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Event Geometry and Anisotropic Flow in Proton-Proton Collisions

Holst, Johannes LU (2021) FYTK02 20211
Theoretical Particle Physics
Abstract
We seek to better understand the relationship between an anisotropic distribution of partons in the initial state of pp-collisions and the amount of elliptic flow. We do this through the implementation of a new model that is based in generating positions for multiparton interactions from the uneven volume created by the overlap of two Gaussians. We then examine how this asymmetric probability density function compares to geometries without intrinsic anisotropy and data from the CMS and ALICE. This is achieved by simulations based in these geometries in a hydrodynamical toy model and in PYTHIA.
Popular Abstract
At the Large Hadron Collider (LHC) in Switzerland protons are being collided at velocities near the speed of light. This is done to study how matter behaves under very hot and dense conditions. Despite the continuous improvement and success of previous models we still lack an exact theory for how the particles that make up protons behave at these high temperatures and densities. \par
One key observation from research at the LHC has been what is known as anisotropic flow. It is a liquid-like motion of the particles flying out of the colliding protons. The reason they behave this way is believed to be because of the collision geometry being lopsided. This means that when the protons collide, they rarely do so head on and the collision... (More)
At the Large Hadron Collider (LHC) in Switzerland protons are being collided at velocities near the speed of light. This is done to study how matter behaves under very hot and dense conditions. Despite the continuous improvement and success of previous models we still lack an exact theory for how the particles that make up protons behave at these high temperatures and densities. \par
One key observation from research at the LHC has been what is known as anisotropic flow. It is a liquid-like motion of the particles flying out of the colliding protons. The reason they behave this way is believed to be because of the collision geometry being lopsided. This means that when the protons collide, they rarely do so head on and the collision region is therefore more closely packed in some directions than others. When the system then expands the particles are sprayed more in some directions than others. \par
Conventionally, flow would be described in terms of Quark-Gluon Plasma (QGP), a state of matter where the particles are deconfined from the proton structure. It is believed to be what constituted the very first microseconds of the universe. QGP explanations of flow are generally combined with hydrodynamical models and widely applied in fairly accurate computer simulations of these events. Although, it is still an open question whether observations of flow necessarily presuppose QGP. \par
The PYTHIA event generator simulates the collective mechanisms associated with flow using the Lund String Model (LSM). The LSM models these flow effects by string shoving, which means describing the particles as being tied up into strings that then interact with each other by pushing, forming ropes and fragmenting into other particles. The reason it is of value to us is because it is not reliant on QGP and hydrodynamics, so if these models are more consistent with experiments it may cause us to reevaluate our theories. \par
In this project a set of different geometries for describing the spatial distributions of particles in the interaction region will be implemented in both hydrodynamical QGP and PYTHIA models. The different geometries rely on a variety of mathematical and empirical descriptions for how interactions are to be distributed in the initial state of the collision. Then computer simulations of anisotropic flow are run based on these probability functions. Finally, the results of each model will be compared to experimental data from the LHC. The purpose of the project is to examine and evaluate the different models and observations. In order to keep improving our understanding of how the world works and how it came to be, we need further research in the comparison between the predictions of our descriptions and the empirical observations. (Less)
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author
Holst, Johannes LU
supervisor
organization
course
FYTK02 20211
year
type
M2 - Bachelor Degree
subject
keywords
high energy physics, particle physics, proton, collision, quark-gluon plasma, flow, anisotropy, anisotropic flow, elliptic flow, initial state, geometry, eccentricity, gaussian, cms, alice, lhc
report number
LU-TP 21-27
language
English
id
9054307
date added to LUP
2021-06-24 09:15:46
date last changed
2021-06-24 09:15:46
@misc{9054307,
  abstract     = {{We seek to better understand the relationship between an anisotropic distribution of partons in the initial state of pp-collisions and the amount of elliptic flow. We do this through the implementation of a new model that is based in generating positions for multiparton interactions from the uneven volume created by the overlap of two Gaussians. We then examine how this asymmetric probability density function compares to geometries without intrinsic anisotropy and data from the CMS and ALICE. This is achieved by simulations based in these geometries in a hydrodynamical toy model and in PYTHIA.}},
  author       = {{Holst, Johannes}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Event Geometry and Anisotropic Flow in Proton-Proton Collisions}},
  year         = {{2021}},
}