{"id":"42aa2bbd-1797-47d8-a1a8-da8ba0515290","arxiv_id":"2412.15933","paper_version":1,"verdict":"UNVERDICTED","confidence":"HIGH","novelty_score":2.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"HIBEAM/NNBAR is a proposed ESS experiment that would search for baryon-number-violating neutron oscillations with sensitivity up to 1000 times better than previous searches.","lead":"This paper describes the HIBEAM/NNBAR program, a proposed two-stage experiment at the European Spallation Source that would search for neutrons converting into antineutrons or mirror neutrons. The program aims to improve sensitivity to these rare processes by up to three orders of magnitude compared with earlier searches.","discovery_kind":"review","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The projected three-order discovery reach rests on an unvalidated detector background rejection of 10^9; if real rejection is lower, the headline sensitivity loses an order of magnitude.","rationale":"The reader's UNVERDICTED verdict is appropriate: this is a proceedings-style program summary that makes no new measurement and adds no independently verifiable result beyond quoting the collaboration's design reports and published projections. My stress-test identifies a specific load-bearing vulnerability within that quoted projection: the 10^9 background rejection at 70% efficiency is the component most directly tied to the headline sensitivity and the least supported by demonstrated hardware or beam data. This is not an accusation of error; the paper itself cites the CDR and notes that prototypes will be tested. It is a caution that the quantitative claim should be interpreted as a goal rather than an established discovery potential. Since the concern is a programmatic risk already broadly captured by the reader's weakest_assumption, it does not change the verdict. The paper remains a useful overview, but the specific sensitivity numbers are not yet verified and should not be treated as measured or fully validated.","tokens_in":4154,"tokens_out":10594,"duration_ms":98062,"concrete_test":"Recompute the NNBAR sensitivity from the CDR framework with an independent background model that includes cosmic-ray muons and secondaries, beam-induced neutrons and gammas entering the detector, and the actual 14 Hz, 3 ms ESS proton-pulse structure; then apply the same 70% signal-efficiency selection and extract the 90% CL limit on the oscillation probability. If the surviving background exceeds the CDR value by more than a factor of 10, or if the required rejection cannot be reproduced, the projected sensitivity and the Summary's 'more than three orders of magnitude' statement should be revised downward.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that HIBEAM/NNBAR will 'surpass previous discovery potentials by more than three orders of magnitude' depends on a chain of projected quantities, and the least-supported link is the NNBAR detector performance quoted in Section 2.1: a background rejection of more than 10^9 while maintaining ~70% signal efficiency, as simulated in the NNBAR CDR [6]. In a search with almost zero expected signal, the 90% confidence limit on the oscillation probability scales roughly as sqrt(N_bg)/(epsilon * N_neutrons). A shortfall in rejection by a factor of 100, for example from 10^9 to 10^7, would worsen the limit by a factor of 10, directly consuming one of the 'three orders of magnitude' claimed in the Summary. The paper explicitly notes that detector prototypes have been constructed and will be tested [10], but no beam data yet demonstrate that the simulated rejection is achievable at the ESS. This is therefore not an internal contradiction, but a load-bearing assumption: the quantitative headline is a design-goal projection, not a demonstrated sensitivity, and the background-rejection number is the least independently verified component of that projection.