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Hard-jet correlations in large and small systems

T0 review · 0 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read This review of recent hard-jet correlation measurements argues that the data now establish a flavor-dependent parton energy loss, with gluon jets quenching more than quark jets in heavy-ion collisions.

desk verdict A solid conference-proceeding review of hard-jet correlations, no new results, with one overstrong flavor-quenching claim and some editing slips. read the letter →

arxiv 2505.00250 v1 pith:XAR32NJU submitted 2025-05-01 nucl-ex

Riccardo Longo This is my paper · ORCID
classification nucl-ex
keywords jetquenchingquark-gluonplasmahard-jetcorrelationsphoton-taggedjetsfractionalenergylossdijetasymmetrycolorfluctuationsnuclearPDFs
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 proceedings review argues that hard-jet correlation measurements have turned several long-standing questions about the quark-gluon plasma into quantitatively constrained results. The central result the author highlights is the comparison of photon-tagged jets with inclusive jets: after removing the shape of the initial parton spectrum through the fractional energy loss $S_{\rm loss}$ and correcting for isospin and for the nuclear modification of parton distributions, gluon-initiated jets lose more energy than quark-initiated jets, as expected from their larger color charge. The same measurements constrain how energy loss depends on path length and jet radius, give a first bounded hint of the medium response opposite the jet, and find no clean evidence for energy loss in small collision systems once selection biases and color fluctuations are taken into account. A sympathetic reader would care because these correlations are the main experimental route from the macroscopic behavior of the quark-gluon plasma down to the microscopic color-charge structure of QCD.

What carries the argument

The load-bearing comparison is between $R_{AA}$ and the fractional energy loss $S_{\rm loss}$ for inclusive versus photon-tagged jets. $S_{\rm loss}$ is constructed so that the effect of the steeply falling transverse-momentum spectrum is removed, allowing the color-charge hierarchy $C_g/C_q \sim 2.25$ to be tested directly; the photon tag selects a mostly quark-initiated sample, while inclusive jets contain a larger gluon-initiated fraction. Supporting this comparison are the dijet momentum imbalance $x_J$ and the recoil-jet yield $I_{AA}$, used as path-length and radius probes, together with the color-fluctuation model used to reinterpret event-activity biases in small collision systems.

What would settle it

Recompute the ATLAS $S_{\rm loss}$ comparison with alternative sets of nuclear parton distribution functions and without the isospin correction: if the gap between photon-tagged and inclusive jets shrinks to zero or reverses under a plausible alternative, the review's central claim is not robust. Alternatively, a future measurement that tags quark jets directly, for instance through $b$-quark jets or jet charge, and finds equal or smaller energy loss for gluon-enriched jets at the same $p_T$ would contradict the claimed hierarchy.

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Extended reading notes

Core claim

The paper's central claim is that the ensemble of hard-jet correlation measurements now supports a flavor-resolved picture of parton energy loss. In large systems, the nuclear modification factor alone is ambiguous because the $R_{AA}$ depends on the slope of the initial transverse-momentum spectrum; the review therefore puts the fractional energy loss $S_{\rm loss}$ at the center, and on that quantity photon-tagged jets, which are mostly quark-initiated, lose less energy than inclusive jets, which carry a larger gluon fraction. The author calls this the strongest confirmation to date of the predicted color-charge hierarchy $C_g/C_q \sim 2.25$. The same data show that narrower, more coherent jets lose less energy, that dijet momentum imbalance and recoil-jet yields across jet radii track path-length and medium-response effects, and that boson-tagged events provide a first constrained hint of a diffusion wake. In small systems, the review's claim is that apparent suppression can be traced to event-activity selection biases and color fluctuations of the projectile rather than to quark-gluon plasma formation.

Load-bearing premise

The conclusions assume that the experimental analyses they cite are correct and unbiased; in particular, extracting the fractional energy loss for photon-tagged jets requires subtracting the different proton-neutron content of the nucleus and the nuclear modification of parton distributions, and if those subtractions are wrong the claimed ordering of gluon versus quark jet quenching would no longer follow.

