Search for elliptic flow in electron-positron to four jets events with Pythia 8
(2026) FYSK04 20261Department of Physics
Particle and nuclear physics
- Abstract
- This project investigates possible elliptic flow in electron-positron collisions, focusing on the event e+e- to W+W- to q1q2q3q4,
which hadronizes into four jets. While collective flow phenomena are typically associated with large systems such as heavy-ion collisions and the formation of quark-gluon plasma, recent theoretical and experimental findings suggest that flow-like effects may also occur in smaller systems. Within the Lund string model, interactions between overlapping strings through the string-shoving mechanism are predicted to generate a negative elliptic flow in ultra-small collision systems.
Electron-positron collision events were simulated using the Pythia 8 event generator, both with and without the string-shoving... (More) - This project investigates possible elliptic flow in electron-positron collisions, focusing on the event e+e- to W+W- to q1q2q3q4,
which hadronizes into four jets. While collective flow phenomena are typically associated with large systems such as heavy-ion collisions and the formation of quark-gluon plasma, recent theoretical and experimental findings suggest that flow-like effects may also occur in smaller systems. Within the Lund string model, interactions between overlapping strings through the string-shoving mechanism are predicted to generate a negative elliptic flow in ultra-small collision systems.
Electron-positron collision events were simulated using the Pythia 8 event generator, both with and without the string-shoving mechanism enabled. The results indicate that the addition of string shoving produces a small but non-negligible deviation of the final-state particle distributions. In particular, the two-particle correlation function show modulation of approximately 1-2%, consistent with the ridge structure associated with elliptic flow. This corresponds to an elliptic flow coefficient of approximately 0.1, agreeing with the literature. Similar sinusoidal modulations were observed in one-particle azimuthal distributions, but of lower magnitude. No significant effects were detected in pseudorapidity or momentum distributions. (Less) - Popular Abstract
- What happens when matter is smashed together at nearly the speed of light? Interestingly, in the world’s largest particle accelerators, collisions can create a new state of matter that behaves like a liquid. This exotic substance, known as quark-gluon plasma, is believed to have filled the Universe just moments after the Big Bang. Typically, this plasma has only been expected to appear in high-energy collisions involving heavy atomic nuclei. But recent research suggests that similar behavior may also emerge in much smaller systems. This raises a question: what is the smallest possible system capable of behaving like a liquid?
This bachelor’s project investigates that question using collisions between electrons and positrons... (More) - What happens when matter is smashed together at nearly the speed of light? Interestingly, in the world’s largest particle accelerators, collisions can create a new state of matter that behaves like a liquid. This exotic substance, known as quark-gluon plasma, is believed to have filled the Universe just moments after the Big Bang. Typically, this plasma has only been expected to appear in high-energy collisions involving heavy atomic nuclei. But recent research suggests that similar behavior may also emerge in much smaller systems. This raises a question: what is the smallest possible system capable of behaving like a liquid?
This bachelor’s project investigates that question using collisions between electrons and positrons (anti-electrons). In particular, the study focuses on events where the collision produces four narrow sprays of particles called jets. According to the Lund string model, this may be the smallest system capable of producing collective motion. In this model, quarks are tied together by very thin, field-like strings. As the strings stretch and break, they create new particles that form jets. The strings can also interact and push against one another through a mechanism known as string shoving. For very small systems, this effect is predicted to generate a kind of opposite (negative) flow, unlike the fluid-like flow predicted for the quark-gluon plasma.
To investigate this idea, computer simulations were performed using the particle physics event generator Pythia 8. Simulated collisions with and without string shoving were compared in order to search for signatures of collective behaviour.
The results show small but significant patterns consistent with flow. In particular, the simulations including string shoving produced a ridge-like structures in particle distributions, generally matching the theoretical predictions of flow. However, the effect was weaker than expected, and no clear changes were observed in the momenta of the produced particles.
These findings suggest that even extremely small particle systems may show behavior usually associated with much larger collisions, challenging the current view on how collective phenomena emerge in high-energy physics. A deeper understanding of these effects could provide new insights into the fundamental properties of strongly interacting matter. Improving our description of these phenomena will also enhance the accuracy of particle collision simulations, which are essential tools in modern physics research. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/student-papers/record/9247031
- author
- Opas, Kacper LU
- supervisor
- organization
- course
- FYSK04 20261
- year
- 2026
- type
- M2 - Bachelor Degree
- subject
- keywords
- elliptic flow, Pythia, QCD, quantum chromodynamics, Lund string, hadronization, string shoving, jet, QGP, quark-gluon plasma
- language
- English
- id
- 9247031
- date added to LUP
- 2026-07-13 09:44:23
- date last changed
- 2026-07-13 09:44:23
@misc{9247031,
abstract = {{This project investigates possible elliptic flow in electron-positron collisions, focusing on the event e+e- to W+W- to q1q2q3q4,
which hadronizes into four jets. While collective flow phenomena are typically associated with large systems such as heavy-ion collisions and the formation of quark-gluon plasma, recent theoretical and experimental findings suggest that flow-like effects may also occur in smaller systems. Within the Lund string model, interactions between overlapping strings through the string-shoving mechanism are predicted to generate a negative elliptic flow in ultra-small collision systems.
Electron-positron collision events were simulated using the Pythia 8 event generator, both with and without the string-shoving mechanism enabled. The results indicate that the addition of string shoving produces a small but non-negligible deviation of the final-state particle distributions. In particular, the two-particle correlation function show modulation of approximately 1-2%, consistent with the ridge structure associated with elliptic flow. This corresponds to an elliptic flow coefficient of approximately 0.1, agreeing with the literature. Similar sinusoidal modulations were observed in one-particle azimuthal distributions, but of lower magnitude. No significant effects were detected in pseudorapidity or momentum distributions.}},
author = {{Opas, Kacper}},
language = {{eng}},
note = {{Student Paper}},
title = {{Search for elliptic flow in electron-positron to four jets events with Pythia 8}},
year = {{2026}},
}