REVIEW
A General Approach To Photon Radiation Off Fermions
High Energy Physics - Phenomenology · Submitted 23 Apr 1999 · 20 references
Pith's verdict: The paper that made massive-fermion dipole subtraction for photons usable; the derivation is solid, with one narrow corner — the mixed ia/ai case at positive P²ia — extended by asserted analytic continuation and never exercised numerically.
T0 · AI review
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Referee report
The paper
The paper claims
A general subtraction recipe removes soft- and collinear-photon singularities from any fermion process, with the singular integrals done once and for all in closed form.
The reading
What carries the argument
The load-bearing object is the pair of auxiliary functions $g^{(sub)}_{ff',�tau}$ and their analytic photon-phase-space integrals $G^{(sub)}_{ff',�tau}$, together with the momentum mappings $�tilde��Phi_{0,ff'}$. The functions $g^{(sub)}$ are fixed only up to non-singular terms by the requirement that they reproduce the soft factor of Eq. (2.3) and the collinear splitting functions of Eq. (2.4); the specific choices in Section 4 are made so that the $z$ and $y$ integrations close. The mappings are constrained to preserve all mass-shell relations and to reduce to the correct limits, $�tilde p_i �to p_i+k$ for a final-state emitter and $�tilde p_a �to x_a p_a$ for an initial-state emitter, while leaving untouched spectator and beam momenta where possible. The finite-mass regularization scheme is what makes the collinear logarithms appear as explicit $�ln(m_f)$ terms in $G^{(sub)}$ rather than as $1/�epsilon$ poles, so the results can be combined directly with virtual corrections computed in the same scheme.
Extended reading notes
Core claim
The paper's claim is that the dipole subtraction formalism of Catani and Seymour, built for massless unpolarized QCD partons, extends to photon radiation off fermions of arbitrary mass and helicity, in a process-independent way. The construction is a sum over ordered pairs of charged fermions, $|M_{sub}|^2 = -Q_f Q_{f'} e^2 g^{(sub)}_{ff',�tau}(p_f,p_{f'},k)|M_0(�tilde��Phi_{0,ff'};�tau �kappa_f)|^2$, in which each term pairs an emitter with a spectator and each term carries a momentum mapping from the $(n+1)$-particle to the $n$-particle phase space. The auxiliary functions $g^{(sub)}$ are chosen to match the known soft limit and the known collinear splitting behaviour, with charge conservation used to avoid double-counting the infrared region across overlapping collinear terms. The author then integrates each $g^{(sub)}$ over the photonic part of phase space analytically, obtaining the closed-form $G^{(sub)}$ of Eqs. (4.10), (4.28), (4.34) and (4.49) for final-final, final-initial, initial-final and initial-initial emitter/spectator pairs, including the spin-flip pieces that vanish in the massless limit but must be retained here. For the mixed and initial-initial cases the analytic result is organized as a convolution over the energy fraction $x$ of the incoming fermion, with the $x�to 1$ singularity split off through a $[�dots]_+$ distribution so that the convolution itself can be evaluated with vanishing photon and fermion masses.
Load-bearing premise
For the mixed and initial-initial cases the construction introduces an arbitrary lower cutoff $x_0$ on the incoming fermion's energy fraction, and the paper asserts without proof that physical results do not depend on it, deferring that check to the numerics; if the endpoint terms $G^{(sub)}(P^2,x_0)$ did not cancel the $x_0$ dependence exactly, the convolutions would silently return cutoff-dependent answers.
Editorial extensions
If this is right
- Real-photonic $O(�alpha)$ corrections to any process with charged fermions and neutral particles can be computed by Monte Carlo without a soft-photon cutoff or an angular collinearity cutoff, removing the need to scan for a plateau in auxiliary parameters.
- Because the subtraction is process-independent and the $G^{(sub)}$ are given in closed form for arbitrary masses, the same code applies across the mass range from the electron to the top quark, and the massless limit is obtained by expansion rather than by a separate derivation.
