REVIEW
Parton distributions incorporating QED contributions
High Energy Physics - Phenomenology · Submitted 2 Nov 2004 · 19 references
Pith's verdict: A careful, honest first global fit with QED-corrected DGLAP; the NuTeV shift and the photon PDF both hold up, though the isospin asymmetry is more input-model than the abstract lets on.
T0 · AI review
Why Pith says this
Highlight any text to comment on it.
Referee report
The paper
The paper claims
Including QED in a global parton fit produces isospin violation that shifts the NuTeV weak mixing angle by about 0.002 and generates a measurable photon distribution in the proton.
The reading
What carries the argument
The machinery is the QED-extended DGLAP system in Eq. (1): quark evolution gains $�tilde P_{qq}$ and $P_{q�gamma}$ terms, gluon evolution is unchanged at this order, and a new photon distribution evolves through $P_{�gamma q}$ and $P_{�gamma�gamma}$. The initial photon distribution is modelled by one-photon emission off valence-like constituent quarks, Eq. (5), and the isospin-violating input is parametrised by $�epsilon$ in Eq. (8), fixed by momentum conservation through Eq. (10). That $�epsilon$ is what translates the QED splitting into a definite valence-quark asymmetry and hence into the NuTeV shift.
Extended reading notes
Core claim
The central claim is that a global parton fit with $�mathcal{O}(�alpha_{�mathrm{QED}})$ DGLAP evolution yields two concrete consequences without spoiling the fit. First, because photons couple to quark charge squared, up-type and down-type quarks evolve differently, producing $u^p�neq d^n$; the induced valence asymmetry shifts the value of $�sin^2�theta_W$ extracted from NuTeV by $-0.0018$ at $Q^2=2$ GeV$^2$, $-0.0021$ at $20$ GeV$^2$, and $-0.0029$ at $20000$ GeV$^2$, in the direction that reduces the anomaly. Second, the evolution generates photon parton distributions $�gamma^{p,n}(x,Q^2)$; convoluting $�gamma^p$ with QED Compton scattering predicts $�sigma(ep�to e�gamma X)=6.2�pm1.2$ pb for the ZEUS cuts, compared with the measured $5.64$ pb. The authors present these as the physical payoff of treating QED collinear logarithms as part of the parton distribution rather than as an ad hoc correction.
Load-bearing premise
The size of the isospin violation, and therefore the NuTeV shift, depends on the model for the initial photon distribution and on using current quark masses ($m_u=6$ MeV, $m_d=10$ MeV) in that model; the paper itself notes that constituent masses of $300$ MeV give $�epsilon=0.0074$ instead of $0.0325$ and can reduce the effect by up to a factor of about four.
Editorial extensions
If this is right
- If the QED-corrected fit is right, about $0.002$ of the $0.005$ NuTeV discrepancy comes from isospin violation generated by QED evolution, leaving less room for new physics.
- The proton and neutron photon distributions become measurable targets at HERA through $eN�to e�gamma X$, giving a direct handle on the QED sector of the parton model.
- Electroweak corrections to hadron-collider cross sections can consistently absorb collinear photon logarithms into QED-corrected parton distributions, avoiding double counting with explicit photon radiation.
- The valence asymmetry is scale-dependent, so the extracted $�sin^2�theta_W$ shift grows with $Q^2$ from $-0.0018$ at $2$ GeV$^2$ to $-0.0029$ at $20000$ GeV$^2$.
Reading between the lines
- A useful cross-check would be to repeat the fit with the photon input from the alternative evolution estimate in Eq. (12) rather than the valence-like model in Eq. (5); the paper says the results are similar, but a full fit would quantify how much of the NuTeV shift is model-independent.
- Because the shift grows with $Q^2$, comparing low-$Q^2$ and high-$Q^2$ extractions of the same weak angle could in principle separate a QED-isospin contribution from a genuine contact interaction.
- The ZEUS agreement is currently a one-bin comparison; extending it to a differential $�gamma^p(x_�gamma,Q^2)$ measurement would turn the photon parton distribution from a prediction into a constrained quantity.
- If constituent quark masses were the right scale for photon radiation, both the photon distribution and the isospin asymmetry would shrink, so the same data that discriminate between $3.6$ pb and $6.2$ pb also bear on the mass-scale choice.
