{"id":"1c31f3f5-bfc5-415c-b112-c004fd79331f","arxiv_id":"1908.07622","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":8,"one_line_summary":"HAWC's first Earth-skimming analysis produced a track-finding algorithm and two high-charge track-like events from the direction of Pico de Orizaba, but no neutrino claim yet.","lead":"This paper reports the first steps toward detecting Earth-skimming tau neutrinos with the HAWC observatory, using the Pico de Orizaba volcano as a shield and target. It describes a track reconstruction algorithm and finds two high-charge track-like events from the volcano's highest-overburden region in 181 days of data.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Tau-shower track-like signal is asserted only in a footnote; without quantitative MC validation or a background rate, the two high-charge tracks lack a neutrino interpretation.","rationale":"After reading the full text, I find the reader's conditional verdict appropriate. The central claim has two layers: (1) HAWC can reconstruct light-speed tracks in EAS-triggered events, and (2) two high-charge tracks from the highest-overburden direction are candidates for Earth-skimming tau neutrinos. Layer (1) is reasonably supported by the algorithm description, the event displays, and the MC-based angular resolution (~3 degrees); this is real positive evidence. Layer (2) fails to close the chain from 'unusually high charge' to 'neutrino': the tau-shower-to-track premise is only asserted in a footnote, and the background rate is not quantified. I agree with the reader that the single most load-bearing assumption is the tau-shower morphology. However, the paper's own language is appropriately cautious ('preliminary', 'Further investigation... in progress'); it does not claim a discovery. Therefore the correct verdict remains conditional: the feasibility result is credible, but the two anomalous events cannot yet be interpreted as neutrino candidates. The concrete test I propose is directly enabled by the cited full MC, so the concern is checkable.","tokens_in":7407,"tokens_out":6626,"duration_ms":146049,"concrete_test":"Run the HAWC full Monte Carlo with injected tau leptons (or tau-decay air showers) of 1-100 PeV arriving from the direction of Cells C and E, exactly as in footnote 2, and process them through the complete Sec. 2 track trigger and filtering chain: 4 PE PMT threshold, at least two PMT hits per pixel, LC<6, HC<=1.5, TL>=4, and the >=75% linear-fit quality requirement. Report the fraction of simulated events that pass all cuts, the distribution of reconstructed <TrackCharge>, and the cut-by-cut rejection factors. If the passage fraction is comparable to that of muon tracks and the charge distribution reaches the observed >1500 PE values, the concern is resolved; if few or none pass, the neutrino interpretation of Figs. 9 and 10 is unsupported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that the two high-charge tracks (Cells C and E, Figs. 6 and 8) are candidate Earth-skimming tau neutrinos depends on the premise, stated in Sec. 2, that an ultra-high-energy tau emerging from the volcano decays into a highly collimated quasi-horizontal shower whose HAWC footprint looks like a single light-speed track. The only support offered is footnote 2: 'This was tested using the standard full MC simulation of HAWC.' No survival fraction, charge distribution, or comparison with the muon-track selection is shown. If the tau-shower footprint is broader than the Sec. 2.3 filters allow (LC<6, HC<=1.5, TL>=4, 75% linear-fit quality), the two events cannot be interpreted as neutrino signals even if they are genuine high-charge tracks. A related gap is that the muon-only expectation for Cells C and E is given only as area-normalized charge distributions (Figs. 3-8); the tail probability of a 1561.7 PE or 1744.8 PE track under the muon-only hypothesis is not evaluated, so the events could be VHE-muon fluctuations or reconstruction artifacts. The tau-shower premise is the more load-bearing because it is the necessary condition for the entire technique, and it is currently backed only by an invisible internal check.