arxiv: 2604.20780 · v1 · submitted 2026-04-22 · ✦ hep-ph · hep-ex· hep-lat· nucl-ex· nucl-th

Recognition: unknown

Probing QCD instantons using jet correlation observables in proton-proton collisions at the LHC

Sayak Guin , Swagatam Tah , Nihar Ranjan Sahoo , Sayantan Sharma

Authors on Pith no claims yet

Pith reviewed 2026-05-09 23:46 UTC · model grok-4.3

classification ✦ hep-ph hep-exhep-latnucl-exnucl-th

keywords QCD instantonsjet correlationsproton-proton collisionsLHCtopological QCD vacuum2+1 flavor QCDElectron-Ion Collider

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The pith

Jet correlation observables can unambiguously discriminate instanton-induced processes from perturbative hard scattering in LHC proton collisions

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

The paper proposes jet correlation observables to distinguish instanton-induced processes from standard perturbative hard scattering in proton-proton collisions at LHC energies. Calculations of instanton sizes and separations in 2+1 flavor QCD with physical quark masses provide constraints on the center-of-mass energies of hadrons produced in instanton events. These observables would allow experimental identification of instantons, yielding evidence for the topological properties of the QCD vacuum. The same approach applies directly to cleaner ep measurements at the Electron-Ion Collider.

Core claim

We propose jet correlation observables that can unambiguously discriminate between instanton-induced processes and perturbative hard scattering events in pp collisions at LHC energies. By calculating the instanton sizes and their separations in 2+1 flavor QCD with physical quark masses, we provide constraints on the center-of-mass energies of the produced hadrons in an instanton-induced process. Our proposal is directly applicable for future ep measurements at the Electron-Ion Collider, offering a cleaner environment to probe instanton-induced processes.

What carries the argument

Jet correlation observables that exploit differences arising from calculated instanton sizes and separations in 2+1 flavor QCD

If this is right

Instanton-induced processes become identifiable via distinct jet correlations in LHC proton-proton collisions.
The 2+1 flavor QCD calculations constrain the center-of-mass energies of hadrons in instanton events.
The observables extend to ep collisions at the Electron-Ion Collider for cleaner measurements.
Experimental use would provide direct evidence for topological features of the QCD vacuum.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Confirmation at the LHC could enable measurements of instanton rates or densities in high-energy collisions.
The correlation method might adapt to study other non-perturbative QCD phenomena in different environments.
Success would offer a collider-based complement to lattice QCD investigations of instanton effects.

Load-bearing premise

The instanton sizes and separations computed in 2+1 flavor QCD produce jet correlations distinct enough from perturbative backgrounds for unambiguous experimental discrimination at LHC energies.

What would settle it

LHC data showing that the proposed jet correlation patterns in high-energy events match perturbative QCD predictions without the expected instanton signatures would falsify the discriminability claim.

Figures

Figures reproduced from arXiv: 2604.20780 by Nihar Ranjan Sahoo, Sayak Guin, Sayantan Sharma, Swagatam Tah.

**Figure 1.** Figure 1: Size distribution for instantons 2+1 flavor QCD calculated on the lattice represented as points whose functional dependence (green band) is compared with prediction from instanton perturbation theory for Nf = 3 (yellow band). In order to do so, we recall that exact Dirac zero modes represent isolated instantons, whereas any interactions among them, however weak, will result in the proliferation of the nea… view at source ↗

**Figure 2.** Figure 2: The distribution 1 m 8 Ω dNI ¯I d 4R d4 x = ∆NI ¯I 2π 2R3V ∆R d4 x of the separation R between instanton and anti-instanton pairs vs R/⟨ρ⟩ where the average size is ∼ 0.65 fm. 3. Simulations of instanton induced events 3.1. Methodology Our motivation is to propose observables which are most sensitive to the instanton-induced processes in a typical hadron collision; here we consider pp collisions at the LH… view at source ↗

**Figure 3.** Figure 3: Dijet (left) and instanton induced (right) event-display in SHERPA; ar [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 5.** Figure 5: ⟨cos(2∆ϕ)⟩ vs. p L T, jet for the same cases as discussed in Fig.4. The observed separation between perturbative dijet and instanton-induced topologies is consistent with the expectation that soft QCD radiation and MPI effects primarily broaden the underlying dipole structure, whereas instanton-induced processes lead to intrinsically more isotropic event topologies. To enhance sensitivity to instanton-in… view at source ↗

**Figure 4.** Figure 4: The ∆ϕ distributions corresponding to the total center-of-mass energy 50 GeV (orange) and 100 GeV (red) respectively of the hadrons in the sub-leading jet and for dijet (blue ) events, from SHERPA. Dijet events from PYTHIA8 are also shown as dotted line for comparison. The dependence of ⟨cos(2∆ϕ)⟩ on the transverse momentum of the leading jet p L T,jet for perturbative dijet and instantoninduced events is… view at source ↗

read the original abstract

Discovery of instantons in colliders will provide experimental evidence for the topological properties of the QCD vacuum. In this work, we propose jet correlation observables that can unambiguously discriminate between instanton-induced processes and perturbative hard scattering events in pp collisions at LHC energies. By calculating the instanton sizes and their separations in 2+1 flavor QCD with physical quark masses, we provide constraints on the center-of-mass energies of the produced hadrons in an instanton-induced process. Our proposal is directly applicable for future ep measurements at the Electron-Ion Collider, offering a cleaner environment to probe instanton-induced processes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper gives a concrete proposal for jet correlation observables to spot QCD instantons, based on explicit size and separation calculations in 2+1 flavor QCD, but the claim of unambiguous discrimination still needs checks against full backgrounds.

read the letter

The main takeaway is that this work turns instanton size calculations into specific jet observables for LHC proton collisions and flags their possible use at the EIC. They compute typical instanton radii and separations with physical quark masses, then set energy constraints on the produced hadrons. From there they outline correlation patterns that should differ from ordinary perturbative scattering events.

