{"id":"4e7e7483-774b-4049-9b72-aa5c97d29f88","arxiv_id":"1908.04635","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A general scalar portal with independent production and decay couplings lets FASER 2 with a 1.5 m radius detector probe scalar masses from a few GeV up to half the Higgs mass.","lead":"This paper shows that a modestly enlarged version of the planned FASER 2 detector could detect new light scalar particles produced in decays of up to 10% of all Higgs bosons at the LHC. It argues that increasing the detector radius from 1 to 1.5 meters would let FASER 2 probe a much wider range of scalar masses and couplings.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Ideal-detector assumption, not the 5% branching ratio, is the least secure pillar of the all-mass FASER 2 claim.","rationale":"The reader's weakest assumption was the fiducial BR(h -> SS) = 0.05. That is a legitimate scaling uncertainty, but it affects all masses roughly uniformly: even at BR = 0.01, the 50-60 GeV region still has about 5 events, so the qualitative FASER2 reach at large mass survives. The low-mass extension, by contrast, has almost no margin at BR = 0.05: unit efficiency and zero background are necessary for the 2.3-event contour. A realistic efficiency of 50% or one background event kills it. Thus the most load-bearing condition for the central claim is the ideal-detector assumption rather than the BR benchmark. The appropriate verdict remains CONDITIONAL, matching the reader's verdict, but for a somewhat different reason. A full Geant4 simulation with the HL-LHC forward flux is the single check that would settle whether the concern lands.","tokens_in":16200,"tokens_out":27950,"duration_ms":296504,"concrete_test":"Implement a Geant4 simulation of the FASER2 vetoed decay vessel (L = 480 m, ldet = 5 m, R = 1.5 m) with the HL-LHC forward neutrino and muon flux from a DPMJET/EPOS-based event generator, and reconstruct the dominant scalar decay modes (bb, cc, tau+tau-, mu+mu-) with the FASER2 tracker/calorimeter response. Compute eps_det and the background count in 3 ab^-1; then redraw the 2.3-event contour of Fig. 8 using Ndet = 2 Nh BR eps_geom <P_decay> eps_det and a Poisson background. If eps_det < 0.8 or background > 0.1 events, the 'all masses down to a few GeV' claim fails.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's headline mass coverage — 'all scalar masses from mh/2 down to a few GeV' for FASER2 with R = 1.5 m — rests not on the fiducial BR = 0.05 but on the background-free, unit-efficiency detector assumption. Using the paper's own numbers for low masses (Nh = 1.7e8, BR = 0.05, eps_geom about 4e-5 at R = 1 m, <P_decay_max> about 3.2e-3), the maximum event count in Eq. (2.15) is O(1); after the radius increase it is only a handful. At the 2.3-event threshold, the low-mass dashed contour in Fig. 8 is a sliver in theta-squared: a reconstruction efficiency below about 80% or one background event removes the few-GeV reach entirely. The high-mass 40-60 GeV reach is more robust because Nmax there is about 27, but the distinctive new conclusion of the paper is precisely the low-mass coverage enabled by the larger radius. The authors explicitly flag both assumptions ('we optimistically assume detector efficiency eps_det = 1' and 'we assume background free experiment'), so this is a declared limitation, not an oversight; nevertheless it is the most load-bearing condition for the strongest claim.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies the sensitivity of the FASER and FASER 2 experiments to a light singlet scalar produced through the Higgs portal. In the model of Eq. (1.1), the cubic coupling α₁ and the quartic coupling α₂ are treated as independent, so the production process h→SS (controlled by α₂) and the decay of S to Standard Model particles (controlled by the mixing angle θ) can be decoupled. The authors derive analytic estimates for the number of scalar decays in the FASER decay vessel, cross-check them against MadGraph simulations, and present sensitivity contours in the (m_S, θ²) plane. With a fiducial branching ratio BR(h→SS)=0.05 and the baseline FASER 2 configuration (R=1 m), they find sensitivity