{"id":"72836ad8-11bf-4fca-8ac9-423049964cb4","arxiv_id":"2412.19890","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"A future nearby supernova could reveal sub-MeV axion-like particles through a delayed ~30 MeV photon signal in Super-Kamiokande and Hyper-Kamiokande, probing couplings down to about 3 x 10^-6.","lead":"This paper predicts that axion-like particles from a nearby supernova would scatter off protons in water Cherenkov detectors, producing a delayed pulse of ~30 MeV photons. If observed, this signal would probe ALP-proton couplings and help separate proton and neutron couplings using the existing oxygen de-excitation signal.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The claimed low-mass reach is unsupported: for m_a ≲ 10^-3 MeV the ALP package arrives during the SN neutrino burst, so the quiescent background rate used in Eqs. (9)-(10) is invalid.","rationale":"The reader's weakest-assumption concern is the external cross-section in Eq. (4), which is indeed load-bearing. However, the most concrete and internally testable problem is that the low-mass end of the claimed parameter region is not a delayed signal at all: for m_a below roughly 10^-3 MeV, the ALP package arrives during the SN neutrino burst, and the quiescent background rate used in the significance calculation is inapplicable. This does not invalidate the core mechanism or the higher-mass projections (m_a ≳ few × 10^-3 MeV), but it does mean the headline range '10^-4 MeV to 1 MeV' is not supported without either removing the overlapping mass window or redoing the background estimate with burst events. The paper should therefore remain conditional, with the stated mass range revised or the analysis extended. No code or data are provided, so the numerical impact cannot be checked from the manuscript, but the proposed concrete test is a straightforward re-analysis using existing SN neutrino burst predictions and SK response.","tokens_in":11205,"tokens_out":13261,"duration_ms":140383,"concrete_test":"Recompute the sensitivity for m_a = 10^-4 to 10^-3 MeV at d_SN = 1 kpc including the expected SN neutrino burst as background: fold a standard IBD event spectrum (e.g., ~7000 events over 10 s in SK) into E_rec = [16,78] MeV and evaluate Z(Δta) from Eq. (9) over the ALP arrival window, or alternatively impose a conservative cut t0_a > 10 s. If the lower mass boundary moves upward from 10^-4 MeV by more than an order of magnitude, the claimed low-mass reach is not robust.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The sensitivity plot in Fig. 4 extends down to m_a = 10^-4 MeV. Using Eq. (7) with the highest ALP energy E_high = 78 MeV, the first ALP arrives at t0_a = 2.01e6 s (d_SN/1 kpc)(m_a/0.1 MeV)^2 (16/78)^2. For m_a = 10^-4 MeV and d_SN = 1 kpc this gives t0_a ≈ 0.08 s, and Eq. (11) gives a package duration Δta ≈ 1.9 s: the entire signal overlaps the ~10 s SN neutrino burst. For m_a = 10^-3 MeV the first ALP arrives at ~8 s and the ~190 s window still begins inside the burst. The background rate ar n_bkg = 9.38×10^-7 s^-1 used in Eqs. (9)-(10) is the quiescent DSNB-search background (Ref. [30]); it excludes the thousands of inverse-beta-decay events that SK will record in the same E_rec = [16,78] MeV window during a galactic SN burst. Thus the condition max[2, 2 sqrt(ar n_bkg Δta)] is not a valid detection criterion for m_a ≲ 10^-3 MeV, and the low-mass portion of the claimed probed region (10^-4 to ~10^-3 MeV) is not supported by the analysis as written.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a new search channel for sub-MeV axion-like particles (ALPs) from a future galactic core-collapse supernova in water Cherenkov detectors, via the process a p -> p gamma on free protons. It computes the ALP flux from an 18 solar-mass proto-neutron-star model at 1 s post-bounce, convolves it with the cross section imported from the authors' companion paper, and estimates photon event rates in Super-Kamiokande and Hyper-Kamiokande in a reconstructed-energy window E_rec = [16, 78] MeV. The paper claims that a SN within about 100 kpc would allow probing ALP masses between 10^-4 MeV and 1 MeV and ALP-proton couplings between 3 x 10^-6 and 4 x 10^-5, and that combining this proton-only signal with the oxygen de-excitation signal around 7 MeV would disentangle the ALP-proton and ALP-neutron couplings.","tokens_in":11528,"tokens_out":8944,"duration_ms":90115,"significance":"If