{"id":"bbce2def-3814-464a-8022-0d4662283bff","arxiv_id":"2504.19693","paper_version":1,"verdict":"REJECT","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"high","formal_verification":"none","parameter_count":6,"one_line_summary":"A new 'emerging photon jet' signature in the hadronic calorimeter could make neutral long-lived particles decaying to photons visible at the HL-LHC.","lead":"This paper proposes a new way to spot long-lived particles that decay into photons deep inside the LHC's hadronic calorimeter, leaving a jet with no tracks and no electromagnetic energy. It demonstrates the idea on a fermiophobic Higgs model and claims discovery-level sensitivity at the HL-LHC.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Table I signal efficiencies are unsupported: the paper applies analytic decay probabilities (Eq. 15) to Delphes events but never models the hf decay vertex, so the HCAL-only photon jet—with zero ECAL energy and no tracks—is an assumed response, not a simulated one.","rationale":"The paper's abstract and conclusion claim discovery-level sensitivity for a new LLP signature, and the demonstration rests entirely on Table I's cut-flow. The reader identified the load-bearing weakness as the absence of any displaced-vertex handling in the Delphes simulation; the manuscript text confirms this by computing decay-region probabilities analytically in Eq. 15 rather than describing a mechanism that places the hf decay inside the HCAL. I agree that this is the single most decisive issue. If standard Delphes deposits photon energy into the ECAL irrespective of production vertex, as its calorimeter parameterization does, then the R_j1_had>950 cut cannot retain the large signal fractions shown in Table I; those fractions are the difference between a 0.6σ and a 6–12σ result. The paper's additional limitations—a single irreducible background, a 10% assumed background uncertainty, and selection cuts optimized on the same samples—are secondary because even a perfect background model cannot rescue an unsimulated signal. The physics of the fermiophobic Type-I 2HDM and the hf lifetime estimate appear reasonable, and the proposed signature is conceptually interesting; however, as written, the central quantitative claim is not supported. The verdict should remain REJECT, with the path to acceptance being a validated displaced-vertex simulation, which is eminently checkable.","tokens_in":21316,"tokens_out":10485,"duration_ms":109999,"concrete_test":"Generate the MH±=200 GeV benchmark signal with the hf lifetime turned on in Pythia (e.g., set the hf width to give cτ≈5 cm) so that Pythia produces displaced γγ vertices, and run the unmodified Delphes HLLHC card. Count how many events with exactly one hf decaying inside the HCAL radius window [L_i_HCAL,L_f_HCAL]=[2.25,3.9] m yield a leading jet that passes R_j1_had>950 and N_charged≤1. If the fraction is decades smaller than the 55.7% quoted for the R>950 step (or if no jet is reconstructed from HCAL-only energy), Table I is an artifact of external weighting. A minimal cross-check is to inject a single γγ pair at a vertex radius 2.5 m into the Delphes HLLHC input and verify the jet's ECAL energy is zero.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central quantitative claim is a detector-level significance of >5σ for pp→H±hf→ℓ±ν γcol J_HCAL. That significance flows from the cut-flow in Table I, where the two most powerful cuts are R_j1_had>950 and N_charged≤1. For these to be meaningful, Delphes must reconstruct the hf→γγ decay taking place inside the HCAL as a jet with essentially zero ECAL energy and no charged constituents. The paper, however, describes no such modeling. The signal events are generated with MadGraph/Pythia, and Delphes with the HLLHC card is run on them; the hf decay region is then chosen externally through the exponential probabilities in Eq. 15 (with L_ECAL, L_i_HCAL, L_f_HCAL boundaries). If the hf is left as a stable particle in the event record, the Delphes photon reconstruction has no object that originates at the HCAL; if the hf is decayed promptly in Pythia, the two photons are standard EM objects whose energy is assigned to the ECAL. In neither case does the reconstruction produce an HCAL-only jet of the kind the selection assumes. The high survival rates of the R>950 cut (13.5–66%) are therefore not credible from the described workflow: a photon-initiated jet in the ECAL would have R≪1, not >950. The 10% background systematic used in Eq. 16 and the single irreducible background pp→ℓν jγ further soften the result, but the load-bearing issue is the simulated signal response. Until a displaced-vertex-capable simulation (or a validated modification of Delphes) is shown to produce the HCAL-only jet, the claimed significance is unsupported.