","cosmological triangle"],"falsifier":"Expose the RECODE germanium detectors to calibrated monoenergetic gamma sources in the 0.3–1 MeV range and measure the fraction of full-energy events. If that efficiency is substantially below 100%, recompute Eq. (6) with it: event rates fall proportionally, and the 100 kg·year coverage of the cosmological triangle may not survive.","tokens_in":12460,"feed_emoji":"⚛️","tokens_out":11013,"duration_ms":116472,"temperature":0.7,"pith_summary":"This paper proposes to use the RECODE reactor experiment—two low-threshold germanium detectors sitting 11 m and 22 m from a 3.4 GW reactor core—to search for axion-like particles (ALPs) in the sub-MeV to few-MeV mass range. The reactor's intense photon flux would produce ALPs through Primakoff and Compton-like scattering, and the detectors would catch them via inverse Primakoff scattering, diphoton decay, or dielectron decay. With a 10 kg·year exposure the authors project sensitivities to ALP-photon and ALP-electron couplings that match or beat existing beam-dump experiments, and with a 100 kg·year upgrade the projected reach fully covers the so-called cosmological triangle, a band of axion parameter space left unexplored by astrophysical and terrestrial searches. If the projections are right, a planned neutrino-coherent-scattering experiment becomes a leading terrestrial probe of light axions.","feed_headline":"Reactor detectors could sweep axions' last unprobed window","feed_subtitle":"Two germanium crystals near a 3.4 GW reactor would beat beam-dump limits; a tenfold upgrade closes the cosmological triangle.","key_machinery":"The governing mechanism is the reactor-to-detector ALP beam: reactor photons scatter off nuclei (Primakoff) and electrons (Compton-like) to make ALPs, which then inverse-Primakoff scatter in the germanium to yield a monoenergetic photon, or decay in flight into two photons or an electron-positron pair. Eq. (6) multiplies this flux by the detector's nuclear surface density, the inverse cross section, the in-detector decay probability, and the exposure time. The 11 m/22 m near-far layout suppresses correlated backgrounds, and the high ratio P/L² = 0.028 GW/m² gives RECODE its flux edge over other reactor neutrino experiments. The cosmological triangle is the unprobed gap in the ALP-photon coup","core_discovery":"The paper claims that a reactor-based experiment can fill the MeV gap in axion searches. RECODE's two 5 kg germanium detectors, 11 m and 22 m from a 3.4 GW reactor, catch ALPs produced by Primakoff and Compton-like scattering via inverse Primakoff, diphoton, or dielectron signals. The event-rate calculation combines reactor photon flux, production cross section, survival probability, and detector signal probability, then smears energies by the detector resolution against a 2 cpkkd background. Projected 90% C.L. contours at 10 kg·year match or beat beam-dump bounds; at 100 kg·year the contour fully covers the cosmological triangle, the unprobed 0.3–0.9 MeV window with gaγ between 1.3×10⁻⁵ and","pith_inferences":["The paper's event-rate formula assumes every ALP-induced interaction is counted; a realistic full-energy detection efficiency, often tens of percent for germanium at these energies, would lower the event rates and likely shrink the claimed coverage of the cosmological triangle.","The reactor photon spectrum is taken from a 1984 exponential approximation said to hold above 0.2 MeV; a measured MeV-range spectrum at the reactor core could shift the ALP production rate and the resulting limits in either direction.","The same near-far germanium configuration could be applied to other weakly-coupled light particles with monoenergetic or two-body final states, such as dark photons or light scalars, by reusing the event-rate machinery with a different production mechanism.","The claimed full coverage of the triangle is a projection based on design background rates; measured backgrounds at the near position inside containment could be higher, which would weaken the upgrade's reach."],"forward_implications":["With 10 kg·year of exposure, RECODE would set the best reactor-based limits on both gaγ and gae in the 0.3–1 MeV window, reaching beyond current beam-dump sensitivities.","At 100 kg·year, a null result would exclude ALP-photon couplings across the entire cosmological triangle, clearing a parameter region that astrophysical and terrestrial constraints have left open.","The same exposure would simultaneously constrain ALP-electron couplings, with projected sensitivity that surpasses existing beam-dump bounds and improves further if the electron-loop diphoton decay is unsuppressed.","Because the near-far geometry suppresses correlated backgrounds, the projected limits scale almost linearly with exposure, so the upgrade path is a straightforward continuation of the first run."],"supporting_citations":[{"why":"Supplies the RECODE detector masses, baselines, and operating parameters that the sensitivity projections use.","marker":"[74]"},{"why":"Provides the analytical reactor photon flux, Eq. (5), that seeds all ALP production rates.","marker":"[82]"},{"why":"Supplies the Standard Model photon cross-section database used to normalize the total reactor cross section.","marker":"[81]"},{"why":"Provides the Primakoff production cross-section and kinematic boundaries used to compute the ALP flux.","marker":"[68]"},{"why":"Gives the previous reactor ALP limits that the projected RECODE sensitivity is directly compared with.","marker":"[67]"},{"why":"Defines the cosmological triangle in axion-photon parameter space that the upgrade is designed to cover.","marker":"[57]"},{"why":"Updates the same cosmological-triangle region and its astrophysical and terrestrial boundaries.","marker":"[58]"},{"why":"Provides representative beam-dump constraints that the projected 10 kg·year limits must beat.","marker":"[47]"},{"why":"Gives the germanium detector energy resolution used to smear the final-state energies in the analysis.","marker":"[83]"}],"fun_headline_variants":["Reactor detectors could close axion search gap","Germanium pair at reactor may beat beam-dump limits","Nuclear plant experiment targets axion blind spot","Reactor-based hunt aims to fill axion's missing window","Two crystals near reactor could unveil axion secrets"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The projections assume every ALP-triggered interaction inside the germanium crystal is counted; the event-rate formula has no detection-efficiency factor, so any real-world loss in reconstructing the photon or electron-positron final state reduces the reach and can weaken the claimed coverage of the cosmological triangle.","fun_headline_variants_meta":{"raw":{"variants":["Reactor detectors could close axion search gap","Germanium pair at reactor may beat beam-dump limits","Nuclear plant experiment targets axion blind spot","Reactor-based hunt aims to fill axion's missing window","Two crystals near reactor could unveil axion secrets"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000201,"raw_usage":{"total_tokens":1221,"prompt_tokens":756,"completion_tokens":465,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":500,"completion_tokens_details":{"reasoning_tokens":389}},"tokens_in":500,"tokens_out":465,"duration_ms":5591,"temperature":1.0,"reasoning_tokens":389,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T12:26:56.257586+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Expose the RECODE germanium detectors to calibrated monoenergetic gamma sources in the 0.3–1 MeV range and measure the fraction of full-energy events. If that efficiency is substantially below 100%, recompute Eq. (6) with it: event rates fall proportionally, and the 100 kg·year coverage of the cosmological triangle may not survive.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the RECODE detector masses, baselines, and operating parameters that the sensitivity projections use."},{"cited_title":"Bechteler, H","cited_arxiv_id":null,"evidence_quote":"Provides the analytical reactor photon flux, Eq. (5), that seeds all ALP production rates."},{"cited_title":"XCOM: Photon Cross Sections Database, 2010,","cited_arxiv_id":null,"evidence_quote":"Supplies the Standard Model photon cross-section database used to normalize the total reactor cross section."},{"cited_title":"A Search of Low-Mass WIMPs with p-type Point Contact Germanium Detector in the CDEX-1 Experiment","cited_arxiv_id":"1601.04581","evidence_quote":"Gives the germanium detector energy resolution used to smear the final-state energies in the analysis."}],"review_version":1}