REVIEW
QCD Analysis of Unpolarized and Polarized Lambda - Baryon Production in Leading and Next-to-Leading Order
High Energy Physics - Phenomenology · Submitted 19 Nov 1997 · 41 references
Pith's verdict: A solid, useful NLO extraction of Lambda fragmentation functions; the only real soft spot is the abstract's word 'compatible' for the polarized scenarios, which Section 3 itself walks back.
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Referee report
The paper
The paper claims
QCD evolution plus a low-scale input fits all unpolarized Lambda production data in e+e- annihilation, and the same machinery yields three spin scenarios that all survive the LEP polarization measurements.
The reading
What carries the argument
The load-bearing object is the pair of fragmentation functions $D_f^Lambda(z,Q^2)$ and $Delta D_f^Lambda(z,Q^2)$ together with their time-like Altarelli-Parisi evolution. The unpolarized functions are extracted from a fit; the polarized ones are not, because the data cannot fix them, so they are parametrized at the input scale as $Delta D_s^Lambda = z^{alpha} D_s^Lambda$ and $Delta D_{u,d}^Lambda = N_u Delta D_s^Lambda$, then evolved with the spin-dependent two-loop time-like splitting functions. The positivity bound $|Delta D_f^Lambda(z,Q^2)| le D_f^Lambda(z,Q^2)$ is what converts the unpolarized fit into a hard ceiling on how large the polarization can be, and it is precisely this ceiling that makes scenarios 1 and 2 fall short of the observed large-$x_E$ polarization. The second piece of machinery is the complete NLO coefficient-function set collected in the appendices for $e^+e^-$ annihilation and for all four polarization combinations of lepton, nucleon, and detected Lambda in semi-inclusive deep-inelastic scattering.
Extended reading notes
Core claim
The paper establishes a working next-to-leading-order QCD description of Lambda production in $e^+e^-$ annihilation, with a radiative-parton-model input $D_f^Lambda(z) = N_f z^{alpha_f}(1-z)^{beta_f}$ at $mu^2 = 0.34$ GeV$^2$ (NLO) fitted to the world data, giving $chi^2 = 103.55$ for 103 points. It then argues, on the basis of the LEP measurements of the longitudinal Lambda polarization at the Z resonance, that the present data constrain only the valence (non-singlet) combination of the spin-dependent fragmentation functions $Delta D_f^Lambda(z,Q^2)$, and not the sea or the gluon. The authors therefore do not claim a unique set of polarized fragmentation functions; they claim three, labelled by the ratio $N_u$ of the light-quark to strange-quark spin transfer at the input scale ($N_u = 0$, $-0.20$, $+1$), each allowed by the positivity condition $|Delta D_f^Lambda| le D_f^Lambda$ and each compatible with the LEP asymmetry. The concrete prediction is that the spin-transfer asymmetry $A^Lambda = g_1^{p/Lambda}/F_1^{p/Lambda}$ in semi-inclusive deep-inelastic scattering differs between these scenarios by more than the expected statistical error, so that future measurements at HERA and HERMES can decide among them.
Load-bearing premise
The unpolarized light-quark fragmentation functions are assumed equal for up, down, and strange quarks at the input scale, an SU(3) flavour-symmetry assumption justified only by naive quark-model arguments and neglect of quark masses.
Editorial extensions
If this is right
- A measurement of the spin-transfer asymmetry $A^Lambda$ in polarized-electron, unpolarized-proton scattering at small Bjorken $x$ would separate the three scenarios, because at small $x$ the asymmetry is driven by the proton's sea and the scenarios differ there by roughly a factor of three or more.
- The same asymmetry measured at fixed-target energies, where valence distributions dominate, probes a different mixture of the light-quark fragmentation functions and therefore supplies complementary rather than redundant information.
- The unpolarized Lambda fragmentation functions extracted here also describe the existing semi-inclusive deep-inelastic scattering data once the data are converted from the Feynman variable to the proper scaling variable $z$, which removes an apparent discrepancy between experiments at different centre-of-mass energies.