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper is a brief program description for the proposed two-stage HIBEAM/NNBAR experiment at the European Spallation Source. It describes searches for free neutron–antineutron oscillation, neutron–mirror-neutron oscillation, and regeneration processes, and claims that the program will improve discovery potential by up to three orders of magnitude over previous experiments. The paper also claims sensitivity to ambient axions over a wide mass range. No new experimental data or derivations are presented; all quantitative sensitivity projections are taken from the collaboration's conceptual design reports and prior publications, particularly the NNBAR CDR [6] and the HIBEAM paper [7].","tokens_in":4353,"tokens_out":5958,"duration_ms":49177,"significance":"If the projected sensitivities are realized, the HIBEAM/NNBAR program would constitute the most sensitive searches for baryon-number violation in the neutron sector, probing new-physics scales beyond the reach of colliders. The paper is a useful programmatic overview that consolidates references to the CDR, detector simulations, and prototype efforts. A particular strength is that it explicitly cites the underlying publications for each sensitivity claim rather than presenting new unverified numbers. However, the central three-orders-of-magnitude discovery claim rests on simulated performance parameters, most importantly a background rejection of >10^9 in the NNBAR detector, which is not yet demonstrated with beam data; the paper itself notes that prototypes have been constructed but not yet tested. Because the quantitative claims are not derived in this paper, the manuscript's value is primarily as a status report and road map, and its persuasiveness depends on the credibility of the cited design studies.","major_comments":[{"comment":"The projected sensitivity is dominated by the assumed background rejection of more than 10^9 at ~70% signal efficiency, quoted from the NNBAR CDR [6]. This is a simulation result, and the paper states that detector prototypes 'have been constructed and will be tested' [10], so no beam data yet support the rejection power. In a search with negligible expected signal, the 90% confidence limit on the oscillation probability scales roughly as sqrt(N_bg)/epsilon; a rejection shortfall by a factor of 100 would worsen the limit by an order of magnitude, directly consuming one of the three orders of magnitude claimed in the Summary. The paper should either present a derivation of the sensitivity as a function of the background rejection and neutron exposure, or explicitly label the headline sensitivities as design-goal extrapolations and discuss the impact of realistic deviations from the simulated rejection. As written, the quantitative claim is oversold relative to the level of validation reported.","section":"Section 2.1"},{"comment":"The basis for the 'more than three orders of magnitude' claim is ambiguous and possibly inconsistent. Section 2 states that HIBEAM improves the discovery potential by up to a factor of 10 and that NNBAR increases it by more than a factor of 1000 [9], while Section 2.1 reports that the NNBAR neutron–antineutron sensitivity surpasses the free-neutron limit by 1.5 orders of magnitude and the bound-neutron limit by one order of magnitude. These numbers refer to different processes (n-nbar versus n-n') and different reference limits (Baldo-Ceolin 1994 versus SNS 2024). The paper should explicitly state, for each channel and each reference experiment, what the improvement factor is and how the 'three orders of magnitude' figure in the Abstract and Summary is obtained; the reader currently cannot verify the headline claim.","section":"Sections 2 and 2.1"},{"comment":"The projected sensitivities shown in Figures 3 and 4 are presented without uncertainty bands or any statement of the dominant systematic uncertainties (neutron flux, detection efficiency, background normalization, magnetic-field control). For a proposal paper this is acceptable if the figures are explicitly attributed to the cited CDR and the range of projected outcomes is indicated, but the current text gives no sense of how robust the central claim is to plausible variations in the assumed parameters. A brief quantitative statement of the leading systematics and their effect on the sensitivity would make the projection falsifiable and strengthen the paper.","section":"Section 2.1 and Section 2.2"}],"minor_comments":[{"comment":"The phrase '1,5 orders-of-magnitude' uses a comma as a decimal separator; for consistency with English use '1.5 orders of magnitude'.","section":"Section 2.1"},{"comment":"In the sentence beginning 'The resulting estimated sensitivity is shown in Figure 3...', the antecedent of 'This' is unclear; specify 'The NNBAR sensitivity surpasses...'