Editorial extensions

If this is right

  • If the flavor hierarchy is real, inclusive jet suppression should increase with the gluon fraction, so measurements at lower $p_T$, forward rapidity, or with different jet radii should show systematically larger $S_{\rm loss}$ than photon-tagged samples.
  • The radius-dependent dijet and recoil-jet results imply that larger jet radii recapture more soft medium-induced radiation, making the ratio of $R_{AA}$ between radii a direct diagnostic of the medium response and a target for the next generation of Monte Carlo generators.
  • The absence of a clean energy-loss signal in $p$+Pb and high-multiplicity $pp$ data, once selection biases are modeled, means future searches for the onset of quenching should use observables insensitive to event-activity selection, such as boson-tagged correlations in $p$+O and O+O collisions.
  • The first bounded hints of a diffusion wake imply that the medium responds collectively to the passing jet; the coming high-statistics data should sharpen the amplitude and width measurement, and a positive detection would validate models that include wake and recoil effects over those without them.

Reading between the lines

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

  • An extension the paper leaves implicit: the $S_{\rm loss}$ flavor test could be repeated with $Z$-tagged and $b$-tagged jets, where the quark fraction and the initial energy scale differ, to map the color-charge hierarchy over a wider kinematic range.
  • If color fluctuations explain both the event-activity bias and the neutral-pion versus direct-photon comparison, then every event-activity-selected observable in $p/d$+A collisions carries a proton-shape prior; a concrete re-analysis of published suppression data with a fluctuating-proton model should come before attributing any residual to final-state energy loss.
  • The tension between the dijet radius trend and the charged-jet radius trend could be resolved by a common jet definition; a same-experiment, same-observable radius scan would be a direct test of whether the difference comes from calorimeter versus track jets or from the dijet selection.
  • The quoted diffusion-wake amplitude of 0.5-0.8 percent is small, but it predicts a definite shape in pseudorapidity; fitting the wake width rather than assuming it would discriminate between weakly and strongly coupled medium-response models.
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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

0 major / 4 minor

Summary. This paper is a conference proceedings based on the author's Hard Probes 2024 talk. It reviews recent hard-jet correlation measurements in large (Pb+Pb) and small (p+Pb, d+Au, pp) systems. The main topics are: (i) flavor dependence of jet quenching, using the ATLAS comparison of photon-tagged and inclusive jet R_AA and S_loss; (ii) color coherence effects probed by jet-substructure R_AA and CMS photon-jet substructure; (iii) path-length dependence from ATLAS dijet asymmetry as a function of jet radius and ALICE semi-inclusive hadron-jet measurements; (iv) medium response and diffusion wake in ATLAS photon-jet and CMS Z-hadron correlations; (v) searches for energy loss in small systems from CMS, STAR, and ALICE; (vi) color-fluctuation effects in p+A collisions from ATLAS and PHENIX; and (vii) nPDF constraints from ATLAS photonuclear and p+Pb dijets and CMS dijet ratios. The paper closes with an outlook on LHC Run 3, the oxygen program, and sPHENIX/STAR upgrades.

Significance. If accurate, the review summarizes a coherent set of recent measurements that strengthen the case for flavor-dependent parton energy loss, color decoherence, path-length dependence, and jet-induced medium response, while consistently finding no energy-loss signature in small systems. The paper is a useful, well-referenced snapshot of the field; its main strength is that it reports external measurements without introducing new derivations and it is explicit about model failures, such as the inability of current models to capture the CMS photon-jet substructure trends and the failure of JETSCAPE/LBT to describe the dijet R-dependence. Its limitations are those of a proceedings: no independent cross-checks of the cited analyses, and a few presentation errors. The central interpretive claim about the 'strongest confirmation' of quark-versus-gluon quenching is inherited from the ATLAS analysis and should be paired with a caveat about the isospin/nPDF corrections on which it rests.

