{"id":"d1aa421f-0bc5-4753-ae47-627f54b3400c","arxiv_id":"1908.06650","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"A template-based likelihood fit that combines HAWC main array and outrigger signals reconstructs gamma-ray shower cores roughly three times more accurately than the previous estimator in Monte Carlo events landing on the outrigger area.","lead":"The HAWC gamma-ray observatory added a sparse ring of 345 small water tanks around its main array, and this paper reports the first combined shower reconstruction for the two detector types. The new fitting method, tested on simulations and one Crab Nebula event, improves the predicted location of the shower core by about three times at high energies.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"All quantitative performance claims rest on an outrigger simulation that the paper itself labels work in progress; until data/MC agreement is shown, the 3-fold core gain and 25% energy resolution are not established.","rationale":"The central claim requires two things: the template likelihood method works, and the outrigger simulation faithfully represents detector response. The paper gives internal evidence for the first: Figure 2 shows convergence to the true core, and Section 4.1 compares LH(MA) with LH(MA+OR), so the 3-fold gain is not solely an algorithmic artifact. The weak link is the second. HAWCSim/GEANT4 is used both to generate templates and to evaluate resolution, so the quoted numbers are self-consistent by construction. The manuscript explicitly says outrigger simulation is 'work in progress,' and the only experimental illustration is a single event selected with main-array criteria. No data/MC comparison of outrigger photoelectron distributions is shown, and no systematic error bars are given. This makes simulation realism the load-bearing assumption. The proposed data/MC KS test on outrigger Npe and zero-hit fractions would settle whether the templates are miscalibrated. Since this is exactly the reader's weakest assumption and the appropriate verdict remains conditional pending such a check, the reader's verdict is unchanged.","tokens_in":5278,"tokens_out":4866,"duration_ms":51031,"concrete_test":"Use the full August 2018 onward outrigger data set and select events reconstructed toward the Crab Nebula (or a high-statistics cosmic-ray sample). For the same energy, core, and zenith bins used in Figures 4 and 5, compare the observed outrigger Npe distribution and zero-signal fraction with HAWCSim predictions using a Kolmogorov-Smirnov or chi-squared test. If the data/MC agreement is poor (e.g., p < 1%), the template probabilities are miscalibrated and the quoted resolutions are not trustworthy; if agreement is good, the concern is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 3 builds the PDF templates from HAWCSim/GEANT4, and Section 4 evaluates the reconstruction on the same simulated events. Section 5 states that 'hit selection and more realistic outrigger simulation is still a work in progress.' This is an explicit admission that the outrigger charge response, zero-signal probability, and hit selection used in the templates are not yet validated against data. Because the likelihood fit uses template probabilities for each Npe/r bin, any mismatch between simulated and real outrigger response (PMT efficiency, saturation, noise, calibration, trigger) shifts the fitted core and energy. Sparse outrigger signals are particularly sensitive to such mismatches, and the paper shows no data/MC comparison, no systematic variations, and no error bars for Figures 4 or 5. Therefore the quoted ~3-fold core improvement above 1 TeV and ~25% energy resolution at high energies could be simulation artifacts rather than instrument performance. The single Crab event in Section 5 is selected with main-array criteria and is not a validation of the outrigger simulation.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports on a new template-based likelihood reconstruction method that combines data from the HAWC main array and its sparse outrigger array to improve shower core and energy reconstruction for gamma-ray induced air showers. The templates are built from HAWCSim/GEANT4 simulations of the detector response, and performance is evaluated on simulated events falling on the outrigger array. The paper claims that the combined reconstruction improves the core resolution by roughly a factor of 3 relative to the existing SFCF estimator above 1 TeV, and that the energy resolution reaches about 25% at the highest energies. A single candidate Crab Nebula event is shown as a first illustration on real data, with the explicit caveat that outrigger hit selection and a more realistic simulation are still work in progress.","tokens_in":5519,"tokens_out":3658,"duration_ms":39151,"significance":"If validated, the method could meaningfully improve HAWC's sensitivity at multi-TeV energies by better constraining shower cores and energies for events that land near the array edge. The paper is transparent about its current limitations, which is a strength, and it builds on a published reconstruction method (Joshi