{"id":"d8d157b5-1495-4107-80b6-3ee4b0d97600","arxiv_id":"1908.10861","paper_version":2,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Fermionic dark matter absorbed by nuclei produces neutral-current recoil peaks, charged-current induced beta decays, and beta endpoint shifts, giving current and future experiments new ways to probe sub-GeV dark matter.","lead":"Dark matter made of fermions could be absorbed by atomic nuclei instead of just bouncing off them, producing distinct recoils, induced beta decays, or shifted beta decay endpoints. Existing and planned detectors such as XENON1T, Super-Kamiokande, CUORE, and PTOLEMY could search for these new signatures.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Charged-current projections in Sec. 4 replace nuclear response by free-nucleon matrix elements and assumed 1 MeV splittings; without a nuclear-structure calculation the induced-beta reach in Figs. 6-8 is not quantitative.","rationale":"The paper's central contribution is to identify fermionic absorption as a distinct class of nuclear signals and to show that existing experiments could be sensitive. The neutral-current kinematics (Section 3) are robust: the delta-function recoil peak at m_chi^2/(2M), coherence, and the rate formula follow from simple kinematics and are internally consistent. The tritium endpoint calculation in Section 5 is also well supported because it reproduces the standard neutrino-capture cross section in the light-mass limit, and the authors give a concrete nuclear correction. The charged-current induced-beta projections, however, are the quantitative pillar of the 'large viable parameter space' claim for masses above ~400 keV. Those projections rely on replacing nuclear matrix elements with free-nucleon amplitudes, multiplying by the Fermi function, and guessing 1 MeV splittings where data are missing. This is not a consistency error, but it is a genuine quantitative uncertainty that the authors explicitly acknowledge. The reader's CONDITIONAL verdict captures this. My read does not move the verdict: the signal taxonomy and the neutral-current and tritium results are unaffected, but the reach plots in Figs. 6 and 7 should not be taken as precise predictions until a nuclear-structure calculation (or measured transition strengths) is folded in. No code or data are provided, so a re-implementation would be needed to even check the algebra; that further supports keeping the verdict conditional.","tokens_in":27874,"tokens_out":6024,"duration_ms":66099,"concrete_test":"Compute the induced beta- rate for a benchmark target, e.g. 131Xe -> 131Cs, with a shell-model or QRPA calculation of the Fermi and Gamow-Teller strengths to all states within ~2 MeV of the daughter ground state, using the operator in Eq. (4.1). Compare the resulting rate versus m_chi and the projected limit in Fig. 6 with the current treatment (free-nucleon matrix element, Fermi function, 1 MeV unmeasured splittings). If the rate or threshold kink locations shift by more than a factor ~3, the projections in Figs. 6 and 7 should be labeled order-of-magnitude estimates, not quantitative sensitivities.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing weak point is the nuclear-response approximation in the charged-current section. Eqs. (4.9)-(4.13) take the full nuclear transition amplitude to be the free-nucleon amplitude (4.11) times the Fermi function, and then sum over Fermi and Gamow-Teller transitions. The text admits twice that this is an approximation: 'the technology exists to compute form factors for scattering between different nuclei generated by general operators, we are unaware of such a computation carried out in the literature' (Sec. 4.1) and, when excited-state data are missing, 'we take the splitting to be 1 MeV' (after Eq. (4.13)). This matters because the induced-beta rate is not governed by a single-nucleon operator: Fermi and Gamow-Teller matrix elements are nuclear many-body overlaps that vary by orders of magnitude between transitions, and the kinematic threshold in Eq. (4.2) depends directly on the daughter excitation energy. Setting unknown splittings to 1 MeV can move thresholds by hundreds of keV for isotopes like 131Xe, directly shifting the mass range and rate shown in Fig. 6; the summed Gamow-Teller strength may be over- or under-counted if the 1 MeV assumption misses collective or fragmented strength. Because the abstract's 'large viable parameter space' claim leans on these projections, the charged-current reach should be treated as an estimate pending a nuclear-structure calculation, exactly as the authors state.