Geometry studies of high energy 16O collisions

Abstract

In this project a model of oxygen, 16O, nuclei, where the nucleons are organised in α-clusters, was implemented in the Pythia Monte Carlo event generator. To study the effects of the new model, some observables, such as the distribution of the number of the wounded nucleons, charged particle multiplicity in the forward and central pseudorapidity region, and the pseudorapidity distribution for different centrality bins and eccentricity, are compared to the model originally implemented in Pythia (Woods-Saxon). Some basic theory that is used in the Pythia framework is also reviewed, as an amalgam from information taken from overview papers on the Glauber Model, Monte Carlo Event generators, the Angantyr model, and the string shoving model.

Popular Abstract

Collisions of matter (particles) sometimes happen naturally, and are sometimes performed in experiments, in order to investigate the fundamental properties of matter, such as its composition, or masses of the produced particles. One example of collisions happening naturally, is collisions of cosmic rays with the atmosphere of the Earth. The main participant of collisions of cosmics with the atmosphere is oxygen. It has been recently hypothesised that the oxygen nuclei have an unusual structure, in comparison to previously studied heavier nuclei.

At higher collision energy, smaller scales become more important. This means that more details of the internal structure are observed. The matter observed in our daily lives, consists of the atoms, which have a nucleus with electrons orbiting it. The nucleus in turn consist of nucleons: protons and neutrons. These particles, as well as other hadrons, are comprised of quarks, elementary particles, which have mass, electric charge, colour charge (called red, green or blue and respective anti-colours) and 1/2-spin. These are intrinsic properties which determine how the particles interact. There are four fundamental forces: electromagnetic, weak, strong and gravity. Corresponding force carriers have been found for the first three. Particles that have electric charge may interact via the electromagnetic force by the exchange of photons. In the same way, the strong interaction affects particles with colour charge, the quarks. The branch of theoretical physics that studies the strong interaction is called Quantum Chromodynamics (QCD).
The force carriers for the strong interaction, gluons, also possess a colour charge (carries both a colour and an anti-colour). That results in the strong force increasing in magnitude with the distance between the particles. It can be imagined as a string stretching between them. That analogy is the basis for the Lund String Model. If the energy stored in the string is large, it breaks by the production of a quark-antiquark pair. After the string breaks, there are two smaller pieces of string, with one quark from the original pair and one quark from the created pair at the ends of each string. The string may break down even further, and the resulting small string fragments are identified as hadrons, which are observed in the final state. This process is called string fragmentation and constitutes the model by which the hadrons are produced. In addition to this, strings can interact with each other. Strings will exert a repulsive force on each other, and the strength of repulsion decreases with the distance between the strings.

We return to the consideration of the collision itself. When two nucleons (or hadrons in general) collide at high energy, their constituents, quarks, can exchange gluons and be knocked out. That causes the string to stretch. Several strings can form simultaneously. This happens in particular in collisions of nuclei, when several nucleons participate. As was stated earlier, the strength of interaction between the strings depends on the distance between them. The internal nucleus geometry is therefore an important factor in determining the properties of the resulting hadrons. This means that an up-to-date structure for oxygen should be used in the models underlying simulations of collisions, in order to correctly predict the experimental results. That is precisely the concern of this project: to implement an up-to-date geometrical model of oxygen in the simulation framework called Pythia.

The model of oxygen currently used within Pythia, has nucleons distributed according to a Woods-Saxon distribution. In the Woods-Saxon model, the density of nucleons depends only on the distance from the centre. The majority of nucleons are bunched up close to the centre of the nucleus, with the density dropping further from the centre. In the new model, there are four distinct clusters of four nucleons (two protons, two neutrons), with their centres located at the vertices of a tetraeder. The coordinates of nucleons within each cluster are distributed according to a Gaussian distribution.

• author: Kuzmenka, Aliaksei
• supervisor: Christian Bierlich
• organization: Theoretical Particle Physics - Undergoing reorganization
• course: FYTK02 20212
• year: 2022
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Ion Collisions, Oxygen
• report number: LU-TP 22-07
• language: English
• id: 9075396
• date added to LUP: 2022-03-01 11:41:35
• date last changed: 2022-03-01 11:41:35