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This proceedings paper reports the first steps toward an Earth-skimming tau-neutrino search with HAWC, using the Pico de Orizaba volcano as a target. The authors describe a simple track reconstruction algorithm that identifies light-speed, straight tracks in HAWC events recorded with the standard EAS trigger, apply a series of filters to reject small inclined showers, and compare the average charge per tank of surviving tracks with single-muon Monte Carlo simulations in solid-angle bins toward the volcano. Using 181 days of data, they find two tracks from the highest-overburden region (cells C and E) with average charges of 1561.7 and 1744.8 PE, which lie beyond the distributions predicted for muons up to 5 TeV. The paper presents these as interesting candidate events and states that further investigation is in progress.","tokens_in":7702,"tokens_out":10857,"duration_ms":530112,"significance":"The proposed technique is novel and potentially valuable: using a large water-Cherenkov EAS array as a neutrino telescope by exploiting a volcanic overburden is an inexpensive way to probe tau neutrinos above 1 PeV. The paper's strengths are the transparent tracking algorithm, the external single-muon MC benchmark that yields a roughly 3-degree angular resolution and is not circular with respect to the tracking validation, the large reduction of the track candidate rate to 0.5 mHz after filtering, and the clear event displays of the two high-charge candidates. If the tau-shower track-like signature were quantitatively validated and a proper background treatment provided, these two events would be a strong motivation for a dedicated search. At present, however, the interpretation of the events as Earth-skimming neutrino candidates is not yet supported by the quantitative evidence in the manuscript.","major_comments":[{"comment":"The premise that an ultra-high-energy tau lepton emerging from the volcano produces a highly collimated, quasi-horizontal shower that appears as a light-speed track in HAWC is the necessary condition for the entire search, yet it is supported only by footnote 2, which states that it 'was tested using the standard full MC simulation of HAWC.' No quantitative result is presented: there is no survival fraction of tau-induced events through the Sec. 2.3 filters (LC<6, HC<=1.5, TL>=4, 75% linear-fit quality), no expected charge distribution, and no comparison with the muon-track selection. Because a broader shower footprint would be rejected by the same cuts, the two high-charge tracks in Figs. 6 and 8 cannot currently be interpreted as neutrino candidates. Please include the tau MC validation (e.g., selection efficiency versus tau energy and decay altitude, and the resulting charge distribution) or explicitly state that the neutrino interpretation is not yet established.","section":"Sec. 2, footnote 2"},{"comment":"The text explicitly states that 'All the cut values used on this analysis were selected based on studying a large number of track candidates in an event display.' Since the same 181-day dataset is used both to tune the LC, HC, TL, and track-quality cuts and to identify the two high-charge events, the selection is partly circular: the events may be outliers by construction of the cuts rather than physically unusual tracks. In addition, the randomized-location test described in the 'Minimum length' paragraph only constrains combinatorial fake tracks (maximum length 3 pixels), not the contamination from small showers that the LC and HC cuts are intended to remove. A blind analysis with cuts fixed before examining the signal region, or a validation in a control region not used to tune cuts, is needed before these events can be claimed as candidates.","section":"Sec. 2.3"},{"comment":"The two high-charge tracks are presented without a statistical significance or an expected background count. Figures 3-8 show charge distributions for mono-energetic muons normalized to equal area, but not the tail probability of observing a track with average charge >=1561.7 PE or >=1744.8 PE under the muon-only hypothesis, and no absolute muon rate per cell is quoted (the global rate after cuts is 0.5 mHz, but the relevant per-cell exposure is not given). Moreover, the muon MC is not weighted by the flux of cosmic-ray muons that actually survive the volcano overburden, so the normalized distributions alone cannot quantify how unlikely these events are. Please provide per-cell expected track counts, p-values for the two events, and a treatment of the muon survival probability through the INEGI overburden profile.","section":"Sec. 3, Figs. 6 and 8"},{"comment":"The extreme charge values (1561.7 and 1744.8 PE per pixel) are central to the paper's claim, but the linearity and saturation behavior of the HAWC PMT charge reconstruction in this regime is not discussed. The charge calibration described in Sec. 2.2 uses a scaling between central and peripheral PMTs, but there is no statement