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper studies the absorption of fermionic dark matter by nuclear targets through dimension-six four-fermion operators. It classifies the signals into neutral-current processes (χ + N → ν + N, producing a nuclear recoil peaked at E_R ≈ m_χ^2/2M) and charged-current processes (χ + N → e^± + N', producing induced β decays in stable isotopes and endpoint shifts in unstable isotopes). Two UV completions are presented: a gauged baryon-number model with χ–ν mixing for the neutral current, and a modified left-right symmetric model for the charged current. The paper computes dark-matter decay constraints from indirect detection, discusses collider and β-decay constraints, and projects sensitivities for a wide set of current and future experiments, including XENON1T, LUX, PandaX-II, CUORE, Borexino, Super-Kamiokande, and PTOLEMY. The central claims are that the neutral-current recoil spectrum is characteristically different from elastic scattering and that a large viable parameter space remains detectable.","tokens_in":28202,"tokens_out":15741,"duration_ms":171655,"significance":"If the quantitative projections hold, the paper opens a genuinely new experimental avenue: sub-MeV to tens-of-MeV fermionic dark matter could be searched for through peaked nuclear recoils, induced β decays with correlated signals, and β-endpoint shifts in existing and proposed detectors. The neutral-current derivation from Eq. (3.3) to Eq. (3.7) is clear and gives a falsifiable prediction, and the charged-current tritium calculation in Sec. 5 reproduces the standard neutrino-capture cross section in the light-mass limit, which is a useful calibration. The paper is also unusually candid about its limitations, explicitly stating that nuclear form factors for these transitions have not been computed in the literature and that unknown excited-state splittings are set to 1 MeV. These admissions are evidence that the charged-current reach should be read as an estimate rather than a precise bound. The novelty and breadth of the proposed signals make the paper a valuable contribution, provided the nuclear-response uncertainties are either quantified or clearly reflected in the main claims.","major_comments":[{"comment":"The charged-current reach projections replace the full nuclear transition amplitude by the free-nucleon amplitude multiplied by the Fermi function, and set unknown daughter excitation energies to 1 MeV. The manuscript itself acknowledges this twice: after Eq. (4.13) it states that the technology exists to compute form factors but that the authors are unaware of such a computation, and in Sec. 4.1 it states that when excited-state data are missing, “we take the splitting to be 1 MeV.” This is load-bearing because the kinematic threshold in Eq. (4.2) depends directly on the daughter excitation energy, and Fermi and Gamow-Teller nuclear matrix elements can vary by orders of magnitude between transitions. The 10-event projections in Figs. 6 and 7 and the abstract's claim of a “large viable parameter space” for charged-current signals therefore rest on an unquantified nuclear-response approximation. I recommend either supplying a nuclear-structure calculation or a careful compilation of measured transition strengths for the isotopes in Table 1, or explicitly demoting the charged-current projections in the abstract and conclusions to order-of-magnitude estimates pending such a calculation.","section":"Sec. 4.1, Eqs. (4.7)–(4.13), Figs. 6–7"},{"comment":"The tritium rate is calibrated to the standard neutrino-capture result, but the same formalism is applied to heavy isotopes without accounting for axial-vector quenching or the redistribution of Gamow-Teller strength. The paper maps λ → sqrt(2.788/3) λ for tritium, citing Ref. [112], but no analogous correction is applied for xenon, tellurium, oxygen, or other targets in Sec. 4. Since the axial contribution enters quadratically in Eq. (4.11), an unquenched g_A can overestimate Gamow-Teller rates by a factor of order (1.2694/1.0)^2 ≈ 1.6 or more, and the fragmented or collective nature of the GT strength can shift the summed rate further. This is part of the same nuclear-response issue as the first comment, but it concerns the operator-level input rather than excitation energies and deserves a separate, explicit treatment or an uncertainty band on the projected limits.","section":"Sec. 5, Eq. (5.4)"}],"minor_comments":[{"comment":"The title contains a spacing typo: “Nuclear T argets” should be “Nuclear Targets.”","section":"Title"},{"comment":"“to this affect” should be “to this effect,” and in the text near Table 1, “displaces the threshold” should be “displays the threshold.”","section":"Sec. 4.1, footnote 7"},{"comment":"“Due to their shear size” should be “Due