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a new long-lived-particle (LLP) signature: a neutral LLP decaying to a collimated photon pair inside the hadronic calorimeter, producing a trackless jet with no ECAL energy ('emerging photon jet'). As a benchmark, the authors use the fermiophobic Type-I two-Higgs-doublet model with an ultralight h_f and study the 'golden channel' pp -> H± h_f -> W± h_f h_f -> l± nu gamma_col J_gamma_HCAL. They compute the h_f decay width and lifetime, constrain the parameter space using the public codes 2HDMC, 2HDME, and HiggsTools, and use MadGraph/Pythia/Delphes to estimate signal and background rates. They report discovery-level sensitivity (>5 sigma) at the HL-LHC over a large fraction of the allowed parameter space.","tokens_in":21593,"tokens_out":8936,"duration_ms":101392,"significance":"The proposed observable is genuinely novel: existing LLP searches for photon decays focus on the ECAL or on displaced vertices, not on electromagnetic showers initiated inside the HCAL. The analytic treatment of Gamma(h_f -> gamma gamma) and the use of external constraint codes are careful, and the paper provides explicit benchmark cross sections and a cut-flow table. If the detector-level modeling were validated, the strategy could be broadly applicable to other neutral LLPs decaying to photons. However, the quantitative claims in Table I and Figure 4 rest entirely on an unvalidated—and, with the stated tools, likely invalid—simulation of the HCAL-origin photon jet. This is the central load-bearing issue of the manuscript.","major_comments":[{"comment":"The background estimate is incomplete. Only the irreducible process pp -> l nu j gamma is simulated. At the HL-LHC, reducible backgrounds such as W+jets with a jet misidentified as a photon, gamma+jets with a jet misidentified as a lepton, and t-tbar production can contribute to the same final state after object reconstruction. The trackless-jet and R_j1_had cuts may suppress some of these sources, but no fake-rate estimates or reducible-background estimates are provided. The quoted significance assumes a 10% systematic uncertainty on the irreducible background alone; this does not cover additional background contributions. The final NB after all cuts in Table I is therefore not established, and the 5-sigma claim depends on this number.","section":"Section III, background paragraph and Table I"}],"minor_comments":[{"comment":"The notation in the significance formula is inconsistent: the second logarithm uses delta_b in the denominator after delta_B was defined in the text. The formula should be harmonized and the variables defined precisely.","section":"Eq. (16)"},{"comment":"The decay probabilities in Eq. (15) use fixed radial distances L_ECAL, L_i_HCAL, and L_f_HCAL. For pseudorapidities up to |eta| < 2.5, the path length through the ECAL and HCAL depends on eta; the approximation of a purely cylindrical geometry should be stated, and its impact on the acceptance should be estimated.","section":"Eq. (15) and detector geometry"},{"comment":"The four selection cuts are optimized on the same simulated samples used to quote the final significances. A discussion of possible overfitting, or a validation on an independent sample, would increase confidence in the quoted significance values.","section":"Table I and Figure 4"},{"comment":"There are several typographical errors, including 'a single photon. 4 due to' in Section II and 'detecter' in footnote 5, which should be corrected in a revised version.","section":"Throughout"}],"recommendation":"reject","confidential_remarks":"The model calculation and the proposed signature are interesting, but the central quantitative result is invalidated by the missing and likely impossible simulation of HCAL-origin EM showers with the stated tools. If the authors can provide a validated displaced-vertex simulation and a more complete background estimate, a resubmission could be considered; in its present form the paper does not support its main claim."