{"id":"fa90d4a1-f3e4-4d13-82d7-58f27d5f071b","arxiv_id":"2505.05952","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":3.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"CODEX-b is a proposed 10-meter cube near LHCb for catching long-lived new particles, and this ESPP paper presents optimized cheaper designs, cost and timeline estimates, and the CODEX-β demonstrator already installed in the cavern.","lead":"This is the CODEX-b collaboration's status report and design review for the European Strategy for Particle Physics, covering the physics case, optimized detector layouts, cost and timeline estimates, and the installed CODEX-β demonstrator. It contains no new measurements, but argues that a compact, non-hermetic detector next to LHCb could be a cost-effective way to search for long-lived particles that other detectors would miss.","discovery_kind":"review","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Zero-background premise remains unvalidated: Fig. 9 sensitivity assumes no backgrounds, while the full MIB/cavern simulation is still under development and CODEX-β has yet to demonstrate the required background level.","rationale":"The central assertion of the paper is that a full 10 × 10 × 10 m^3 CODEX-b can be built in the UXA cavern with performance comparable to the baseline at reduced cost and instrumentation. That assertion has two independent supports: the geometry and vertex-efficiency optimizations in Sections III B–C and Table I, and the background-control design in Section III D. I examined both. The geometry support is weakened by an internal tension: Table I lists relative vertex reconstruction efficiencies for the reduced scenarios 2 and 3 as low as 0.22–0.42 for the b→sS' benchmark at m_S = 2.5–4 GeV, which is below the '50–90%' range quoted in the text and would shift the dark-Higgs reach in Fig. 9 substantially, not by a 'small' logarithmic amount. This is a real but secondary concern: scenario 1 retains 0.81–0.94 relative efficiencies with a smaller module reduction, and the final design can in principle be re-optimized per benchmark before freezing. The background premise is more load-bearing because it underpins every sensitivity curve in Fig. 9 and the 'zero background environment' advantage asserted in Section I. The paper's own status statements show this premise is not yet established: the full simulation including MIB is under development, and CODEX-β goal 2 is to demonstrate zero-background operation. The CODEX-β data-taking beginning in 2025 provides a direct near-term test. If Table V's prediction is confirmed to within uncertainties, the simulation framework gains credibility; if it is not, the zero-background advantage and all background-free projections fail together. This does not change the reader's CONDITIONAL verdict; it sharpens the condition that must be met before those projections can be treated as reliable. I agree with the reader's identification of the weakest assumption.","tokens_in":18549,"tokens_out":6915,"duration_ms":72667,"concrete_test":"Use the first ~15 fb^-1 of Run 3 CODEX-β data to measure the four-or-more-track event rate (Ekin > 0.4 GeV per track) inside the 2 × 2 × 2 m^3 volume and compare it with the Table V prediction of (9.34 ± 2.10) × 10^4 events. If the observed rate is consistent with the prediction, the neutral-flux simulation framework is validated and confidence in the full-detector zero-background design increases. If the observed rate exceeds the prediction by more than a factor of two, or if an irreducible muon-induced neutral-secondary component is seen that would scale nonlinearly with the proposed 4.5 m Pb shield, the zero-background design basis is not established and the Fig. 9 sensitivity curves must be recomputed with background included.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing premise is the Section I claim of 'a zero background environment' at the CODEX-b site, which is the basis for the background-free sensitivity curves of Fig. 9 and for design choices that omit internal shielding. The support for this premise is the Geant4 shielding study in Section III D (3 m concrete plus 4.5 m Pb, with a single-layer active veto), together with earlier flux measurements [6]. The paper itself states that a full simulation including LHCb, cavern infrastructure, and machine-induced background is only 'being developed' (Section III D), and lists 'demonstrating that CODEX-b can be operated as a zero-background experiment' as an uncompleted CODEX-β goal (Section VII A). The dangerous channel is neutral secondaries, especially K0_L, produced within about 1 m of the far side of the shield by muons that stop in the shield and are invisible to the veto. If the true rate of such secondaries, or of soft cavern or MIB backgrounds, exceeds the simulation, the zero-background assumption fails. The paper provides a quantitative, testable anchor: Table V predicts (9.34 ± 2.10) × 10^4 four-or-more-track neutral events in the 2 × 2 × 2 m^3 CODEX-β volume for Run 3 at 15 fb^-1 with only the concrete wall. Before that prediction and the full-detector shield simulation are validated against CODEX-β data, the Fig. 9 curves are unverified upper limits rather than demonstrated sensitivities.