the calculation holds, this is a valuable phenomenological forecast: the delayed, roughly 30 MeV photon signal sits in a low-background window, and its exclusive dependence on the proton coupling is a genuinely useful complement to the oxygen channel. The paper is explicit about its modeling choices and identifies a concrete region of parameter space between existing bounds, so it gives falsifiable predictions for SK and HK observing strategies. The main quantitative claims are not yet established, however, because of the validity of the background assumption at low masses, the unvalidated central amplitude, and the only qualitative treatment of the coupling-disentangling procedure.","major_comments":[{"comment":"Section III.1, Eqs. (7)–(11) and Fig. 4: the claimed probed region extends down to m_a = 10^-4 MeV, but the detection criterion in Eq. (10) is not valid there. Using Eq. (7) with E_high = 78 MeV and d_SN = 1 kpc gives a first-ALP arrival time t0_a ≈ 0.08 s for m_a = 10^-4 MeV, and Eq. (11) gives Δt_a ≈ 1.9 s; for m_a = 10^-3 MeV the corresponding numbers are ≈ 8 s and ≈ 190 s. Both windows overlap with the ~10–20 s burst of SN neutrinos that a galactic supernova will produce, during which Super-Kamiokande will record a large number of inverse-beta-decay and other events in E_rec ∈ [16, 78] MeV. The background rate nbar_bkg = 9.38 × 10^-7 s^-1 is the quiescent DSNB-search rate from Ref. [30] and does not include burst events. Hence Eq. (10) is not a valid 95% C.L. condition for m_a ≲ 10^-3 MeV, and the low-mass portion of the SK/HK regions in Fig. 4 is unsupported; the same caveat applies to the oxygen-channel curves in Fig. 6 via Eq. (12). The analysis should either restrict to masses whose entire arrival window starts after the neutrino burst or include a time-dependent burst-background model.","section":"§III.1, Eqs. (7)–(11), Fig. 4"},{"comment":"Section III, Eq. (4): the whole event-rate calculation is proportional to the squared amplitude for a p → p γ that is imported from the authors' companion paper [29]. The manuscript does not re-derive this amplitude, provide an independent numerical check, or estimate the effect of nuclear binding on protons in water at the energies of interest (E_a ≈ 10–100 MeV). Since any multiplicative error in Eq. (4) directly rescales all predictions in Eqs. (6)–(10) and all contours in Figs. 4 and 6, the derivation or a cross-check of the amplitude should be included, and the free-proton approximation should be justified.","section":"§III, Eq. (4)"},{"comment":"Section III.2, Fig. 6: the claimed ability to disentangle g_ap and g_an is only sketched qualitatively. The oxygen-channel sensitivity is obtained by extrapolating Ref. [28] to a fixed ALP energy window [E_low, E_high] = [9.55, 28] MeV, but the paper does not report the oxygen event-rate formula or the systematic uncertainties in the nuclear matrix elements, and Fig. 5 is schematic. To turn the claim into a demonstrated capability, the authors should define a two-coupling likelihood and show, for a representative SN distance and ALP parameters, the expected joint confidence regions or the precision with which each coupling could be extracted.","section":"§III.2, Fig. 6"}],"minor_comments":[{"comment":"Eq. (7): the algebraic expression t_a ≃ d_SN m_a^2/(2 E_a) is dimensionally inconsistent; the correct relation is t_a ≃ d_SN m_a^2/(2 E_a^2), which is what the numerical prefactor implements. Please fix the equation.","section":"Eq. (7)"},{"comment":"Section III.1: the sentence 'we consider E_low_a = 16 MeV and E_low_a = 78 MeV' should read E_low_a = 16 MeV and E_high_a = 78 MeV.","section":"§III.1"},{"comment":"Section III.1, Eq. (10): the criterion Nγ ≥ max[2, 2√(nbar_bkg Δt_a)] is a Gaussian approximation for a Poisson process with small counts; the 95% C.L. boundary should be checked with a Poisson or profile-likelihood treatment wherever Nγ is of order 2.","section":"§III.1, Eq. (10)"},{"comment":"Fig. 5: because the ordinate is in arbitrary units, the claimed relative size of the oxygen and a p → p γ peaks cannot be verified; a quantitative version with a common scale, or with the ratio stated in the caption, would be more informative.","section":"Fig. 5"},{"comment":"Section II: the forecast is based entirely on a single 18 M_sun profile evaluated at 1 s post-bounce and integrated over 0.5–2 s. A brief