{"id":"352b9c63-2017-475a-8fb3-31461bc9a977","arxiv_id":"2508.18047","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Leptoquark decays can dominate the production of heavy vectorlike tau partners at the LHC, offering new mono- and di-lepton search channels.","lead":"This paper studies a possible new way to produce heavy partner particles of the tau lepton at the LHC, where leptoquarks decay into these partners. It predicts that this mechanism could make the partners much easier to see than previous searches, and maps out where the HL-LHC could discover them.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The near-100% Br(LQ→τ2 q) assumption is still the key unvalidated premise; Sec. V's 10% contamination bound is a branching-fraction estimate, not a signal-region simulation.","rationale":"The paper is a competent, standard cut-based collider study: FeynRules/MadGraph/Pythia/Delphes chain, K factors for the main backgrounds and for sLQ pair production, and clearly presented cutflows. The central observation that t-channel LQ exchange can dominate VLL pair production at large LQ mass is supported by the benchmark cross sections. The reader's conditional verdict is appropriate. I agree that the branching-ratio assumption is the most fragile input, and I add that the paper's own mitigation in Sec. V is not a detector-level check: a 0.3 SM coupling implies Br(LQ → SM lepton + jet) ≈ 8%, but the contamination rate in the selected signal regions is not equal to this branching fraction, and the PP/SP signal is simultaneously reduced. Testing this with a full two-coupling simulation is straightforward and would settle whether the contours in Figs. 4 and 5 need revision. Because this is an addressable phenomenological simplification rather than an internal inconsistency, the verdict remains CONDITIONAL; the caveat should be made explicit in a revision, and the Sec. V statement should either be upgraded to a simulated bound or softened.","tokens_in":19355,"tokens_out":27759,"duration_ms":324837,"concrete_test":"Extend the existing UFO model to include a small SM lepton-quark coupling, e.g. λ_qℓ = 0.3 with λ_qτ2 = 1 for eU1 and for S1. Generate the complete processes pp → LQ LQ, pp → LQ τ2 + j, and pp → τ2 τ2 with both LQ decay modes, run them through Pythia8/Delphes3 with the same detector card, apply the exact Table III selection for the mono- and di-lepton channels, and compare (i) the fraction of selected events originating from LQ → SM lepton + jet decays and (ii) the 2σ/5σ contours in the M_LQ–λ plane with the Br = 1 results. If the contamination exceeds 10% or the contours shift by more than about 20% in λ, the branching-ratio assumption must be treated as part of the model rather than a harmless default.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central reach in Figs. 4 and 5 is computed under two explicit assumptions: LQ couplings to SM quark-lepton pairs are negligible (Sec. II after Table I), and the LQ→τ2 q branching ratio is nearly 100% (Sec. III A). These are not derived from a UV model, and the only numerical control offered in Sec. V, the statement that ℓqqℓ ≲ 0.3 keeps contamination below 10%, is a branching-ratio estimate for constructively interfering eU1/eS1 in the dilepton channel only. It does not establish that direct LQ→SM lepton + jet decays contaminate the actual mono- and di-lepton signal regions of Table III at the claimed level, because those events have different kinematics and selection efficiencies, and it does not account for the reduction of the PP and SP signal by Br² or Br. Since the discovery and exclusion contours are the paper's central claim, this free parameter assumption is load-bearing. It should be either derived from an explicit 4321-type flavour structure or validated by a full simulation with both decay modes included.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a scenario in which a heavy vectorlike tau (τ2) is produced at the LHC through interactions with leptoquarks (LQs) that couple preferentially to first-generation quarks and the VLL. It considers the six scalar/vector LQ representations that can couple to a singlet VLL, computes pair, single, indirect (t-channel) and interference contributions to τ2 pair production, and performs a Delphes-based cut-and-count analysis of mono-lepton and di-lepton final states at the HL-LHC (14 TeV, 3 ab−1). The main output is a set of 2σ exclusion and 5σ discovery contours in the M_LQ–λ and M_LQ–M_τ2 planes, obtained under the assumptions that LQ couplings to SM quarks and leptons are negligible and that Br(LQ→τ2 q) ≈ 1. The central claim is that t-channel LQ exchange can make τ2 pair production much larger than the conventional electroweak production for O(1) couplings, opening a new probe of both LQs and VLLs.","tokens_in":19584,"tokens_out":10426,"duration_ms":104656,"significance":"The framework is timely and the proposed signatures are concrete and falsifiable. If the branching-ratio assumption is justified, the analysis demonstrates that the HL-LHC can probe LQ masses up to several TeV and coupling values well below the current sensitivity of standard LQ searches. The authors provide a reproducible simulation chain (FeynRules/MadGraph/Pythia/Delphes), report detailed cutflows and background cross