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This ESPP contribution from the CODEX-b collaboration summarizes the scientific case, simulation tools, detector design options, costs, timeline, and the status of the CODEX-β demonstrator for the proposed CODEX-b long-lived particle detector at LHCb. The central claim is that a full 10×10×10 m^3 detector can be realized in the nominal UXA cavern location using ATLAS Phase-II RPC modules in non-hermetic optimized configurations (scenarios 1–3) with 350–500 modules instead of the 800-module baseline, with vertex reconstruction efficiencies of 50–90% relative to the baseline and thus \"performance comparable\" to the original proposal, at substantially lower cost. The sensitivity projections (Fig. 9) are reproduced from the collaboration's earlier proposal and assume 100% vertex reconstruction efficiency and a zero-background environment, with background suppression to be provided by 3 m of concrete plus 4.5 m of lead and an active veto. The paper reports that CODEX-β is installed at IP8 and will be used to validate background rates, RPC performance, and LHCb readout integration.","tokens_in":18810,"tokens_out":7555,"duration_ms":74227,"significance":"If the central feasibility claim holds—in particular, once the zero-background environment is demonstrated and the optimized geometries achieve the quoted vertex reconstruction efficiencies—the paper makes a strong, cost-effective case for a transverse LLP detector complementary to the ATLAS/CMS/LHCb and forward programs. The manuscript's strengths are its transparency: it explicitly states the 100% vertex-efficiency assumption, quantifies the impact of realistic 40–90% efficiencies, lists the conservative assumptions in the background simulation, and provides a testable prediction (Table V) for CODEX-β. The construction cost and FTE estimates are grounded in the built CODEX-β demonstrator and are itemized with a stated contingency. The deterministic optimization framework of ref. [50] is a substantive methodological asset, although no code is bundled with this document. The main caveat is that the physics reach shown in Fig. 9 is an upper limit pending validation of the zero-background assumption; with that caveat stated, the document is a useful ESPP input rather than a claim of demonstrated sensitivity.","major_comments":[{"comment":"The central claim that \"a full detector may be realized ... with performance comparable to the original baseline proposal\" is not fully supported by the numbers in Table I. The text states that \"the studied configurations can achieve 50–90% relative vertex reconstruction efficiency,\" but Table I shows scenario 2 with relative efficiencies of 0.33(5) at m_S = 2.5 GeV and 0.22(4) at m_S = 4.0 GeV for the b→sS benchmark, and scenario 3 with 0.42(6) at 4.0 GeV. Because the baseline curves in Fig. 9 assume 100% vertex reconstruction efficiency, a relative efficiency of 0.22 corresponds to an absolute efficiency of only about 10–20% for realistic baseline efficiencies of 40–90%, which is not a small shift in the logarithmic sensitivity plots. The authors should either restrict the comparability claim to the benchmarks and scenarios that actually meet the 50–90% range, or quantify the effect of the lower efficiencies on the Fig. 9 curves.","section":"Section IV A, Table I"},{"comment":"The zero-background premise that underlies the Fig. 9 sensitivity curves is not yet validated. Section III D states that a full simulation including LHCb, cavern infrastructure, and machine-induced background \"is being developed,\" and Section VII A lists \"demonstrating that CODEX-b can be operated as a zero-background experiment\" as an uncompleted goal of CODEX-β. Section IV A itself calls the baseline curves \"plausible approximate upper limits,\" but Fig. 9 is presented as the reach without this caveat; moreover, Section VIII's statement that \"most of the fundamental assertions from its 2017 proposal\" have been confirmed is in tension with the unvalidated zero-background assertion. The authors should attach the upper-limit caveat to every appearance of Fig. 9, and should state explicitly what would change in the full-detector design if CODEX-β measures a non-zero background at a rate comparable to the Table V prediction rather than zero.","section":"Section III D; Fig. 9; Section VII A"},{"comment":"The treatment of neutral secondaries produced by muons stopping in the shield—the channel identified as the most dangerous background—rests on a single-layer active veto that does not use tracking information, together with simulation assumptions (100% detector response to neutral secondary decays, no angular exploitation, and no inclusion of non-zero angles of incidence) that have not been checked against data. Table V provides a quantitative, testable anchor: (9.34 ± 2.10) × 10^4 four-or-more-track neutral events are predicted in the CODEX-β volume for Run 3 at 15 fb^-1 with only the concrete wall. The paper should identify this as the key validation milestone and specify what design changes would be required (e.g., segmented/directional vetoes or additional internal shielding) if the CODEX-β data exceed this prediction.","section":"Section III D"}],"minor_comments":[{"comment":"The main text refers