only near m_S≈40–60 GeV; increasing the detector radius to 1.5 m is claimed to extend the reach down to a few GeV. The analysis assumes 100% detection efficiency, 100% visible branching, and a background-free experiment, all of which are explicitly flagged by the authors.","tokens_in":16404,"tokens_out":10045,"duration_ms":96471,"significance":"If the central claim holds, the paper provides a concrete, quantitative argument for a modest modification of the FASER 2 geometry (R: 1 m → 1.5 m) that would turn the experiment from a marginal probe of this Higgs-portal scenario into a broad-coverage discovery machine. The analytic framework in Section 2 and the appendices is transparent and largely reproducible, and the cross-check against MadGraph adds credibility to the kinematic treatment. The authors are also explicit about their optimistic assumptions (ε_det=1, BR_visible=1, zero background), which is rare and helpful. The paper's main value is a design-oriented sensitivity projection, not a new theoretical constraint; its conclusions are conditional on BR(h→SS)=0.05 and on the ideal-detector assumptions.","major_comments":[{"comment":"The treatment of the h→SS multiplicity is internally inconsistent. Eq. (2.1) defines N_S = 2 N_h BR(h→SS), but Eq. (2.15) computes N_events^(max) = N_h BR_fid ε_geom ⟨P_decay⟩ with no factor of 2, and Eq. (B.14) writes N_det = N_S BR(h→SS) ∫ f P_decay, which double-counts the branching ratio if N_S already contains it. The numerical estimate in §2.1 (N_naive_S ≈ 33) also does not match N_h = 1.7×10^8 and BR_fid = 0.05, which would give ≈19; it corresponds instead to BR ≈ 0.1. Since the sensitivity threshold is 2.3 events, this factor-of-two ambiguity shifts the quoted mass reach and must be resolved. Please state explicitly whether N_h or N_S enters each formula and correct Eq. (2.15) and Eq. (B.14) accordingly.","section":"§2.1, Eq. (2.15), Eq. (B.14)"},{"comment":"The low-mass reach of the R=1.5 m configuration rests on O(1) events. Using the paper's own numbers (N_h = 1.7×10^8, BR = 0.05, ε_geom ≈ 4×10^-5 for m_S ≲ 30 GeV, ⟨P_decay⟩ ≈ 3.2×10^-3), the maximum event count is ≈1 (or ≈2 with the factor of two from the previous comment). The dashed contour in Fig. 8 for the R=1.5 m geometry is therefore a narrow sliver in θ², and it disappears entirely if the detection efficiency is below about 80% or if a single background event is present. Because the abstract and conclusion present the \"all masses from m_h/2 down to a few GeV\" result without this caveat, please add an explicit robustness statement and, if possible, show curves for ε_det < 1 and for a background of one event.","section":"§3, Fig. 8, Eq. (2.15)"},{"comment":"All quoted sensitivity scales linearly with BR(h→SS), yet the paper moves among the current bound (BR_inv < 0.19), the HL-LHC projection (0.05–0.15), and the adopted fiducial value 0.05. The abstract's statement that \"about 10% of all Higgs bosons\" can be converted to scalars may be misread as the value used in the sensitivity analysis, which is actually 5%. Please state explicitly in the abstract and in the caption of Fig. 8 that all mass ranges correspond to BR(h→SS)=0.05 and give the trivial rescaling rule (N_det ∝ BR) so that the reader can evaluate other benchmarks.","section":"§1.1, §2.1, Conclusions"}],"minor_comments":[{"comment":"There is a typo: \"trough such operators\" should be \"through such operators\".","section":"§1, p.1"},{"comment":"The affiliation of the Leiden authors contains a typo: \"Intituut-Lorentz\" should be \"Instituut-Lorentz\".","section":"Title page"},{"comment":"The sentence \"all major decay channels have > 2 charged tracks\" should be \"≥ 2 charged tracks\", since the dimuon final state has exactly two charged tracks.","section":"§2.3"},{"comment":"The bound BR_inv < 0.19 is cited to Ref. [73], which is a CMS search for heavy neutral leptons; the correct reference for the invisible Higgs decay constraint is Ref. [34] (CMS, Phys. Lett. B793 (2019) 520).","section":"Conclusions, Ref. [73]"},{"comment":"The same symbol L is used for integrated luminosity and for the distance to the detector, which is confusing; please use e.g. ℒ for