statement of how the sensitivity contours would change with a different progenitor mass or a later emission time would materially strengthen the robustness of the claimed reach.","section":"§II"}],"recommendation":"major_revision","confidential_remarks":"The main risk beyond the public report is that Ref. [29], which supplies the central amplitude, is a companion paper by the same authors and is not yet published in peer-reviewed form; the present manuscript is therefore not self-contained on the key physics input. I would encourage the editor to require the derivation or an independent check in the revision. The paper is otherwise within the scope of the journal and the forecast is presented in a falsifiable way."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read the Alonso-González et al. paper on delayed ALP signals in SK from a future supernova. The core idea is genuinely new: for sub-MeV ALPs that couple to protons, the a p → p γ scattering produces photons at ~30 MeV that arrive delayed relative to the neutrino burst, and because this channel is proton-only, combining it with the oxygen de-excitation signal could disentangle C_ap and C_an. That is a clever extension of their earlier diffuse-flux work, and the paper is clear about its inputs.\n\nThe forecast is plausible for the middle of the mass range. The flux machinery, the SN profile, and the detection formalism are standard, and the paper is honest that it is inheriting the cross section from the companion paper and the background from an SK DSNB search. The single-SN time-delay analysis is a real extension, and the idea that the two channels can be separated by energy and timing is attractive.\n\nThe main problem is the low-mass part of the claimed reach. The background rate n_bkg = 9.38×10^-7 s^-1 comes from the DSNB search and describes quiescent SK running. For m_a ≲ 10^-3 MeV, the ALP package arrives during the ~10 s neutrino burst. At m_a = 10^-4 MeV, the first ALP arrives at ~0.08 s and the package lasts ~1.9 s; at 10^-3 MeV it starts at ~8 s. In that window SK is recording thousands of inverse beta decay events in the same 16–78 MeV reconstructed-energy range. The significance condition Z = N_γ/sqrt(n_bkg Δt_a) is therefore not valid there, and the sensitivity curve extending to 10^-4 MeV is unsupported. The abstract's mass range should be revised or the analysis must include the burst background.\n\nSecondary issues: the squared amplitude in Eq. (4) is load-bearing and not re-derived here, and the oxygen-channel estimate in Sec. III.2 is a sketch based on an extrapolation from Ref. [28]. Both are worth checking, but they are not fatal.\n\nThis is a worthwhile paper for the SN axion community. It deserves serious peer review, with the expectation of a major revision on the background treatment. If the low-mass claim gets fixed, it becomes a solid contribution. I would bring it to a reading group.","headline":"A promising delayed ALP signal in SK with a real low-mass problem: the claimed reach below ~10^-3 MeV is built on a quiescent background that does not apply during the SN neutrino burst.","tokens_in":12083,"tokens_out":3038,"would_cite":true,"duration_ms":28913,"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":"Axion-like particles from a future galactic supernova would scatter off protons in water and produce a delayed, easily identifiable photon signal near 30 MeV in Super-Kamiokande.","keywords":["axion-like particles","supernova","Super-Kamiokande","proton scattering","water Cherenkov detector","delayed signal","ALP-nucleon couplings","oxygen de-excitation"],"falsifier":"A first-principles or measured value of the $a\\,p\\to p\\,\\gamma$ cross section at ALP energies of tens of MeV that differs substantially from Eq. (4) — for example because nuclear binding in oxygen or proton form factors change the amplitude — would shift the predicted photon rates and the probed coupling region by the same factor; a concrete check is to recompute the cross section including the proton's electromagnetic form factors and in-medium corrections for protons in water, and then compare the resulting event spectrum in the 16--78 MeV window with the paper's predictions.","tokens_in":1853,"feed_emoji":"💥","tokens_out":7347,"duration_ms":99520,"temperature":0.7,"pith_summary":"The paper shows that axion-like particles (ALPs) produced in a nearby core-collapse supernova