repeatedly to \"top left plot of fig. 9\" and \"bottom row of fig. 9,\" but Fig. 9 appears only in Appendix C; the cross-references should be adjusted or the figure should be moved into the main body.","section":"Section II A; Appendix C"},{"comment":"The gas mixture is spelled \"ECO65\" in most places but \"EC065\" in Section VI; the spelling should be made consistent.","section":"Section VI"},{"comment":"The caption says the efficiencies are \"cτ-averaged,\" but the averaging range and weighting are not specified; please state the lifetime range used.","section":"Table I"},{"comment":"Table VI lists columns \"baseline scenario 1 scenario 2,\" while the main text defines scenarios 1–3; the mapping should be clarified or the table columns renamed to match the main text.","section":"Appendix B, Table VI"},{"comment":"The simulation requirements (600 MeV minimum track momentum, 2 cm hit separation, 10 cm vertex resolution) are stated only after they are used in the optimization discussion; moving them to the start of the simulation description would improve readability.","section":"Section III C"}],"recommendation":"major_revision","confidential_remarks":"This is essentially a project status report and white paper for the ESPP, drawing heavily on the collaboration's own prior documents (refs. [4,5,50,52]). That is appropriate for the genre, but the journal should consider whether a paper with no new measurements and with a central feasibility claim that depends on an unvalidated background premise fits its usual scope. The referee sees no citation-practice concerns. The main risk is that the Fig. 9 reach curves, if republished without the upper-limit caveat, could be mistaken for demonstrated sensitivity; the revision should make the caveat unavoidable."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read as an ESPP contribution — which is what it is — this paper does its job. The new material is the three reduced-instrumentation scenarios with relative vertex efficiencies, plus concrete cost and manpower estimates. The physics-reach curves are updated from the collaboration's EOI and optimization papers, which is transparent and appropriate for a strategy document. The optimization framework in Section III is a real computational contribution, and the cost tables are specific enough to be useful.\n\nThe paper is honest about its assumptions. It states that the baseline sensitivity curves assume 100% vertex reconstruction efficiency, that realistic configurations give 40–90%, and that the curves should be treated as approximate upper limits. It explicitly lists zero-background operation as an uncompleted CODEX-β goal, and it gives a quantitative, testable anchor in Table V for CODEX-β's expected neutral background rate. That is the right degree of candor for a proposal.\n\nThe soft spot is the zero-background premise itself. The introduction asserts it, and Fig. 9 assumes it, but the full simulation including LHCb, the cavern, and machine-induced background is still under development, and CODEX-β has not yet taken data. The stress-test concern is correct: until the CODEX-β background measurement validates the simulation, the sensitivity curves are optimistic upper limits, not demonstrated sensitivities. This is not hidden, and it is not fatal — the paper says CODEX-β is meant to close exactly this gap — but it does make the central viability claim conditional. There is also a smaller caveat: the 'performance comparable to baseline' for the reduced scenarios is established relative to an idealized baseline, and engineering constraints could move the efficiency numbers further. The paper acknowledges this by calling the scenarios illustrations, so it is a minor point.\n\nThe right audience is the LLP community and the ESPP process. Anyone comparing transverse LLP detectors will get a clear, up-to-date picture of CODEX-b's status, cost, and open questions. I would take it seriously and send it to peer review; the open background question is concrete and testable. The one thing I would ask in a referee report is that the zero-background validation be framed explicitly as a gating milestone for the sensitivity projections.","headline":"A transparent and useful ESPP status update for CODEX-b, with genuinely new cost and design-scenario detail, but the zero-background premise is still an assumption awaiting CODEX-β data.","tokens_in":19663,"tokens_out":4402,"would_cite":true,"duration_ms":43035,"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":"CODEX-b, a proposed 10-meter cubic detector beside LHCb, can be built with roughly half its planned RPC panels while keeping performance comparable to the original baseline design.","keywords":["long-lived particles","CODEX-b detector","displaced vertex searches","resistive plate chambers","background suppression","detector geometry optimization","LHCb","high-luminosity LHC"],"falsifier":"Watch the CODEX-β demonstrator's first Run 3 data: it measures the neutral multitrack and soft-background rates in the shielded cavern, which are compared to the simulated predictions of Section III D. If the measured rates, scaled to the full 10×10×10 m³ volume behind the 4.5 m lead shield and active veto, exceed the zero-background expectation by even a handful