minor comments (4)
  1. [Section 2.1] The sentence beginning 'These data provide the strongest confirmation to date of larger jet quenching for gluon jets compared with quark jets' uses a superlative that is not supported within the review, which does not compare against other flavor-sensitive measurements (e.g., b-jet or open-heavy-flavour tagged jets) and does not discuss the sensitivity of the S_loss ordering to the ATLAS isospin and nuclear-PDF corrections; suggest replacing 'strongest' with 'strong' and adding a one-sentence caveat that the conclusion inherits the assumptions of the ATLAS correction procedure.
  2. [Section 3.2] The sentence 'The analysis aimed to suppress centrality bias by comparing two probes similarly affected by such correctly interpreting, attributing any residual modification to final-state effects' is garbled; it should read something like 'by comparing two probes that are similarly affected by the centrality selection, so that any residual modification can be attributed to final-state effects.'
  3. [Section 3.2 / Fig. 7] The PHENIX ratio RdAu(pi0)/RdAu(gamma_dir) is shown in panel (b) of Figure 7, but the text refers to it twice as 'Fig. 7c' (once in the sentence about decreasing with the number of binary collisions and once in the color-fluctuation-model comparison); correct these cross-references.
  4. [Section 2.2] The sentence 'These findings become even more compelling when compared to ALICE's radial scan of charged-jet R_AA, where R=0.6 jets were more suppressed than R=0.2 jets' is vague because the ATLAS and ALICE radial trends point in opposite directions; suggest rewording to emphasize that the apparent tension motivates a unified interpretation, which the following sentence partly does.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the paper is a conference summary that attributes all physics claims to external measurements and model references.

full rationale

This proceedings makes no new derivation and fits no parameters. Its central statements, including the claim that photon-tagged jet S_loss provides 'the strongest confirmation to date of larger jet quenching for gluon jets compared with quark jets,' are explicitly attributed to the ATLAS measurement and analysis in Refs. [3,4], including the isospin and nPDF corrections performed there. The review adds only commentary and comparison, not an independent reduction of an output to an input. The color fluctuation model discussion in Section 3.2 relies on Refs. [23,26], which are external published papers; although the acknowledgments thank D. Perepelitsa for discussions, the present author is not an author of those cited works, so this is not a self-citation chain. No equation in the paper defines a quantity in terms of the quantity it is used to predict, and no fitted parameter is renamed as a prediction. Concerns about the reliance on the experimental correction procedure are correctness or robustness concerns about external analyses, not circularity of this paper. The derivation chain, to the extent one exists, is a chain of citations to independent experimental results, which the rules treat as legitimate evidence. Therefore the circularity score is 0.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

This is a review/proceeding. It introduces no free parameters and no invented entities. The central content is a summary of cited experimental results, and the background assumptions are the validity and accurate representation of those references, plus the standard theoretical models used to interpret them.

assumptions (3)
  • domain assumption The cited experimental measurements by ATLAS, CMS, ALICE, STAR, and PHENIX are accurate and correctly interpreted in the review.
    All substantive conclusions, e.g., the quark versus gluon quenching ordering in Section 2.1, are inherited from these papers.
  • domain assumption Perturbative QCD predicts that gluon jets lose about 9/4 times more radiative energy than quark jets (Section 2.1, Ref. [2]).
    This expectation motivates the interpretation of the photon-tagged versus inclusive jet RAA comparison, but the review does not derive it.
  • domain assumption The color fluctuation model of Alvioli et al. (Ref. [23]) reproduces the event-activity and nuclear-breakup data without tuning.
    This model is used in Section 3.2 to explain the PHENIX RdAu ratio and ATLAS ZDC trends.

