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REVIEW 2 major objections 4 minor 20 references

Collins asymmetries in pion-in-jet production support Collins-function universality and give EIC a cleaner path to transversity, including sea quarks.

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2026-07-10 20:35 UTC pith:5CCZRE7H

load-bearing objection Solid conference summary of the authors' own recent letters: STAR postdiction supports Collins universality under the hybrid scheme, and EIC predictions remain useful even after WW photons. the 2 major comments →

arxiv 2607.06821 v1 pith:5CCZRE7H submitted 2026-07-07 hep-ph hep-exnucl-th

Collins effect in pion-in-jet production in polarized pp and ep collisions

Carlo Flore , Umberto D'Alesio , Marco Zaccheddu This is my paper
classification hep-ph hep-exnucl-th
keywords Collins asymmetrypion-in-jettransversityCollins functionhybrid TMDElectron-Ion ColliderWeizsäcker-Williamsuniversality
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper claims that a hybrid transverse-momentum-dependent calculation, using transversity and Collins fragmentation functions taken from semi-inclusive deep-inelastic scattering and electron-positron data, already describes STAR measurements of Collins azimuthal asymmetries for pions inside jets in polarized proton-proton collisions. That agreement is presented as evidence that the Collins function is universal across processes. The same framework is then used to predict the analogous asymmetries for lepton-proton collisions at Electron-Ion Collider energies. Although quasireal-photon exchange (Weizsäcker-Williams) roughly doubles the unpolarized cross section, quark-initiated channels still dominate both the numerator and the denominator of the asymmetry. Because gluons therefore play only a marginal role, the lepton-proton process is argued to give cleaner access to the transversity distribution, including its poorly known sea-quark piece, than the proton-proton case.

Core claim

A hybrid TMD approach with collinear initial-state distributions and Collins fragmentation functions extracted from SIDIS and e+e- data reproduces STAR Collins asymmetries in polarized pp collisions, supporting universality of the Collins function; the same framework predicts that, even after sizable Weizsäcker-Williams photon contributions, quark channels continue to dominate at EIC kinematics, so that ℓp pion-in-jet asymmetries furnish a cleaner probe of transversity, including its sea-quark component.

What carries the argument

Hybrid TMD factorization for hadron-in-jet production: the Collins azimuthal asymmetry is formed as a convolution of collinear transversity (and unpolarized PDFs) with a polarized partonic hard scattering and the Collins TMD fragmentation function, while TMD effects are confined to the fragmentation sector; the Weizsäcker-Williams photon flux is added to open additional partonic channels without spoiling quark dominance.

Load-bearing premise

The assumption that initial-state transverse-momentum effects can be ignored and TMD physics kept only in the fragmentation of the jet, for the jet and hadron momenta considered here.

What would settle it

A precise EIC measurement of the Collins asymmetry for charged pions inside jets at forward rapidity (where the calculation predicts 4-8 percent for π-) that systematically falls outside the predicted 2σ bands once the Weizsäcker-Williams contribution is included.

Watch this falsifier — get emailed when new claim-graph text bears on it.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 4 minor

Summary. The manuscript studies Collins azimuthal asymmetries for pion-in-jet production in polarized pp and ℓp collisions within a hybrid TMD framework that treats the initial state collinearly while confining TMD effects to the fragmentation sector. Using the transversity distribution and Collins function extracted from SIDIS and e^{+}e^{-} data (Ref. [6]), the authors first recall a good description of STAR pp data at √s = 200 and 510 GeV, interpreting this as support for Collins-function universality and TMD factorization in the process. They then present LO and LO+Weizsäcker-Williams predictions for EIC kinematics (√s = 45, 105, 141 GeV), showing that the quasireal-photon contribution enhances the unpolarized cross section by factors of ~1.5–2 while leaving quark-initiated channels dominant (80–90 %). Consequently, the Collins asymmetries remain sizable (up to ~8–10 %) and the process is argued to provide cleaner access to transversity, including its poorly constrained sea-quark component.