Original abstract
We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${\cal O}(\alpha_{\rm QED})$ corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p \neq d^n$, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2\theta_W$ anomaly. A second consequence is the appearance of photon parton distributions $\gamma(x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to e\gamma X$; our predictions are in agreement with the present 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 perform a global parton analysis of deep inelastic and related hard-scattering data, including ${\cal O}(\alpha_{\rm QED})$ corrections to the parton evolution. Although the quality of the fit is essentially unchanged, there are two important physical consequences. First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p \neq d^n$, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2\theta_W$ anomaly. A second consequence is the appearance of photon parton distributions $\gamma(x,Q^2)$ of the proton and the neutron. In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to e\gamma X$; our predictions are in agreement with the present data.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We perform a global parton analysis of deep inelastic and related hard-scattering data, including ${\cal O}(\alpha_{\rm QED})$ corrections to the parton evolution. | contribution 0.59 | 0.96 | 0.07 | asserted 0.98 |
| s1 | Although the quality of the fit is essentially unchanged, there are two important physical consequences. | result 0.53 | 0.67 | 0.10 | asserted 1.00 |
| s2 | First, the different DGLAP evolution of u and d type quarks introduces isospin violation, i.e. $u^p \neq d^n$, which is found to be unambiguously in the direction to reduce the NuTeV $\sin^2\theta_W$ anomaly. | result 0.71 | 0.92 | 0.43 | asserted 1.00 |
| s3 | A second consequence is the appearance of photon parton distributions $\gamma(x,Q^2)$ of the proton and the neutron. | result 0.99 | 0.79 | 0.12 | asserted 1.00 |
| s4 | In principle these can be measured at HERA via the deep inelastic scattering processes $e N \to e\gamma X$; our predictions are in agreement with the present data. | result 0.61 | 0.75 | 0.08 | hedged possibility 0.60 |
What was examined, and what was not
- arXiv listing abstract v1
- 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/0411040 by the authors.
References
How later papers used it
107 later papers cite it
11 later papers by other authors are listed below: 2 used its method or tool, 8 used its data or values and 1 compared its own results with it.
Uses its method or tool 2 of 10 papers with this label
-
Used a collinear approach with this paper's photon PDFs to calculate inelastic contributions.
Can the diphoton enhancement at 750 GeV be due to a neutral technipion? (2016) · 0.99 · paraphrased: the licence does not allow quoting
-
Followed this paper for MRST and used the envelope spanned by assuming current quark masses and constituent quark masses.
Doubly Charged Higgs Boson Production at Hadron Colliders (2019) · 0.88 · paraphrased: the licence does not allow quoting
Uses its data or values 8 of 13 papers with this label
-
Used this paper's MRST2004QED set of parton density functions to compute all the hadron-level cross sections.
Precision electroweak calculation of the production of a high transverse-momentum lepton pair at hadron colliders (2007) · 0.85 · paraphrased: the licence does not allow quoting
-
Used this paper's standard parameterization of the parton distribution functions as an input.
Parton distribution functions and quark orbital motion (2007) · 0.94 · paraphrased: the licence does not allow quoting
-
Used this paper's MRST2004 parton distribution functions in its calculations.
Charged current reactions in the NuSOnG and a test of neutrino-W couplings (2008) · 0.83 · paraphrased: the licence does not allow quoting
-
Used this paper's MRST set, along with other sets, for the photon PDF inside the proton.
NNLO QCD$\oplus$QED corrections to Higgs production in bottom quark annihilation (2019) · 0.82 · paraphrased: the licence does not allow quoting
-
Used this paper's standard PDF parameterization as the unpolarized input.
Images of Quark Intrinsic Motion in Covariant Parton Model (2010) · 0.81 · paraphrased: the licence does not allow quoting
-
Used this paper's MRST2004 parton distribution functions in its calculations.
Probe of anomalous neutrino couplings to W and Z in medium energy setup of a beta-beam facility (2008) · 0.86 · paraphrased: the licence does not allow quoting
-
Reported a luminosity ratio for this paper's MRST2004 among other sets.
Constraints on 750 GeV colorless Q-onia from running couplings (2016) · 0.91 · paraphrased: the licence does not allow quoting
-
Used this paper's standard PDF parameterization as the input.
Quark intrinsic motion and the link between TMDs and PDFs in covariant approach (2009) · 0.92 · paraphrased: the licence does not allow quoting
Compares with it 1 of 9 papers with this label
-
Compared its own result with the QED evolution used in this paper's determination of the MRST2004QED parton distributions.
Parton distribution functions with QED corrections (2015) · 1.00 · paraphrased: the licence does not allow quoting
How the literature uses it
91 arXiv papers cite this in 141 sentences
Papers citing it · 79 sentences without a confident label
Builds on it 1 paper
-
Similar to the DGLAP evolution equation for PDFs in QCD [ref] [equation] with $i,j=q,q̅,g$, DGLAP-type evolution equations for PDFs in the joint QED and QCD factorization approach have the same form as that in Eq. ([ref]), but, the parton indices $i,j$ need to be extended to include leptons, $l,l̅,γ$, with $l=e,μ,τ$ to include the evolution in both QCD and QED, as well as the mixing evolution from QCD to QED or vise versa [ref] .
