A
bstract
We discuss extensions of multi-jet matrix element and parton shower merging approaches, to also include next-to-leading order accuracy. Specifically, we generalise the so-called CKKW-L ...prescription and the recently developed unitarised matrix element + parton shower (UMEPS) scheme. Endowing tree-level merging methods with NLO corrections greatly enhances the perturbative accuracy of parton shower Monte Carlo programs.
To generalise the CKKW-L approach, we augment the Nils-Lavesson-Leif-Lönnblad (NL
3
) scheme, which was previously developed for e
+
e
−
-annihilation, with a careful treatment of parton densities. This makes the application of the NL
3
method to hadronic collisions possible. NL
3
is further updated to use for more readily accessible next-to-leading order input calculations.
We also extend the UMEPS scheme to NLO accuracy. The resulting approach, dubbed unitarised next-to-leading order + parton shower (UNLOPS) merging, does not inherit problematic unitarity-breaking features of CKKW-L, and thus allows for a theoretically more appealing definition of NLO order merging.
Both schemes have been implemented in PYTHIA8. We present results for the merging of W- and Higgs-production events, where the zero- and one-jet contribution are corrected to next-to-leading order simultaneously, and higher jet multiplicities are described by tree-level matrix elements. We find that NL
3
and UNLOPS yield a very similar description for W production. For Higgs production however, UNLOPS produces more stable results.
The implementation of the NLO merging procedures is completely general and can be used for higher jet multiplicities and other processes, subject to the availability of programs able to correctly generate the corresponding partonic states to leading and next-to-leading order accuracy.
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A
bstract
We present an implementation of the so-called Ckkw-l merging scheme for combining multi-jet tree-level matrix elements with parton showers. The implementation uses the ...transverse-momentum-ordered shower with interleaved multiple interactions as implemented in P
YTHIA
8. We validate our procedure using e
+
e
−
-annihilation into jets and vector boson production in hadronic collisions, with special attention to details in the algorithm which are formally sub-leading in character, but may have visible effects in some observables. We find substantial merging scale dependencies induced by the enforced rapidity ordering in the default P
YTHIA
8 shower. If this rapidity ordering is removed the merging scale dependence is almost negligible. We then also find that the shower does a surprisingly good job of describing the hardness of multi-jet events, as long as the hardest couple of jets are given by the matrix elements. The effects of using interleaved multiple interactions as compared to more simplistic ways of adding underlying-event effects in vector boson production are shown to be negligible except in a few sensitive observables. To illustrate the generality of our implementation, we also give some example results from di-boson production and pure QCD jet production in hadronic collisions.
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Pythia8 is the only general purpose event generator able to produce complete fully exclusive hadronic final states in collisions involving heavy ions. The model that allows this is called Angantyr ...and will be discussed in this contribution. The contribution also discusses models based on interactions between strings in an extention of the Lund model that are able to describe some collective effects in proton–proton collisions. In particular, a reformulation of the so-called shoving will be described, which also works for heavy ion collisions as generated by the Angantyr model. Preliminary results for the ridge effects are presented for proton–proton and proton–lead collisions.
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We present a microscopic model for collective effects in high multiplicity proton–proton collisions, where multiple partonic subcollisions give rise to a dense system of strings. From lattice ...calculations we know that QCD strings are transversely extended, and we argue that this should result in a transverse pressure and expansion, similar to the flow in a deconfined plasma. The model is implemented in the Pythia8 Monte Carlo event generator, and we find that it can qualitatively reproduce the long range azimuthal correlations forming a near-side ridge in high multiplicity proton–proton events at LHC energies.
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A
bstract
We present a new model for building up complete exclusive hadronic final states in high energy nucleus collisions. It is a direct extrapolation of high energy pp collisions (as described by ...P
ythia
), and thus bridges a large part of the existing gap between heavy ion and high energy physics phenomenology. The model is inspired by the old Fritiof model and the notion of wounded nucleons. Two essential features are the treatment of multi-parton interactions and diffractive excitation in each
NN
sub-collision. Diffractive excitation is related to fluctuations in the nucleon partonic sub-structure, and fluctuations in both projectile and target are here included for the first time. The model is able to give a good description of general final-state properties such as multiplicity and transverse momentum distributions, both in p
A
and
AA
collisions. The model can therefore serve as a baseline for understanding the non-collective background to observables sensitive to collective behaviour. As P
ythia
does not include a mechanism to reproduce the collective effects seen in pp collisions, such effects are also not reproduced by the present version of Angantyr. Effects of high string density, shown to be able to reproduce e.g. higher strangeness ratios and the ridge in pp, will be added in future studies.