to their sheer size.”","section":"Sec. 6"},{"comment":"The experiment name is written inconsistently as “Panda-XII” in Fig. 6 and “PandaX-II” in Table 3 and elsewhere; please unify the notation.","section":"Fig. 6 and Table 3"},{"comment":"Eq. (4.4) uses the symbol E_R for both the outgoing electron energy and the nuclear recoil energy; renaming the electron kinetic energy, for example E_e, would avoid confusion with the recoil variable E_R used throughout Sec. 3.","section":"Eq. (4.4)"},{"comment":"The charged-current rate in Eq. (4.5) contains a factor ρ_χ/(2m_χ) while the neutral-current rate in Eq. (3.6) uses ρ_χ/m_χ. Please clarify whether the extra factor of 1/2 accounts for a symmetric χ/anti-χ abundance or for a chiral spin average, and how it is consistent with Eq. (5.4), where the 1/2 is absent.","section":"Eqs. (3.6) and (4.5)"}],"recommendation":"major_revision","confidential_remarks":"The paper is well within the scope of JHEP and the neutral-current part is solid. The main issue is that the charged-current quantitative claims outrun the nuclear physics input, and the manuscript itself admits the missing computations. A revision that either adds a nuclear-structure treatment or clearly labels the charged-current projections as order-of-magnitude estimates, including in the abstract, would address the concern. The factor-of-2 normalization question in Eqs. (4.5) vs. (5.4) should also be resolved in the revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear Colleague,\n\nQuick take: this is a genuinely useful catalog of fermionic dark matter absorption signals on nuclei, with a clean neutral-current recoil peak and a sensible taxonomy of charged-current induced beta decays and endpoint shifts. It is not a precision phenomenology paper. The charged-current projections are order-of-magnitude estimates, and the authors mostly say so.\n\nWhat is new: the extension of their earlier electron-target absorption framework to nuclear targets, the sharp m_chi^2/2M recoil peak, the induced beta decay classification with threshold table, and the tritium endpoint-shift sensitivity. The derivation from Eq. (3.3) to (3.7) is internally consistent, and the tritium calculation reproduces the standard neutrino capture cross section in the light-mass limit. For a mapping paper of this kind, that is enough.\n\nSoft spots: the charged-current nuclear response is the weakest link. Eqs. (4.9)-(4.13) replace the many-body transition amplitude with the free-nucleon amplitude times a Fermi function, and when excited-state data are missing they set splittings to 1 MeV. That is not a small correction: for 131Xe the threshold can shift by hundreds of keV depending on the actual daughter excitation energy, and the Gamow-Teller sum can be over- or under-counted by an order of magnitude in rate. The text admits this twice, so the authors are not hiding it, but it means Figs. 6-8 should not be cited as quantitative reach. The neutral-current UV benchmark also requires fine-tuning of epsilon and the 3-nu operator for m_chi above roughly an MeV; again, the paper labels those contours as fine-tuning and notes model dependence. That does not break the central classification claim.\n\nNo code or data is included, so reproducibility is a re-implementation exercise. That is a minor ding for a theory paper. The citation pattern looks fair: the heavy reliance on [39] and [40] is justified because those are the direct predecessors.\n\nWho this is for: direct detection phenomenologists and experimental collaborations looking for new sub-GeV dark matter signatures. It will likely be a background reference for future absorption searches. It deserves serious peer review, but the referee should push for an explicit caveat that the charged-current reach is schematic and, ideally, a pointer to nuclear-structure calculations.\n\nRecommendation: send it to review with a request for minor-to-moderate revision. Cite it if you work on sub-GeV dark matter; bring it to reading group only if someone is starting an absorption search.","headline":"A well-organized catalog of nuclear fermionic absorption signals with clean neutral-current kinematics, but the charged-current reach figures rest on uncomputed nuclear-structure input and should be treated as order-of-magnitude estimates.","tokens_in":28806,"tokens_out":1970,"would_cite":true,"duration_ms":20484,"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":"Fermionic dark matter can be caught by absorption, not just scattering","keywords":["fermionic dark matter","dark matter absorption","nuclear recoil","induced beta decay","beta endpoint shift","four-fermion operators","direct detection","neutrino experiments"],"falsifier":"Measure