- Because the polarized gluon fragmentation function is generated radiatively even from a vanishing input, a maximal gluon input at the low scale noticeably enhances the light-quark polarization at small $z$, so the gluon sector is indirectly accessible through the sea.
- The contribution of the Lambda fragmentation functions to the momentum sum rule comes out at only a few percent, consistent with the expectation that the sum rule is dominated by pions and kaons.
Reading between the lines
- The paper's own diagnosis suggests a way out it does not pursue: if the assumed flavour symmetry $D_u^Lambda = D_d^Lambda = D_s^Lambda$ were broken, the positivity ceiling on the large-$x_E$ polarization would loosen, and a strange-quark-dominated scenario could fit the LEP points without being pushed to the positivity limit.
- The secondary-Lambda contamination from $Sigma^0 to Lambda gamma$ decays, which the authors explicitly set aside, is a systematic that could bias the extracted normalization at large $z$; a data set with the $Sigma^0$ contribution subtracted would be a cleaner input for the same fit.
- Because the three scenarios are constructed to bracket the allowed range rather than to represent physical mechanisms, a future measurement that lands outside the bracket would be more informative than one that merely picks a scenario, since it would indicate that the input-scale parametrization itself is too restrictive.
- The same machinery applies directly to other spin-1/2 hyperons with self-analyzing weak decays, so the method generalizes beyond the Lambda even though the data do not yet exist.
Original abstract
We analyze experimental data for the production of Lambda baryons in e^+e^- annihilation in terms of scale dependent, QCD evolved, Lambda fragmentation functions. Apart from the vast majority of the data for which the polarization of an observed Lambda was not determined, we also consider the recent LEP measurements of the longitudinal polarization of Lambda's produced on the Z-resonance. Such data correspond to spin-dependent fragmentation functions for the Lambda. We point out that the present data are insufficient to satisfactorily fix these. We therefore suggest several different sets of fragmentation functions, all compatible with present data, and study the prospects for conceivable future semi-inclusive deep-inelastic scattering experiments to discriminate between them. We provide the complete next-to-leading order QCD framework for all the processes we consider.
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 analyze experimental data for the production of Lambda baryons in e^+e^- annihilation in terms of scale dependent, QCD evolved, Lambda fragmentation functions. Apart from the vast majority of the data for which the polarization of an observed Lambda was not determined, we also consider the recent LEP measurements of the longitudinal polarization of Lambda's produced on the Z-resonance. Such data correspond to spin-dependent fragmentation functions for the Lambda. We point out that the present data are insufficient to satisfactorily fix these. We therefore suggest several different sets of fragmentation functions, all compatible with present data, and study the prospects for conceivable future semi-inclusive deep-inelastic scattering experiments to discriminate between them. We provide the complete next-to-leading order QCD framework for all the processes we consider.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We analyze experimental data for the production of Lambda baryons in e^+e^- annihilation in terms of scale dependent, QCD evolved, Lambda fragmentation functions. | contribution 0.81 | 0.92 | 0.07 | asserted 0.99 |
| s1 | Apart from the vast majority of the data for which the polarization of an observed Lambda was not determined, we also consider the recent LEP measurements of the longitudinal polarization of Lambda's produced on the Z-resonance. | method 0.46 | 0.26 | 0.04 | asserted 1.00 |
| s2 | Such data correspond to spin-dependent fragmentation functions for the Lambda. | interpretation 0.53 | 0.11 | 0.04 | asserted 1.00 |
| s3 | We point out that the present data are insufficient to satisfactorily fix these. | limitation 0.40 | 0.45 | 0.22 | asserted 0.98 |
| s4 | We therefore suggest several different sets of fragmentation functions, all compatible with present data, and study the prospects for conceivable future semi-inclusive deep-inelastic scattering experiments to discriminate between them. | contribution 0.53 | 0.87 | 0.48 | hedged possibility 0.92 |
| s5 | We provide the complete next-to-leading order QCD framework for all the processes we consider. | contribution 0.64 | 0.92 | 0.08 | asserted 1.00 |
What was examined, and what was not
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- figures and tables
- references and citing sentences
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Figures
Showing 3 of the paper's 9 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:hep-ph/9711387 by the authors.