sections, and make explicit predictions that can be tested by the ATLAS and CMS collaborations. The main weakness is the lack of a UV derivation or a dedicated simulation of the assumed LQ branching ratios; this limits the robustness of the claimed reach but is addressable in a revision.","major_comments":[{"comment":"The near-100% branching ratio for LQ→τ2 q is assumed (Sec. II, after Table I, and Sec. III.A, page 4) without a UV motivation. The PP and SP contributions in Eq. (8) and the low-mass parts of Figs. 4 and 5 depend directly on this Br, and the only numerical control offered in Sec. V (an ℓqqℓ coupling ≲ 0.3 for <10% contamination in the dilepton channel of eU1/eS1) is a branching-ratio estimate rather than a signal-region simulation; it does not cover the mono-lepton channel, does not account for Br²/Br suppression of the PP/SP signals, and does not include the different kinematics of LQ→SM lepton+jet events. Please either embed the model in an explicit flavour structure that guarantees the hierarchy or simulate both decay modes simultaneously and propagate the branching fraction through Eq. (8) into the contours.","section":"Sec. II and Sec. III.A"},{"comment":"The signal cross sections for SP, IP, and II are used at LO, and the only higher-order correction applied is a K-factor of 1.58 for sLQ pair production; vLQ PP is also left at LO. Since the vertical shaded bounds in Fig. 4 are derived from PP alone with x/y→0, the vLQ PP treatment directly affects the claimed model-independent mass limits. Please quantify the NLO uncertainty on vLQ PP (and ideally on SP/IP/II) or show that the contours are insensitive to a conservative K-factor range.","section":"Sec. III, page 4, and Fig. 4"},{"comment":"The scans extend to λ = 3.5, where the LQ width from the τ2 q coupling alone is Γ/M ∼ λ²/(16π) ≈ 0.24, so the narrow-width approximation used for the on-shell LQ decays in PP and SP is not valid and the coupling is in a nonperturbative regime. The authors should either truncate the scan at a perturbative value (e.g., λ ≲ 1 or at least specify the width used in the simulation), include finite-width/off-shell effects, or explicitly demonstrate that the large-λ contours are controlled by the t-channel IP contribution and are therefore insensitive to the NWA.","section":"Sec. III.A and Fig. 4"}],"minor_comments":[{"comment":"The text states that events are required to contain at least one AK4 jet and at least one fatjet, but this requirement is not listed among the cuts C1–C4 in Table III; please include it explicitly or clarify that it is part of the preselection.","section":"Sec. IV, Table III"},{"comment":"The caption uses parentheses to denote vLQ numbers but does not specify which vLQ species is used (e.g., U1 for S1, eU1 for eS1); please make the association explicit.","section":"Table IV"},{"comment":"The coupling notation (e.g., y^RR_{10,13}) is not defined in the table caption; a brief explanation of subscripts/superscripts would improve readability.","section":"Table I"},{"comment":"The 10% contamination statement would benefit from the explicit formula it comes from; as written, it is a single unquantified number that the reader cannot verify.","section":"Sec. V, last paragraph"},{"comment":"Reference [60] is cited as having considered a 4321 benchmark with VLL production via an off-shell LQ, but the listed reference is a CMS experimental search for pair-produced VLLs; this citation appears incorrect and should be checked.","section":"Introduction, page 1"},{"comment":"Since σII is negative for destructive interference (as noted for S1 and U1), the text should state explicitly that σII is a signed quantity.","section":"Eq. (8), Sec. V"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the scope of the journal and presents an interesting search strategy. The main concerns are the unvalidated branching-ratio assumption and the LO treatment of vLQ production. The authors should also check the citation of Ref. [60], which appears to point to an experimental paper rather than the theory benchmark described in the text. I recommend major revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Punchline: this is a competent extension of the group's existing leptoquark-to-right-handed-neutrino program to vectorlike tau partners. The new piece is showing that t-channel LQ exchange can dominate over electroweak production of a singlet VLL at the HL-LHC, and giving 2-sigma and 5-sigma contours in mono- and dilepton channels. But the whole reach sits on one unvalidated assumption: the LQ decays almost 100% of the time to tau2 plus a quark. If that branch is not near unity, the contours move a lot. The paper states the assumption clearly, but does not protect the claim with a UV model or a full simulation of competing decays.\n\nWhat is good: the study is systematic. It enumerates all LQ representations that can couple to a singlet VLL, separates PP/SP/IP/II contributions, explains the interference signs, provides a transparent cutflow with event tables, and gives HL-LHC projections. The simulation stack is standard and the background treatment is more careful than much of the phenomenology literature. The paper is honest about what it assumes; the problematic branching assumption is stated explicitly in Sec. III A, and Sec. V makes at least a crude attempt to bound contamination. That honesty should count in its favor.