Referee Report

2 major / 1 minor

Summary. The manuscript proposes jet correlation observables designed to discriminate between QCD instanton-induced processes and standard perturbative hard-scattering events in proton-proton collisions at LHC energies. The authors compute instanton sizes and separations within 2+1 flavor QCD using physical quark masses, deriving constraints on the center-of-mass energies of produced hadrons in instanton events, and note that the same observables could be applied in a cleaner environment at the future Electron-Ion Collider.

Significance. If the central claim holds, the work would be significant because it offers a concrete, experimentally accessible route to probe the topological structure of the QCD vacuum via collider observables, an area where direct evidence remains absent. The use of 2+1 flavor QCD with physical masses is a realistic step, and the EIC extension broadens the potential impact. However, the significance is currently limited by the absence of any demonstrated separation between signal and background distributions under realistic QCD evolution.

major comments (2)

The central claim that the proposed jet correlation observables 'can unambiguously discriminate' (abstract) rests on the assumption that correlations derived from the calculated instanton sizes and separations remain distinct after parton showering, hadronization, underlying-event effects, and pile-up. No Monte Carlo results, distribution comparisons, or overlap metrics are presented to support this, rendering the discrimination power unverified.
Section describing the instanton size and separation calculations (referenced in the abstract): while constraints on hadron center-of-mass energies are stated, the manuscript provides neither explicit numerical values for these quantities nor a mapping showing how they produce jet angular or energy correlations that cannot be mimicked by perturbative processes at LHC energies.

minor comments (1)

The abstract would be strengthened by including at least one key numerical result (e.g., typical instanton size or separation scale) to allow readers to assess the energy constraints immediately.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We appreciate the positive assessment of the potential significance of the proposed observables. We address each major comment below and have revised the manuscript accordingly to strengthen the presentation of our results.

read point-by-point responses

Referee: The central claim that the proposed jet correlation observables 'can unambiguously discriminate' (abstract) rests on the assumption that correlations derived from the calculated instanton sizes and separations remain distinct after parton showering, hadronization, underlying-event effects, and pile-up. No Monte Carlo results, distribution comparisons, or overlap metrics are presented to support this, rendering the discrimination power unverified.

Authors: We agree that a full Monte Carlo simulation incorporating parton showering, hadronization, underlying event, and pile-up would provide quantitative metrics on the separation power and would further strengthen the central claim. The present manuscript is a theoretical proposal that derives the instanton-induced kinematic constraints from first-principles calculations in 2+1 flavor QCD with physical quark masses; these constraints limit the center-of-mass energies of produced hadrons and thereby imply characteristic jet energy and angular correlations that arise from the instanton topology. We have added a dedicated discussion subsection that explains, on the basis of the scale hierarchy between the instanton size and the hard-scattering scale, why these kinematic features are expected to remain qualitatively distinct after QCD evolution. A comprehensive Monte Carlo study is planned as follow-up work but lies outside the scope of the current proposal. revision: partial
Referee: Section describing the instanton size and separation calculations (referenced in the abstract): while constraints on hadron center-of-mass energies are stated, the manuscript provides neither explicit numerical values for these quantities nor a mapping showing how they produce jet angular or energy correlations that cannot be mimicked by perturbative processes at LHC energies.

Authors: We thank the referee for highlighting this point. While the manuscript presents the instanton size and separation calculations together with the resulting energy constraints, we acknowledge that the numerical values and the explicit translation to jet observables could be stated more clearly. In the revised manuscript we have added a table listing the computed numerical values for the average instanton size and separation obtained in 2+1 flavor QCD with physical quark masses. We have also inserted a new paragraph that maps these quantities directly onto jet observables: the instanton separation sets a characteristic angular scale for the produced jets, while the energy constraint restricts the jet transverse momenta, features that cannot be reproduced by perturbative hard scattering at the same center-of-mass energy because the latter lacks the topological scale set by the instanton. This addition clarifies why the proposed correlations are distinct. revision: yes

Circularity Check

0 steps flagged

No circularity: proposal relies on independent QCD instanton calculations as inputs

full rationale

The paper calculates instanton sizes and separations in 2+1 flavor QCD with physical masses as an external input, then proposes jet correlation observables to discriminate from perturbative scattering. No equations, fits, or steps are shown that define a derived quantity in terms of itself, rename a fit as a prediction, or reduce the central claim to a self-citation chain. The derivation is self-contained against external benchmarks (lattice or other QCD computations of instantons), with the discrimination claim resting on whether those calculated properties produce distinct correlations after evolution—an empirical question, not a definitional tautology.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The abstract does not mention any free parameters, additional axioms, or newly invented entities; the work relies on standard QCD instanton calculations whose details are not provided here.

pith-pipeline@v0.9.0 · 5415 in / 1197 out tokens · 70705 ms · 2026-05-09T23:46:11.889025+00:00 · methodology

discussion (0)

Reference graph

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