et al., JCAP 2019). The central performance claims, however, rest entirely on a Monte Carlo closure test: the same HAWCSim simulation is used both to produce the PDF templates and to generate the test events. There is no data/MC comparison for outrigger observables, no systematic uncertainty treatment, and the only experimental demonstration is a single selected event. The stated improvement is therefore not yet established for real detector data.","major_comments":[{"comment":"The performance evaluation is a Monte Carlo closure test: the PDF templates are constructed from HAWCSim (Section 3) and the test events are generated with the same detector simulation (Section 4). The figures carry no error bars, no systematic variations are studied, and no comparison to real outrigger data is shown. The quoted ~3-fold core resolution gain and ~25% energy resolution could be artifacts of the assumed outrigger response (PMT efficiency, saturation, noise, calibration, trigger). I request a quantitative statement of statistical and systematic uncertainties and at least one data/MC comparison of basic outrigger observables such as hit multiplicity, Npe distributions, zero-signal fraction, or trigger rates.","section":"Sections 3 and 4, Figures 4 and 5"},{"comment":"The headline claim that 'the outrigger array improves the core resolution by ~3 fold in comparison to SFCF' conflates the gain from the new likelihood method with the gain from adding outrigger information. The text itself notes that LH with only the main array already outperforms SFCF, so the marginal improvement attributable to the outriggers is only visible in the difference between the LH MA and LH MA+OR curves, which is not separately quantified. Please report the outrigger-specific improvement relative to LH with MA alone, with its energy dependence and statistical uncertainty.","section":"Section 4.1, Figure 4"},{"comment":"The experimental validation is a single Crab-candidate event selected with main-array criteria, and the text states that 'hit selection and more realistic outrigger simulation is still a work in progress.' This is a direct admission that the outrigger response model used to build the templates has not been validated against data. A single event can demonstrate that the fitting procedure runs on real data, but it cannot validate the quoted resolutions. To support the central claim, the paper needs a systematic data/MC comparison of outrigger response and an estimate of the resulting systematic shifts in reconstructed core and energy.","section":"Section 5, Figures 6 and 7"},{"comment":"The energy bias is large at low energies and only converges to zero above about 10 TeV, while the resolution at 10 TeV is about 50% and improves to about 25% at the highest energies. The paper should state explicitly the energy range over which the reconstruction is claimed to be valid, and it should report the number of Monte Carlo events per bin so the reader can judge the statistical significance of the quoted resolution at the highest energies, where event counts are typically low.","section":"Section 4.2, Figure 5"}],"minor_comments":[{"comment":"The figures lack error bars or confidence bands; adding them would make the energy dependence of the improvement, and its statistical significance, much clearer.","section":"Figures 4 and 5"},{"comment":"The caption refers to 'fractional energy bias' and 'energy resolution' but does not define the plotted quantity in terms of log10(Ereco) - log10(Etrue) on the vertical axis; please add the units or an explicit formula.","section":"Figure 5 caption"},{"comment":"The abstract says the outrigger array increases the instrumented area by a factor of 4, while Section 2 says by a factor of 4-5; please make these consistent.","section":"Abstract and Section 2"},{"comment":"The abstract says 'the first combined experimental data set,' but the paper shows only a single event; 'the first combined experimental event' would be more accurate.","section":"Section 5"},{"comment":"The paper would benefit from a brief statement that the method itself is described in detail in the JCAP 2019 reference [4], so that the reader knows which parts are new in this contribution.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"This manuscript is essentially an ICRC proceedings contribution, and its length and scope are appropriate for that venue. For a full journal paper, the Monte-Carlo-only closure test and the single-event illustration do not meet the evidentiary bar for the headline performance claims. The issues are fixable in a revised version that adds error bars, a data/MC comparison, and a clear separation of the likelihood-method gain from the outrigger-array gain."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read this as a status report, not a performance paper. The new thing is the first application of the template-based likelihood reconstruction from [4] to the combined HAWC main + outrigger array, with MC core/energy resolution plots and one Crab event as a feel-good example. The method itself is already published; the contribution here is the extension and the first performance estimates.