REVIEW 2 major objections 4 minor 35 references
Measurement of the average very forward energy as a function of the track multiplicity at central pseudorapidities in proton-proton collisions at $\sqrt{s} =$ 13 TeV
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read This paper reports that in 13 TeV proton-proton collisions, the average very forward energy rises with central track multiplicity, and while every tested Monte Carlo generator reproduces the total energy, all of them overestimate the…
desk verdict A solid first measurement correlating CASTOR very-forward energy with central multiplicity at 13 TeV; the central hadron-fraction claim is real but somewhat overstated relative to the quoted intercalibration systematic. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The measurement is carried by two pieces of apparatus-level machinery. The first is the CASTOR calorimeter, a quartz-tungsten sampling calorimeter whose first two channels form a 20-radiation-length electromagnetic section and whose remaining twelve channels form a hadronic section of 10 interaction lengths total; its electron and hadron response, including noncompensation, was measured in test beams and is simulated with GEANT4. The second is a 'forward folding' procedure: four-dimensional migration matrices, constructed from full detector simulations of PYTHIA 8 CUETP8M1, PYTHIA 8 4C+MBR, EPOS LHC, and SIBYLL 2.1, map every generator's true central multiplicity and forward energy onto reconstructed track multiplicity and CASTOR energy, so that any model can be compared to the data at detector level without unfolding. Five matrices are provided, four generator-specific plus one averaged, and the spread among them is assigned as a systematic uncertainty.
What would settle it
Shifting the CASTOR intercalibration to its quoted extremes—electromagnetic energy down 8% and hadronic up 15%—and re-extracting the hadronic fraction would show whether data still disagree with all generators, and a dedicated test-beam comparison of electron-versus-pion response against GEANT4 would identify which calibration is right.
Extended reading notes
Core claim
The paper establishes a detector-level measurement of the average total, electromagnetic, and hadronic energy in the CASTOR calorimeter at $-6.6 < \eta < -5.2$ as a function of the reconstructed charged-track multiplicity at $|\eta| < 2$, using an unbiased low-luminosity 13 TeV dataset. The total energy rises steeply at low multiplicity and flattens at high multiplicity; all generators reproduce this rise, with SIBYLL 2.1 the closest. The electromagnetic component is generally well described, while most generators overestimate the hadronic component. The ratio of hadronic to electromagnetic energy is lower in data than in all tested models, and this shortfall is largest for PYTHIA 8 CP5 and SIBYLL 2.3c, the most recent tunes. The ratio is approximately independent of track multiplicity, and the paper argues that this disagreement rules out the simplest explanation of the air-shower muon deficit, namely that models simply need more hadronic energy in the forward region.
Load-bearing premise
The result depends on the assumption that the CASTOR intercalibration corrections (electromagnetic energy down by up to 8%, hadronic up by up to 15%) correctly convert the measured section energies into the true particle-level electromagnetic and hadronic energies, so if the shower simulation behind those corrections is wrong, the claimed excess of electromagnetic energy in data could change or disappear.
Editorial extensions
If this is right
- The average total very forward energy rises with central track multiplicity, and all tested generators reproduce this rise reasonably well, showing that underlying-event tunes from central rapidity extrapolate to the far forward region for the total energy.
- All tested generators overestimate the hadronic fraction of the very forward energy, meaning they produce too many long-lived hadrons relative to neutral pions and photons in this phase space.
- The two newest model tunes, SIBYLL 2.3c and PYTHIA 8 CP5, disagree most strongly with the measured electromagnetic-to-hadronic ratio, so retuning them on these data is a direct next step.
- Because the data put even more energy into the electromagnetic channel than the models, the cosmic-ray muon deficit cannot be cured by simply shifting more energy into hadronic production in the forward region.
- The forward-folding matrices are published in a RIVET plugin, so any future generator or tune can be compared to this dataset without a full detector simulation.
Reading between the lines
- The ratio of hadronic to electromagnetic energy is roughly constant across track multiplicities, so a single energy-independent adjustment to the neutral-to-charged pion ratio in fragmentation might bring models into agreement with data.