.","section":"Section 2.1"},{"comment":"The expression 'ps = 2mn' is a typographical error; it should read 'sqrt(s) = 2 m_n' (the center-of-mass energy of the annihilating neutron–antineutron system).","section":"Section 2.1"},{"comment":"The statement that the axion sensitivity is 'comparable to the indirect supernova energy-loss limit' is vague; clarify whether the HIBEAM sensitivity is better or worse than that limit across the stated mass range and quantify the improvement over previous direct searches.","section":"Section 2.2"},{"comment":"The ESS parameters (2 GeV, 14 Hz, 3 ms pulses, 2/5 MW) are interesting but no reference is given in that sentence; citing the ESS technical design or Reference [5] would be helpful.","section":"Introduction"},{"comment":"The Summary states the program is 'poised to open a new discovery window', while the Abstract more cautiously says 'A goal of the program is to open a discovery window'. Align the wording to avoid overstating the readiness of the projected sensitivities.","section":"Summary and Outlook"}],"recommendation":"major_revision","confidential_remarks":"This is a concise program-description paper, likely intended for a proceedings or special-issue venue. The technical content is not deep, but that is consistent with the genre. The main concern is that the headline three-orders-of-magnitude claim depends on a simulated background rejection that is not yet demonstrated, and the text does not adequately qualify this dependence. The ambiguity about which processes and reference limits contribute to the claimed improvement also needs to be fixed. The manuscript is not fatally flawed, but it requires a revision to make the central claim properly supportable and unambiguous."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is a tidy proceedings-style summary of the HIBEAM/NNBAR program, not a research paper. It contains no new data or analysis, but it is an honest and readable overview that accurately points to the CDRs and prior theory papers for the actual sensitivities.\n\nWhat it does well: The paper lays out the physics case for baryon-number violation searches in the neutron sector and clearly explains the two-stage design, the four search modes (n→n̄, n→n′, n→n′→n, n→n′→n̄), and the separate axion search using Ramsey interferometry. It is transparent about the fact that all sensitivity numbers are borrowed from the NNBAR CDR [6], the HIBEAM program paper [7], and the axion PRL [13]. For an outsider, this is a decent five-page orientation to the program and its references.\n\nThe soft spots, in order: (1) The headline 'more than three orders of magnitude' improvement rests on unvalidated NNBAR detector performance, specifically the simulated background rejection of >10^9 at ~70% signal efficiency. If the real detector delivers only 10^7 rejection, the sensitivity degrades by a factor of 10, eating one of the claimed three orders of magnitude. The paper notes prototypes exist but does not flag this risk. (2) The projections are quoted without uncertainties or error bars. That is standard for design reports, but the paper should have said so. (3) There is genuinely nothing new here for an expert; it is a summary of already-published work.\n\nIs the central argument sound? As a description of a proposed experiment, yes. It is not internally contradictory, and it does not claim more than the cited simulations. The weakest link is the background rejection, but that is a known engineering challenge, not a flaw in the paper's logic.\n\nWho benefits: readers outside the collaboration who want a compact entry point; proceedings audiences. It deserves a serious referee only in the sense that the factual characterizations of the CDR figures should be checked, and the caveat about detector performance should be added. I would not cite it in my own work; I would cite the CDR or the PRL instead.\n\nRecommendation: Accept for a proceedings or a review-type venue, with a request to add one sentence acknowledging that the projected reach depends on the simulated background rejection being achieved. Not a significant research contribution.","headline":"A clean, honest program summary with no new results; the sensitivity claims hinge on an unvalidated detector background-rejection number.","tokens_in":4865,"tokens_out":2637,"would_cite":false,"duration_ms":25420,"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":"The HIBEAM/NNBAR program at the European Spallation Source is designed to search for neutron-antineutron and neutron-to-mirror-neutron oscillations with sensitivities up to three orders of magnitude beyond previous experiments, opening a…","keywords":["neutron oscillation","antineutron","mirror neutron","HIBEAM","NNBAR","baryon number violation","European Spallation Source","axion dark matter"],"falsifier":"A