| 1. |
- Adare, A., et al.
(författare)
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Cold Nuclear Matter Effects on J/psi Yields as a Function of Rapidity and Nuclear Geometry in d plus A Collisions at root S-NN=200 GeV
- 2011
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Ingår i: Physical Review Letters. - 1079-7114. ; 107:14
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements of J/psi yields in d + Au collisions at root S-NN = 200 GeV recorded by the PHENIX experiment and compare them with yields in p + p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/psi rapidity (-2.2 < y < 2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. In order to remove model dependent systematic uncertainties we also compare the data to a simple geometric model. The forward rapidity data are inconsistent with nuclear modifications that are linear or exponential in the density weighted longitudinal thickness, such as those from the final state breakup of the bound state.
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| 2. |
- Bierlich, Christian, et al.
(författare)
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Effects of color reconnection on hadron flavor observables
- 2015
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Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - 1550-2368. ; 92:9
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Tidskriftsartikel (refereegranskat)abstract
- We present a series of observables for soft inclusive physics, and utilize them for comparison between two recently developed color reconnection models: the new color reconnection model in Pythia and the DIPSY rope hadronization model. The observables are ratios of identified hadron yields as a function of the final-state activity, as measured by the charged multiplicity. Since both considered models have a nontrivial dependence on the final-state activity, the above observables serve as excellent probes to test the effect of these models. Both models show a clear baryon enhancement with increasing multiplicity, while only the DIPSY rope model leads to a strangeness enhancement. Flowlike patterns, previously found to be connected to color reconnection models, are investigated for the new models. Only Pythia shows a p(perpendicular to)-dependent enhancement of the Lambda/K ratio as the final-state activity increases, with the enhancement being largest in the mid-p(perpendicular to) region.
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| 3. |
- Christiansen, Jesper Roy, et al.
(författare)
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Color reconnection at future e(+) e(-) colliders
- 2015
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 75:9
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Tidskriftsartikel (refereegranskat)abstract
- The effects of color reconnection (CR) at e(+) e(-) colliders are revisited, with focus on recently developed CR models. The new models are compared with the LEP2 measurements for e(+) e(-) -> W+W- -> q(1)(q) over bar (2)q(3)(q) over bar4 and found to lie within their limits. Prospects for constraints from new high- luminosity e(+) e(-) colliders are discussed. The novel arena of CR in Higgs decays is introduced, and it is illustrated by shifts in angular correlations that would be used to set limits on a potential CP-odd admixture of the 125 GeV Higgs state.
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| 4. |
- Christiansen, Jesper Roy
(författare)
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Colour Reconnection and Weak Showers
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consider the improvement of the simulation tools used to describe high energy particle collisions. These simulation programs are normally referred to as event generators. The emphasis is on two specific parts of the event generation: the introduction of radiation of weak bosons associated with the collision and a new model for colour reconnection. Each collision contains significant additional radiation of particles, but earlier models have only included QCD and QED radiation. This thesis presents a first attempt at including also the weak radiation. The inclusion of weak emissions allows for a better understanding of already observed data. The weak radiation is also included in the common framework for including multi-jet matrix elements into the description of the radiation. Colour Reconnection (CR) address the question: between which partons do colour strings form? The new model incorporates three principles, the SU(3) colour rules from QCD, a space-time causality requirement, and a minimization of the potential energy. It also introduces a new type of reconnections producing junction structures. The new model is able to explain the lambda production simultaneously at both LEP and LHC. The effects of CR, both the new model as well as older models, are also considered at potential future electron-positron colliders. Comparisons between the new model and a string rope model is carried out, which include suggestions for relevant observables.
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| 5. |
- Christiansen, Jesper Roy
(författare)
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Colour reconnection - Models and tests
- 2015
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Ingår i: Proceedings of Science. - 1824-8039. ; 22-29-July-2015
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress on colour reconnection within the Pythia framework is presented. A new model is introduced, based on the SU(3) structure of QCD and a minimization of the potential string energy. The inclusion of the epsilon structure of SU(3) gives a new baryon production mechanism and makes it possible simultaneously to describe hyperon production at both e+e- and pp colliders. Finally, predictions for e+e-colliders, both past and potential future ones, are presented.
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| 6. |
- Christiansen, Jesper Roy, et al.
