Colour and logarithmic accuracy in final-state parton showers

Journal of High Energy Physics ◽

10.1007/jhep03(2021)041 ◽

2021 ◽

Vol 2021 (3) ◽

Author(s):

Keith Hamilton ◽

Rok Medves ◽

Gavin P. Salam ◽

Ludovic Scyboz ◽

Gregory Soyez

Keyword(s):

Matrix Element ◽

Matrix Elements ◽

Computationally Efficient ◽

Final State ◽

Logarithmic Accuracy ◽

Type Classification ◽

Diagram Type

Abstract Standard dipole parton showers are known to yield incorrect subleading-colour contributions to the leading (double) logarithmic terms for a variety of observables. In this work, concentrating on final-state showers, we present two simple, computationally efficient prescriptions to correct this problem, exploiting a Lund-diagram type classification of emission regions. We study the resulting effective multiple-emission matrix elements generated by the shower, and discuss their impact on subleading colour contributions to leading and next-to-leading logarithms (NLL) for a range of observables. In particular we show that the new schemes give the correct full colour NLL terms for global observables and multiplicities. Subleading colour issues remain at NLL (single logarithms) for non-global observables, though one of our two schemes reproduces the correct full-colour matrix-element for any number of energy-ordered commensurate-angle pairs of emissions. While we carry out our tests within the PanScales shower framework, the schemes are sufficiently simple that it should be straightforward to implement them also in other shower frameworks.

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Initial state radiation in the Herwig 7 angular-ordered parton shower

Journal of High Energy Physics ◽

10.1007/jhep01(2022)026 ◽

2022 ◽

Vol 2022 (1) ◽

Author(s):

Gavin Bewick ◽

Silvia Ferrario Ravasio ◽

Peter Richardson ◽

Michael H. Seymour

Keyword(s):

Parton Shower ◽

Phenomenological Study ◽

Initial State Radiation ◽

Centre Of Mass ◽

Initial State ◽

Final State ◽

State Radiation ◽

Optimal Values ◽

Logarithmic Accuracy ◽

Intrinsic Transverse Momentum

Abstract We study the simulation of initial-state radiation in angular-ordered parton showers in order to investigate how different interpretations of the ordering variable affect the logarithmic accuracy of such showers. This also enables us to implement a recoil scheme which is consistent between final-state and initial-state radiation. We present optimal values of the strong coupling and intrinsic transverse momentum to be used in each version of the parton shower, tuned using Z0-boson production at the LHC at 7 TeV. With these tuned showers, we perform a phenomenological study of the Drell-Yan process at several centre-of-mass energies.

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Spin correlations in final-state parton showers and jet observables

The European Physical Journal C ◽

10.1140/epjc/s10052-021-09378-0 ◽

2021 ◽

Vol 81 (8) ◽

Author(s):

Alexander Karlberg ◽

Gavin P. Salam ◽

Ludovic Scyboz ◽

Rob Verheyen

Keyword(s):

Spin Correlation ◽

Final State ◽

Point Energy ◽

Spin Correlations ◽

Correlation Algorithm ◽

Phenomenological Studies ◽

Quantum Mechanical Effects ◽

Jet Observables ◽

The Impact ◽

Logarithmic Accuracy

AbstractAs part of a programme to develop parton showers with controlled logarithmic accuracy, we consider the question of collinear spin correlations within the PanScales family of parton showers. We adapt the well-known Collins–Knowles spin-correlation algorithm to PanScales antenna and dipole showers, using an approach with similarities to that taken by Richardson and Webster. To study the impact of spin correlations, we develop Lund-declustering based observables that are sensitive to spin-correlation effects both within and between jets and extend the MicroJets collinear single-logarithmic resummation code to include spin correlations. Together with a 3-point energy correlation observable proposed recently by Chen, Moult and Zhu, this provides a powerful set of constraints for validating the logarithmic accuracy of our shower results. The new observables and their resummation further open the pathway to phenomenological studies of these important quantum mechanical effects.

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N-jettiness in electroweak high-energy processes

Journal of High Energy Physics ◽

10.1007/jhep01(2022)007 ◽

2022 ◽

Vol 2022 (1) ◽

Author(s):

Junegone Chay ◽

Taewook Ha ◽

Taehyun Kwon

Keyword(s):

High Energy ◽

Hard Scattering ◽

Initial State ◽

Final State ◽

High Energies ◽

State Radiation ◽

The Standard Model ◽

Energy Processes ◽

Effective Theories ◽

Logarithmic Accuracy

Abstract We study N-jettiness in electroweak processes at extreme high energies, in which the mass of the weak gauge bosons can be regarded as small. The description of the scattering process such as e−e+ → μ−μ+ + X is similar to QCD. The incoming leptons emit initial-state radiation and the resultant particles, highly off-shell, participate in the hard scattering, which are expressed by the beam functions. After the hard scattering, the final- state leptons or leptonic jets are observed, described by the fragmenting jet functions or the jet functions respectively. At present, electroweak processes are prevailed by the processes induced by the strong interaction, but they will be relevant at future e−e+ colliders at high energy. The main difference between QCD and electroweak processes is that the initial- and final-state particles should appear in the form of hadrons, that is, color singlets in QCD, while there can be weak nonsinglets as well in electroweak interactions. We analyze the factorization theorems for the N-jettiness in e−e+ → μ−μ+ + X, and compute the factorized parts to next-to-leading logarithmic accuracy. To simplify the comparison with QCD, we only consider the SU(2)W gauge interaction, and the extension to the Standard Model is straightforward. Put it in a different way, it corresponds to an imaginary world in which colored particles can be observed in QCD, and the richer structure of effective theories is probed. Various nonzero nonsinglet matrix elements are interwoven to produce the factorized results, in contrast to QCD in which there are only contributions from the singlets. Another distinct feature is that the rapidity divergence is prevalent in the contributions from weak nonsinglets due to the different group theory factors between the real and virtual corrections. We verify that the rapidity divergence cancels in all the contributions with a different number of nonsinglet channels. We also consider the renormalization group evolution of each factorized part to resum large logarithms, which are distinct from QCD.

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