- Extending this measurement to other collision energies (e.g., 5.02 or 7 TeV) would test whether the hadronic overestimate grows with energy, which would sharpen its relevance for ultra-high-energy cosmic-ray modeling.
- The forward-folding matrices could be applied to generators with different parton-shower or color-reconnection schemes to isolate which modeling ingredient controls the hadronic energy fraction.
- A particle-level measurement using unfolding, rather than forward folding, would confirm that the effect is not introduced by the detector-response correction itself.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper reports a CMS measurement of the average total, electromagnetic, and hadronic energy reconstructed in the CASTOR calorimeter at pseudorapidities -6.6 < eta < -5.2, as a function of the charged-track multiplicity at |eta| < 2, in proton-proton collisions at sqrt(s) = 13 TeV. The analysis uses 0.22 inverse nanobarns of low-luminosity 2015 data, an unbiased bunch-crossing trigger, and a modified pixel tracking algorithm for the solenoid-off configuration. The data are compared with PYTHIA 8 tunes, EPOS LHC, SIBYLL 2.1/2.3c, QGSJET-II.04, and HERWIG 7.1. For models without full detector simulation, the paper introduces a forward-folding method based on four-dimensional migration matrices and makes the matrices available in a RIVET plugin. The central physics claim is that all tested generators overestimate the fraction of energy going into hadrons, and the paper connects this to the muon deficit in ultra-high-energy air shower simulations.
Significance. If the result holds, it is a valuable and novel measurement: it is the first to correlate very-forward CASTOR energy with central charged multiplicity at 13 TeV, and it provides new constraints on underlying-event modelling, forward particle production, and cosmic-ray interaction models. The analysis has clear strengths: an unbiased trigger, quantified vertex and pileup rejection with residual uncertainties, a detector-level presentation that avoids unfolding, a forward-folding procedure validated against full detector simulation to better than 1%, and a public RIVET plugin with the migration matrices. The main physics conclusion about the hadronic energy fraction, however, depends on the relative electromagnetic/hadronic calibration of CASTOR, and the paper does not currently demonstrate the robustness of the universal 'all generators' statement to that systematic uncertainty.
major comments (2)
- [Section 3, Section 5, Fig. 3, Abstract] The universal claim that all generators overestimate the fraction of energy going into hadrons is not shown to survive the quoted intercalibration systematic uncertainty. Section 3 states that dedicated simulation studies allow a maximum decrease of the electromagnetic energy by 8% and a corresponding increase of the hadronic energy by 15%, and that these shifts are anticorrelated. Applied together, they lower the measured E_em/E_had ratio by about 20% (0.92/1.15), i.e., in the direction of all model predictions. The paper states in Section 5 that 'all model predictions are lower than the data' and in Section 6 that the models 'cannot explain the muon deficit', but it does not show whether every generator remains below the data after applying this correlated shift. Please quantify the comparison after applying the intercalibration shifts, or show residuals relative to the full asymmetric systematic band, and qualify the abstract and Section 6 statements if any model becomes consistent with data within this uncertainty.
- [Section 5, Fig. 3] The claim that all generators overestimate the hadronic energy fraction is presented without a quantitative significance or goodness-of-fit measure. Since the abstract makes a universal statement about every tested generator, the paper should report, for each generator and multiplicity bin, the size of the discrepancy relative to the total systematic uncertainty, and state explicitly whether the discrepancy is dominated by the intercalibration uncertainty or by the model predictions themselves. This would make the central claim reproducible and would clarify how much weight the muon-deficit conclusion can bear.
minor comments (4)
- [Abstract] The phrase 'fraction of energy going into hadrons' refers to the energy reconstructed in the hadronic section of a non-compensating calorimeter, not directly to the particle-level hadronic energy. Because the comparison is forward-folded this is internally consistent, but the wording risks overinterpretation; please state explicitly in the abstract or in Section 5 that this is a detector-level quantity.
- [Fig. 3] The axis label of Fig. 3 should unambiguously define the ratio, preferably as <E_em^reco>/<E_had^reco>, directly on the axis; the current layout is ambiguous and the caption alone does not remove the ambiguity.