References
How the literature uses it
42 arXiv papers cite this in 87 sentences
Papers citing it · 62 sentences without a confident label
Uses its method or tool 5 papers
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In [ref] we used the fragmentation function set developed by [ref] and the following model: [equation]
Update on Counting Valence Quarks at RHIC · Introduction · 0.93
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SIDIS coefficient functions are implemented up to NLO [ref] , while NNLO corrections are included via the approximate threshold resummation formalism [ref] .
Status and Developments in Polarised Parton Distribution Functions · MAPPDFpol1.0 at NNLO · 0.91
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Here, for $⟨𝒫̅_T^sin2ϕ⟩$ and $⟨𝒫̅_N^sin2ϕ⟩$, we took $H_1T^Λ/D_1^Λ ∼G_1L^Λ/D_1^Λ ∼z_Λ^α$ where $α>0$ in the DSV parameterization [ref] , leading to the monotonic behaviors with increasing $z_Λ$.
Longitudinal and transverse polarizations of $\Lambda$ hyperon in unpolarized SIDIS and $e^+e^-$ annihilation · A rough numerical estimate · 0.83
Uses its data or values 3 papers
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For both unpolarized and longitudinally polarized fragmentation functions for $Λ$, we use the leading-order set from [ref] , whose three different scenarios are plotted in fig.
Polarized jet fragmentation functions · Example 1: Longitudinally polarized $Λ$ · 0.98
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Neglecting the mass differences between the up, down and strange quarks ($m=0.36$ GeV), we obtain the light flavors fragmentation function $H̃$ as follows [equation] In order to obtain the numerical result for $H̃$, we should choose the values for the parameters as Ref. [ref] [equation] the parameters were obtained by fitting the model result of $D_1^Λ(z)$ in the same model to the deFlorian-Stratmann-Vogelsang (DSV) parametrization [ref] at the model scale $Q^2=0.23 GeV^2$.
Predicting the $\sin\phi_S$ Single-Spin-Asymmetry of $\Lambda$ Production off transversely polarized nucleon in SIDIS · Model calculation of fragmentation function in qgq correlator · 0.84
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For the collinear helicity FFs for $Λ/Λ$, we take the parameterizations from [ref] , in which three scenarios are provided, each with a different assumption for the source of contribution to $Λ$ longitudinal polarization.
Polarized fragmenting jet functions in Inclusive and Exclusive Jet Production · Back-to-back electron-jet production with longitudinally polarized $Λ$ in jet · 0.99
Compares with it 3 papers
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In figure [ref], we compare the NLO FFs extracted in this work to those from DSV [ref] , AKK08 [ref] , and NPC23 [ref] at the scale $Q=5$ GeV.
First next-to-next-to-leading-order extraction of fragmentation functions for Lambda hyperons · Comparison with other fragmentation function extractions · 1.00
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[ref] and references therein).
Polarized hyperon production in single-inclusive electron-positron annihilation at next-to-leading order · Unpolarized cross section at NLO · 0.97
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As mentioned before, they agree with the results of Ref. [ref] .
Fragmentation contributions to transverse nucleon spin observables in semi-inclusive deep-inelastic scattering at NLO · Unpolarized cross section · 1.00
Cites it as background 12 papers
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The first determination of collinear FFs for $$ production was obtained by the DSV group [ref] .
First next-to-next-to-leading-order extraction of fragmentation functions for Lambda hyperons · Introduction · 0.97
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Longitudinal spin dependent $Λ/Λ̅$ fragmentation functions were first determined (see also [ref] for earlier study) by analyzing the LEP data [ref] to NLO [ref] , which was not able to constrain the valence fragmentation functions for all flavors; vastly different helicity FFs motivated by different scenarios in fig.
Polarized jet fragmentation functions · Example 1: Longitudinally polarized $Λ$ · 0.86
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More generally, the polarization transfer will depend on the fraction of the jet momentum carried by the baryon, [equation] and is described by the so-called spin-dependent (or polarized) fragmentation functions [ref] .
Prospects for measuring quark polarization and spin correlations in $b\bar b$ and $c\bar c$ samples at the LHC · Polarization Retention in Baryons · 0.81
Discussion
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