REVIEW
Saturation in Diffractive Deep Inelastic Scattering
High Energy Physics - Phenomenology · Submitted 15 Mar 1999 · 32 references
Pith's verdict: The diffractive companion to the Golec-Biernat–Wüsthoff saturation fit: the parameter-free diffractive normalization is a real result, but the agreement is shown only by eye and the 'no further parameters' line is softer than the abstract suggests.
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Referee report
The paper
The paper claims
The same dipole cross section that fits inclusive small-$x$ scattering, with its three parameters already fixed, also accounts for HERA's diffractive deep inelastic data without any new fit — and it predicts that the diffractive and the dif
The reading
What carries the argument
The saturating dipole cross section $hat{sigma}(x,r^2) = sigma_0{1-exp[-r^2/(4R_0^2(x))]}$ with $R_0(x) = (x/x_0)^{lambda/2}$ GeV$^{-1}$, together with its Fourier-conjugate unintegrated gluon distribution $alpha_s F(x,l_t^2) = (3sigma_0/4pi^2) R_0^2(x), l_t^2 e^{-R_0^2(x)l_t^2}$. The Gaussian form in $l_t^2$ is what makes every $l_t$ integral in the diffractive formulae converge without an external cutoff, and it is the same exponent that supplies the $e^{-Q^2R_0^2}$ factor in the gluon distribution $xG(x,Q^2)$. The diffractive cross section is then built from $hat{sigma}^2$ at zero momentum transfer, divided by the slope $B_D$, with the $t$-dependence assumed exponential.
Extended reading notes
Core claim
The authors argue that diffractive deep inelastic scattering is dominated by the saturation region of the dipole cross section, while inclusive scattering at large $Q^2$ is dominated by the color-transparency region $r sim 2/Q$. Because the diffractive integrand peaks at $r sim 2R_0(x)$, where $hat{sigma}$ has already flattened to $sigma_0$, the diffractive cross section is cut off by the saturation scale itself rather than by an arbitrary infrared cutoff. Working through the small-$r$ approximation of the photon wave function, they obtain $sigma^D/sigma approx (sigma_0/8pi B_D), 1/ln[R_0^2(x)Q^2]$, i.e. an approximately constant ratio, in contrast to the $x^{-2lambda}$ behavior that a non-saturated $hat{sigma} propto r^2$ ansatz would give. Numerically they evaluate three contributions — transverse $qbar q$, longitudinal $qbar q$, and $qbar q g$ — using the parameters $sigma_0 = 23.03$ mb, $lambda = 0.288$, $x_0 = 3.04 imes 10^{-4}$ from the inclusive fit, with $B_D = 6$ GeV$^{-2}$ taken from HERA and $alpha_s = 0.2$ fixed, and compare to the H1 and ZEUS $x_{I P}F_2^D$ data in $beta$, in $x_{I P}$, and through the effective Pomeron intercept. They report agreement, in particular at moderate and large $beta$, and note that the relative size of the three contributions is set by QCD color factors rather than fitted.
Load-bearing premise
The small-$beta$ part of the prediction rests on the $qbar q g$ term computed in a leading-log$(Q^2)$ approximation with strongly ordered transverse momenta, an approximation the authors themselves say does not exactly satisfy the kinematic constraints and whose error is not quantified.
Editorial extensions
If this is right
- The diffractive structure function's normalization is a prediction of the inclusive fit, so a discrepancy in overall size would be a failure of the model rather than a free parameter to absorb.
- The ratio $sigma^D/sigma$ stays nearly flat in $W$ and $Q^2$ across the measured range, with only a mild rise at small diffractive mass from the longitudinal, higher-twist piece.
- The effective Pomeron intercept extracted from the $x_{I P}$ slope rises with $Q^2$ and depends on the diffractive mass, tracking the longitudinal contribution at low mass and being pulled down at small $beta$ by the $ln(Q^2/k^2)$ in the $qbar q g$ term.
- Soft contributions to diffraction are suppressed relative to hard ones, so a large fraction of the diffractive cross section comes from $r < 2R_0$ — the basis for calling diffractive deep inelastic scattering a semi-hard process.
- Because the gluon distribution implied by the model scales rather than evolves, the $Q^2$ dependence of the data is claimed to be accounted for without adding DGLAP evolution, though the authors note evolution could be added and would give moderate improvement.
Reading between the lines
- If the diffractive normalization really is fixed by the inclusive fit, then a future measurement of the diffractive-to-inclusive ratio at much smaller $x$ or higher $Q^2$ is a sharper test of saturation than $F_2$ itself, since the two channels weight different regions of the same dipole cross section.
