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REVIEW 4 major objections 5 minor 8 references

Prospects of Earth-skimming neutrino detection with HAWC

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read 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.

desk verdict 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. read the letter →

arxiv 1908.07622 v1 pith:NFZHHCPM submitted 2019-08-20 physics.ins-det hep-ex

Hermes León Vargas (for the HAWC Collaboration) This is my paper · ORCID
classification physics.ins-dethep-ex
keywords Earth-skimmingneutrinostauHAWCobservatorywaterCherenkovdetectorstrackreconstructionPicodeOrizabaultra-high-energymuonbackground
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

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.

What carries the argument

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.

What would settle it

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.

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Extended reading notes

Core claim

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.

Load-bearing premise

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.

Editorial extensions

If this is right

  • 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.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • 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.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

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.

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 (4)
  1. [Sec. 2, footnote 2] 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.
  2. [Sec. 2.3] 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.
  3. [Sec. 3, Figs. 6 and 8] 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.
  4. [Sec. 2.2 and Sec. 3] 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.
minor comments (5)
  1. [Figs. 1, 9, 10] The captions of the event displays use 'CXPE40', which is not defined anywhere in the text; please define it or remove it.
  2. [Fig. 2] 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.
  3. [Sec. 2.2] 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.
  4. [Sec. 2.1] 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).
  5. [Sec. 3] 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.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the tracking validation and charge comparisons use external muon MC and observables that are not inputs to the selection.

full rationale

The derivation chain is not circular under the stated criteria. The tracking algorithm is validated against external single-muon Monte Carlo simulations (Sec. 2.2: positive muons from 100 GeV to 100 TeV injected instead of air showers), so the angular-resolution benchmark does not use the data being interpreted. The LC, HC, track-quality, and minimum-length filters (Sec. 2.3) are functions of event topology (NWCDs, NPixelA, TL, linear-fit quality), not of the track charge that is later compared; selecting the cuts by event display may be data-driven, but it does not force the two high-charge events to be outliers because track charge is not a filter input. The charge distributions in Figs. 3-8 compare data to independently simulated mono-energetic muon samples, and the two large-charge tracks are outliers with respect to those Monte Carlo predictions; they are not fitted parameters renamed as predictions. The tau-shower premise (Sec. 2) is an assumption supported only by a footnote to an internal full-MC test and by self-citation [7]; this is an under-documented validation and a legitimate correctness or robustness risk, but the paper does not equate the premise with its conclusion by construction. No equation or fitted parameter reduces to the target result, so the appropriate finding is no significant circularity.

Assumptions & free parameters 8 free parameters · 4 assumptions · 0 invented entities

The central claims rest on the accuracy of HAWC's MC simulation, the volcano profile, and the assumption that tau-induced showers appear as tracks. Several analysis thresholds were tuned on data or MC, and these count as free parameters rather than independently fixed inputs.

free parameters (8)
  • PMT charge threshold = 4 PEs
    Threshold for defining an active PMT; chosen as a compromise between noise and disk usage (Section 2.1, footnote 3).
  • Track propagation time window = ±40 ns
    Allowed time difference between two pixels for propagation at light speed; optimized using MC simulations of muons (Section 2.2).
  • Maximum direction change = 30 degrees
    Allowed deviation from the initial track direction when adding pixels; optimized using MC simulations (Section 2.2).
  • Minimum track length = TL >= 4 pixels
    Set above the maximum length of fake tracks found in randomized background (Section 2.3).
  • Low Charge activity cut = LC < 6
    Selected by studying track candidates in event displays (Section 2.3).
  • High Charge activity cut = HC <= 1.5
    Selected by studying track candidates in event displays (Section 2.3).
  • Shower size cut = Fewer than 100 active PMTs
    Events with more than 100 active PMTs are discarded to maintain a manageable trigger rate; value optimized for rate and disk space (Section 2.1).
  • Track trigger minimum hits = More than 9 PMT hits
    Minimum number of PMT hits for a track trigger to be stored (Section 2.1).
assumptions (4)
  • domain assumption Standard HAWC detector simulation accurately models PMT charge and timing.
    The MC muon charge distributions are used as the reference for interpreting data (Section 2.2 and Figures 3-8).
  • domain assumption INEGI volcano elevation profile correctly represents the rock overburden of Pico de Orizaba.
    Cell definitions and overburden estimates in Figure 2 rely on INEGI data (Section 3).
  • domain assumption The EAS trigger can record isolated track-like signals within its 1.5 microsecond window.
    The analysis uses the standard EAS trigger, assuming it can contain isolated tracks even in the presence of a small shower (Section 2.1).
  • ad hoc to paper An ultra-high-energy tau-induced shower produces a track-like signature in HAWC.
    This is the detection principle for this search, adapted from [7] and tested only in internal full MC (Section 2, footnote 2).