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe genuinely new thing here is the signature: a neutral LLP decaying to a photon pair inside the hadronic calorimeter, giving a trackless jet with no ECAL energy. As far as I can tell, that specific case hasn't been dedicatedly studied before, and it's a reasonable direction for LLP searches. The model section is careful: the Type-I 2HDM setup, the fermiophobic condition, the decay width, and the parameter scan with 2HDMC/2HDME/HiggsTools all look standard and believable. They also correctly identify the upper bound on M_{A/H±} around 335 GeV from unitarity/perturbativity, and the universal cτ curve as a function of tβ is a nice observation.\n\nBut the detector-level analysis has a load-bearing problem. The paper uses MadGraph+Pythia+Delphes with the HLLHC card and then applies the decay probabilities of Eq. 15 as an analytical wrapper. Nothing in the described workflow places the hf decay vertex inside the HCAL. If hf decays promptly in Pythia, the photons hit the ECAL; if it's left stable, there are no photons at all. In neither case does the reconstruction produce a jet with zero ECAL energy and no tracks. The cut R_j1_had > 950 assumes that jet exists. Efficiencies like 13.5–66% after that cut are not something standard Delphes can produce from the stated setup. The paper doesn't mention any private modification to Delphes, Pythia, or the event record, so the quantitative sensitivity is unsupported as written.\n\nThe rest is softer. The background is just one irreducible pp→ℓν jγ process; no fakes from W+jets or ttbar are estimated. The cuts are optimized on the same samples used for the final significance, which is a mild optimism source. The 10% background systematic is asserted. These are fixable with more work, but they don't matter until the simulation question is resolved.\n\nThe idea itself may still be viable. A dedicated simulation with a proper displaced-vertex treatment (e.g., a modified Delphes or a full simulation with a custom calorimeter response) could validate it. But as it stands, the discovery-level claim doesn't hold up.\n\nRecommendation: send to peer review with a request for major revision. The novelty is sufficient and the model part is sound. A good referee should ask the authors to redo the signal simulation with genuinely displaced vertices and to expand the background estimate. If they can't reproduce the HCAL-only jet in simulation, the paper remains a proposal, not a projection.","headline":"Novel HCAL photon-jet signature with a solid model part, but the detector simulation as described cannot produce the purported signal, so the discovery claims are unsupported.","tokens_in":22218,"tokens_out":2800,"would_cite":false,"duration_ms":28992,"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":"A neutral long-lived particle that decays into photons inside the hadronic calorimeter would leave a trackless, ECAL-free jet, and this paper argues this 'emerging photon jet' reaches 5-sigma discovery at the HL-LHC over a broad region of…","keywords":["long-lived particles","emerging photon jet","hadronic calorimeter","fermiophobic Higgs","Type-I 2HDM","HL-LHC","trackless jets","diphoton decay"],"falsifier":"Generate $h_f\\to\\gamma\\gamma$ events with the decay vertex forced to a radius between about 2.25 and 3.9 m and run them through the same fast detector simulation used for the analysis. If the reconstructed leading jet does not show $E_{\\rm HCAL}/E_{\\rm ECAL}>950$ with at most one charged subparticle—for instance if the photon pair is reconstructed as a photon or deposits energy in the ECAL—then the central signature is not produced as claimed, and the quoted efficiencies, including the $5\\sigma$ contours, would not hold.","tokens_in":21007,"feed_emoji":"⚛️","tokens_out":13175,"duration_ms":129217,"temperature":0.7,"pith_summary":"The paper proposes a new way to look for neutral long-lived particles at the LHC: instead of searching for displaced tracks or displaced vertices, look for a jet of electromagnetic energy that appears entirely inside the hadronic calorimeter (HCAL), with no charged tracks and effectively no energy in the electromagnetic calorimeter (ECAL); the authors call this an emerging photon jet in the HCAL. They argue that such a jet is produced whenever a neutral LLP decays to a photon pair inside the HCAL, and they make the case concrete with the fermiophobic Higgs boson of the Type-I two-Higgs-doublet model, an ultralight scalar that decays exclusively to two photons and can live long enough to reach the calorimeter. In the golden