- The prediction that the average transverse momentum of the diffractive final state grows as $x$ falls is separable from the ratio test: an explicit measurement of the $k_t$ spectrum in diffraction would probe the mechanism rather than just the normalization.
- The suppression of large dipoles by saturation suggests the same blackening argument should apply to diffractive vector-meson production and to single diffraction in hadron–hadron scattering, where the profile function in central collisions is the analogous quantity.
- The authors leave diffractive charm production for later work; since charm masses change the wave-function weighting toward smaller $r$, a charm diffractive measurement would test whether the same $R_0(x)$ sets the scale for heavy quarks too.
Original abstract
We successfully describe the HERA-data on diffractive deep inelastic scattering using a saturation model which has been applied in our earlier analysis of the inclusive $ep$-scattering data. No further parameters are needed. Saturation already turned out to be essential in describing the transition from large to small values of $Q^2$ in inclusive scattering. It is even more important for diffractive processes and naturally leads to a constant ratio of the diffractive versus inclusive cross sections. We present an extensive discussion of our results as well as detailed comparison with data.
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 successfully describe the HERA-data on diffractive deep inelastic scattering using a saturation model which has been applied in our earlier analysis of the inclusive $ep$-scattering data. No further parameters are needed. Saturation already turned out to be essential in describing the transition from large to small values of $Q^2$ in inclusive scattering. It is even more important for diffractive processes and naturally leads to a constant ratio of the diffractive versus inclusive cross sections. We present an extensive discussion of our results as well as detailed comparison with data.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We successfully describe the HERA-data on diffractive deep inelastic scattering using a saturation model which has been applied in our earlier analysis of the inclusive $ep$-scattering data. | contribution 0.60 | 0.92 | 0.54 | asserted 1.00 |
| s1 | No further parameters are needed. | result 0.72 | 0.70 | 0.10 | asserted 1.00 |
| s2 | Saturation already turned out to be essential in describing the transition from large to small values of $Q^2$ in inclusive scattering. | background 0.67 | 0.60 | 0.14 | asserted 1.00 |
| s3 | It is even more important for diffractive processes and naturally leads to a constant ratio of the diffractive versus inclusive cross sections. | interpretation 0.78 | 0.68 | 0.49 | asserted 1.00 |
| s4 | We present an extensive discussion of our results as well as detailed comparison with data. | contribution 0.96 | 0.77 | 0.04 | asserted 0.73 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:hep-ph/9903358 by the authors.
References
How later papers used it
92 later papers cite it
4 later papers by other authors are listed below: 1 used its method or tool and 3 used its data or values.
Uses its method or tool 1 of 11 papers with this label
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Interpolated between the small beta, large Nc formula used in another work and the finite beta, large Q2 form used in this paper.
Diffractive structure functions in nuclei (2009) · 0.97 · paraphrased: the licence does not allow quoting
Uses its data or values 3 of 5 papers with this label
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The kinematic dependence of $Q_F$ is given phenomenologically by the relation [ref] [equation] where $Q_p,0^2 = 0.152$GeV$^2$, $λ=0.215$, $σ_0≡2πB_G$ denotes the effective transverse area of the nucleon, with $B_G = 0.156$ fm$^2$ .
Baryon stopping and charge deposition in heavy-ion collisions due to gluon saturation (2024) · Parametrical formula · 0.85
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Used this paper's value of sigma_0 = 23.03 mb together with its R_0(x) expression.
Higgs Hadroproduction at Large Feynman x (2007) · 0.95 · paraphrased: the licence does not allow quoting
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Superimposed the saturation scale from the GBW model and this paper onto its contour plots to illustrate sensitivity to saturation effects.
Unitarity Constraints on Semi-hard Jet Production in Impact Parameter Space (2008) · 0.80 · paraphrased: the licence does not allow quoting
How the literature uses it
85 arXiv papers cite this in 170 sentences
Papers citing it · 103 sentences without a confident label
Contests it 1 paper
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Note that the form eq:dip-adjoint differs from the one used in Ref. [ref] , where the adjoint dipole does not have the right saturation limit.
Nuclear enhancement and suppression of diffractive structure functions at high energies · Computing the diffractive structure function · contest 0.74
Builds on it 4 papers
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The model was extended to describe diffraction in [ref] .
A unitarized model of inclusive and diffractive DIS with Q2-evolution · Introduction · 0.85
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We first consider a simple impact parameter dependent generalization of the Golec-Biernat Wusthoff (GBW) model [ref] , where the fundamental dipole gluon distribution is given by [equation]
The McDIPPER: A novel saturation-based 3+1D initial state model for Heavy Ion Collisions · GBW Model · 0.96
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In particular, we first study the Golec-Biernat Wusthoff (GBW) model [ref] , extended to include a spatially dependent target and angular correlations.
