REVIEW
Ioffe-time distributions instead of parton momentum distributions in description of deep inelastic scattering
High Energy Physics - Phenomenology · Submitted 19 Oct 1994 · 23 references
Pith's verdict: The framework is borrowed, but the sum rule evaluation is new and the u-quark agreement is a real check — the z ≤ 4 claim just runs past the authors' own convergence bound.
T0 · AI review
Why Pith says this
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Referee report
The paper
The paper claims
Parton distributions are better computed in coordinate space than in momentum space, and a QCD sum rule shows it for the proton's up quark.
The reading
What carries the argument
The load-bearing object is the Ioffe-time distribution itself, $Q_{ig!rm val}(z,ig!mu^2) = ig!int_0^1 du, ig!cos(uz), q_V(u,ig!mu^2)$, identified with a proton matrix element of a nonlocal light-cone operator carrying a path-ordered exponential. Two pieces of machinery make it usable. First, the coordinate-space evolution equation $ig!mu^2 ig!partial_{ig!mu^2} Q_{ig!rm val}(z;ig!mu^2) = -ig!frac{ig!alpha_s}{2ig!pi}C_F ig!int_0^1 du, K(u), Q_{ig!rm val}(uz;ig!mu^2)$, which shows that the scale dependence at separation $z$ is fixed entirely by the distribution at smaller separations, so the short-distance expansion is visible in the equation itself, unlike the case for fragmentation functions. Second, the sum rule: a double Borel transform in $p_1^2$ and $p_2^2$ with continuum subtraction, plus a Ward identity relating the three-point correlator at $ig!Delta = 0$ to the derivative of the two-point correlator, which enforces the normalization $ig!int du, q_V(u) = N_u = 2$ and $N_d = 1$. The same Ward identity converts the most important bilocal power corrections into contact terms, that is, into ordinary local condensate contributions.
Extended reading notes
Core claim
The central claim is that coordinate-space (Ioffe-time) parton distributions are the natural target for nonperturbative QCD, and that a standard QCD sum rule can actually deliver them for the proton's valence quarks. Using a three-point correlator of two nucleon interpolating currents with the nonlocal vector operator $P e^{ig!int A}$-dressed $ig!bar{Psi}(ig!Delta)ig!slashed{n}[ig!Delta;0]ig!Psi(0)$, the authors obtain sum rules whose coefficient functions are smooth functions of $z$, in contrast to the momentum-space version where the same terms become $ig!delta(u)$, $ig!delta(1-u)$ and derivatives thereof. Keeping perturbative, dimension-4, dimension-6 and dimension-8 contributions, the up-quark Ioffe-time distribution agrees with the Glück–Reya–Vogt fit up to $z ig!simeq 4$, quoted as light-cone distances of order 1 to 2 fm in the proton rest frame; the abstract says about 1 fm while the text says nearly 2 fm, reflecting a factor-of-two convention between light-cone and physical longitudinal distance that the paper states explicitly. The down-quark distribution comes out much shorter-ranged than the fit. The authors attribute this to a $ig!cos z$ term from the diagram in which one quark carries the whole proton momentum, to the absence of the dimension-6 bilocal correction in that channel, and to strong cancellation between dimension-6 and dimension-8 condensate contributions that makes the result sensitive to the normalization point.
Load-bearing premise
That truncating the operator product expansion at dimension 8, and dropping the bilocal power corrections that contain explicit gluon fields (a neglect the authors state in the Appendix), leaves the up-quark prediction numerically intact at $z$ near 4.
Editorial extensions
If this is right
- If coordinate-space distributions are the right interface, lattice QCD and instanton-vacuum models can supply parton distributions at moderate $z$ without the singular-term problem that afflicts momentum-space sum rules.
- The coordinate-space evolution equations give a practical alternative to the usual momentum-space GLAP equations, with the large-$z$ tail being the part that moves most between 4 and 20 GeV$^2$.
- Matching the calculated small-$z$ region onto the Regge asymptotic form $ig!sim z^{ig!alpha+1}$ at $z ig!sim 5$ to 8 would reconstruct the full Ioffe-time distribution and hence, by Fourier transform, the momentum-space distribution.
- The same construction applies to the polarized gluon distribution, where knowledge of $ig!Delta G(z)$ up to its maximum at $z ig!sim 6$ would fix the total gluon polarization to roughly 50 percent accuracy.
- For the down quark, the failure points at the need for a different interpolating current, radiative corrections, or a large bilocal correction behaving as $z^2$ in coordinate space.
Reading between the lines
- The argument is really about conditioning: a Fourier transform turns a delta-function-dominated expansion into a smooth one, so the same operator product expansion is far better behaved in $z$ than in $u$. This could be quantified by comparing the convergence of the two expansions term by term, which the paper does not do.