StringSpinner is a plugin allowing for the introduction of spin effects in the hadronization part of the PYTHIA 8 event generator. In this first version the implementation is limited to handling ...pesudoscalar mesons in events without parton showers. The spin effects are generated by propagating the quark polarization along the fragmentation chain according to the rules of the quantum mechanical string+P03 model. By using parametrizations for the transversity parton distribution functions, StringSpinner allows to simulate the polarized semi inclusive deep inelastic scattering process on protons and neutrons. The instructions to use StringSpinner and to modify the parametrizations of the transversity distribution functions are presented, as well as an example of the calculation of the Collins asymmetries as obtained from simulations of the transversely polarized semi inclusive deep inelastic scattering process.
Program Title: StringSpinner
CPC Library link to program files:https://doi.org/10.17632/37jrpmyz3t.1
Developer's repository link:https://gitlab.com/albikerbizi/stringspinner.git
Licensing provisions: GNU GPL v2 or later
Programming language: C++, Fortran
Nature of problem: To analyse and interpret the experimental data on the study of the nucleon spin structure, as well as to make predictions for future experiments simulations are needed.
Solution method: Introduce spin effects in the hadronization part of a complete Monte Carlo event generator, largely used in the community of high energy particle physicists.
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The Sudakov veto algorithm for generating emission and no-emission probabilities in parton showers is revisited and some oversampling and reweighting techniques are suggested, mainly to improve ...statistics. Specifically we consider the generation of Sudakov form factors in algorithms of CKKW type for matrix element/parton shower merging, both at tree- and loop-level, and the generation of highly suppressed splittings inside standard parton showers.
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Quark spin effects in hadronization were recently included in the Pythia 8 Monte Carlo event generator for the simulation of the deep inelastic scattering (DIS) process off a polarized proton or ...neutron target. The spin effects were activated via the external StringSpinner package, which is based on the string+P03 model of polarized hadronization, and were restricted to the production of pseudoscalar mesons in string fragmentation. The struck quark polarization could either be calculated using parametrizations of the transversity PDFs or be freely specified. In this article we present a major extension of StringSpinner that includes vector meson production in the Pythia 8 string fragmentation. The spin-dependent emission and decay processes of the vector mesons are described using the string+P03 model, which systematically accounts for the quark-spin degree of freedom in the fragmentation chain. The new package works with the version 8.3 of the Pythia event generator and it allows for a more complete and quantitative simulation of the final states in polarized semi-inclusive DIS events. Also, it can be used to study the transverse-spin effects for the production of vector mesons. After describing the new version of StringSpinner, examples of its usage in semi-inclusive DIS events are presented.
Program Title: StringSpinner
CPC Library link to program files:https://doi.org/10.17632/37jrpmyz3t.2
Developer's repository link:https://gitlab.com/albikerbizi/stringspinner.git
Licensing provisions: GNU GPL v2 or later
Programming language:C++, Fortran
Journal reference of previous version: Comput. Phys. Commun. 272 (2022) 108234.
Does the new version supersede the previous version?: Yes
Reasons for the new version: Vector meson emission and decay is required for a more complete simulation of the DIS final states.
Summary of revisions: The spin-dependence of the string fragmentation is extended to include the production and the decay of polarized vector mesons.
Nature of problem: Vector meson contribute strongly to the final hadrons produced in DIS, and they can produce non-trivial effects on the spin-dependent observables.
Solution method: Extend the previous version of StringSpinner to include polarized vector meson production and decay using a more complete model of polarized hadronization.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
9.
Effects of overlapping strings in pp collisions Bierlich, Christian; Gustafson, Gösta; Lönnblad, Leif ...
The journal of high energy physics,
03/2015, Volume:
2015, Issue:
3
Journal Article
Peer reviewed
Open access
A
bstract
In models for hadron collisions based on string hadronization, the strings are usually treated as independent, allowing no interaction between the confined colour fields. In studies of ...nucleus collisions it has been suggested that strings close in space can fuse to form “colour ropes”. Such ropes are expected to give more strange particles and baryons, which also has been suggested as a signal for plasma formation. Overlapping strings can also be expected in
pp
collisions, where usually no phase transition is expected. In particular at the high LHC energies the expected density of strings is quite high. To investigate possible effects of rope formation, we present a model in which strings are allowed to combine into higher multiplets, giving rise to increased production of baryons and strangeness, or recombine into singlet structures and vanish. Also a crude model for strings recombining into junction structures is considered, again giving rise to increased baryon production. The models are implemented in the dipsy MC event generator, using Pythia8 for hadronization, and comparison to
pp
minimum bias data, reveals improvement in the description of identified particle spectra.
We investigate the space–time picture of string evolution and hadron production in a fully string-based model for high energy collisions involving heavy-ions. We find that although the density of ...strings is quite large at the time of hadronization in a central heavy-ion collision, the initial overlap between them right after the collisions is not necessarily large. We also find that when including string-string interactions using the so-called shoving model, the density of strings is decreased which should dampen the rapid increase in string tension in the rope hadronization with multiplicity that we found in a previous paper.
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