\n\nWhat is good: the paper is honest about where it stands. It labels the figures 'preliminary', discusses the low-energy bias, notes that the outrigger simulation and hit selection are work in progress, and does not oversell the Crab event as validation. The MC core improvement is internally consistent: the likelihood method with main array alone already beats the current SFCF estimator, and adding outriggers gives another gain at 1-10 TeV. That trend is physically plausible because the outriggers constrain the core for events landing on them.\n\nThe soft spot is the one the stress-test flags, and it is real. The PDF templates and the test events come from the same HAWCSim/GEANT4 simulation, and there is no data/MC comparison. So the ~3-fold core improvement and the ~25% high-energy resolution are closure results. They could shift once the outrigger charge response, zero-signal probability, and hit selection are calibrated against real data. The single Crab event is selected with main-array criteria and cannot validate the outrigger response. Missing error bars on Figures 4 and 5 make it impossible to judge the significance of the differences, and only one MC realization is used. These are not manufactured flaws; the paper itself admits them.\n\nIs the central argument okay? For a proceedings paper, yes. The authors are careful to say the realistic outrigger simulation is work in progress. The reconstruction method is a legitimate extension of an established program, not a new algorithm. The paper deserves a serious referee if it ever gets submitted as a journal article, but it should not be accepted for publication until the performance claims are backed by data/MC agreement and error bars. For now, treat it as a useful status report and cite it as such, not as a measured instrument performance.\n\nI'd bring it to a reading group only if people care about HAWC's upgrade. Otherwise, skim the figures and move on.","headline":"A clean status report for the HAWC outrigger upgrade: the first combined reconstruction results are shown, but the performance numbers are closure tests of one simulation, and the paper says so itself.","tokens_in":5988,"tokens_out":4013,"would_cite":true,"duration_ms":34986,"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":"Outrigger array improves HAWC shower core resolution by about threefold.","keywords":["HAWC","outrigger array","air shower reconstruction","water Cherenkov detector","template-based likelihood","core resolution","energy resolution","gamma-ray astronomy"],"falsifier":"Take a sample of real events landing on the outrigger array and compare reconstructed cores against independently known core positions, for example from shower timing planes or a coincident detector; if the data discrepancy distribution is not consistent with the simulated 68 percent containment radii, the simulation-template assumption is wrong. A calibration check of single-tank photoelectron response against laser or muon signals would also reveal whether the outrigger response model is realistic.","tokens_in":5120,"feed_emoji":"🔭","tokens_out":5681,"duration_ms":58800,"temperature":0.7,"pith_summary":"The paper is trying to establish that adding a sparse outrigger array to HAWC and fitting a Monte Carlo template to the combined lateral distribution of photoelectrons can fix the main weakness of the main array at multi-TeV energies: poorly constrained shower cores and hence poor energy estimates. If correct, the outrigger array increases the area over which high-energy showers are well contained, improves core resolution by roughly a factor of three relative to the standard HAWC core estimator above 1 TeV, and delivers roughly 25 percent energy resolution at the highest energies. The proof so far is based on simulated gamma-ray showers, with one experimental Crab Nebula event shown as a first demonstration. The paper's own qualification is that hit selection and a fully realistic outrigger simulation are still work in progress.","feed_headline":"Outrigger array sharpens HAWC core location by 3x","feed_subtitle":"Combined template fit on 345 outrigger tanks plus the main array reaches ~25% energy resolution on multi-TeV gamma rays.","key_machinery":"The central mechanism is a template-based maximum-likelihood lateral-distribution fit: for a fixed assumed arrival direction, the algorithm predicts the probability distribution of photoelectrons in each tank as a function of distance from the shower axis, using PDFs from a full air-shower simulation plus the detector simulation, binned in primary energy, Xmax, and zenith angle. It then minimizes the negative log-likelihood of the observed photoelectrons per tank, including zero-signal outriggers, to jointly estimate the energy and core location. This is what lets detectors with different sizes, photomultiplier gains, and signal scales be combined naturally in one fit.","core_discovery":"The paper claims that a template-based likelihood fit using the HAWC main array and the 345 outrigger tanks together reconstructs the shower core for outrigger-landing gamma-ray showers about three