channel, one such scalar decays in the ECAL as a collimated photon and the other in the HCAL as an emerging photon jet, together with a lepton and missing transverse momentum. Their detector-level analysis of this final state reports discovery-level significance at the high-luminosity LHC for a broad region of the model's allowed parameter space, with benchmark significances of about $7.6\\sigma$, $12.0\\sigma$, and $4.1\\sigma$ for charged-Higgs masses of 100, 200, and 300 GeV at $3\\ \\mathrm{ab}^{-1}$. If correct, the paper's main payoff is a previously unexplored search strategy that applies to any neutral LLP decaying to photons, not only to this particular Higgs model.","feed_headline":"Trackless photon jets reach 5-sigma discovery at the HL-LHC","feed_subtitle":"A long-lived Higgs decaying inside the hadron calorimeter leaves a trackless, ECAL-free jet—enough for 5-sigma reach.","key_machinery":"The central object is the emerging photon jet in the HCAL, $J^\\gamma_{\\rm HCAL}$: a jet-like object built from an electromagnetic shower initiated by a photon pair inside the hadronic calorimeter, with no associated inner-detector tracks and no ECAL deposits. The argument is carried by the combination of (i) the loop-induced diphoton decay $h_f\\to\\gamma\\gamma$, whose width scales as $m_{h_f}^3$ and makes an ultralight $h_f$ long-lived; (ii) the exponential radial-decay probabilities that assign one decaying scalar to the ECAL branch ($\\gamma_{\\rm col}$, a collimated photon pair reconstructed as a single photon) or the HCAL branch ($J^\\gamma_{\\rm HCAL}$) depending on the decay radius $d_{\\rm rad}=c\\tau\\,p_T/m_{h_f}$; and (iii) four cut variables that strip away the $W+j+\\gamma$ background, above all the ratio $R_{j_1}^{\\rm had}=E_{\\rm HCAL}/E_{\\rm ECAL}>950$, which exploits the fact that ordinary QCD jets deposit significant energy in the ECAL from prompt $\\pi^0$ decays while the HCAL-emerging jet does not.","core_discovery":"On the paper's own terms, the central discovery is that a neutral LLP decaying to photons inside the hadronic calorimeter leaves an experimentally recognizable object—an emerging photon jet, $J^\\gamma_{\\rm HCAL}$—that is essentially free of Standard-Model background once a few simple kinematic requirements are imposed. For the ultralight fermiophobic Higgs $h_f$ with $m_{h_f}=0.5$ GeV in the Type-I 2HDM, the analysis starts from $pp\\to H^\\pm h_f\\to W^\\pm h_f h_f$, requires one $h_f$ to decay inside the ECAL and be reconstructed as a single collimated photon $\\gamma_{\\rm col}$, and the other to decay inside the HCAL, with decay probabilities set by the exponential radial-decay law. After basic object cuts and $E_T^{\\rm miss}>50$ GeV, four variables separate signal from the irreducible $\\ell^\\pm\\nu j\\gamma$ background: the HCAL-to-ECAL energy ratio of the leading jet must exceed 950, the jet must contain at most one charged subparticle, the reconstructed $W$ must lie within $\\Delta R<2$ of the leading jet or photon, and the leading jet must carry most of the hadronic activity. The resulting significances at $3\\ \\mathrm{ab}^{-1}$ with a 10% background uncertainty are $7.6\\sigma$, $12.0\\sigma$, and $4.1\\sigma$ for $M_{H^\\pm}=100,200,300$ GeV, and scanning the full theoretically and experimentally allowed parameter space yields $5\\sigma$ discovery across a broad region, with the charged Higgs mass bounded below about 335 GeV.","pith_inferences":["Because the four selection variables are defined entirely at the object level, the same search could be run for any photon-decaying LLP—such as axion-like particles or dark scalars—once the production channel is reweighted; the HCAL/ECAL ratio and trackless-jet requirements would carry over.","The geometric window from the ECAL outer radius near 2 m to the HCAL outer radius near 3.9 m implies the signature is most sensitive to proper lifetimes around the centimetre-to-metre scale, so this search naturally complements inner-detector displaced-vertex searches at shorter lifetimes.","A data-driven estimate of the trackless-jet background, for example from a control sample of prompt isolated photons or from $W+$jets events with a late photon conversion, would test whether the residual background is as small as the simulation reports."],"forward_implications":["Any neutral LLP that decays predominantly to photons and whose decay length places it inside the HCAL becomes, in principle, searchable with the same trackless-jet