Diffractive dijet production in impact parameter dependent saturation models · Introduction · 0.96
Uses its method or tool 11 papers
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In different works this component has been evaluated in different limits, we shall here use the approach of [ref] and interpolate between the small $β$, large $N_c$ formula used in [ref] and the finite $β$, large $Q^2$ form used in [ref] .
Diffractive structure functions in nuclei · Method · 0.97
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To keep the model as simple as possible, we have chosen the form of the dipole amplitude $N(x,r)$ given by the Golec-Biernat and Wusthoff model [ref] ,
Dissociative production of vector mesons at electron-ion colliders · Dipole-target cross section · 0.99
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For this, we will use the simplest model available, the Golec-Biernat Wusthoff (GBW) model [ref] , where the fundamental dipole gluon distribution is given by a Gaussian distribution around the saturation scale,
Baryon stopping and charge deposition in heavy-ion collisions due to gluon saturation · Parametrical formula · 0.94
Uses its data or values 4 papers
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The kinematic dependence of $Q_F$ is given phenomenologically by the relation [ref] [equation] where $Q_p,0^2 = 0.152$GeV$^2$, $λ=0.215$, $σ_0≡2πB_G$ denotes the effective transverse area of the nucleon, with $B_G = 0.156$ fm$^2$ .
Baryon stopping and charge deposition in heavy-ion collisions due to gluon saturation · Parametrical formula · 0.85
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Here $R_0(x)=0.4(x/x_0)^0.144$ fm (with $x_0 = 3 ×10^-4$), and $σ_0 = 23.03$ mb [ref] .
Higgs Hadroproduction at Large Feynman x · The Unintegrated Gluon Density · 0.95
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A simple starting point for our estimate is the GBW fit [ref] , where the proton saturation scale (in the fundamental representation, which is convenient for DIS) is parametrized as [equation] The result of the fit including charm quarks gives $λ= 0.277$ and $x_0 = 0.41 ·10^-4$, with the one redundant parameter chosen as $Q_0 = 1$, while the fit without charm makes $$ approximately 30% larger.
Wilson line correlator in the MV model: relating the glasma to deep inelastic scattering · Discussion · 0.93
Compares with it 10 papers
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By also recalling the expression of the Pomeron UGD, that is, pomugddef with the scalar function $G(K_⊥,x,Y_ℙ)$ from Gscalarnew, one can check that our above result for the $qq̅g$ contribution to the diffractive structure function is indeed equivalent (up to an overall normalisation factor, due to the different conventions) to that originally obtained in [ref] (see Eq. (39) in [ref] ).
Gluon dipole factorisation for diffractive dijets · The gluon contribution to the diffractive structure function · 1.00
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As we explain in detail in Appendix [ref], we also find agreement with the results of Refs. [ref] , although in the gluon case there is an overall factor 2 difference due to an inconsistent parameterization of the dipole S-matrix in the adjoint representation in [ref] , see also the discussion in [ref] .
Semi-inclusive Diffractive Deep Inelastic Scattering at Small-$x$ · Unpolarized DPDFs in a saturation model · 0.85
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The contribution of exclusive dijets to the diffractive structure function is obtained by integrating 2jetTMD over $K_⊥^2$ up to the maximally allowed value of order $∼Q^2$, cf. K2max: [equation] The r.h.s. features the quark diffractive PDF $xq_ℙ(x, x_ℙ, Q^2)$, as obtained from the color dipole picture after isolating the leading-twist contribution: [equation] It is easy to verify that the structure of the integrand, that can be read from Eqs. [ref] and [ref], is fully consistent with the respective results in [ref] (see e.g. Eq. (25) in [ref] ).
TMD factorisation for diffractive jets in photon-nucleus interactions · Multiple scattering and parton saturation in the Pomeron · 0.97
Cites it as background 20 papers
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The (2+1)–jet process with a semi-hard gluon has been discussed in detail in [ref] (see also Refs. [ref] for earlier related work), with results that will be briefly summarised in the next subsection.
TMD factorisation for diffractive jets in photon-nucleus interactions · Diffractive (2+1)–jets with a soft quark · 0.88
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One prominent example is the Impact Parameter Saturation (IPSat) model [ref] , which is based on the particularly simple Golec-Biernat and Wusthoff (GBW) [ref] dipole saturation model, but contains on one hand, a DGLAP extension towards higher \(Q\) [ref] and, on the other hand, an impact parameter dependence in the dipole dynamics.
Forward dijet production at the LHC within an impact parameter dependent TMD approach · Introduction · 0.86
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This wave function has been discussed in [ref] .
Unitarity bound for gluon shadowing · Gluon shadowing from the Balitsky-Kovchegov (BK) equation. · 0.95
Discussion
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