times more accurately than HAWC's existing SFCF core estimator above 1 TeV, with the gain around a factor of three at 10 TeV and about two at the highest energies. The same combined fit estimates an energy resolution that starts near 50 percent at 10 TeV and improves to about 25 percent at the highest energies, with energy bias converging to zero above 10 TeV. These results are established on simulated events using the collaboration's detector simulation, and the paper also shows a candidate Crab event from real data as a proof of concept.","pith_inferences":["If the gains survive real-data calibration, the outrigger array will matter most for sources above a few tens of TeV, where the main array's core uncertainty currently limits spectral measurements; this is an extension because the paper only projects the improvement on simulations.","Because the method fits in Xmax as well as energy, the same machinery could be applied to cosmic-ray composition studies, although the paper demonstrates only gamma-ray showers.","A natural test is to run the same fit on simulated cosmic-ray showers: if the core improvement is similar, the outrigger array may also sharpen background rejection, which the paper does not explicitly quantify."],"forward_implications":["Events whose cores land in the outrigger area above 1 TeV will have their cores located with a 68 percent containment radius roughly three times smaller than with HAWC's current SFCF estimator.","Energy reconstruction is stable, with bias near zero above about 10 TeV, and resolution improves from about 50 percent at 10 TeV to about 25 percent at the highest energies.","With the outrigger array, the instrumented footprint grows by a factor of 4 to 5, so a larger fraction of high-energy showers is well contained inside the detector.","Better core locations should propagate into better arrival-direction, energy, and gamma-hadron separation for the same events, improving HAWC's sensitivity at multi-TeV energies."],"supporting_citations":[{"why":"Defines the template-based gamma-ray reconstruction likelihood that the combined main-plus-outrigger fit applies.","marker":"[4]"},{"why":"Supplies the HAWC detector simulation used to build the photoelectron probability templates.","marker":"[5]"},{"why":"Provides the detector-simulation toolkit that underlies the tank response model.","marker":"[6]"},{"why":"Describes the outrigger array hardware whose response the simulation must reproduce.","marker":"[3]"}],"fun_headline_variants":["Outrigger array triples HAWC core location precision","HAWC plus outriggers: 3x better core, 25% energy","Combined fit of HAWC and outriggers improves core 3x","345 outrigger tanks sharpen HAWC core 3x","Outrigger-enhanced HAWC hits 25% energy resolution"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The quoted performance assumes that the simulation of how outrigger tanks convert shower particles into photoelectrons is accurate enough that templates made from it describe real showers.","fun_headline_variants_meta":{"raw":{"variants":["Outrigger array triples HAWC core location precision","HAWC plus outriggers: 3x better core, 25% energy","Combined fit of HAWC and outriggers improves core 3x","345 outrigger tanks sharpen HAWC core 3x","Outrigger-enhanced HAWC hits 25% energy resolution"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000654,"raw_usage":{"total_tokens":2949,"prompt_tokens":849,"completion_tokens":2100,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":465,"completion_tokens_details":{"reasoning_tokens":2007}},"tokens_in":465,"tokens_out":2100,"duration_ms":15004,"temperature":1.0,"reasoning_tokens":2007,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T12:37:59.620271+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Take a sample of real events landing on the outrigger array and compare reconstructed cores against independently known core positions, for example from shower timing planes or a coincident detector; if the data discrepancy distribution is not consistent with the simulated 68 percent containment radii, the simulation-template assumption is wrong. A calibration check of single-tank photoelectron response against laser or muon signals would also reveal whether the outrigger response model is realistic.","supporting_citations":[{"cited_title":"A template-basedγ-ray reconstruction method for air shower arrays,","cited_arxiv_id":null,"evidence_quote":"Defines the template-based gamma-ray reconstruction likelihood that the combined main-plus-outrigger fit applies."},{"cited_title":"Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory,","cited_arxiv_id":null,"evidence_quote":"Supplies the HAWC detector simulation used to build the photoelectron probability templates."},{"cited_title":"GEANT4: A Simulation toolkit,","cited_arxiv_id":null,"evidence_quote":"Provides the detector-simulation toolkit that underlies the tank response model."},{"cited_title":"HAWC High Energy Upgrade with a Sparse Outrigger Array,","cited_arxiv_id":null,"evidence_quote":"Describes the outrigger array hardware whose response the simulation must reproduce."}],"review_version":1}