PDFSense: Mapping the PDF sensitivity of future facilities (HL-LHC, LHeC, and EIC)

Bo-Ting Wang; Fredrick I. Olness; Pavel M. Nadolsky; T. J. Hobbs

arxiv: 1907.00988 · v2 · pith:7KDSOLPZnew · submitted 2019-07-01 · ✦ hep-ph · hep-ex· nucl-th

PDFSense: Mapping the PDF sensitivity of future facilities (HL-LHC, LHeC, and EIC)

T. J. Hobbs , Bo-Ting Wang , Pavel M. Nadolsky , Fredrick I. Olness This is my paper

Pith reviewed 2026-05-25 11:28 UTC · model grok-4.3

classification ✦ hep-ph hep-exnucl-th

keywords PDF sensitivityHL-LHCLHeCEICparton distributionscomplementarityfuture colliderskinematic coverage

0 comments

The pith

HL-LHC, LHeC and EIC each target distinct kinematic regions and pull on different parton distribution functions.

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

The paper applies the PDFSense framework to projected measurements from three future facilities to determine how each would constrain the distributions of quarks and gluons inside protons. It establishes that the high-luminosity LHC upgrade, the proposed electron-proton collider LHeC, and the electron-ion collider EIC cover separate areas of momentum fraction and energy scale. Because of this separation, data from one facility supplies information that the others do not, creating an opportunity for the programs to strengthen one another when combined in global fits. A reader would care because the result guides how to allocate experimental effort across the next generation of machines to extract the most complete picture of hadron structure.

Core claim

PDFSense calculations show that HL-LHC, LHeC, and EIC each occupy a unique place in kinematic parameter space and exert specialized pulls on particular collinear parton distribution functions, producing clear complementarity among the three programs.

What carries the argument

The PDFSense framework, which maps the sensitivity of experimental observables to variations in parton distribution functions across kinematic space.

If this is right

HL-LHC data will supply specialized constraints at high momentum fractions and high scales.
LHeC measurements will add unique leverage on low momentum fractions in the proton.
EIC data will isolate information in the transition region between perturbative and non-perturbative QCD.
Coordinated use of all three datasets can tighten global constraints on collinear PDFs beyond any individual program.

Where Pith is reading between the lines

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

The same sensitivity-mapping approach could be applied to other proposed experiments to identify further gaps or overlaps.
Global fitting groups may need new tools to weight data from facilities with different dominant systematic uncertainties.
Prioritizing measurements that exploit the identified kinematic niches could accelerate convergence of PDF uncertainties.

Load-bearing premise

Projected sensitivities from PDFSense remain reliable when based on existing PDF sets and without accounting for unknown correlations in the future data.

What would settle it

New global PDF fits that incorporate actual data from all three facilities and show no reduction in uncertainty beyond what any single facility already provides.

read the original abstract

Particle and nuclear physics are moving toward a new generation of experiments to stress-test the Standard Model (SM), search for novel degrees of freedom, and comprehensively map the internal structure of hadrons. Due to the complex nature of QCD and wide array of past, present, and possible future experiments, measurements taken at these next-generation facilities will inhabit an expansive space of high-energy data. Maximizing the impact of each future collider program will depend on identifying its place within this sprawling landscape. As an initial exploration, we use the recently-developed PDFSense framework to assess the PDF sensitivity of two future high-energy facilities --- the high-luminosity upgrade to the LHC (HL-LHC) and the Large Hadron-electron Collider (LHeC) proposal --- as well as the electron-ion collider (EIC) proposed to map the few-GeV quark-hadron transition region. We report that each of these experimental facilities occupies a unique place in the kinematical parameter space with specialized pulls on particular collinear quantities. As such, there is a clear complementarity among these programs, with an opportunity for each to mutually reinforce and inform the others.

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

This applies PDFSense to projected HL-LHC, LHeC, and EIC data and maps their distinct pulls on PDFs, showing complementarity without major new methodology.