(författare)
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Merging weak and QCD showers with matrix elements
- 2016
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 76:1
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Tidskriftsartikel (refereegranskat)abstract
- We present a consistent way of combining associated weak boson radiation in hard dijet events with hard QCD radiation in Drell-Yan-like scatterings. This integrates multiple tree-level calculations with vastly different cross sections, QCD- and electroweak parton-shower resummation into a single framework. The new merging strategy is implemented in the PYTHIA event generator and predictions are confronted with LHC data. Improvements over the previous strategy are observed. Results of the new electroweak-improved merging at a future 100 TeV proton collider are also investigated.
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| 7. |
- Christiansen, Jesper Roy, et al.
(författare)
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String formation beyond leading colour
- 2015
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; :8
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Tidskriftsartikel (refereegranskat)abstract
- We present a new model for the hadronisation of multi-parton systems, in which colour correlations beyond leading N-C are allowed to influence the formation of confining potentials (strings). The multiplet structure of SU(3) is combined with a minimisation of the string potential energy, to decide between which partons strings should form, allowing also for "baryonic" configurations (e.g., two colours can combine coherently to form an anticolour). In e(+)e (-) collisions, modifications to the leading-colour picture are small, suppressed by both colour and kinematics factors. But in pp collisions, multi-parton interactions increase the number of possible subleading connections, counteracting their naive 1/N-C(2) suppression. Moreover, those that reduce the overall string lengths are kine-matically favoured. The model, which we have implemented in the PYTHIA 8 generator, is capable of reaching agreement not only with the important < p perpendicular to >(n(charged)) distribution but also with measured rates (and ratios) of kaons and hyperons, in both ee and pp collisions. Nonetheless, the shape of their p perpendicular to spectra remains challenging to explain.
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| 8. |
- Christiansen, Jesper Roy, et al.
(författare)
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Weak gauge boson radiation in parton showers
- 2014
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Ingår i: Journal of High Energy Physics. - 1029-8479. ; 04, 115:4, s. 37-115
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Tidskriftsartikel (refereegranskat)abstract
- The emission of W and Z gauge bosons off quarks is included in a traditional QCD + QED shower. The unitarity of the shower algorithm links the real radiation of the weak gauge bosons to the negative weak virtual corrections. The shower evolution process leads to a competition between QCD, QED and weak radiation, and allows for W and Z boson production inside jets. Various effects on LHC physics are studied, both at low and high transverse momenta, and effects at higher-energy hadron colliders are outlined.
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| 9. |
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| 10. |
- Sjöstrand, Torbjörn, et al.
(författare)
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An introduction to PYTHIA 8.2
- 2015
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 191, s. 159-177
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Tidskriftsartikel (refereegranskat)abstract
- The PYTHIA program is a standard tool for the generation of events in high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multiparticle final state. It contains a library of hard processes, models for initial- and final-state parton showers, matching and merging methods between hard processes and parton showers, multiparton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and several interfaces to external programs. PYTHIA 8.2 is the second main release after the complete rewrite from Fortran to C++, and now has reached such a maturity that it offers a complete replacement for most applications, notably for LHC physics studies. The many new features should allow an improved description of data. New version program summary Program title: PYTHIA 8.2 Catalogue identifier: ACTU_v4_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ACTU_v4_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU General Public Licence, version 2 No. of lines in distributed program, including test data, etc.: 478360 No. of bytes in distributed program, including test data, etc.: 14131810 Distribution format: tar.gz Programming language: C++. Computer: Commodity PCs, Macs. Operating system: Linux, OS X; should also work on other systems. RAM: 10 megabytes Classification: 11.2. Does the new version supersede the previous version?: Yes Catalogue identifier of previous version: ACTU_v3_0 Journal reference of previous version: Comput Phys. Comm. 178 (2008) 852 Nature of problem: High-energy collisions between elementary particles normally give rise to complex final states, with large multiplicities of hadrons, leptons, photons and neutrinos. The relation between these final states and the underlying physics description is not a simple one, for two main reasons. Firstly, we do not even in principle have a complete understanding of the physics. Secondly, any analytical approach is made intractable by the large multiplicities. Solution method: Complete events are generated by Monte Carlo methods. The complexity is mastered by a subdivision of the full problem into a set of simpler separate tasks. All main aspects of the events are simulated, such as hard-process selection, initial- and final-state radiation, beam remnants, fragmentation, decays, and so on. Therefore events should be directly comparable with experimentally observable ones. The programs can be used to extract physics from comparisons with existing data, or to. study physics at future experiments. Reasons for new version: Improved and expanded physics models. Summary of revisions: Hundreds of new features and bug fixes, allowing improved modelling. Restrictions: Depends on the problem studied. Running time: 10-1000 events per second, depending on process studied. (C) 2015 Elsevier B.V. All rights reserved.
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