Investigation of event-shape classifiers for proton-proton collisions with the ALICE experiment

Abstract

This project uses proton-proton collision data from the ALICE experiment at CERN to investigate the differences between three ways to classify events, known as event shape classifiers. The event shape classifiers that were investigated are RT , spherocity and flattenicity.

The motivation behind this is the consistent use of these classifiers to select events when investigating the heavy-ion like behavior observed in high-multiplicity proton-proton collisions. A comprehensive understanding of the types of events that are being selected by these classifiers is essential, and providing the community with such an understanding is the
goal of this project.

Two-particle angular correlation functions were created in classifier bins to investigate the event shape. While all three classifiers can select more isotropic events (dominated by the underlying event, where QGP-like effects are more likely to be found) and more jet-like events, the jet-like effects are present for all event shape classes.

The results indicate several differences between the event selection being done by the three classifiers. These differences include the bias towards events with an average of 90 degrees between the tracks for higher spherocities, and a bias against events where two or more tracks belong to the same flattenicity cell for lower flattenicities. Flattenicity appears to select the most isotropic events out of the three classifiers.

Popular Abstract

The universe consists of elementary particles that come together to form what we recognize as ordinary matter. You may know that atoms are made up of protons and neutrons. However, these are not the smallest constituents we know of. Protons and neutrons are what we call hadrons, and they are made up of quarks and gluons. The gluons act as a glue, keeping the quarks together inside the hadrons. In particle accelerators, such as the Large
Hadron Collider (LHC) in Switzerland, protons and heavy ions can be accelerated and collided with the goal of ripping them apart and studying the products.

One example of interesting research that can be done in such colliders is quark-gluon plasma (QGP) research. The QGP is a state of matter that we believe the universe consisted of just after the Big Bang, and it can also exist inside neutron stars. The QGP is the result of highly energetic environments, where there is so much energy that the quarks and gluons are not as bound to each other as they normally are, and they exist semi-freely in a "soup" of quarks and gluons. Due to the high energies when colliding heavy ions such as Pb (lead), it is possible to, for a brief moment, create a QGP at a collider like the LHC. One can then study the way that particles behave after the collision to extract information about the QGP.

A QGP should not be able to be created inside proton-proton collisions due to the smallness of these systems. However, scientists have observed several signatures that were previously assigned to be signatures of the QGP, in a certain type of proton-proton collisions known as "high-multiplicity" proton-proton collisions. High multiplicity means that a lot of particles are created in the collision.

A hot topic in the field of heavy-ion physics is understanding why we see these QGP-like signatures in such high-multiplicity proton-proton collisions. Recently scientists have begun to explore the effect that the event shape may have on this behavior. The event shape refers to how the particles after the collision are spread out. There are three major classifiers that are currently being used to select different shapes of collisions (so-called event shape classifiers): RT , spherocity and flattenicity. The problem is that people are using these classifiers to classify collisions, but there has been no study on what the difference between the collisions that are being selected by these classifiers is, or what types of collisions are being selected by each of them.

The goal of this project is to make a study of the types of collisions that are being selected by each of the classifiers, and how they differ from each other. This was done by analyzing proton-proton collision data from one of the experiments at the LHC: the ALICE (A Large
Ion Collider Experiment) experiment, which detects the products of both heavy-ion collisions and proton-proton collisions. This is an important contribution to the ongoing research on how event shapes are correlated with the heavy-ion like behavior in high multiplicity proton-proton collisions.

• author: Bonnevier Wallstedt, Tilde
• supervisor: Alice Ohlson
• organization: Department of Physics; Particle and nuclear physics
• course: FYSM63 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Particle physics, ALICE, heavy-ion physics, proton-proton collisions, event shapes, event shape classifiers
• language: English
• id: 9160009
• date added to LUP: 2024-06-11 13:53:36
• date last changed: 2024-06-11 13:53:36