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Cite this review

Pith. "Pith review of Hard-jet correlations in large and small systems." pith.science (2026). https://pith.science/paper/XAR32NJU

@misc{pith2026250500250,
  author       = {Pith},
  title        = {Pith review of: Hard-jet correlations in large and small systems},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/XAR32NJU}},
  note         = {Machine review of arXiv:2505.00250}
}
abstract

Hard-jet correlations probe parton energy loss and the microscopic structure of the quark-gluon plasma formed in ultra-relativistic heavy-ion collisions. The correlation of high-$p_\mathrm{T}$ jets with other jets, hadrons, or electroweak bosons, offers differential sensitivity to medium-induced effects such as momentum broadening, color decoherence, and medium response in different types of nuclear reactions. Such correlations can also be used to study cold nuclear matter effects arising in $p$+A collisions. This proceeding summarizes recent advances achieved by studying hard-jet correlations in large and small systems discussed at Hard Probes 2024, complementing the experimental jet overview.

Figures

Figures reproduced from arXiv: 2505.00250 by the authors.

Figure 1
Figure 1. (a) The RAA of photon-tagged jets (filled squares) as a function of pT for 0-10% Pb+Pb events, overlaid with that of inclusive jets (open circles) in the same centrality range [3]. (b) Yields of photon-tagged jets as a function of pT in Pb+Pb events for different centrality classes and the differential cross-section in pp events (circles). (c) ATLAS S loss extraction for inclusive and γ-tagged jets. In recent years,… view at source ↗
Figure 2
Figure 2. (a) The RAA of inclusive jets as a function of the groomed jet radius measured by ATLAS [7] for 0-10% Pb+Pb events. (b) Ratio of the normalized γ-jets yields of Pb+Pb to pp data measured by CMS [8] as a function of groomed jet radius, for events characterized by large photon-jet imbalance (xJγ > 0.4) in 0-30%Pb+Pb events. (c) Same as (b) but for less quenched jet selections (xJγ > 0.8). Carrying out an RAA measureme… view at source ↗
Figure 3
Figure 3. (a) ATLAS dijet asymmetry for jets of R=0.2, 0.3, 0.4, 0.5, 0.6, compared with JETSCAPE (LBT+MATTER) predictions (b) Ratio between ATLAS R pair AA for large (R=0.6) and small (R=0.2) jet radii, for both leading and subleading jets. The results compared to predictions from LBT and JETSCAPE. (c) Ratio between RAA for large (R=0.6) and small (R=0.2) charged track jets measured by ALICE. Additional insights into the pat… view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: (a) Underlying event-corrected pT distributions for pp and central Pb+Pb collisions and corresponding IAAfrom ALICE. (b) IAA for jets with R=0.2, 0.4 and 0.5 for different jet pT selections as a function of the azimuthal separation of the trigger hadron and recoil jet,…
Figure 5
Figure 5. Figure 5: (a) ATLAS [16] signal/background ratio in γ-jets as a function of |∆η(jet, track)| for most 0-10% Pb+Pb collisions and 0.3 < xJγ < 0.6 selection. (b) CMS distribution of ∆ϕch,Z in Z-tagged events [17]. (c) CMS Z-hadron data [17] in different centrality selections, over…
Figure 6
Figure 6. Figure 6: (a) CMS xJ ratio between two given track multiplicity selections, compared with Pythia8+EPOS MC [18] (b) STAR per-trigger jet spectra for trigger and recoil sides, for both high and low event activity selections [20]. (c) ALICE comparison of hadron+jet azimuthal correl…
Figure 7
Figure 7. Figure 7: (a) ATLAS RCP as a function of the Bjorken-x of the proton, xp [21] (b) PHENIX RdAu(π 0 )/RdAu(γdir) from d+Au collisions at 200 GeV, compared with results from the color fluctuation model [26]. (c) ATLAS average ZDC (blue circles) and forward calorimeter transverse en…
Figure 8
Figure 8. Figure 8: (a) (x,Q 2 ) coverage of data included in EPPS21 [32] together with new coverage of new results from ATLAS and CMS discussed in this proceeding. (b) ATLAS triple-differential extraction of the dijet cross-section in photo-nuclear events [29]. (c) CMS forward-to-backwar…

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Reviewed August 16, 2026 · model on record in the stance chip above.