Significance. If the hybrid factorization and LO treatment remain adequate, the work supplies a nontrivial cross-process test of Collins universality (STAR postdictions) and concrete, falsifiable EIC predictions that highlight the kinematic reach for sea transversity. Strengths include the quantitative STAR comparison with 2σ uncertainty bands, the explicit inclusion of the WW photon flux and its partonic channels, the clear separation of quark versus gluon contributions in the denominator, and the transparent acknowledgment that TMD evolution and NLO jet fragmentation are left for future work. These elements make the paper a useful bridge between existing SIDIS/e^{+}e^{-} extractions and forthcoming EIC measurements.

major comments (2)
  1. [Secs. 2–3, Eqs. (1)–(3)] Secs. 2–3 and the schematic convolutions after Eq. (1) and in Eqs. (2)–(3): the central claim that STAR agreement supports Collins universality (and that EIC ℓp data will cleanly access transversity) rests on the hybrid collinear-initial + TMD-fragmentation approximation remaining valid at the relevant p_jT ~ 10–40 GeV. The paper states that TMD evolution plays only a “marginal role” and defers a more refined treatment, yet no quantitative estimate of evolution or NLO jet-fragmentation corrections is provided for the STAR or EIC kinematics. A short assessment (or reference to existing studies) of how such corrections would shift the z or j_T shapes would make the universality and “cleaner-access” statements more robust.
  2. [Sec. 3, Fig. 3] Fig. 3 and the accompanying text on small-x (large negative η_j) asymmetries: the strong suppression is a direct consequence of the valence-like transversity parametrization of Ref. [6]. While the paper correctly notes that this region is sensitive to the sea, the claim that ℓp processes “allow for a clearer access … including its sea-quark component” would be strengthened by a brief illustration of how a non-zero sea component (within present uncertainties) would alter the predicted asymmetries.
minor comments (4)
  1. [Fig. 2] Several figures (especially Fig. 2) contain repeated axis labels and panel titles that appear to be copy-paste artifacts; cleaning them would improve readability.
  2. [Sec. 1] The hard scale is set to μ = p_jT throughout; a one-sentence remark on the sensitivity of the results to this choice (or to a variation around it) would be useful for readers.
  3. Minor typographical inconsistencies appear in the text (e.g., missing spaces around mathematical symbols, occasional OCR-like artifacts such as “ℓp” vs. “ep”). A careful proof-reading pass is recommended.
  4. [Eq. (5)] The photon-flux formula (Eq. 5) and the choice Q^{2}_max = 1 GeV^{2} are standard, but a brief citation or justification for the numerical cutoff would help non-specialist readers.

Circularity Check

1 steps flagged

Mild self-citation of the authors' own SIDIS/e+e- extraction and prior hybrid-factorization papers; STAR agreement remains an independent cross-process test and EIC results are genuine predictions, so no reduction by construction.

specific steps
  1. self citation load bearing [Sec. 1 (after Eq. 1) and Sec. 2 (Fig. 1 discussion)]
    "we adopted the parametrization of h_1^q and H_1^{\perp q} from Ref. [6]. ... our postdictions based on Ref. [6] provide a good description of the STAR data ... The main outcomes of this study are the good overall data-theory agreement, ... a support for the universality of the Collins FF"

    The numerical inputs that produce the STAR postdiction (and thereby the claimed support for universality) are taken from the authors' own overlapping extraction [6]; the hybrid factorization framework itself is likewise developed in their prior works. This is ordinary self-citation of a data-driven fit rather than a closed definitional loop, but it is load-bearing for the central claim.

full rationale

The paper takes transversity and Collins functions from a global fit to SIDIS and e+e- data (Ref. [6], overlapping authors), inserts them into a hybrid collinear-initial + TMD-fragmentation calculation, and obtains postdictions for STAR pp data plus predictions for EIC lp kinematics (including WW photon contributions). The STAR comparison is a genuine external benchmark: the functions were not fitted to those data, so numerical agreement is not forced by construction and can legitimately be read as support for Collins universality. The EIC numbers are true forward predictions. The only circularity-adjacent element is ordinary self-citation of the extraction and of the authors' earlier pp/ep analyses that define the hybrid scheme; this is load-bearing for the numerical inputs but does not make the central claims tautological. No self-definitional loops, no fitted-parameter-renamed-as-prediction, no uniqueness theorems imported from the authors, and no ansatz smuggled via citation appear. Score 2 reflects the mild self-citation without elevating it to partial circularity of the result.

Axiom & Free-Parameter Ledger

2 free parameters · 4 axioms · 0 invented entities

The calculation rests on standard TMD factorization assumptions, a published global fit for the two TMD functions, and conventional collinear PDFs/FFs. No new particles or forces are introduced. The free parameters live entirely inside the adopted extraction of Ref. [6]; the present paper does not refit them.