luminosity and d (or L_dec) for the distance.","section":"Table 1"},{"comment":"The caption says the sensitivity estimates assume 100% reconstruction efficiency, but the main text also assumes zero background and 100% visible branching. These two additional assumptions should be stated in the caption as well, since the figure is the basis of the paper's headline claim.","section":"§3, Fig. 8 caption"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the scope of JHEP and addresses a timely design question for FASER 2. The main issues are internal consistency of the event-rate formulas (factor-of-two and BR normalization) and the fragility of the headline low-mass reach to the explicitly assumed ideal detector. Both are fixable with clarifications and a robustness study, so I do not recommend rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper does one clean thing: it takes the general scalar portal with independent cubic and quartic couplings, so h->SS production is set by alpha and decay by theta, and works out what FASER 2 can see. The analytic estimates are transparent, the MadGraph checks are honest, and the main figures are easy to read. The production-decay decoupling is real, and the geometric-acceptance calculation, which is the core of the paper, is done carefully. It is a legitimate, modest contribution to the forward-search program.\n\nThe soft spots are the ones the authors themselves flag. The fiducial BR(h->SS)=0.05 is an assumption, not a result: if the invisible width is not dominated by h->SS, or if HL-LHC does not reach 5%, the reach shrinks proportionally. The abstract's \"about 10%\" is also not derived anywhere; the current 95% bound is 19%, so 10% looks like a conservative round number, not a computed ceiling. More important, the \"all masses down to a few GeV\" claim for R=1.5 m depends on 100% detection efficiency, 100% visible BR, and zero background. The stress-test note is right: with their own numbers the low-mass event count is O(1), so the dashed contour is a sliver in theta^2; a reconstruction efficiency of 80% or a single background event removes it. The 40-60 GeV region is sturdier and carries the paper's fallback conclusion. So the design recommendation to move from R=1 m to 1.5 m is reasonable, but it should be presented as \"under ideal conditions,\" not as a robust projection.\n\nMinor: the conclusion cites [73] for BR_inv < 0.19, but [73] is a CMS heavy-neutral-lepton search; the correct references are [34,35]. The authors also cite their own Ref. [33] and the earlier FASER dark-Higgs paper [56], so the novelty claim is honest.\n\nWho is this for? People doing forward LLPs and scalar-portal phenomenology. It deserves a serious referee: the physics is sound as a conditional study, and the ideal-detector caveat is a reason to ask for a detector-level follow-up, not a reason to reject. I'd accept it, and I'd push the authors to add a realistic efficiency and background estimate before the radius-increase advice is taken by FASER's collaboration.","headline":"Clear, honest conditional sensitivity study of h->SS at FASER 2; the wide low-mass reach rests on ideal detector assumptions that need a real efficiency/background estimate before being sold as a design driver.","tokens_in":16999,"tokens_out":3965,"would_cite":true,"duration_ms":42059,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"If the invisible Higgs decay rate is at the 5 percent level, FASER 2 can discover the dark scalars produced in h -> SS decays, and a modest doubling of its geometric acceptance extends that reach down to a few GeV in scalar mass.","keywords":["Higgs portal","dark scalar","long-lived particles","FASER 2","invisible Higgs decay","forward physics","intensity frontier","h to SS decay"],"falsifier":"If the HL-LHC measures the invisible Higgs branching ratio to be below about 5% (or attributes it to channels other than h -> SS), the fiducial event count drops below 2.3 and the quoted mass ranges no longer hold; equivalently, a run of FASER 2 with R = 1.5 m that finds zero displaced scalar decays while BR(h -> SS) is measured at 5% would refute the sensitivity