- For processes with only massive quarks as QCD partons, the same results carry over to gluon radiation by replacing $Q_q^2�alpha$ with $4�alpha_s/3$ and translating the infrared regulator into the dimensional-regularization pole.
- Worked results are supplied for $�gamma�gamma �to t�bar t(�gamma)$, $e^-�gamma �to e^-�gamma(�gamma)$ and $�mu^+�mu^- �to �nu_e�bar�nu_e(�gamma)$, covering the final-final, mixed and initial-initial cases respectively.
Reading between the lines
- The same machinery should apply to any neutral-boson-mediated process with many charged fermions, since the subtraction is a sum over emitter/spectator pairs and the pair sum is fixed by charge conservation; the practical bottleneck becomes the number of momentum mappings to implement, not new analytic integrals.
- The treatment of cuts is the part most likely to be reused incorrectly: the paper argues that cuts applied to the subtraction function must be applied identically to its integrated counterpart, which for angular cuts means cutting the reconstructed angles of the reduced momenta rather than the original ones. That prescription, more than the algebra, is what a reimplementation has to get right.
- Because the collinear logarithms are kept as explicit $�ln(m_f)$ terms, the formalism can be used to study the size of mass-singular initial-state corrections directly, which is the regime where electroweak corrections at future lepton colliders grow large; that is a natural next application the paper does not carry out.
Original abstract
Soft or collinear photon emission potentially poses numerical problems in the phase-space integration of radiative processes. In this paper, a general subtraction formalism is presented that removes such singularities from the integrand of the numerical integration and adds back the analytically integrated contributions that have been subtracted. The method is a generalization of the dipole formalism of Catani and Seymour, which was formulated for NLO QCD processes with massless unpolarized particles. The presented formalism allows for arbitrary mass and helicity configurations in processes with charged fermions and any other neutral particles. Particular attention is paid to the limit of small fermion masses, in which collinear singularities cause potentially large corrections. The actual application and the efficiency of the formalism are demonstrated by the discussion of photonic corrections to the processes \gamma \gamma --> t \bar t (\gamma), e^- \gamma --> e^- \gamma (\gamma), and \mu^+ \mu^- --> \nu_e \bar\nu_e (\gamma).
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
Soft or collinear photon emission potentially poses numerical problems in the phase-space integration of radiative processes. In this paper, a general subtraction formalism is presented that removes such singularities from the integrand of the numerical integration and adds back the analytically integrated contributions that have been subtracted. The method is a generalization of the dipole formalism of Catani and Seymour, which was formulated for NLO QCD processes with massless unpolarized particles. The presented formalism allows for arbitrary mass and helicity configurations in processes with charged fermions and any other neutral particles. Particular attention is paid to the limit of small fermion masses, in which collinear singularities cause potentially large corrections. The actual application and the efficiency of the formalism are demonstrated by the discussion of photonic corrections to the processes \gamma \gamma --> t \bar t (\gamma), e^- \gamma --> e^- \gamma (\gamma), and \mu^+ \mu^- --> \nu_e \bar\nu_e (\gamma).
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Soft or collinear photon emission potentially poses numerical problems in the phase-space integration of radiative processes. | background 1.00 | 0.06 | 0.03 | hedged possibility 0.98 |
| s1 | In this paper, a general subtraction formalism is presented that removes such singularities from the integrand of the numerical integration and adds back the analytically integrated contributions that have been subtracted. | contribution 0.75 | 0.94 | 0.59 | asserted 1.00 |
| s2 | The method is a generalization of the dipole formalism of Catani and Seymour, which was formulated for NLO QCD processes with massless unpolarized particles. | background 0.73 | 0.11 | 0.06 | asserted 1.00 |
| s3 | The presented formalism allows for arbitrary mass and helicity configurations in processes with charged fermions and any other neutral particles. | contribution 0.87 | 0.86 | 0.22 | asserted 1.00 |
| s4 | Particular attention is paid to the limit of small fermion masses, in which collinear singularities cause potentially large corrections. | contribution 0.46 | 0.29 | 0.05 | asserted 0.69 |
| s5 | The actual application and the efficiency of the formalism are demonstrated by the discussion of photonic corrections to the processes \gamma \gamma --> t \bar t (\gamma), e^- \gamma --> e^- \gamma (\gamma), and \mu^+ \mu^- --> \nu_e \bar\nu_e (\gamma). | contribution 0.56 | 0.93 | 0.17 | asserted 1.00 |
What was examined, and what was not
- arXiv listing abstract v2
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- P abstract
- S sections
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Figures
Figures reproduced from arXiv:hep-ph/9904440 by the authors.