of the maximum calibrated charge, the saturation point, or the systematic uncertainty at these values. If the PMT response is not linear at several thousand PE, the average charges, and therefore the conclusion that these tracks lie beyond the 5 TeV muon expectation, could be incorrect. Please state the dynamic range and linearity limits of the charge measurement and verify the two events against the calibration.","section":"Sec. 2.2 and Sec. 3"}],"minor_comments":[{"comment":"The captions of the event displays use 'CXPE40', which is not defined anywhere in the text; please define it or remove it.","section":"Figs. 1, 9, 10"},{"comment":"The axes of Fig. 2 are not labeled: the horizontal axis appears to be azimuth and the vertical axis elevation, but this is not stated in the caption, and the color scale for the overburden is not explicitly described in the text.","section":"Fig. 2"},{"comment":"The quoted average angular resolution of approximately 3.0 degrees is given without a distribution or an uncertainty; a plot of the angle between injected and reconstructed muons would allow the reader to judge the probability of migration between the 6-by-4 degree cells.","section":"Sec. 2.2"},{"comment":"The text says that more than one track-triggered event is allowed in the same shower event, but it is not explained how multiple track candidates in the same event are treated in the final analysis (for example, whether all candidates are kept or only the first one).","section":"Sec. 2.1"},{"comment":"The data means quoted in Figs. 3-8 (for example, 83.4 PE for Cell C) are not compared with the muon MC means in the text; a short table listing the cell overburden, the number of tracks, the data mean, and the 5 TeV muon mean would make the comparison easier to follow.","section":"Sec. 3"}],"recommendation":"major_revision","confidential_remarks":"This appears to be a proceedings contribution from ICRC 2019. The main risk is that the central empirical claim—two candidate neutrino events—is presented before the tau-shower signature and the muon background are quantitatively validated. A revised version that adds the missing tau MC validation, a proper background/significance treatment, and a brief discussion of PMT charge linearity would be suitable for a proceedings or a technical report. The paper should not be read as a claim of neutrino detection until those points are addressed."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"First thing to know: this is exactly what it says on the tin—a status report from HAWC on whether Earth-skimming tau neutrinos could be seen in the existing EAS-triggered data. It does not claim a detection, and it is honest about being preliminary. What is actually new is the concrete track-trigger and fitting algorithm, plus 181 days of charge-deposit distributions from six angular bins pointing at Pico de Orizaba. The tracking validation is credible: single-muon MC from 100 GeV to 100 TeV gives about 3 degrees angular resolution, fake tracks are bounded by randomizing PMT positions, and the post-cut rate drops to 0.5 mHz. The muon MC is external to the fit, so the tracking performance is not circular.\n\nThe soft spots are the ones you would expect at this stage. The whole physics case rests on the statement that an ultra-high-energy tau escaping the volcano produces a quasi-horizontal, highly collimated shower whose HAWC footprint looks like a single light-speed track. The support is footnote 2: 'This was tested using the standard full MC simulation of HAWC.' No survival fraction, charge distribution, or comparison with the muon-track selection is shown. That is load-bearing and currently invisible; if tau-induced showers are not track-like, the two high-charge events cannot be interpreted as neutrino candidates. The second soft spot is that the two interesting tracks in cells C and E were selected using cuts tuned on the same data—the authors say so—and the paper gives no tail probability for a 1561.7 PE or 1744.8 PE track under the muon-only hypothesis. So the events are outliers by construction of the cuts and could be muon fluctuations. The paper does not overclaim them; it calls them interesting cases under investigation.\n\nThe stress-test note is directionally right, but I want to be fair: this is a conference proceedings, not a discovery claim. The circularity is a known limitation of a first look, not a hidden flaw. The missing tau-shower MC support is the real issue, and it should be fixed before anyone uses this as more than a feasibility hint.