Multiparton Interaction Models and Parton Shower Studies

• 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.

• 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.

Subject headings

NATURVETENSKAP  -- Fysik -- Subatomär fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Subatomic Physics (hsv//eng)

Keyword

Multiparton Interactions

Parton Showers

Phenomenological Models

QCD Phenomenology

Publication and Content Type

dok (subject category)

vet (subject category)