of events per nominal year, the design basis fails and the Fig. 9 sensitivity curves become optimistic upper limits rather than projections.","tokens_in":18359,"feed_emoji":"⚛️","tokens_out":19264,"duration_ms":167408,"temperature":0.7,"pith_summary":"This paper argues that CODEX-b—a proposed 10×10×10 m³ box of resistive-plate chambers placed about 25 m sideways from the LHCb interaction point to catch long-lived particles (LLPs) that decay far from the collision—can be built with performance comparable to the original baseline design while using roughly half the tracking panels and about half the money. This matters because a transverse detector covers a slice of LLP parameter space—light masses and long lifetimes—that the main LHC experiments and forward detectors cover poorly, and CODEX-b would do it with existing technology at a modest scale. The feasibility claim is grounded in an optimization framework showing that 350 RPC panels (or 250 plus scintillator) recover 50–90% of the baseline vertex-reconstruction efficiency, cutting the estimated cost from about €51M to about €24M and shrinking build time and crew. The paper also reports that the small demonstrator CODEX-β is already installed in the LHCb cavern, with its first job being to prove the site really is a zero-background environment—the assumption on which the whole physics case rests.","feed_headline":"CODEX-b can match its planned reach with 350 panels instead of 800","feed_subtitle":"Optimized detector layouts preserve the physics case while cutting the estimated bill from €51M to €24M.","key_machinery":"The argument is carried by three mechanisms. The first is a fast-simulation and deterministic optimization framework—set-theoretic with branch-and-bound, scanning exponentially many panel configurations in linear time—whose efficiency-versus-panel-count curves show strong negative curvature: vertex-reconstruction efficiency plateaus well before hermetic coverage, so panels beyond a few hundred add little. The second is the background-suppression scheme: the 3 m concrete UXA wall (about 7 nuclear interaction lengths) and 4.5 m of lead (about 25 interaction lengths) kill primary hadron fluxes, while an active veto layer inside the shield catches the muons that stop short of the detector after producing neutral secondaries in the shielding material itself. The third is technology transfer: CODEX-b adopts the ATLAS Phase-II triplet RPC modules (1.03 × 1.88 m² BIS7-type singlets), and the 2×2×2 m³ CODEX-β demonstrator—42 such singlets already installed at IP8—provides the measured per-module cost, build time, and LHCb-integration experience from which all full-detector estimates are scaled.","core_discovery":"CODEX-b (COmpact Detector for EXotics at LHCb) is a proposed 10×10×10 m³ cube of resistive-plate chambers (RPCs), placed about 25 m transverse to the LHCb interaction point IP8 and shielded from collision debris by the existing 3 m concrete UXA wall plus a new 4.5 m lead shield with an embedded active veto. The paper's central assertion is that the full detector can be realized in the nominal location with performance comparable to the original baseline proposal: non-hermetic layouts using 350 RPC triplet panels—or 250 panels augmented with scintillator—retain 50–90% of the baseline vertex-reconstruction efficiency, which shifts the logarithmic sensitivity curves of Fig. 9 only slightly. These leaner configurations cost about €24M instead of €51M and need correspondingly less installation time and manpower, so the reduced-instrumentation design is presented not as a compromise but as the realistic route to construction. The case rests on the site being a zero-background environment, which the installed CODEX-β demonstrator is designed to validate during Run 3 before the collaboration commits to the full detector.","pith_inferences":["The geometry-optimization approach—searching exponentially many layouts in linear time—should transfer to other proposed large-volume LLP detectors whose costs are dominated by hermetic tracking coverage, not just to CODEX-b.","If the zero-background claim holds, the physics case generalizes beyond the benchmark plots: CODEX-b would probe any LLP model decaying to two charged tracks with masses near 0.1–10 GeV and $c\\tau$ from meters to hundreds of meters, a region where the main LHC experiments are trigger- and background-limited.","The CODEX-β plan to reconstruct $K^0_S$ decay vertices and extract the $K^0_S$ lifetime and boost distribution could turn a background-calibration measurement into a standalone physics result and a cross-check of the same quantities measured at the interaction point.","Because installation can proceed during LHC operation, a partially instrumented CODEX-b could start taking useful data—especially for shorter-lived LLPs—before the full cubic volume is complete, yielding physics return during construction."],"forward_implications":["The full CODEX-b detector can be installed in its nominal cavern location with 350 RPC triplet panels—or 250 panels plus scintillator—retaining 50–90% of the baseline vertex-reconstruction efficiency at a cost near €24M instead of €51M.","The baseline sensitivity curves of Fig. 9 remain plausible approximate upper limits for the leaner