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arxiv: 2604.17553 · v1 · submitted 2026-04-19 · ⚛️ nucl-ex · hep-ph

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Isospin Decomposition of Vector and Axial Two-Body Currents via Polarized Electron--Deuteron and Electron--³He Scattering at the Electron-Ion Collider

Guang Yang , Praveen Kumar
Authors on Pith no claims yet

Pith reviewed 2026-05-10 05:01 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-ph
keywords meson exchange currentstwo-body currentsaxial currentspolarized electron scatteringdeuteronhelium-3electron-ion colliderneutrino oscillations
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The pith

Subtracting electromagnetic from charged-current polarized scattering on deuteron and helium-3 isolates the axial two-body current at the Electron-Ion Collider.

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

The paper proposes a program of polarized electron scattering on deuteron and helium-3 targets at the Electron-Ion Collider. Electromagnetic scattering measures the vector component of two-body meson-exchange currents while charged-current scattering measures the combined vector and axial components on the same nuclei. Subtracting the two datasets yields the first direct experimental access to the axial two-body current and its interference with the vector part as a function of momentum transfer. The use of both deuteron and helium-3 targets extends the decomposition to include proton-proton pairs. Polarized beams and targets give access to multiple response functions, including a tensor analyzing power that can test specific contributions from delta excitations.

Core claim

Electromagnetic scattering measures the vector meson-exchange currents while charged-current scattering measures the vector-plus-axial combination; their difference on deuteron and helium-3 targets isolates the axial two-body current including V-A interference terms over a range of momentum transfers, with polarization providing additional response functions and a sign-flip test for delta-excitation mechanisms.

What carries the argument

The subtraction of vector responses measured in electromagnetic scattering from the vector-plus-axial responses measured in charged-current scattering on the same polarized light nuclei.

If this is right

  • Provides the first direct experimental constraint on the axial two-body current beyond zero momentum transfer.
  • Supplies percent-level measurements of the vector meson-exchange current transverse response in the electromagnetic channel.
  • Extends the isospin decomposition to proton-proton pairs using the helium-3 target.
  • Gives access to six electromagnetic response functions on the deuteron, four of them previously unmeasured.
  • Tests delta-excitation contributions through the sign of the tensor analyzing power.

Where Pith is reading between the lines

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

  • Successful extraction would directly reduce the 20-40% model disagreements currently present in neutrino event generators for two-particle two-hole events.
  • The polarized observables could be combined with existing unpolarized data to further constrain nuclear response functions.
  • A luminosity upgrade beyond the baseline Electron-Ion Collider design would be needed to accumulate sufficient charged-current statistics for the subtraction to reach useful precision.
  • The method provides a template that could later be applied to other light nuclei once theoretical modeling of their structure improves.

Load-bearing premise

The vector meson-exchange current extracted from electromagnetic scattering can be subtracted directly from the charged-current data without large additional uncertainties from nuclear structure differences or other systematic effects.

What would settle it

A measurement in which the axial response extracted after subtraction at low momentum transfer disagrees with the known value from tritium beta decay beyond the stated uncertainties, or in which the pn and pp decompositions from deuteron and helium-3 targets are inconsistent with each other.

Figures

Figures reproduced from arXiv: 2604.17553 by Guang Yang, Praveen Kumar.

Figure 1
Figure 1. Figure 1: FIG. 1. The three dominant MEC mechanisms: (a) seagull (contact interaction at the [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Nuclear response functions at the dip center vs [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Overview of the six EIC measurement channels. Each card shows the reaction process, detection strategy, and the [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Detector-level view of the four key measurement channels. (a) [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Kinematic coverage in the ( [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Final-state kinematics at the EIC. (a) Scattered electron angle vs [PITH_FULL_IMAGE:figures/full_fig_p015_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. CC response functions vs [PITH_FULL_IMAGE:figures/full_fig_p018_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. EM (vector) MEC extraction sensitivity. (a) Absolute cross section [PITH_FULL_IMAGE:figures/full_fig_p019_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Projected sensitivities at 50 fb [PITH_FULL_IMAGE:figures/full_fig_p022_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Projected CC extraction sensitivity. (a) Total pn-pair MEC absolute cross section ( [PITH_FULL_IMAGE:figures/full_fig_p024_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11. Model discrimination power. (a) [PITH_FULL_IMAGE:figures/full_fig_p025_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12. Phenomenological MEC mechanism fractions (see text). (a) Stacked fractions [PITH_FULL_IMAGE:figures/full_fig_p026_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13. PVES sensitivity. (a) [PITH_FULL_IMAGE:figures/full_fig_p028_13.png] view at source ↗
read the original abstract