\n\nBottom line: the paper is worth a serious referee because it opens a new channel and ships a reproducible algorithm plus real data. A referee should ask for the tau-shower MC validation, a background expectation for the two events, and ideally a blind analysis before any neutrino claim is made. As it stands, I would read it as a plausible first step, not as evidence of neutrinos.","headline":"Honest status report from HAWC on Earth-skimming tau neutrinos: new tracking algorithm and 181 days of data, but the neutrino interpretation of the two high-charge tracks relies on an unquantified internal MC check.","tokens_in":8222,"tokens_out":2680,"would_cite":false,"duration_ms":126260,"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":"The paper reports two track-like events from the base of Pico de Orizaba whose light output exceeds the simulated muon ceiling, presenting them as candidate Earth-skimming tau neutrinos.","keywords":["Earth-skimming neutrinos","tau neutrinos","HAWC observatory","water Cherenkov detectors","track reconstruction","Pico de Orizaba","ultra-high-energy neutrinos","muon background"],"falsifier":"Run the full Monte Carlo of 1–100 PeV tau leptons emerging from the actual Pico de Orizaba profile through the same track trigger and cuts; if none of those simulated tau events produces a track with average pixel charge above the simulated 5 TeV muon range, or if the simulated tau tracks fail the length and straightness cuts, then the two observed events cannot be neutrino candidates and revert to unexplained muon background.","tokens_in":7193,"feed_emoji":"🌋","tokens_out":6657,"duration_ms":61061,"temperature":0.7,"pith_summary":"This paper reports the first steps toward using the HAWC water-Cherenkov array, normally a gamma-ray telescope, as a particle tracker that watches for neutrinos skimming through the Pico de Orizaba volcano. Using 181 days of ordinary shower-triggered data, the author selects fast, straight, light-speed track-like events and compares their light output with simulated muons crossing the detector. Two tracks that point back through the volcano's thickest rock deposit far more light than any simulated muon up to 5 TeV: 1561.7 and 1744.8 photoelectrons per pixel. The paper argues these are candidate ultra-high-energy tau neutrinos, whose charged lepton escapes the mountain and decays into a compact shower that looks like a track. If true, HAWC would have a working, hardware-free path toward detecting neutrinos above 1 PeV.","feed_headline":"Two bright tracks may be Earth-skimming neutrinos","feed_subtitle":"They point through Pico de Orizaba's thickest rock, outshining every simulated muon below 5 TeV.","key_machinery":"The central object is the track-like event selector: a two-stage algorithm that first identifies, within ordinary 1.5-microsecond shower-triggered snapshots, sequences of neighboring water tanks activated in a light-speed chain, and then fits those pixel sequences to straight lines while allowing the direction to bend by up to 30 degrees. Each of HAWC's 300 tanks acts as one pixel, and a pixel is accepted only if more than one photomultiplier sees light, which suppresses single-PMT noise. The average charge deposited per pixel serves as the energy estimator, and Monte Carlo muons from 10 GeV to 100 TeV calibrate what charge each overburden cell should produce. The volcano profile from elevation data defines the solid-angle cells and their average rock widths, so each track can be associated with a specific slant depth.","core_discovery":"The paper's central finding is that after applying simple track-selection cuts the 181-day HAWC data set contains two track-like events whose reconstructed directions point to the highest-overburden face of the volcano and whose average charge deposits lie well outside the range expected for muons with energies up to 5 TeV. The tracking method treats each water tank as a pixel, requires at least two photomultiplier hits per tank, connects signals propagating at the speed of light, and uses the average charge per pixel as an energy proxy. The selected sample, after cuts on shower contamination, track length, and straightness, has a rate of 0.5 mHz. The two exceptional events are presented as candidates for Earth-skimming tau neutrinos; the paper is explicit that further investigation is in progress.","pith_inferences":["Because the paper only compares against simulated muons, a natural next step is to simulate tau-induced showers with the same detector response and confirm the track hypothesis; this is the single most direct test of the claim.","The