References
How later papers used it
114 later papers cite it
1 later paper by other authors is listed below: 1 used its method or tool.
Uses its method or tool 1 of 91 papers with this label
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Employed the dipole-subtraction method for QCD and its extension to QED from this paper to handle the associated IR divergences.
Complete NLO corrections to ${\rm W}^+{\rm W}^+$ scattering and its irreducible background at the LHC (2017) · 1.00 · paraphrased: the licence does not allow quoting
How the literature uses it
111 arXiv papers cite this in 211 sentences
Papers citing it · 70 sentences without a confident label
Builds on it 3 papers
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To this end, based on the results presented in Refs. [ref] , the implementation of the Catani–Seymour dipole-subtraction formalism has been extended to account for QED corrections [ref] .
Automation of NLO QCD and EW corrections with Sherpa and Recola · The framework · 0.92
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In this paper, we have presented an extension of the dipole subtraction formalism introduced in Refs. [ref] to massive initial-state particles for NLO (SUSY)-QCD calculations which allows the analytic cancellation of infrared singularities between the virtual and real corrections.
The dipole formalism for massive initial-state particles and its application to dark matter calculations · Summary and outlook · 0.98
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The dipole splitting functions of [ref] have been generalised further in [ref] to incorporate also polarisation.
Soft Photon Radiation in Particle Decays in SHERPA · Approximations for real emission matrix elements · 0.92
Uses its method or tool 88 papers
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It is based on the ACOT massive factorization theorem [ref] and massive dipole subtraction method (DSM) [ref] , which however had to be reformulated in order to match with the former.
Fully massive Scheme for Jet Production in DIS · Introduction · 0.89
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The results presented here were produced using the parton-level Monte Carlo program [ref] , in which the subtraction of IR divergences in the real radiation is realised with the Catani–Seymour dipole formalism [ref] .
NLO QCD and EW corrections to semileptonic vector-boson scattering at the LHC · Details and validation of the calculation · 1.00
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We perform the subtraction of IR divergences in the Catani–Seymour (CS) dipole formalism [ref] .
NLO EW and QCD corrections to polarized ZZ production in the four-charged-lepton channel at the LHC · Calculation details · 1.00
Compares with it 1 paper
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Furthermore we define for brevity [equation] The contribution from the collinear integral is simpler and we find [equation] The results in dimensional regularization can be compared with the results in mass regularization given in Eq. (4.10) of [ref] .
Photon Radiation with MadDipole · Final-final $f→f+γ$ dipole in dimensional regularization · 0.99
Cites it as background 5 papers
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The factorization properties of such amplitudes are well-known [ref] .
Final-state QED Multipole Radiation in Antenna Parton Showers · Emission kernel · 0.87
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Various general NLO subtraction schemes have been proposed (see [ref] ), and several (semi)automated implementations are available [ref] .
An alternative subtraction scheme for next-to-leading order QCD calculations · Introduction · 0.93
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The various methods that were proposed in the literature can be broadly classified into two types: the phase-space-slicing [ref] and the subtraction formalisms [ref] .
Techniques for the treatment of IR divergences in decay processes at NLO and application to the top-quark decay · Introduction · 0.90
Discussion
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