- If the finite correlation length of the vacuum is what pushes nonperturbative strength away from $u ig!to 1$, then momentum-space sum rule calculations truncated at local operators will systematically over-weight the endpoint region, a bias that could be checked against existing fits.
- The paper's own 'matching window' question, whether a region exists where both the small-$z$ expansion and the Regge asymptotics apply, is empirically answerable with present fits; the figures suggest $z ig!sim 5$ to 8, which is already where the sum rule is failing, so the window may be narrow or absent in the down-quark channel.
- A useful independent test would be to compute the same Ioffe-time distributions with a different nonperturbative method and see whether the up/down asymmetry of agreement persists; if it does, it is a property of the interpolating current rather than of the coordinate-space formulation.
Original abstract
We argue that parton distributions in coordinate space provide a more natural object for nonperturbative methods compared to the usual momentum distributions in which the physics of different longitudinal distances is being mixed. To illustrate the advantages of the coordinate space formulation, we calculate the coordinate space distributions for valence quarks in the proton using the QCD sum rule approach. A remarkable agreement is found between the calculated and the experimentally measured u-quark distribution up to light-cone distances $\Delta^- = \Delta^0 - \Delta^3$ of order $\sim 1$ fm in the proton rest frame. The calculation for valence d quarks gives much worse results; the reasons for this discrepancy are discussed.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
We argue that parton distributions in coordinate space provide a more natural object for nonperturbative methods compared to the usual momentum distributions in which the physics of different longitudinal distances is being mixed. To illustrate the advantages of the coordinate space formulation, we calculate the coordinate space distributions for valence quarks in the proton using the QCD sum rule approach. A remarkable agreement is found between the calculated and the experimentally measured u-quark distribution up to light-cone distances $\Delta^- = \Delta^0 - \Delta^3$ of order $\sim 1$ fm in the proton rest frame. The calculation for valence d quarks gives much worse results; the reasons for this discrepancy are discussed.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We argue that parton distributions in coordinate space provide a more natural object for nonperturbative methods compared to the usual momentum distributions in which the physics of different longitudinal distances is being mixed. | contribution 0.47 | 0.26 | 0.67 | asserted 0.60 |
| s1 | To illustrate the advantages of the coordinate space formulation, we calculate the coordinate space distributions for valence quarks in the proton using the QCD sum rule approach. | method 0.51 | 0.95 | 0.07 | asserted 0.97 |
| s2 | A remarkable agreement is found between the calculated and the experimentally measured u-quark distribution up to light-cone distances $\Delta^- = \Delta^0 - \Delta^3$ of order $\sim 1$ fm in the proton rest frame. | result 1.00 | 0.91 | 0.37 | asserted 1.00 |
| s3 | The calculation for valence d quarks gives much worse results; the reasons for this discrepancy are discussed. | limitation 0.73 | 0.90 | 0.05 | asserted 0.99 |
What was examined, and what was not
- arXiv listing abstract v2
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:hep-ph/9410318 by the authors.
References
How the literature uses it
47 arXiv papers cite this in 73 sentences
Papers citing it · 33 sentences without a confident label
Uses its method or tool 1 paper
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For the non-local condensate we adopt the model proposed in [ref] :
Light-Cone Distribution Amplitudes of the Ground State Bottom Baryons in HQET · QCD Sum Rules · 1.00
Cites it as background 33 papers
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Then taking the time component of Eq. [ref] we get [equation] The correlators defined in Eqs. [ref] and [ref] are known in the literature as (bare) Ioffe-time distribution (ITD) and pseudodistribution (pseudo-ITD) respectively [ref] .
Neural-network analysis of Parton Distribution Functions from Ioffe-time pseudodistributions · Introduction · 0.83
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As a function of Lorentz invariant variables, $z^2$ and $≡zp_z$ (known as the Ioffe time [ref] or the quasi light-front distance [ref] ), $𝔐$ can be expressed in terms of invariant amplitudes, $ℳ_sp^(+)$ and $ℳ_pp$ [ref] ,
Gluon helicity distribution in the nucleon from lattice QCD and machine learning · Introduction · 0.85
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All invariant amplitudes are functions of the invariant interval $z^2$ and the Lorentz invariant $(p ·z) ≡- ν$ [ref] called the Ioffe time due to its relation, up to a sign and normalization of the target's mass, with the original variable from DIS cross section analyses [ref] .
Towards the determination of the gluon helicity distribution in the nucleon from lattice quantum chromodynamics · Theoretical Background of Polarized Gluon pseudo-Distribution · 0.98
Discussion
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