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Cite this review

Pith. "Pith review of Prospects of Earth-skimming neutrino detection with HAWC." pith.science (2026). https://pith.science/paper/NFZHHCPM

@misc{pith2026190807622,
  author       = {Pith},
  title        = {Pith review of: Prospects of Earth-skimming neutrino detection with HAWC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/NFZHHCPM}},
  note         = {Machine review of arXiv:1908.07622}
}
read the original abstract

Searches for Earth-skimming neutrinos using volcanoes have not yet been achieved, but it is a promising technique for the detection of neutrinos above 1 PeV. The HAWC observatory is located in the vicinity of the highest mountain in Mexico, the Pico de Orizaba volcano, which shields the detector from quasi-horizontal very high-energy (VHE) muons. The large amount of shielding, up to 8 km of rock, enables the suppression of the large VHE muon background and makes neutrino detection possible. In this work we present the first steps towards the implementation of the Earth-skimming technique for the indirect measurement of tau neutrinos with HAWC. The results include the description of the charged lepton tracking reconstruction algorithm developed for this study and the initial analysis of the background of VHE muons using half a year of data.

Figures

Figures reproduced from arXiv: 1908.07622 by the authors.

Figure 1
Figure 1. Display of the signals produced by an event acquired with the EAS trigger. The event was afterwards identified as containing a track by our track finding algorithms. See the text for details. 290 300 310 320 330 340 ] o f [ 0 2 4 6 8 10 12 14 16 18 o ] 20 [ q 1000 2000 3000 4000 5000 6000 7000 <Width> [m] INEGI Profile Geometric model A B C D E F G H I J K L [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 3
Figure 3. Average charge de￾posit for tracks in the cell I. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 > [PEs] Charge <Track −3 10 −2 10 −1 10 1 Normalized counts Cell J 10 GeV, Mean: 32.8 100 GeV, Mean: 45.9 1 TeV, Mean: 75.4 5 TeV, Mean: 136.9 100 TeV, Mean: 373.7 Data, Mean: 65.6 Preliminary [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 6
Figure 6. Average charge de￾posit for tracks in the cell C. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 > [PEs] Charge <Track −3 10 −2 10 −1 10 1 Normalized counts Cell D 10 GeV, Mean: 32.8 100 GeV, Mean: 44.6 1 TeV, Mean: 73.5 5 TeV, Mean: 132.5 100 TeV, Mean: 385.6 Data, Mean: 71.3 Preliminary [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figures from the paper (1 more)
Figure 9
Figure 9. Figure 9: Event display of a track-like signal whose reconstructed direction is consistent with the region with largest overburden in the Pico de Orizaba volcano (cell C in figure 2). x [m] −60 −40 −20 0 20 40 60 80 100 120 y [m] 180 200 220 240 260 280 300 320 hit time [ns] 0 1…

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Works this paper leans on

8 extracted references · 5 canonical work pages

  1. [1]

    A. U. Abeysekara et al. (HAWC Collaboration), Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory, The Astrophysical Journal 843:39 (2017) [arXiv:1701.01778]

    arXiv 2017

  2. [2]

    INEGI, Continuo de Elevaciones Mexicano 3.0 (CEM 3.0), https://www.inegi.org.mx/app/geo2/elevacionesmex/ (2013)

    work page 2013

  3. [3]

    Fargion, A

    D. Fargion, A. Aiello, R. Conversano, Horizontal Tau air showers from mountains in deep valley. Traces of UHECR neutrino tau, in proceedings of 26th International Cosmic Ray Conference [arXiv:astro-ph/9906450]

    arXiv

  4. [4]

    J. L. Feng, P. Fisher, F. Wilczek and T.M. Yu,Observability of Earth-Skimming Ultrahigh Energy Neutrinos, Physical Review Letters 88 (2002) 161102 [arXiv:hep-ph/0105067]

    work page Pith review arXiv 2002

  5. [5]

    Limits on the flux of tau neutrinos from 1 PeV to 3 EeV with the MAGIC telescopes

    M.L. Ahnen et al. (MAGIC Collaboration), Limits on the flux of tau neutrinos from 1 PeV to 3 EeV with the MAGIC telescopes, Astroparticle Physics 102 (2018) 77-88 [arXiv:1805.02750]

    work page Pith review arXiv 2018

  6. [6]

    Aab et al

    A. Aab et al. (The Pierre Auger Collaboration), An improved limit to the diffuse flux of ultra-high energy neutrinos from the Pierre Auger Observatory, Phys. Rev. D 91 (2015) 092008 [arXiv:1504.05397]

    arXiv 2015

  7. [7]

    Capability of the HAWC gamma-ray observatory for the indirect detection of ultra-high energy neutrinos

    H. León Vargas, A. Sandoval, E. Belmont and R. Alfaro, Capability of the HAWC Gamma-Ray Observatory for the Indirect Detection of Ultrahigh-Energy Neutrinos, Advances in Astronomy 2017 (2017) 1932413 [arXiv:1610.04820]

    work page Pith review arXiv 2017

  8. [8]

    A. U. Abeysekara et al. (HAWC Collaboration), Data acquisition architecture and online processing system for the HAWC gamma-ray observatory, Nuclear Inst. and Methods in Physics Research, A 888 (2018) 138-146 [arXiv:1709.03751]. 7

    work page Pith review arXiv 2018

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