and coupled to protons would scatter off free protons in water Cherenkov detectors through $a\\,p\\to p\\,\\gamma$, producing photons peaking near 30 MeV. Because ALPs are massive, their arrival at Earth is delayed and spread out relative to the supernova neutrino burst, giving a distinctive time window for a low-background search. Applying this to a future galactic supernova, the authors find Super-Kamiokande and Hyper-Kamiokande could probe ALP masses between $10^{-4}$ MeV and 1 MeV and ALP-proton couplings between $3\\times 10^{-6}$ and $4\\times 10^{-5}$ for source distances up to about 100 kpc. They argue that combining this proton-only channel with the ~7 MeV oxygen de-excitation signal would disentangle ALP-proton from ALP-neutron couplings.","feed_headline":"Supernova axions would fire a delayed 30 MeV photon burst","feed_subtitle":"A proton-scattering signal in Super-Kamiokande could probe axion masses below 1 MeV and separate proton from neutron couplings.","key_machinery":"The load-bearing object is the scattering process $a\\,p\\to p\\,\\gamma$ and its differential cross section, Eq. (4), which converts an incoming ALP into a photon of energy $E_\\gamma\\sim E_a$ through proton intermediate states. The kinematics of this two-body process fix the photon energy for a given ALP energy, and the ALP's nonzero mass makes its time of flight energy-dependent, so the whole ALP package arrives in a delayed window $\\Delta t_a$ proportional to $(d_{\\rm SN}/1\\,{\\rm kpc})(m_a/0.1\\,{\\rm MeV})^2$. That time window, combined with the low background in the reconstructed-energy region 16--78 MeV, sets the sensitivity through the significance condition $N_\\gamma \\ge \\max\\left(2,\\,2\\sqrt{\\bar{n}_{\\rm bkg}\\Delta t_a}\\right)$. The second object is the oxygen de-excitation channel at ~7 MeV, which depends on both proton and neutron couplings and provides the complementary handle for disentangling the two couplings.","core_discovery":"The central claim is that a future galactic supernova would act as an ALP factory whose ALPs, after a delayed flight, would be detected through $a\\,p\\to p\\,\\gamma$ scattering on free protons in water, with a photon spectrum peaking around 30 MeV where the Super-Kamiokande background is very small. The authors compute the ALP flux from an 18 solar-mass progenitor using production via nucleon-nucleon bremsstrahlung and pion-ALP conversion, including trapping and absorption in the proto-neutron star, and convolve it with the proton scattering cross section. They find that a supernova at 1 kpc would let Super-Kamiokande and Hyper-Kamiokande probe the previously unexcluded region between SN 1987A cooling and event bounds, SNO solar-axion limits, and the diffuse galactic ALP flux; at 10 kpc the probed region shrinks but remains substantial, and the most distant observable supernova would be around 100 kpc. They also show the SN 1987A signal in this channel would have been too weak to constrain.","pith_inferences":["Because the delay scales as $m_a^2$ and can reach years for the lightest masses at kiloparsec distances, the observable mass range depends on the experiment's lifetime; a detector that starts taking data only after the neutrino burst would still capture most of the ALP package, while one decommissioned before the package arrives would miss it entirely.","The same time-of-flight delay could confirm an ALP interpretation independently of the overall rate: the photon arrival times should track $E_a^{-2}$ with a single mass parameter, a correlation that is testable event by event.","If a future supernova produces both peaks, the ratio of the 30 MeV to 7 MeV photon rates would directly measure $g_{ap}^2$ relative to the combination of $g_{ap}$ and $g_{an}$ entering oxygen excitation, turning a single event into a coupling-ratio measurement."],"forward_implications":["A galactic supernova at 1 kpc would allow Super-Kamiokande to probe ALP masses from $10^{-4}$ MeV to 1 MeV and couplings from $3\\times 10^{-6}$ to $4\\times 10^{-5}$, filling the gap between existing SN 1987A, SNO, and diffuse-flux bounds.","Hyper-Kamiokande, with roughly eight times the fiducial mass, extends the reach toward smaller couplings.","The $a\\,p\\to p\\,\\gamma$ channel is purely proton-coupled, so a detected ~30 MeV peak would establish that ALPs couple to protons.","Combining the ~30 MeV peak with the ~7 MeV oxygen