the low-lying Fermi and Gamow-Teller strengths for the relevant transitions, e.g. $^{131}$Xe to $^{131}$Cs and $^{125}$Te to $^{125}$I, and compare the true excited-state energies to the 1 MeV default. If the matching excited states lie much higher or the transition strengths are much smaller than assumed, the projected induced-$\\beta$ reach below about 1 MeV would not materialize at the quoted thresholds.","tokens_in":27570,"feed_emoji":"☢️","tokens_out":12015,"duration_ms":116065,"temperature":0.7,"pith_summary":"Dark matter that interacts as a fermion can be captured by a nucleus rather than merely scattering off it, and the paper argues that this absorption process should be observable with detectors that already exist or are being built. Neutral-current absorption liberates the dark matter's rest energy, pushing a nucleus back at a recoil energy $E_R = m_\\chi^2/2M$ that is about $10^6$ times larger than an elastic-scattering recoil for the same mass, so lighter dark matter becomes accessible. Charged-current absorption triggers $\\beta$-like transitions: stable isotopes are driven into $\\beta$ decays, and already-radioactive isotopes get a shifted endpoint in their $\\beta$ spectrum. The paper builds complete UV models for both operator classes and shows that, even after accounting for the unavoidable decays these interactions cause, a large region of parameter space survives. If the argument is right, searches for scattering are looking at only one of several ways dark matter can reveal itself in a detector.","feed_headline":"Absorbed dark matter prints a sharp peak on nuclear recoil spectra","feed_subtitle":"The dark matter's full rest mass, not just its tiny kinetic energy, becomes the signal in existing detectors.","key_machinery":"The machinery is a set of dimension-6 four-fermion operators that do not conserve dark matter number. In the neutral-current case, a $Z'$-mediated model with a small $\\chi$--$\\nu$ mixing angle generates $\\frac{1}{\\Lambda^2}(\\bar n\\gamma^\\mu n+\\bar p\\gamma^\\mu p)\\bar\\chi\\gamma_\\mu P_R\\nu$; the kinematic identity $E_R^0 = m_\\chi^2/2M$ turns the dark matter rest mass into a peaked, velocity-independent nuclear recoil, and the Helm form factor $F(q)$ plus coherent $A^2$ enhancement sets the rate. In the charged-current case, a right-handed $W_R$ model generates $\\frac{1}{\\Lambda^2}[\\bar p\\gamma^\\mu(1+\\lambda\\gamma_5)n][\\bar e\\gamma_\\mu P_R\\chi]$; the Fermi function $F(Z,E_e)$ and the Fermi ($\\Delta I=0$) and Gamow-Teller ($\\Delta I=\\pm1$) selection rules determine which nuclear transitions contribute. The two model classes also fix the dark matter decay channels that indirect-detection searches constrain, which is what limits the parameter space to light dark matter.","core_discovery":"The central claim is that the two classes of four-fermion, dark-matter-number-violating operators produce distinct nuclear signals that current experiments can test. For the neutral-current operator of the form $[\\bar\\chi\\Gamma_i\\nu][\\bar n\\Gamma_j n]$ and $[\\bar\\chi\\Gamma_i\\nu][\\bar p\\Gamma_j p]$, absorption yields a recoil spectrum concentrated in a peak at $E_R^0 = m_\\chi^2/2M$, with a rate enhanced by $A^2 F(q)^2$; the spectrum is characteristically different from the falloff of elastic scattering. For the charged-current operator $[\\bar\\chi\\Gamma_i e][\\bar n\\Gamma_j p]$, absorption above the threshold $m_\\chi > M_{A,Z+1}+m_e - M_{A,Z}$ produces induced $\\beta^-$ decays in otherwise stable isotopes, with multiple correlated signals, and for already-unstable isotopes it shifts the kinematic endpoint of the $\\beta$ spectrum without any threshold. The paper claims that current detectors can reach $m_\\chi$ down to roughly 350 keV through induced $\\beta$ decays, and that a tritium-based experiment with about a kilogram-year of exposure could reach the lightest fermionic dark matter consistent with phase-space bounds, near 190 eV.","pith_inferences":["Because the neutral-current recoil is a sharp peak, null results from past exposures can be reinterpreted as bounds on $\\sigma_{NC}$ without any new hardware; the paper does not itself run those re-analyses.","The charged-current reach depends on nuclear response data the paper flags as missing; measuring actual Fermi and Gamow-Teller strength distributions for the listed daughter nuclei would sharpen or move the projected thresholds.","In asymmetric dark matter scenarios where only $\\chi$ or only $\\bar\\chi$ survives, only one of the $\\beta^-$ or $\\beta^+$ channels would fire, so a null search in the other channel could bound the asymmetry.","If absorption signals are found, the same operators imply dark matter