- [Table 1] The ranges of the pileup-rejection uncertainty (1-8% for total and electromagnetic energies, 1-10% for hadronic energy) and the resulting total uncertainties (18-19%, 18-20%, 20-26%) are not fully explained; a sentence on why the hadronic component is more affected would be helpful.
- [Section 4] The notation k_ij^lm for the migration matrix is introduced compactly; a brief definition of the four indices in one place would improve readability, since the same indices are used in Eq. (1) and in the bin description.
Circularity Check
No significant circularity: the measurement is a detector-level comparison with explicitly model-dependent but non-self-referential forward folding; the central claim is an empirical observation, not an input to the analysis.
full rationale
This is a data measurement paper, not a derivation. The central claim is that all tested Monte Carlo generators predict a lower electromagnetic-to-hadronic energy ratio in the CASTOR region than observed in data. Nothing in the paper defines the data quantity in terms of the model predictions, and no model parameter is fitted to the measured ratio. The forward-folding procedure is an explicit detector-response mapping: the paper states that 'all known detector effects are applied to a given model prediction or theoretical calculation' and that the migration matrix is built from full detector simulation. The model dependence of the matrix is handled as a systematic uncertainty, with the central matrix being an average of four models and the spread among the four variants quoted as the associated uncertainty. For the generator-level-only models (SIBYLL 2.3c, QGSJETII.04, PYTHIA8 CP5, HERWIG 7.1), the folded predictions are not produced from those same models, so the comparison is not self-referential. For the four full-simulation models, the predictions come directly from the full detector simulation, not from the averaged matrix, and the paper reports that forward folding and full simulation agree to better than 1%. The abstract's 'all generators overestimate the fraction of energy going into hadrons' is an empirical comparison of reconstructed energies, not a tautology. The skeptic's concern about the intercalibration systematic (8% electromagnetic decrease and 15% hadronic increase) is a legitimate calibration-robustness question, but it is not circularity: it concerns whether the measured ratio would still lie above all model predictions after applying a systematic shift, not whether the result is equivalent to its own inputs by construction. Self-citations to prior CMS CASTOR papers are used for detector calibration constants and are external inputs, not load-bearing conclusions being proven. Therefore, no circular step can be exhibited, and the appropriate score is 0.
Assumptions & free parameters
assumptions (4)
- domain assumption GEANT4 detector simulation of CASTOR correctly models the calorimeter response, including noncompensation and shower containment for hadrons and electrons or photons over GeV to TeV energies.
- domain assumption The average of migration matrices from PYTHIA 8 CUETP8M1, PYTHIA 8 4C+MBR, EPOS LHC, and SIBYLL 2.1 adequately describes detector response for any generator-level prediction.
- domain assumption The generator-level event selection (Nch >= 1 and xi > 10^-6) reproduces the detector-level event selection (one HF tower above 5 GeV and Ntracks >= 1).
- domain assumption Energy deposited in the CASTOR electromagnetic and hadronic sections can serve as good estimators of particle-level electron/photon and hadron energies.
Cite this review
Pith. "Pith review of Measurement of the average very forward energy as a function of the track multiplicity at central pseudorapidities in proton-proton collisions at $\sqrt{s} =$ 13 TeV." pith.science (2026). https://pith.science/paper/PLZJR5H3
@misc{pith2026190801750,
author = {Pith},
title = {Pith review of: Measurement of the average very forward energy as a function of the track multiplicity at central pseudorapidities in proton-proton collisions at $\sqrts =$ 13 TeV},
year = {2026},
howpublished = {\url{https://pith.science/paper/PLZJR5H3}},
note = {Machine review of arXiv:1908.01750}
}
abstract
The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $-$6.6 $<$ $\eta$ $<-$5.2 in proton-proton collisions at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. The results are presented as a function of the charged particle multiplicity in the region $|\eta|$ $<$ 2. This measurement is sensitive to correlations induced by the underlying event structure over a very wide pseudorapidity region. The predictions of Monte Carlo event generators commonly used in collider experiments and ultra-high energy cosmic ray physics are compared to the data. All generators considered overestimate the fraction of energy going into hadrons.
Figures
Reference graph
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