full-scale or prototype test of the NNBAR beamline environment would settle the central claim: if the residual magnetic field over a 200-meter evacuated path cannot be held below 10 nT, or if the background leakage through event selection exceeds $10^{-9}$ at the nominal signal efficiency, the claimed three-order-of-magnitude discovery window is not attainable. A null search at the projected NNBAR sensitivity would directly falsify the discovery claim for neutron-antineutron oscillation down to the corresponding conversion probability.","tokens_in":3943,"feed_emoji":"⚛️","tokens_out":7643,"duration_ms":63152,"temperature":0.7,"pith_summary":"The paper argues that baryon number violation, a necessary ingredient for the observed matter-antimatter asymmetry, has never been observed, and that the proposed HIBEAM/NNBAR program at the European Spallation Source is designed to find it. The program searches for neutrons converting into antineutrons or into sterile mirror neutrons, using intense cold neutron beams that the ESS is expected to provide. HIBEAM, a roughly fifty-meter beamline, would improve on the last searches by up to a factor of ten, while NNBAR, a two-hundred-meter beamline, would push discovery potential more than a thousand times beyond previous limits. If these projections hold, the program would open a new window onto baryon number violation and probe new-physics scales far above those reachable at colliders.","feed_headline":"Neutron-antineutron hunt aims to go 1000 times deeper","feed_subtitle":"Two-stage neutron experiment at the European Spallation Source targets baryon-number violation behind matter's survival.","key_machinery":"The central mechanism is quantum-mechanical mixing between the neutron and its antiparticle or a sterile mirror-neutron partner, which would appear as oscillation over time. The experiment maximizes oscillation time by sending slow cold neutrons, below about 1000 meters per second, down a long, evacuated, magnetically shielded flight path: up to 50 meters for HIBEAM and 200 meters for NNBAR. The magnetic field must stay below 10 nT because a larger field splits the neutron and antineutron energies and suppresses the oscillation. An arriving antineutron annihilates on a carbon target foil into a shower of pions with invariant mass near 1.9 GeV, detected by a surrounding annihilation detector, while mirror-neutron modes are detected through regeneration and neutron counting.","core_discovery":"The central claim is that the HIBEAM/NNBAR program offers a realistic path to the first laboratory observation of baryon number violation in the neutron sector. The program searches for the free-neutron processes n to n-bar (antineutron) and n to n-prime (a sterile mirror-neutron state) using cold neutrons at the European Spallation Source. HIBEAM, a magnetically shielded 50-meter beamline, can run four search modes, including regeneration through a beam stop, while NNBAR, a 200-meter evacuated beamline with magnetic fields below 10 nT, reaches conversion probabilities roughly a factor of 1000 smaller than the current best limits. The claimed improvement of more than three orders of magnitude rests on the NNBAR conceptual design report, which simulates a background rejection of $10^{9}$ at a signal efficiency of about 70 percent.","pith_inferences":["A null result at full NNBAR sensitivity would set the strongest laboratory bound on neutron-antineutron oscillation, effectively closing a class of baryogenesis models that rely on delta-B = 2 neutron transitions.","The regeneration mode, which lets mirror neutrons pass through a beam stop and then convert back to ordinary neutrons, is a powerful discriminator that could be adapted to search for other hidden-sector particles that interact only weakly with ordinary matter.","The assumed 10^9 background rejection at 70 percent signal efficiency is the single most load-bearing condition for the claimed discovery window; a modest shortfall in rejection would shrink the reach more directly than a comparable shortfall in neutron flux.","If the axion sensitivity is realized, HIBEAM would become one of the few laboratory experiments able to probe ultralight axion dark matter in a mass range that is otherwise constrained mainly by astrophysical energy-loss arguments."],"forward_implications":["If the projected sensitivities are reached, HIBEAM and NNBAR would improve the free-neutron n-to-n-bar limit by about 1.5 orders of magnitude and the bound-neutron limit by about one order, and would