is unstable and should decay at a calculable rate; the decay products would be a complementary indirect-detection signature tied to the same parameter point."],"forward_implications":["Neutral-current absorption gives detectors a mono-energetic nuclear recoil at $m_\\chi^2/2M$, so the dark matter mass can be read off from the peak position in a single experiment.","The same operator makes heavier targets with larger exposures, including neutrino detectors, competitive with dedicated direct-detection experiments because the signal does not rely on low thresholds for light dark matter.","Charged-current absorption of stable isotopes provides a multi-channel signature—energetic electron, nuclear recoil, gamma from the excited daughter, and a possible secondary beta decay—so a single event can be confirmed by correlated observables.","Induced $\\beta^-$ searches in xenon, tellurium, and oxygen reach dark matter masses around a few hundred keV to tens of MeV, while hydrogen targets for induced $\\beta^+$ are the best probe above roughly 2 MeV.","A tritium experiment with a kilogram-year exposure could detect fermionic dark matter at masses below the threshold for induced beta decays, down toward the 190 eV phase-space floor."],"supporting_citations":[{"why":"The companion paper that introduced fermionic dark matter absorption as a direct-detection signal, extended here to nuclear targets.","marker":"[40]"},{"why":"Established the induced-decay route with a Dysprosium target for sterile neutrino dark matter, which this paper generalizes to other isotopes and models.","marker":"[39]"},{"why":"Supplies the nucleon-level vector and axial couplings and the Coulomb Fermi function used in the charged-current rate calculation.","marker":"[58]"},{"why":"Provides the Helm form factor that shapes the neutral-current recoil-rate spectrum at finite momentum transfer.","marker":"[70]"},{"why":"Review of direct-detection formalism used for comparing the absorption recoil spectrum with the elastic scattering spectrum.","marker":"[71]"},{"why":"The tritium-based proposal whose exposure anchors the projection for beta-endpoint-shift searches.","marker":"[99]"},{"why":"Computation of neutrino capture on tritium that the paper reproduces in the light-dark-matter limit.","marker":"[46]"},{"why":"Diffuse X-ray and gamma-ray constraints on dark matter decay used to bound the neutral-current model's parameter space.","marker":"[49]"},{"why":"Cosmological bounds on invisible dark matter decays used to constrain the $\\chi\\to 3\\nu$ channel.","marker":"[50]"}],"fun_headline_variants":["Fermionic dark matter absorption yields sharp recoil peak","Induced beta decay reveals absorbed fermionic dark matter","Fermionic dark matter absorption shifts beta endpoints","New nuclear recoil peak from fermionic dark matter absorption","Absorbed dark matter triggers induced beta decays"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The induced-decay projections rest on the assumption that a free-nucleon interaction, after a simple Coulomb correction, describes the whole nucleus, and that any missing excited-state energy is set to 1 MeV.","fun_headline_variants_meta":{"raw":{"variants":["Fermionic dark matter absorption yields sharp recoil peak","Induced beta decay reveals absorbed fermionic dark matter","Fermionic dark matter absorption shifts beta endpoints","New nuclear recoil peak from fermionic dark matter absorption","Absorbed dark matter triggers induced beta decays"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000277,"raw_usage":{"total_tokens":1677,"prompt_tokens":998,"completion_tokens":679,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":614,"completion_tokens_details":{"reasoning_tokens":603}},"tokens_in":614,"tokens_out":679,"duration_ms":6959,"temperature":1.0,"reasoning_tokens":603,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T10:31:47.501753+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the low-lying Fermi and Gamow-Teller strengths for the relevant transitions, e.g. $^{131}$Xe to $^{131}$Cs and $^{125}$Te to $^{125}$I, and compare the true excited-state energies to the 1 MeV default. If the matching excited states lie much higher or the transition strengths are much smaller than assumed, the projected induced-$\\beta$ reach below about 1 MeV would not materialize at the quoted thresholds.","supporting_citations":[{"cited_title":"Direct Search for keV Sterile Neutrino Dark Matter with a Stable Dysprosium Target","cited_arxiv_id":"1609.04671","evidence_quote":"Established the induced-decay route with a Dysprosium target for sterile neutrino dark matter, which this paper generalizes to other isotopes and models."}],"review_version":1}