free parameters (2)
  • parameters of the transversity and Collins TMD functions of Ref. [6]
    The entire numerical prediction is driven by the functional forms and fitted coefficients of h1^q and H1^perp taken from the simultaneous reweighting of SIDIS and e+e- data; those coefficients are free parameters of the prior fit and are not re-determined here.
  • Q^2_max = 1 GeV^2 cutoff in the WW photon flux
    The upper virtuality cut that defines the quasireal-photon approximation is chosen by hand; changing it alters the size of the WW contribution shown in Fig. 2.
axioms (4)
  • domain assumption Hybrid TMD factorization: initial-state partons are collinear while the Collins TMD FF describes the hadron-in-jet transverse momentum.
    Stated in the abstract and Sec. 1; used for all cross-section formulae (Eqs. 2-3 and the WW extension).
  • domain assumption Universality of the Collins function between SIDIS, e+e- and hadron-in-jet processes.
    The central interpretive claim; tested a posteriori by the STAR comparison but assumed when the same functions are inserted into the pp and ep formulae.
  • standard math No gluon transversity for a spin-1/2 target, so only the q gamma -> q g channel contributes to the polarized numerator under WW.
    Invoked in Sec. 3 after Eq. (4); follows from angular-momentum conservation and is standard.
  • domain assumption Hard scale mu = p_jT and LO accuracy for both the hard scattering and the jet fragmentation.
    Explicitly chosen in Sec. 1 and used throughout the numerical results; higher-order corrections are deferred to future work.

pith-pipeline@v1.1.0-grok45 · 12742 in / 2611 out tokens · 35613 ms · 2026-07-10T20:35:42.946714+00:00 · methodology

0 comments
read the original abstract

We study Collins azimuthal asymmetries for pion-in-jet production in polarized proton-proton and lepton-proton collisions. We adopt a hybrid transverse momentum dependent approach, with a collinear configuration for the initial state, and employ the transversity and Collins fragmentation functions extracted from semi-inclusive deep inelastic scattering and $e^+e^-$ annihilation data. After recalling the good description of the STAR data in $pp$ collisions, which supports the universality of the Collins function, we present predictions for Electron-Ion Collider kinematics, both at leading order and by including the quasireal photon exchange in the Weizs\"acker-Williams approximation. This contribution is sizable but does not spoil the dominance of quark-initiated channels. This implies that $\ell p$ processes allow for a clearer access to the transversity distribution, including its sea-quark component.

Figures

Figures reproduced from arXiv: 2607.06821 by Carlo Flore, Marco Zaccheddu, Umberto D'Alesio.

Figure 1
Figure 1. Figure 1: Collins asymmetry 𝐴 sin(𝜙𝑆 −𝜙 𝐻 𝜋 ) 𝑈𝑇 for 𝑝 ↑ 𝑝 → jet 𝜋 ±𝑋 as a function of 𝑧, compared to STAR data at √ 𝑠 = 200 (left) [16] and 510 GeV (right) [17]. Postdictions are obtained using the transversity and Collins functions of Ref. [6]. The shaded grey area marks the region not covered by the SIDIS and 𝑒 + 𝑒 − data used in the extraction. Figure taken from Ref. [15]. factorization in this class of processe… view at source ↗
Figure 2
Figure 2. Figure 2: Unpolarized cross sections for ℓ 𝑝 → jet 𝜋 ±𝑋 as a function of 𝜂 𝑗 at √ 𝑠 = 45, 105, 141 GeV, with the corresponding 𝑝 𝑗𝑇 integration ranges. Upper panels: LO and LO+WW results; middle panels: ratio of the full (LO+WW) result to the LO piece; lower panels: relative contribution of quark- and gluon-induced channels to the total cross section. Figure taken from Ref. [18]. where 𝑄 2 max = 1 GeV2 and 𝑄 2 min (… view at source ↗
Figure 3
Figure 3. Figure 3: Collins asymmetries for ℓ 𝑝↑ → jet 𝜋 ±𝑋 as a function of 𝜂 𝑗 : LO (dashed lines, gray bands) and LO+WW (solid lines, colored bands). From left to right: √ 𝑠 = 45, 105, 141 GeV, with the corresponding 𝑝 𝑗𝑇 ranges. Uncertainty bands at 2𝜎 CL. Figure taken from Ref. [18]. the asymmetry is almost flat, around 2 − 3%, whereas for 𝜋 + it increases with 𝑧, reaching about 8% in the forward region. This reflects th… view at source ↗
Figure 4
Figure 4. Figure 4: Collins asymmetries for ℓ 𝑝↑ → jet 𝜋 ±𝑋 as a function of 𝑧: LO (dashed lines, gray bands) and LO+WW (solid lines, colored bands). From left to right: √ 𝑠 = 45, 105, 141 GeV. Upper panels: forward rapidities; lower panels: backward rapidities. Uncertainty bands at 2𝜎 CL. Figure taken from Ref. [18]. Acknowledgments C.F. is supported by the European Union’s Horizon Europe research and innovation programme un… view at source ↗

discussion (0)

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