Multiparton Interaction Models and Parton Shower Studies

Abstract

This thesis involves phenomenological models for describing high energy

particle collisions. One class of these models, multiparton interactions

(MPI), deals with the fact that when colliding two hadrons, it is possible

for more than one pair of their constituents to interact. The other deals

with parton showers, which are used to fill enhanced regions of phase space

and give fully exclusive final states. These two components, when part of

an event generation framework, help in the simulation of complete high

energy collision events.



Paper I presents a study of an extension to the MPI model, enhanced

screening, where the amount of colour screening in an incoming hadron is

increased in those events with a large amount of activity.



Paper II presents another extension to the MPI model, rescattering.

Commonly, in MPI models, interactions are between pairs of partons which

originate from the incoming hadrons. With rescattering, an MPI can instead

involve partons which come from previous interactions or shower

branchings.



Paper III studies the first emission of the parton shower. First, the

interface to a next-to-leading generator, POWHEG-hvq, is examined.

Second, a dampening of the first shower emission, based on general matrix

element arguments is studied.



Paper IV again deals with parton showers. Minor changes to the framework

are outlined, before a kinematic comparison of the shower is made against

2 to 3 QCD real emission matrix elements. Finally, tunes of the

generator are made to both Tevatron and LHC data.



Paper V examines an extension to the impact parameter formalism of the MPI

model, where the size of an incoming proton is varied depending on the

x value of a parton being taken from it.

Abstract (Swedish)

Popular Abstract in English

Experiments, such as the Large Hadron Collider (LHC) in CERN, Switzerland,

collide particles together at high energies, in the hope of probing the

fundamental constituents and forces of nature. On the theoretical side, the

Standard Model (SM) is currently the best description we have. It is a

theory that encompasses the strong, weak and electromagnetic forces,

although excludes gravity. This thesis deals with topics related to Monte

Carlo event generators; tools which, based on the theory, simulate these

particle collisions, and give us an idea of what to expect at particle

physics experiments.



When protons are made to collide, as at the LHC, much of the physics will

be dominated by the strong force. The part of the SM that describes this is

known as Quantum Chromo Dynamics (QCD). It is not currently possible to

solve this theory exactly, and many approximate methods are used. This

thesis, in part, relates to parton showers, which describe the phenomenon

that quarks and gluons (collectively known as partons, and the constituents

of hadrons, such as the proton), can radiate other quarks and gluons, and

in fact do so copiously under certain conditions.



The other part of this thesis deals with multiparton interaction (MPI)

models. MPI is a consequence of the composite nature of hadrons.

For example, a proton is made up from three quarks, and will also contain

many gluons which in turn can split into further quark-antiquark pairs.

When two protons collide, it is hoped that one parton from each will

interact together, but it is also possible for more than one pair to do so.

These MPI affect the results from collider experiments, and so it is vital

to have good models for them, such that their effect can be understood and

disentangled from the data.

• author: Corke, Richard
• supervisor: Torbjörn Sjöstrand
• opponent: Professor Cacciari, Matteo, LPTHE, UPMC Univ. Paris 6, France and Université Paris Diderot, France
• organization: Theoretical Particle Physics - Has been reorganised
• publishing date: 2011
• type: Thesis
• publication status: published
• subject: Subatomic Physics
• keywords: Multiparton Interactions, Parton Showers, Phenomenological Models, QCD Phenomenology
• pages: 211 pages
• defense location: Lecture Hall F, Department of Astronomy and Theoretical Physics, Sölvegatan 14A, Lund, Sweden
• defense date: 2011-03-11 10:15:00
• ISBN: 978-91-7473-087-6
• language: English
• LU publication?: yes
• id: 6f55eea6-7de0-4ece-9792-73ed6f5b3b78 (old id 1787237)
• date added to LUP: 2016-04-04 10:13:12
• date last changed: 2018-11-21 20:57:30