Factorized QED and QCD Contribution to Deeply Inelastic Scattering · Numerical Results and Impacts · 0.88
Uses its method or tool 9 papers
-
In this work, we adopt the isospin symmetric approximation (ISA) to relate the proton and neutron quark PDFs, [equation] Due to the QED evolution effect, a small isospin violation will be generated through the $γ→qq̅$ splitting [ref] , which will be properly addressed later with the CT18qed approach [ref] .
The Photon Content of the Neutron · The inelastic photon PDF with the LUXqed approach: CT18lux · 0.81
-
In particular, DY processes were analyzed using NNLOjet [ref] , Pythia [ref] , Herwig [ref] , DYNNLO [ref] , FEWZ [ref] .
Angular coefficients of the Drell-Yan process across different rapidity and kinematical ranges · Introduction · 0.91
-
To calculate inelastic contributions we use collinear approach with photon PDFs [ref] .
Can the diphoton enhancement at 750 GeV be due to a neutral technipion? · $2 →1$ subprocess · 0.99
Uses its data or values 12 papers
-
The set of parton density functions used to compute all the hadron-level cross sections is $MRST2004QED$ [ref] .
Precision electroweak calculation of the production of a high transverse-momentum lepton pair at hadron colliders · Numerical results · 0.85
-
The relation ([ref]) can be written as [equation] where [equation] At the same time Eq.([ref]) can be replaced by [equation] which after inserting from Eq. ([ref]) gives [equation] This equality together with the relation ([ref]) give [equation] or equivalently [equation] Now, using the input on the $q(x)$ [ref] and $Δq(x)$ [ref] (LO at the scale $4GeV^2$) one can calculate the distributions $ΔP_q,P_q,P_q^±$ and related structure functions $Δf_q,f_q$ and $f_q^±$.
Parton distribution functions and quark orbital motion · Momentum distribution from structure function $g_1$ · 0.95
-
In our calculations parton distribution functions of Martin, Roberts, Stirling and Thorne (MRST2004) [ref] have been used.
Charged current reactions in the NuSOnG and a test of neutrino-W couplings · cross sections and numerical analysis · 0.83
Compares with it 7 papers
-
We also compare our PDFs with the first generation QED PDFs, i.e., MRST2004qed [ref] , CT14qed [ref] , and NNPDF2.3qed [ref] in Fig. [ref], which shows a significant improvement in precision, thanks to the LUXqed formalism.
The Photon Content of the Neutron · Conclusions · 1.00
-
This reduced the uncertainty by a factor of about forty compared to previous photon PDF determinations, such as MRST2004qed [ref] and NNPDF23_qed [ref] , which relied on fits to LHC data and/or modelling.
The photon parton distribution function: updates and applications · Introduction · 1.00
-
At this point we compare to the QED evolution used in the determination of the MRST2004QED parton distributions [ref] .
Parton distribution functions with QED corrections · Comparison with MRST2004QED · 1.00
Cites it as support 1 paper
-
Therefore, off-peak Drell-Yan production provides important constraints on the photon PDF [ref] for PDF fits which account for QED corrections [ref] .
The PDF4LHC report on PDFs and LHC data: Results from Run I and preparation for Run II · High and low mass Drell-Yan production · 0.96
Cites it as background 26 papers
-
Currently, from literature we observe that only the photon content of the proton has been determined based either on model assumptions [ref] , the MRST2004QED set, or on a fit to data [ref] , the NNPDF2.3QED, but no attempt to estimated the lepton PDFs has ever been tried.
Parton distribution functions with QED corrections · Lepton PDFs · 0.87
-
While weak [ref] , QED [ref] and EW (weak+QED) [ref] corrections to $$ production at hadron colliders have been known for quite some time and the effect of Sudakov logarithms has been quantified and studied, the impact of photon-induced subprocesses has been addressed only in [ref] for only the $gγ$ channel at LO and using the only PDF set with the photon distribution available at that time, i.e., MRST2004QED [ref] .
The impact of the photon PDF and electroweak corrections on $t \bar t$ distributions · Introduction · 0.93
-
Solutions to the evolution equations including corrections at LO in QED were presented in [ref] and different fits of the parton distributions including these corrections were performed in [ref] .
QED corrections to parton distributions and Altarelli-Parisi splitting functions in the polarized case · Introduction · 0.95
Discussion
Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.