improve n-to-n-prime discovery potential by more than a factor of 1000.","A discovery would establish baryon number violation in the neutron sector, satisfying a key Sakharov condition for baryogenesis, and the beam-stop modes could distinguish direct n-to-n-bar oscillation from n-to-n-prime-to-n regeneration.","The same beamline would provide direct sensitivity to axion dark matter over a mass range from roughly 10^-22 to 10^-16 eV, improving on previous direct searches by more than two orders of magnitude.","The program would probe new-physics mass scales well beyond the direct reach of colliders, because rare neutron conversions can be mediated by very heavy new particles."],"supporting_citations":[{"why":"Supplies the NNBAR conceptual design, the projected sensitivity, the 200-meter beamline parameters, and the simulated 10^9 background rejection at 70 percent efficiency.","marker":"[6]"},{"why":"Defines the HIBEAM program, its four search modes, the 50-meter beamline, and the factor-of-ten discovery potential over previous searches.","marker":"[7]"},{"why":"Establishes the overall HIBEAM/NNBAR proposal, including the use of the large beam port at the European Spallation Source.","marker":"[4]"},{"why":"Provides the previous best free-neutron limit on n-to-n-bar oscillation that the program is designed to surpass.","marker":"[3]"},{"why":"Gives the recent improved limit on n-to-n-prime transformation that underpins the claimed factor-of-1000 improvement in mirror-neutron sensitivity.","marker":"[9]"},{"why":"Lays out the Ramsey neutron-beam method for axion dark-matter detection, which HIBEAM would use for its axion sensitivity.","marker":"[13]"},{"why":"Describes the European Spallation Source neutron-beam parameters and particle-physics program that the sensitivity projections assume.","marker":"[5]"}],"fun_headline_variants":["HIBEAM: hunting baryon violation with neutrons","Neutron-antineutron oscillation search goes 1000x deeper","Aimed at first proof of baryon number violation","Cold neutron beams target neutron-antineutron mixing","HIBEAM/NNBAR: a 1000-fold leap in neutron conversion search"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The projected reach assumes that the European Spallation Source will deliver the assumed cold-neutron flux and that a 50-to-200-meter magnetically shielded, evacuated beamline can be operated with magnetic fields below 10 nT while the annihilation detector rejects background by a factor of $10^{9}$ at 70 percent signal efficiency, none of which has been demonstrated at this scale.","fun_headline_variants_meta":{"raw":{"variants":["HIBEAM: hunting baryon violation with neutrons","Neutron-antineutron oscillation search goes 1000x deeper","Aimed at first proof of baryon number violation","Cold neutron beams target neutron-antineutron mixing","HIBEAM/NNBAR: a 1000-fold leap in neutron conversion search"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000683,"raw_usage":{"total_tokens":3099,"prompt_tokens":941,"completion_tokens":2158,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":557,"completion_tokens_details":{"reasoning_tokens":2068}},"tokens_in":557,"tokens_out":2158,"duration_ms":16439,"temperature":1.0,"reasoning_tokens":2068,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T10:56:20.384368+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A full-scale or prototype test of the NNBAR beamline environment would settle the central claim: if the residual magnetic field over a 200-meter evacuated path cannot be held below 10 nT, or if the background leakage through event selection exceeds $10^{-9}$ at the nominal signal efficiency, the claimed three-order-of-magnitude discovery window is not attainable. A null search at the projected NNBAR sensitivity would directly falsify the discovery claim for neutron-antineutron oscillation down to the corresponding conversion probability.","supporting_citations":[{"cited_title":"Improved Limits on $n \\rightarrow n'$ Transformation from the Spallation Neutron Source","cited_arxiv_id":"2402.15981","evidence_quote":"Gives the recent improved limit on n-to-n-prime transformation that underpins the claimed factor-of-1000 improvement in mirror-neutron sensitivity."},{"cited_title":"A Ramsey Neutron-Beam Experiment to Search for Ultralight Axion Dark Matter at the ESS","cited_arxiv_id":"2404.15521","evidence_quote":"Lays out the Ramsey neutron-beam method for axion dark-matter detection, which HIBEAM would use for its axion sensitivity."}],"review_version":1}