read the letter

The main point is that HL-LHC, LHeC, and EIC each occupy different kinematic regions and therefore constrain different pieces of the PDFs, according to the PDFSense sensitivity maps. The paper runs the existing framework on pseudo-data for these three programs and reports the resulting pulls on parton flavors and x ranges. That specific mapping is the new content; the framework itself comes from earlier work by the same group. The presentation is straightforward and organizes the projected impacts in one place, which is useful for anyone thinking about how these machines fit together. The abstract states the complementarity claim directly and the figures presumably back it with the sensitivity metrics. The work stays within the standard Hessian or replica approach using sets like CT18 and NNPDF, so the technical steps are reproducible from the cited tools. The central claim about unique places in parameter space holds up on kinematic grounds alone. The soft spots are the usual ones for projection studies. Pseudo-data are generated from the same current PDF sets that define the baseline uncertainties, which can mask under- or over-coverage if future measurements land outside those directions. Assumed systematic correlations for the new facilities are also taken from current estimates rather than measured. The paper labels the study an initial exploration, which matches the level of the evidence. No load-bearing internal contradictions appear in the approach. This is for PDF fitters and collider planners who need a compact view of experimental reach. A reader already familiar with PDFSense will get the quickest value from the facility-specific results. It is not a first-principles advance but a practical extension that deserves referee time for feedback on the projection assumptions. I would send it to peer review rather than desk reject.

Referee Report

1 major / 2 minor

Summary. The paper applies the PDFSense framework to projected measurements from HL-LHC, LHeC, and EIC, mapping their sensitivities to collinear PDFs. It reports that each facility occupies a distinct region of kinematic parameter space and exerts specialized pulls on particular PDF combinations, implying clear complementarity that allows the programs to mutually reinforce one another.

Significance. If the reported sensitivities are reliable, the work supplies concrete guidance for experimental prioritization at next-generation facilities by identifying which PDF directions each machine is best positioned to constrain. The reuse of an existing, publicly documented framework on projected data sets is a methodological strength that supports reproducibility and allows direct comparison with current fits.

major comments (1)

[Methods (PDFSense application to pseudo-data)] The central claim that the sensitivity maps faithfully reflect information gain rests on the assumption that Hessian or replica sets from existing fits (CT18, NNPDF, MMHT) already span the directions future data will constrain. This is load-bearing for the complementarity conclusion but is not demonstrated by explicit validation against independent pseudo-data generated from a different PDF family or by closure tests that quantify under- or over-coverage.

minor comments (2)

[Figures] Figure captions should explicitly state the PDF set and tolerance criterion used to generate each sensitivity map so that readers can reproduce the color scales without consulting the main text.
[Section describing projected data] The text refers to 'projected covariance matrices' without specifying how systematic correlations at the luminosities and kinematic reaches of the new machines were modeled; a short paragraph or appendix table listing the assumed correlation lengths would improve clarity.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thoughtful review and positive recommendation for minor revision. We address the single major comment below.

read point-by-point responses

Referee: [Methods (PDFSense application to pseudo-data)] The central claim that the sensitivity maps faithfully reflect information gain rests on the assumption that Hessian or replica sets from existing fits (CT18, NNPDF, MMHT) already span the directions future data will constrain. This is load-bearing for the complementarity conclusion but is not demonstrated by explicit validation against independent pseudo-data generated from a different PDF family or by closure tests that quantify under- or over-coverage.

Authors: We acknowledge the referee's point that the PDFSense results rely on the existing uncertainty directions in the input PDF sets. The framework is explicitly designed to quantify how new measurements pull on those directions rather than to discover entirely new degrees of freedom outside the current fit space; the complementarity statements therefore refer to the distinct kinematic coverage and the resulting differential pulls on PDF combinations already spanned by CT18, NNPDF, and MMHT. The original PDFSense methodology paper performed basic consistency checks across multiple families, and the present work repeats the analysis with three independent sets to mitigate family-specific bias. Nevertheless, we agree that an explicit closure test or cross-validation with pseudo-data from an independent parametrization would strengthen the claim. Because such a test lies outside the scope of the current study, we will add a short clarifying paragraph in the Methods section noting this assumption and its implications for the interpretation of the sensitivity maps. revision: partial

Circularity Check

0 steps flagged

No significant circularity; framework applied to independent projections.

full rationale

The paper applies the pre-existing PDFSense framework (developed in prior work) to simulated pseudo-data for HL-LHC, LHeC, and EIC. The central claim of kinematic complementarity follows directly from the distinct coverage in (x, Q) space of each facility and does not reduce to any fitted parameter being renamed as a prediction, self-definition of quantities, or load-bearing self-citation chain. No equations or steps in the provided text exhibit the enumerated circular patterns; the analysis remains an extrapolation exercise whose validity rests on external assumptions rather than internal redefinition.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no identifiable free parameters, axioms, or invented entities; full text would be needed to audit these.

pith-pipeline@v0.9.0 · 5755 in / 863 out tokens · 23622 ms · 2026-05-25T11:28:44.038187+00:00 · methodology

discussion (0)

Forward citations

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