prediction.","tokens_in":15978,"feed_emoji":"⚛️","tokens_out":8504,"duration_ms":76816,"temperature":0.7,"pith_summary":"This paper argues that the quartic Higgs-portal coupling can be large enough that a few to ten percent of LHC Higgs bosons decay invisibly to a pair of new singlet scalars, h -> SS, and that the forward detector FASER 2 is well placed to catch the scalars that decay hundreds of meters downstream. With the fiducial branching ratio BR(h -> SS) = 0.05, the planned FASER 2 geometry should discover scalars with masses between about 40 GeV and m_h/2, and a modest doubling of geometric acceptance — enlarging the decay-vessel radius from 1 m to 1.5 m — extends the reach down to a few GeV. This is possible because the cubic and quartic portal couplings are treated as independent, so the production rate is set by a coupling that invisible-Higgs-decay limits only weakly constrain, while the scalar's decay is controlled by a separate small mixing angle. The result matters because it turns a poorly constrained part of the Higgs portal into a concrete, near-term search target for a proposed LHC forward experiment.","feed_headline":"FASER 2 could spot Higgs-made scalars down to a few GeV","feed_subtitle":"A detector-radius increase from 1 to 1.5 meters opens nearly the full scalar mass range.","key_machinery":"The load-bearing object is the effective triple-Higgs interaction (α/2) $S^{2}$ h, produced after electroweak symmetry breaking by independent cubic and quartic portal terms in the Lagrangian (1.1). It allows Higgs bosons to decay to a pair of scalars at a rate set by the quartic coupling while the scalar's subsequent decay is set by the separate mixing angle θ; the paper combines this with the forward boost of high-energy Higgs bosons, a geometric-acceptance integral over the scalar angular distribution, and the decay probability P_decay = $e^{{-L/l_decay}}$ - $e^{{-(L+l_det)/l_decay}}$ to predict event rates in the FASER 2 decay vessel.","core_discovery":"The central claim is that the process h -> SS, driven by the quartic portal coupling, can dominate all other scalar production channels at the LHC while the scalar's decay remains controlled by the tiny Higgs-mixing angle, and this decoupling makes the FASER 2 event rate factorize into a geometric acceptance times a decay probability. For a fiducial invisible branching ratio of 5%, the paper computes that FASER 2 in its baseline configuration expects at least 2.3 events only for scalar masses in the upper part of the allowed range, approximately 40 GeV ≲ m_S ≲ m_h/2, while increasing the detector radius from 1 m to 1.5 m opens the full range from a few GeV up to m_h/2, with the upper end set by the kinematic threshold and the lower end by geometry. The analytic estimates are checked against Monte Carlo simulations of Higgs production, and the sensitivity curves assume a background-free experiment with 100% detection efficiency.","pith_inferences":["If the actual invisible branching ratio is closer to the current upper bound of 19% than to the fiducial 5%, the predicted FASER 2 event count scales up by roughly a factor of four, which would extend the reach to smaller values of θ^2 than shown in the paper's sensitivity plot.","The same production/decay decoupling should also change the expected sensitivity of other planned long-lived-particle detectors, whose published reach is usually computed with the production and decay couplings tied together; re-running those analyses with h -> SS as the dominant channel is a natural next step.","A detector-simulation study that replaces the 100% reconstruction-efficiency assumption with realistic tracking and vertexing efficiencies, and that estimates backgrounds near the beam line, would turn the paper's sensitivity contours into firm discovery projections; the paper explicitly leaves that verification to future work.","The same forward-boost and geometric-acceptance machinery could be applied to B-meson production of scalars to quantify the reach below a few GeV, a region the paper identifies but does not compute."],"forward_implications":["At BR(h -> SS) = 0.05 and with the baseline 1 m radius, FASER 2 will have no sensitivity below roughly 40 GeV, so