designs, so the projected reach for dark photons, dark Higgs scalars, axion-like particles, and heavy neutral leptons does not materially shrink.","If CODEX-β demonstrates zero-background operation and successful LHCb readout integration, the collaboration expects to finalize a Technical Design Report and begin staged construction, with a full install targeted for around 2030.","A recirculating eco-gas system would cut the dominant operational cost—the gas itself—by a large factor, with the final eco-mixture expected to be substantially cheaper than today's ECO65.","Because CODEX-b events are read out together with LHCb, any discovered LLP decay would come with matched vertex and flavor information from the main detector, aiding the interpretation of a signal."],"supporting_citations":[{"why":"Original CODEX-b proposal that fixes the 10×10×10 m³ baseline geometry, the transverse location 25 m from IP8, and the initial background-suppression concept.","marker":"[4]"},{"why":"Expression of interest defining the benchmark models, the 3 m concrete plus 4.5 m lead shielding baseline, and the sensitivity projections that Fig. 9 reproduces.","marker":"[5]"},{"why":"The 2018 in-situ background measurement campaign at the proposed site that the zero-background claim builds on and CODEX-β is designed to validate.","marker":"[6]"},{"why":"ATLAS Phase-II muon spectrometer design that supplies the triplet RPC module adopted as CODEX-b's tracking technology.","marker":"[49]"},{"why":"The fast-simulation and deterministic optimization framework from which the reduced-panel layouts and efficiency-versus-panel-count results are taken.","marker":"[50]"},{"why":"Simulation toolkit used for the shielding and neutral-secondary background studies.","marker":"[51]"},{"why":"CODEX-β technical design report that provides the per-module cost, build-time, and personnel figures scaled up for the full detector.","marker":"[52]"},{"why":"The existing LHCb scintillator veto detector whose design experience carries over to the CODEX-b active shield.","marker":"[54]"}],"fun_headline_variants":["CODEX-b proves 350 panels can match its planned reach","CODEX-b slashes panel count to 350, halving cost to €24M","CODEX-b: fewer panels, lower cost, same long-lived particle hunt","CODEX-b's lean design retains physics with 350 panels"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the site truly runs at zero background—the concrete and lead shield plus an active veto must keep muon-induced neutral secondaries and soft cavern noise so rare that nothing mimics an LLP decay over the experiment's lifetime; this is simulated in Section III D but not yet measured, and Section VII A lists demonstrating it as the first goal of the installed CODEX-β demonstrator.","fun_headline_variants_meta":{"raw":{"variants":["CODEX-b proves 350 panels can match its planned reach","CODEX-b slashes panel count to 350, halving cost to €24M","CODEX-b: fewer panels, lower cost, same long-lived particle hunt","CODEX-b's lean design retains physics with 350 panels"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000671,"raw_usage":{"total_tokens":3026,"prompt_tokens":884,"completion_tokens":2142,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":500,"completion_tokens_details":{"reasoning_tokens":2062}},"tokens_in":500,"tokens_out":2142,"duration_ms":17270,"temperature":1.0,"reasoning_tokens":2062,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T22:51:39.417226+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Watch the CODEX-β demonstrator's first Run 3 data: it measures the neutral multitrack and soft-background rates in the shielded cavern, which are compared to the simulated predictions of Section III D. If the measured rates, scaled to the full 10×10×10 m³ volume behind the 4.5 m lead shield and active veto, exceed the zero-background expectation by even a handful of events per nominal year, the design basis fails and the Fig. 9 sensitivity curves become optimistic upper limits rather than projections.","supporting_citations":[{"cited_title":"Background studies for the CODEX-b experiment: measurements and simulation","cited_arxiv_id":"1912.03846","evidence_quote":"The 2018 in-situ background measurement campaign at the proposed site that the zero-background claim builds on and CODEX-β is designed to validate."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"ATLAS Phase-II muon spectrometer design that supplies the triplet RPC module adopted as CODEX-b's tracking technology."},{"cited_title":"Geometry Optimization for Long-lived Particle Detectors","cited_arxiv_id":"2211.08450","evidence_quote":"The fast-simulation and deterministic optimization framework from which the reduced-panel layouts and efficiency-versus-panel-count results are taken."},{"cited_title":"Agostinelli et al., GEANT4: A Simulation toolkit , Nucl","cited_arxiv_id":null,"evidence_quote":"Simulation toolkit used for the shielding and neutral-secondary background studies."},{"cited_title":"Aielli et al., Technical design report for the CODEX-β demonstrator, 2406.12880","cited_arxiv_id":null,"evidence_quote":"CODEX-β technical design report that provides the per-module cost, build-time, and personnel figures scaled up for the full detector."}],"review_version":1}