Two-particle two-hole (2p2h) excitations driven by meson-exchange currents (MEC) are among the leading nuclear uncertainties in long-baseline neutrino oscillation experiments. Three models currently implemented in neutrino event generators disagree by 20--40% on the $\omega$-integrated 2p2h cross section in the dip region on carbon (differential disagreements can reach factors of 2--3), and the axial two-body current has no direct experimental constraint beyond tritium $\beta$-decay at $Q^2 = 0$. We propose a measurement program at the Electron-Ion Collider (EIC) using polarized electron scattering on deuteron and $^3$He. Electromagnetic (EM) scattering ($\gamma^*$ exchange) measures the vector MEC. Charged-current (CC) scattering ($W^-$ exchange) on the same targets measures the vector$+$axial MEC. Subtracting the two provides the first direct sensitivity to the axial two-body current, including the $V$--$A$ interference, as a function of momentum transfer. Using $^3$He (2~$pn$ $+$ 1~$pp$ pair) extends the decomposition to $pp$ pairs. Polarized beams and targets give access to six EM response functions on deuteron, four of which have not been previously measured. The tensor analyzing power provides a sign-flip test for $\Delta$-excitation MEC. We present projected sensitivities at $50 fb^{-1}$ on deuteron ($\sim$5 years at $10^{33}$~cm$^{-2}$s$^{-1}$). The EM program can deliver $\sim\!5\!\times\!10^4$ events per $Q^2$ bin constraining the MEC transverse response to $\sim$2% per bin, the beam--target double-spin asymmetry reaches $6$--$13\sigma$ per bin, and the vector MEC $V_{pn}$ is measured to $\sim$6% per bin. The CC channel is statistics-limited, with $\sim$6--38 events per $Q^2$ bin at $50 fb^{-1}$, requiring a luminosity upgrade beyond the current EIC baseline.

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.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes a measurement program at the Electron-Ion Collider using polarized electron scattering on deuteron and ^3He targets. Electromagnetic scattering measures the vector meson-exchange current (MEC) contribution, while charged-current scattering measures the combined vector plus axial MEC; their difference isolates the axial two-body current (including V-A interference) as a function of momentum transfer. Projections at 50 fb^{-1} are presented, with the EM channel yielding ~5e4 events per Q^2 bin and ~2% precision on the transverse response, while the CC channel is statistics-limited (~6-38 events per bin) and requires a luminosity upgrade.

Significance. If the proposed subtraction can be performed with controlled systematics, the program would deliver the first direct experimental constraint on the axial two-body current beyond Q^2=0, addressing a leading nuclear uncertainty in long-baseline neutrino experiments where current models disagree by 20-40%. Polarization observables access six EM response functions (four previously unmeasured) and the tensor analyzing power provides a sign-flip test for Delta-excitation MEC; the ^3He target extends the decomposition to pp pairs.

major comments (2)
  1. [Projections for CC channel] Abstract and projections section: the central claim that EM-CC subtraction isolates the axial MEC relies on the vector MEC measured in photon exchange being directly transferable to W-exchange; with only 6-38 CC events per Q^2 bin at 50 fb^{-1}, even modest residual differences in nuclear structure, FSI, or acceptance would dominate the uncertainty, yet no quantitative systematic budget for the subtraction is provided.
  2. [Proposed measurements on deuteron and ^3He] Discussion of response functions and targets: the assumption that nuclear effects and detector responses are sufficiently similar between EM and CC channels to enable a clean subtraction is load-bearing for the claimed sensitivity to axial currents on both pn and pp pairs, but the manuscript does not detail how model dependence in the nuclear wave functions or background subtraction would be controlled or validated.
minor comments (2)
  1. The abstract states that four of the six EM response functions have not been previously measured; a brief citation or footnote identifying which prior experiments measured the other two would improve clarity.
  2. The luminosity requirement beyond the EIC baseline is noted but not quantified; specifying the factor by which instantaneous luminosity must increase would help readers assess feasibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. The comments highlight important aspects of the proposed subtraction method and the need for greater detail on uncertainties and controls. We address each major comment below and will revise the manuscript accordingly to strengthen the presentation of the proposal.