de-excitation peak, whose rate depends on both couplings, would let the two couplings be extracted separately.","The SN 1987A data would not have seen this signal: even in the most favorable case the significance would have been only $Z\\simeq 0.5$."],"supporting_citations":[{"why":"Supplies the $a\\,p\\to p\\,\\gamma$ amplitude in Eq. (4) and the diffuse-flux method that this work adapts to a single supernova.","marker":"[29]"},{"why":"Provides the proto-neutron star temperature, density, and composition profiles used to compute the ALP flux.","marker":"[17]"},{"why":"Gives the oxygen de-excitation cross section and event estimates used for the ~7 MeV channel and the comparison in Fig. 6.","marker":"[28]"},{"why":"Provides the SN 1987A cooling and event bounds that define the parameter region this work aims to probe.","marker":"[20]"},{"why":"Defines the optimized 16--78 MeV reconstructed-energy window and the background rate $\\bar{n}_{\\rm bkg}$ used in the significance estimate.","marker":"[30]"},{"why":"Supplies the low-energy solar-neutrino analysis and its background rate used for the oxygen channel.","marker":"[46]"},{"why":"Specifies the Hyper-Kamiokande detector mass and characteristics used to rescale the Super-Kamiokande sensitivity.","marker":"[31]"},{"why":"Introduces the oxygen excitation channel from SN 1987A axions that the disentangling strategy builds on.","marker":"[32]"}],"fun_headline_variants":["Supernova axions may ignite 30 MeV flashes in Super-K","30 MeV photon bursts could reveal supernova axion-proton couplings","Delayed 30 MeV photons from supernova axions could split proton vs neutron couplings","Super-K could catch 30 MeV axion sparks from a nearby supernova","Future supernova could expose axion-proton coupling via 30 MeV delayed photons"],"cache_read_input_tokens":14208,"weakest_assumption_plain":"The prediction assumes that the $a\\,p\\to p\\,\\gamma$ squared amplitude taken from the companion paper correctly describes ALP scattering on free protons in water at these energies; the paper does not re-derive that amplitude, and every event-rate and sensitivity result scales with it.","fun_headline_variants_meta":{"raw":{"variants":["Supernova axions may ignite 30 MeV flashes in Super-K","30 MeV photon bursts could reveal supernova axion-proton couplings","Delayed 30 MeV photons from supernova axions could split proton vs neutron couplings","Super-K could catch 30 MeV axion sparks from a nearby supernova","Future supernova could expose axion-proton coupling via 30 MeV delayed photons"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000976,"raw_usage":{"total_tokens":4170,"prompt_tokens":995,"completion_tokens":3175,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":611,"completion_tokens_details":{"reasoning_tokens":3075}},"tokens_in":611,"tokens_out":3175,"duration_ms":23118,"temperature":1.0,"reasoning_tokens":3075,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T23:49:12.114064+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A first-principles or measured value of the $a\\,p\\to p\\,\\gamma$ cross section at ALP energies of tens of MeV that differs substantially from Eq. (4) — for example because nuclear binding in oxygen or proton form factors change the amplitude — would shift the predicted photon rates and the probed coupling region by the same factor; a concrete check is to recompute the cross section including the proton's electromagnetic form factors and in-medium corrections for protons in water, and then compare the resulting event spectrum in the 16--78 MeV window with the paper's predictions.","supporting_citations":[{"cited_title":"Probing a diffuse flux of axion-like particles from galactic supernovae with neutrino water Cherenkov detectors","cited_arxiv_id":"2412.09595","evidence_quote":"Supplies the $a\\,p\\to p\\,\\gamma$ amplitude in Eq. (4) and the diffuse-flux method that this work adapts to a single supernova."},{"cited_title":"Cross section for supernova axion observation in neutrino water Cherenkov detectors","cited_arxiv_id":"2306.17055","evidence_quote":"Gives the oxygen de-excitation cross section and event estimates used for the ~7 MeV channel and the comparison in Fig. 6."},{"cited_title":"Engel, D","cited_arxiv_id":null,"evidence_quote":"Introduces the oxygen excitation channel from SN 1987A axions that the disentangling strategy builds on."}],"review_version":1}