Skip to main content

Lund University Publications

LUND UNIVERSITY LIBRARIES

Colour Reconnection and Weak Showers

Christiansen, Jesper Roy LU (2015)
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... (More)
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. (Less)
Abstract (Swedish)
Popular Abstract in Danish

Ved CERN (Conseil Européen pour la Recherche Nucléaire) accelerer man protoner

og bly-ioner op til næsten lysets hastighed, for derefter at kollidere dem med hinanden. Målet med dette er at forstå, hvordan de mindste ting i universet er bygget op, og hvordan de interagerer. Alle tidligere eksperimentelle data er kombineret til en enkelt teori, kendt som StandardModellen (SM). Håbet er, at de energirige partikelkollisioner vil vise os noget, der ligger udover SM, men før vi kan finde noget nyt, er vi nødt til at være sikre på, at vi forstår SM fuldstændigt.



SM er en yderst elegant teori; den kan dog desværre ikke løses analytisk. For at sammenholde modellen med... (More)
Popular Abstract in Danish

Ved CERN (Conseil Européen pour la Recherche Nucléaire) accelerer man protoner

og bly-ioner op til næsten lysets hastighed, for derefter at kollidere dem med hinanden. Målet med dette er at forstå, hvordan de mindste ting i universet er bygget op, og hvordan de interagerer. Alle tidligere eksperimentelle data er kombineret til en enkelt teori, kendt som StandardModellen (SM). Håbet er, at de energirige partikelkollisioner vil vise os noget, der ligger udover SM, men før vi kan finde noget nyt, er vi nødt til at være sikre på, at vi forstår SM fuldstændigt.



SM er en yderst elegant teori; den kan dog desværre ikke løses analytisk. For at sammenholde modellen med eksperimentel data er man derfor nødt til at lave

nogle approksimationer og antagelser. En af de mest udbredte måder at sammenligne data med teoretiske forudsigelser, er ved hjælp af computersimuleringer, kaldet Monte Carlo simuleringer. Disse programmer simulerer partikelkollisioner ud fra den bedst tilgængelige forståelse af teorien. De kan sammenlignes direkte med de data, der kommer fra rigtige

kollisioner, og afvigelser vil være indikationer på fysik udover SM. Denne

afhandling omhandler forbedringer af simuleringerne, sådan at vi kan være

mere sikre på at potentielle afvigelser virkelig er ny fysik. Det er specielt to områder indenfor simuleringerne, der bliver berørt, den svage partonkaskade og farve omkoblinger.



Når en ladet partikel bliver accelereret udsender den elektromagnetisk stråling i form af fotoner, bremsstrahlung. En kollision mellem to partikler kan ses som en yderst ekstrem acceleration, derfor er store mængder ekstra stråling

forventet. Beskrivelsen af denne ekstra stråling kaldes partonkaskaden. Udover

at udsende fotoner er det også muligt at udsende gluoner. Disse svarer til fotoner for den stærke kernekraft. Da den stærke kernekraft er mange gange stærkere end den elektromagnetiske kraft dominerer gluon-emissioner totalt billedet. Hidtil har udsendelse af den svage kernekrafts ``fotoner'' (W og Z) været ignoreret. Dette skyldes, at de er mange gange tungere end protonen. Nu er kollisionsenergien dog blevet så høj, at de bør inkluderes. Inklusion af disse partikler er netop dækket af en svag partonkaskade og er et af hovedemnerne i denne afhandling. Inklusionen af den svage partonkaskade gør det muligt at beskrive produktion af W og Z, som den tidligere partonkaskade ikke kunne forklare.



Den stærke kernekraft er så stærk, at den binder de mest fundamentale

partikler (kvarker) sammen til større partikler kaldet hadroner. Et eksempel

på en hadron er protonen, som består af to op-kvarker og en ned-kvark. Et af

spørgsmålene som en computersimulering må forklare er, hvordan disse kvarker

bindes sammen til hadroner. Denne proces kaldes hadronisering. Billedet man

forestiller sig er, at der mellem to kvarker er spændt en streng. Denne streng

kan så fragmenteres til de forskellige hadroner. Det fungerer fint så længe

man bare har to kvarker, da det er oplagt, hvor strengen befinder sig. Men hvis

der er mange kvarker, er der også mange forskellige kombinationsmuligheder.

Valget mellem disse streng-kombinationer går under navnet farve omkoblinger. I

denne afhandling beskrives en nyudviklet model for beskrivelse af dette. Modellen giver en bedre beskrivelse af de forskellige typer af hadroner og deres energier sammenlignet med eksperimentelle data.



Begge tilføjelser er implementeret i det lokalt udviklede Monte Carlo simuleringsprogram, Pythia. Dette er en af de mest udbredte simuleringsprogrammer i verdenen og dermed vil de nye modeller direkte kunne

indgå i sammenligningerne mellem teori og data. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Gieseke, Stefan, Karlsruhe Institute of Technology
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Merging, Weak interactions, Parton Showers, Event generators, hadron collisions, Colour Reconnection, Fragmentation, Fysicumarkivet A:2015:Christiansen
pages
244 pages
publisher
Department of Astronomy and Theoretical Physics, Lund University
defense location
Lundmarksalen, Sölvegatan 27, Lund
defense date
2015-11-13 13:00:00
ISBN
978-91-7623-500-3
978-91-7623-501-0
language
English
LU publication?
yes
id
9b59fb39-3efe-4455-9356-b861ea042dbd (old id 8057650)
date added to LUP
2016-04-04 10:01:24
date last changed
2020-04-29 12:04:12
@phdthesis{9b59fb39-3efe-4455-9356-b861ea042dbd,
  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. <br/><br>
<br/><br>
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.<br/><br>
<br/><br>
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.}},
  author       = {{Christiansen, Jesper Roy}},
  isbn         = {{978-91-7623-500-3}},
  keywords     = {{Merging; Weak interactions; Parton Showers; Event generators; hadron collisions; Colour Reconnection; Fragmentation; Fysicumarkivet A:2015:Christiansen}},
  language     = {{eng}},
  publisher    = {{Department of Astronomy and Theoretical Physics, Lund University}},
  school       = {{Lund University}},
  title        = {{Colour Reconnection and Weak Showers}},
  year         = {{2015}},
}