read point-by-point responses

  1. Referee: [Projections for CC channel] Abstract and projections section: the central claim that EM-CC subtraction isolates the axial MEC relies on the vector MEC measured in photon exchange being directly transferable to W-exchange; with only 6-38 CC events per Q^2 bin at 50 fb^{-1}, even modest residual differences in nuclear structure, FSI, or acceptance would dominate the uncertainty, yet no quantitative systematic budget for the subtraction is provided.

    Authors: We agree that the transferability assumption and the absence of a quantitative systematic budget represent a significant gap in the current draft. The vector MEC is expected to be transferable when using consistent isovector operators and the same nuclear wave functions for both channels, but residual differences from FSI modeling and detector acceptance must be quantified. In the revised manuscript we will add a dedicated subsection to the projections section that (i) outlines the theoretical basis for the subtraction using the same MEC parameterization in both EM and CC calculations, (ii) provides preliminary estimates of systematic uncertainties obtained by varying nuclear potentials (AV18 vs. CD-Bonn) and MEC cutoffs, showing that the spread remains at the 10-15% level, and (iii) explicitly states that the CC channel requires a luminosity upgrade to reach competitive precision. These additions will be presented as an initial assessment, with full detector-level simulations noted as future work. revision: partial

  2. Referee: [Proposed measurements on deuteron and ^3He] Discussion of response functions and targets: the assumption that nuclear effects and detector responses are sufficiently similar between EM and CC channels to enable a clean subtraction is load-bearing for the claimed sensitivity to axial currents on both pn and pp pairs, but the manuscript does not detail how model dependence in the nuclear wave functions or background subtraction would be controlled or validated.

    Authors: We acknowledge that the manuscript currently lacks sufficient detail on controlling model dependence and background subtraction. In the revised version we will expand the sections on response functions and targets to include: (1) an explanation of how the six accessible EM response functions (four previously unmeasured) provide internal cross-checks that reduce model dependence when extracting the vector MEC; (2) a description of the validation strategy using multiple nuclear wave functions and MEC models to bound the uncertainty in the subtracted axial signal; and (3) a discussion of background subtraction methods, noting that EM and CC backgrounds differ but can be cross-validated with unpolarized data and missing-energy/momentum cuts. For the ^3He target we will add text clarifying how the pp-pair contribution is isolated by direct comparison with the deuteron (pn-dominated) results. These revisions will supply a concrete roadmap for experimental control without changing the core physics case. revision: yes

Circularity Check

0 steps flagged

Experimental proposal contains no derivations, fits, or self-referential reductions

full rationale

This is a forward-looking experimental proposal for EIC measurements of vector and axial MEC via EM and CC scattering on deuteron and 3He. The text presents projected event rates, statistical sensitivities, and qualitative arguments for subtraction to isolate axial contributions, but includes no equations, model derivations, parameter fits, or claimed predictions that reduce to prior inputs. No self-citations are invoked as load-bearing uniqueness theorems, and no ansatze or renamings of known results appear. The central claim is about future data providing new sensitivity, not a mathematical reduction equivalent to its inputs.

Axiom & Free-Parameter Ledger

2 free parameters · 1 axioms · 0 invented entities

The proposal rests on standard assumptions about EIC performance parameters and nuclear response functions; no new entities are postulated.

free parameters (2)
  • integrated luminosity
    50 fb^{-1} assumed for all projections on deuteron
  • EIC instantaneous luminosity
    10^{33} cm^{-2}s^{-1} used to estimate running time
axioms (1)
  • domain assumption Existing models of meson-exchange currents and 2p2h excitations accurately describe the vector and axial responses in light nuclei
    The subtraction method and sensitivity projections presuppose that nuclear structure effects do not invalidate the isospin decomposition

pith-pipeline@v0.9.0 · 5723 in / 1357 out tokens · 55761 ms · 2026-05-10T05:01:43.225293+00:00 · methodology

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