selection; the discovery claim does not depend on the details of the fermiophobic Higgs sector.","Within the fermiophobic Type-I model, the full experimentally allowed parameter space is testable at the HL-LHC, because the charged Higgs mass is capped near 335 GeV and $H^\\pm\\to W^\\pm h_f$ is essentially 100%.","The four-variable cut flow reduces the irreducible $\\ell^\\pm\\nu j\\gamma$ background by roughly three orders of magnitude, from 193 fb at the basic level to about 0.06 fb, which is what makes the otherwise small signal cross sections visible.","For the $M_{H^\\pm}=300$ GeV benchmark the predicted $4.1\\sigma$ excess falls short of the usual $5\\sigma$ discovery threshold but would constitute strong evidence; higher luminosity or a small reduction in the assumed 10% background systematic would push it over."],"supporting_citations":[{"why":"Maps the LLP search landscape and motivates specialized signatures for neutral long-lived particles.","marker":"[4]"},{"why":"The closest existing experimental search for trackless jets from LLPs, framing the new HCAL-emerging photon jet as an extension.","marker":"[62]"},{"why":"Defines the fermiophobic Type-I parameter space and the cutoff-scale constraints used to select viable model points.","marker":"[134]"},{"why":"Supplies the loop-induced partial width for $h_f\\to\\gamma\\gamma$, whose $m_{h_f}^3$ scaling produces the long lifetime.","marker":"[144]"},{"why":"Provides the normalized $h_fH^+H^-$ coupling entering the diphoton width calculation.","marker":"[145]"},{"why":"Supports the reconstruction of a collimated photon pair as a single ECAL photon, which defines the $\\gamma_{\\rm col}$ branch of the golden channel.","marker":"[147]"},{"why":"Supplies the fast detector simulation and the high-luminosity LHC configuration used for all reconstructed signal and background objects.","marker":"[154]"},{"why":"Provides the profile-likelihood significance formula behind every quoted sigma value.","marker":"[159]"}],"fun_headline_variants":["Photon jets inside calorimeter expose long-lived particles at HL-LHC","New LLP search: photon jets emerging in hadron calorimeter","5-sigma reach with hadronic photon jets from long-lived decays","Trackless photon jets: a fresh probe for LLP decays at the LHC","Emerging photon jets in HCAL yield discovery-level sensitivity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the fast detector simulation treats a photon pair that materializes inside the hadronic calorimeter as depositing essentially all its energy in the HCAL, with no ECAL deposit and no charged tracks; if the simulation instead routes that energy to the ECAL or reconstructs the pair as a photon, the main selection cut $R_{j_1}^{\\rm had}>950$ fails and the signal rates collapse.","fun_headline_variants_meta":{"raw":{"variants":["Photon jets inside calorimeter expose long-lived particles at HL-LHC","New LLP search: photon jets emerging in hadron calorimeter","5-sigma reach with hadronic photon jets from long-lived decays","Trackless photon jets: a fresh probe for LLP decays at the LHC","Emerging photon jets in HCAL yield discovery-level sensitivity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000326,"raw_usage":{"total_tokens":1960,"prompt_tokens":1218,"completion_tokens":742,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":834,"completion_tokens_details":{"reasoning_tokens":651}},"tokens_in":834,"tokens_out":742,"duration_ms":7244,"temperature":1.0,"reasoning_tokens":651,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T05:46:43.339847+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Generate $h_f\\to\\gamma\\gamma$ events with the decay vertex forced to a radius between about 2.25 and 3.9 m and run them through the same fast detector simulation used for the analysis. If the reconstructed leading jet does not show $E_{\\rm HCAL}/E_{\\rm ECAL}>950$ with at most one charged subparticle—for instance if the photon pair is reconstructed as a photon or deposits energy in the ECAL—then the central signature is not produced as claimed, and the quoted efficiencies, including the $5\\sigma$ contours, would not hold.","supporting_citations":[{"cited_title":"Probing Light Fermiophobic Higgs Boson via diphoton jets at the HL-LHC","cited_arxiv_id":"2310.17741","evidence_quote":"Supports the reconstruction of a collimated photon pair as a single ECAL photon, which defines the $\\gamma_{\\rm col}$ branch of the golden channel."}],"review_version":1}