two high-charge tracks could also be explained by an underestimated tail of high-energy muons from very inclined showers; checking whether their rate grows with exposure and whether their arrival directions cluster near the volcano would discriminate the two interpretations.","The same track-finding chain could be applied to any mountain seen from a dense water-Cherenkov array, not just Pico de Orizaba; other arrays with similar modular geometry might adopt the technique."],"forward_implications":["HAWC can search for >1 PeV tau neutrinos using data already being recorded for gamma-ray astronomy, with no detector modifications.","The two candidate events motivate a longer exposure: if they are atmospheric-muon fluctuations, their rate should fall with the muon background model, while a true neutrino signal would accumulate with livetime.","A dedicated Monte Carlo of tau-induced showers, run through the same selection, would convert these two events into either a flux constraint or evidence for a diffuse ultra-high-energy neutrino flux.","The same track-charge calibration can be extended to higher muon energies to sharpen the boundary between ordinary muons and neutrino-induced showers."],"supporting_citations":[{"why":"This reference first proposed the HAWC Earth-skimming technique and supplies the geometric effective-area model and the expectation that ultra-high-energy tau showers appear as tracks.","marker":"[7]"},{"why":"This reference describes the HAWC data acquisition and trigger system that defines the 1.5-microsecond event window and the 25 kHz trigger rate this analysis uses.","marker":"[8]"},{"why":"This reference provides the INEGI elevation model of Pico de Orizaba used to define the overburden cells and compute average rock widths.","marker":"[2]"},{"why":"This reference describes the HAWC detector geometry and photomultiplier configuration that underpins the per-tank pixel definition and charge calibration.","marker":"[1]"}],"fun_headline_variants":["Two tracks through Pico de Orizaba's thick rock","HAWC's volcano shield reveals two neutrino candidates","Earth-skimming neutrinos? HAWC sees two bright tracks","Volcano-skimming tracks: two events at HAWC"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The result stands or falls on the assumption that an ultra-high-energy tau particle escaping the mountain produces a shower compact enough to look exactly like a fast muon track in HAWC, a check the paper says was made with the full simulation but does not show quantitatively.","fun_headline_variants_meta":{"raw":{"variants":["Two tracks through Pico de Orizaba's thick rock","HAWC's volcano shield reveals two neutrino candidates","Earth-skimming neutrinos? HAWC sees two bright tracks","Volcano-skimming tracks: two events at HAWC"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00031,"raw_usage":{"total_tokens":1724,"prompt_tokens":856,"completion_tokens":868,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":472,"completion_tokens_details":{"reasoning_tokens":799}},"tokens_in":472,"tokens_out":868,"duration_ms":9304,"temperature":1.0,"reasoning_tokens":799,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T12:00:44.313744+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the full Monte Carlo of 1–100 PeV tau leptons emerging from the actual Pico de Orizaba profile through the same track trigger and cuts; if none of those simulated tau events produces a track with average pixel charge above the simulated 5 TeV muon range, or if the simulated tau tracks fail the length and straightness cuts, then the two observed events cannot be neutrino candidates and revert to unexplained muon background.","supporting_citations":[{"cited_title":"Capability of the HAWC gamma-ray observatory for the indirect detection of ultra-high energy neutrinos","cited_arxiv_id":"1610.04820","evidence_quote":"This reference first proposed the HAWC Earth-skimming technique and supplies the geometric effective-area model and the expectation that ultra-high-energy tau showers appear as tracks."},{"cited_title":"Data Acquisition Architecture and Online Processing System for the HAWC gamma-ray observatory","cited_arxiv_id":"1709.03751","evidence_quote":"This reference describes the HAWC data acquisition and trigger system that defines the 1.5-microsecond event window and the 25 kHz trigger rate this analysis uses."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference provides the INEGI elevation model of Pico de Orizaba used to define the overburden cells and compute average rock widths."}],"review_version":1}