a null result there would not constrain the model.","Enlarging the FASER 2 radius to 1.5 m extends the sensitivity to every scalar mass from a few GeV up to m_h/2, including the region where existing prompt-decay searches are blind because the scalars have cτ_S ~ O(100) m.","The maximal scalar mass that can be probed is set by the kinematic threshold m_S < m_h/2, not by the usual lifetime-vs-production trade-off, because the production and decay couplings are independent.","Moving the detector closer to the interaction point increases the event rate roughly as L^3, making detector placement a powerful design lever for the scalar portal.","Even if HL-LHC does not discover the invisible Higgs decay directly, FASER 2 can still discover the scalars through their displaced decays, since the signal is not missing energy but reconstructed decay products."],"supporting_citations":[{"why":"Supplies the current experimental upper bound BR_inv < 0.19 that leaves room for a sizable h -> SS branching ratio.","marker":"[34]"},{"why":"Provides the projected HL-LHC sensitivity of BR_inv around 0.05 that the paper adopts as the fiducial branching ratio.","marker":"[62]"},{"why":"Supplies the scalar decay branching ratios and lifetimes used to compute the decay probability in the FASER 2 volume.","marker":"[10]"},{"why":"Gives the event-count formula N_det = N_S * eps_geom * P_decay * eps_det and the analytic sensitivity framework the paper adapts.","marker":"[51]"},{"why":"Defines the FASER experiment's geometry, location, and the baseline parameters of FASER 2 used in the acceptance calculation.","marker":"[55]"},{"why":"Provides the Monte Carlo event generator used to simulate Higgs boson production at the LHC.","marker":"[71]"},{"why":"Supplies the simulation setup for loop-induced Higgs production that yields the pL and pT distributions of the Higgs bosons.","marker":"[72]"},{"why":"Provides the resummed Higgs pT spectrum used to cross-check the simulated forward Higgs rate, agreeing within about 30 percent.","marker":"[75]"}],"fun_headline_variants":["FASER 2 radius boost opens full Higgs-scalar mass window","Wider FASER 2 catches scalar rain from Higgs decays","Tweaking FASER 2's radius unveils hidden scalar spectrum","Bigger FASER 2 detector probes Higgs portal scalars from GeV to half Higgs","FASER 2 with 1.5m radius sees all kinematically allowed scalars"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the invisible Higgs-decay rate is at least 5 percent and comes entirely from h -> SS; if the true rate is smaller or shared with other channels, all quoted event numbers and mass reach drop proportionally.","fun_headline_variants_meta":{"raw":{"variants":["FASER 2 radius boost opens full Higgs-scalar mass window","Wider FASER 2 catches scalar rain from Higgs decays","Tweaking FASER 2's radius unveils hidden scalar spectrum","Bigger FASER 2 detector probes Higgs portal scalars from GeV to half Higgs","FASER 2 with 1.5m radius sees all kinematically allowed scalars"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000855,"raw_usage":{"total_tokens":3674,"prompt_tokens":861,"completion_tokens":2813,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":477,"completion_tokens_details":{"reasoning_tokens":2709}},"tokens_in":477,"tokens_out":2813,"duration_ms":18870,"temperature":1.0,"reasoning_tokens":2709,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:36:09.975443+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"If the HL-LHC measures the invisible Higgs branching ratio to be below about 5% (or attributes it to channels other than h -> SS), the fiducial event count drops below 2.3 and the quoted mass ranges no longer hold; equivalently, a run of FASER 2 with R = 1.5 m that finds zero displaced scalar decays while BR(h -> SS) is measured at 5% would refute the sensitivity prediction.","supporting_citations":[{"cited_title":"Resummation ambiguities in the Higgs transverse-momentum spectrum in the Standard Model and beyond","cited_arxiv_id":"1510.08850","evidence_quote":"Provides the resummed Higgs pT spectrum used to cross-check the simulated forward Higgs rate, agreeing within about 30 percent."}],"review_version":1}