\n\nSoft spots, in order of size. First, the near-100% branching assumption. The Sec. V bound (lqq_l coupling below 0.3 keeps dilepton contamination below 10%) is a branching-fraction estimate for constructively interfering eU1 and eS1 in the dilepton channel only. It does not demonstrate that direct LQ to SM lepton plus jet decays leave the mono-lepton signal regions clean, and it ignores the fact that PP and SP signals are suppressed by Br^2 and Br respectively. Since the contours are the paper's main product, this needs to be either derived from a 4321-type flavour structure or checked with a simulation that includes both decay modes. Second, the largest scanned couplings (lambda up to 3.5) are in a regime where the narrow-width approximation for the LQ is questionable, and the paper does not discuss this. Third, signal cross sections for SP/IP are leading order, with a K-factor applied only to scalar LQ pair production; for the IP-dominated high-mass reach this could be a modest but real systematic. None of this kills the central idea, but it means the absolute reach numbers are provisional.\n\nVerdict: this paper deserves a serious referee. It is a clear, useful contribution to LQ and VLL searches, and the main production mechanism is credible within the stated assumptions. A revision that derives or simulates the branching ratio, and adds a caveat on the high-coupling regime, would make the quantitative claims solid. I would take it to a reading group and I would cite it if I were working in this area.","headline":"Useful and systematic HL-LHC study of LQ-mediated vectorlike tau production, but the advertised reach rests on an unvalidated near-100% Br(LQ -> tau2 + quark) assumption.","tokens_in":20184,"tokens_out":2553,"would_cite":true,"duration_ms":28481,"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":"Leptoquarks could dominate vectorlike tau production at LHC.","keywords":["vectorlike leptons","leptoquarks","indirect production","monolepton final state","dilepton final state","HL-LHC prospects","third-generation tau partner","beyond Standard Model"],"falsifier":"Measure the leptoquark branching fractions: if the decay to an ordinary charged lepton and a quark, $BR(\\ell q \\to \\ell q)$, is anywhere near the size of the decay to $\\tau_2$ plus a quark rather than being smaller by an order of magnitude or more, the predicted mono- and di-lepton signals and the derived HL-LHC contours do not hold. A direct null result from a dedicated HL-LHC search in the mass–coupling region the paper maps as a $5\\sigma$ discovery would also falsify the enhancement claim.","tokens_in":19110,"feed_emoji":"⚛️","tokens_out":11653,"duration_ms":108103,"temperature":0.7,"pith_summary":"Leptoquarks are colour-charged bosons that normally appear in LHC searches through decays to an ordinary quark and lepton, while vectorlike leptons are usually sought through electroweak pair production. This paper studies the case where the two sectors are coupled: leptoquarks decay almost entirely to a quark and a heavy vectorlike partner of the tau, so the copious QCD-like production of leptoquarks becomes a production channel for the heavy lepton. The authors show that this indirect, t-channel mechanism can overwhelm conventional electroweak production when the leptoquark–quark–VLL coupling is of order 0.3–1, and that with the full HL-LHC dataset of $3~\\mathrm{ab}^{-1}$ the mono-lepton and di-lepton final states can exclude or discover the framework for leptoquark masses up to several TeV. If right, this links two otherwise separate search programmes and exposes a parameter region that existing leptoquark bounds, derived from Standard Model decay modes, do not cover.","feed_headline":"Leptoquarks could dominate vectorlike tau production at LHC","feed_subtitle":"If so, the HL-LHC can discover or rule out the idea for leptoquark masses up to several TeV.","key_machinery":"The carrier of the argument is the coupling of a leptoquark (a colour-charged boson connecting a quark and a lepton) to a third-generation singlet vectorlike tau $\\tau_2$. For such a state, the possible LQ species are the scalars $S_1$, $\\widetilde S_1$, $R_2$ and the vectors $U_1$, $\\widetilde U_1$, $V_2$, each with a renormalisable interaction listed in Table I. The mechanism is the coupling-power hierarchy among the production modes: pair production (PP) is essentially coupling-independent, single production (SP) and the indirect–QED interference (II) scale as $\\lambda^2$, and indirect production (IP) via t-channel LQ exchange scales as $\\lambda^4$, so IP wins at large $\\lambda$ and high LQ mass. The paper assumes $M_{\\ell q} > M_{\\tau_2}$, so the LQ decays on shell to $\\tau_2$ plus a quark with branching ratio near 100%.","core_discovery":"The central claim is stated directly in the abstract: LQ-mediated processes enhance VLL production at the LHC. Concretely, when a leptoquark couples to a first-generation quark and a weak-singlet vectorlike tau $\\tau_2$, t-channel leptoquark exchange between two initial-state quarks produces $\\tau_2$ pairs with a cross section proportional to the fourth power of the coupling $\\lambda$; for $\\lambda$ in the range roughly 0.3 to 1, this indirect production dominates the electroweak $Z/\\gamma^*$ channel at large leptoquark mass. The interference between the two amplitudes is destructive for some leptoquark species and constructive for others, which shifts the reach, and the paper works through all six singlet-VLL-capable leptoquarks. With the $\\tau_2$ decaying to $W\\nu$ or $Z\\tau$, the mono-lepton final state has slightly better sensitivity than the di-lepton final state, and the HL-LHC can give $2\\sigma$ exclusion or $5\\sigma$ discovery contours in the mass–coupling plane up to several TeV.","pith_inferences":["Reading beyond the paper: if the enhancement is real, recasting old leptoquark limits from SM-decay searches will miss the VLL-decay region entirely; dedicated VLL-plus-jet searches are needed to cover it.","The $\\lambda^4$ dependence of indirect production suggests a way to extract the coupling: measuring $\\tau_2$-pair production at two LQ masses, or comparing the IP-dominated high-mass rate with the PP-dominated low-mass rate, could constrain $\\lambda$ without relying on the assumed branching ratio for the decay.","The same mechanism should transfer to other flavours, such as a vectorlike muon partner coupled to second-generation quarks; the parton luminosities and backgrounds would shift, but the enhancement logic is flavour-generic.","If the $\\tau_1$–$\\tau_2$ mixing is smaller than the value implied by an SM-tau-sized off-diagonal term, the decays become displaced, turning these prompt signatures into long-lived-particle searches; the paper notes this possibility without quantifying it."],"forward_implications":["The mono-lepton channel is the most sensitive single search, so lepton-plus-jets-plus-missing-energy analyses are the first place to look for this class of models.","For order-one couplings, the HL-LHC reach extends to leptoquark masses of several TeV, well beyond the roughly 1.5–2 TeV limits that assume leptoquarks decay directly to Standard Model fermions.","Existing leptoquark mass limits do not constrain the parameter region where leptoquarks decay mainly to vectorlike taus, so that region is currently unexplored and should be searched with VLL-plus-jet topologies.","The $\\lambda^4$ scaling of indirect production makes the production-rate dependence on coupling qualitatively different from electroweak production, providing a kinematic handle to separate the two mechanisms if a signal appears.","The constructive-interference leptoquark species (notably $\\widetilde U_1$) give a stronger reach than their destructive-interference partners, so searches are best targeted at those states first."],"supporting_citations":[{"why":"Supplies the scalar-leptoquark production decomposition (PP, SP, IP, II) and the LHC limits this work builds on.","marker":"[49]"},{"why":"Supplies the analogous vector-leptoquark production framework and exclusion limits for the vector-LQ species.","marker":"[54]"},{"why":"Gives the current LHC mass bounds for third-generation vectorlike leptons, the baseline the new production mechanism is compared with.","marker":"[28]"},{"why":"Describes the existing 4321-model search with QED-produced VLLs, the benchmark this paper replaces with on-shell LQ decays to VLLs.","marker":"[60]"},{"why":"Establishes the related method of producing heavy fermion pairs through second-generation leptoquark exchange.","marker":"[5]"},{"why":"Motivates the coexistence of leptoquarks and vectorlike leptons in UV-complete gauge models.","marker":"[57]"}],"fun_headline_variants":["Leptoquarks could be main source of vectorlike taus","Leptoquark t-channel dominates vectorlike tau pairs","HL-LHC may expose leptoquark-driven tau partners","Leptoquark exchange boosts vectorlike tau yield"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the leptoquark decays to a vectorlike tau plus a quark with nearly 100% branching fraction, which in turn requires the leptoquark's couplings to ordinary quark–lepton pairs to be negligible.","fun_headline_variants_meta":{"raw":{"variants":["Leptoquarks could be main source of vectorlike taus","Leptoquark t-channel dominates vectorlike tau pairs","HL-LHC may expose leptoquark-driven tau partners","Leptoquark exchange boosts vectorlike tau yield"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00071,"raw_usage":{"total_tokens":3158,"prompt_tokens":868,"completion_tokens":2290,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":484,"completion_tokens_details":{"reasoning_tokens":2222}},"tokens_in":484,"tokens_out":2290,"duration_ms":19479,"temperature":1.0,"reasoning_tokens":2222,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T16:58:11.347281+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the leptoquark branching fractions: if the decay to an ordinary charged lepton and a quark, $BR(\\ell q \\to \\ell q)$, is anywhere near the size of the decay to $\\tau_2$ plus a quark rather than being smaller by an order of magnitude or more, the predicted mono- and di-lepton signals and the derived HL-LHC contours do not hold. A direct null result from a dedicated HL-LHC search in the mass–coupling region the paper maps as a $5\\sigma$ discovery would also falsify the enhancement claim.","supporting_citations":[],"review_version":2}