Factorizing Charm Production in Proton-Proton Collisions with PYTHIA

Abstract

In this thesis, proton-proton collisions at 7 TeV are simulated using PYTHIA. Charm and anticharm quarks are identified as being produced from direct, meaning that the charm and anticharm are pair produced by a hard scattering, or indirect interactions where the charm and anticharm pair is produced from the splitting of a single gluon. This data is used to generate a pT spectrum and correlation functions for rapidity and the azimuthal angle. The relative contributions of the direct and indirect processes to the histograms are then tuned to better match the experimental data taken from the LHCb. The goal is to obtain a more accurate fraction of direct to indirect interactions that produce charm quarks in proton-proton collisions.

The direct to indirect production fraction was determined to be 0.54, which is significantly lower than the fraction estimated by PYTHIA, as that was 1.28. The overall shapes of the correlation functions and pT spectra did follow the LHCb data; however, for small values of |∆ϕ| and |∆y|, the correlation functions had lower magnitudes compared to the experimental data. Furthermore, the simulated PYTHIA data created more charm and anticharm quarks at low pT compared to what was measured in the LHCb, causing some of the discrepancy. This suggests that further refinement is needed to make the simulation more accurate to model charm and anticharm production from proton-proton collisions properly.

Popular Abstract

Our solar system is approximately 4.5 billion years old, our galaxy is 13.6 billion years old, and the Universe is estimated to be around 13.8 billion years old. Therefore, the Quark Gluon Plasma (QGP), a state created only a few moments after the Big Bang, may seem insignificant as it only lasted as long as the blink of an eye. However, many theoretical models of the expansion and evolution of the Universe rely on the very early stages, much like how the stability of a Jenga tower relies on the stability of the foundation. Therefore, being able to understand and accurately model the QGP is vital.

As many know, all matter is made up of protons, neutrons, and electrons. But the protons and neutrons consist of even smaller particles called quarks. These quarks are held together to form protons and neutrons using a ‘glue’ particle, called gluons. In the first few moments of the Universe, all known matter was contained inside a space smaller than a grain of rice. Therefore, understandably, the conditions were extremely hot and dense. Due to these conditions, larger particles could not form, so the quarks moved around freely, resulting in a state called the QGP.

Since the QGP lasted for only a brief moment billions of years ago, and as we do not have access to a time machine, one may wonder how we can conduct experiments on it. Luckily for us, similar conditions can be created by colliding particles that are moving close to the speed of light. However, even with the best accelerators, the QGP state can only be created for brief moments. As a result, to get any meaningful data, a probe is needed. The probes contain information about the QGP, but they can survive much longer, so they can be studied properly. One such probe is the charm quark, a type of quark that is created early in the collision.

One of the properties that is studied in this thesis is whether the charm quarks are created directly or indirectly. Determining this is no walk in the park, as within each collision, there are hundreds, if not thousands, of subprocesses that occur. To determine how each charm quark is created, one has to use the same method that is used when checking our ancestry, by constructing a family tree. For each charm quark, the parent particles are determined, and the parents of those, up to the first generation. The charm is considered to have been formed directly if the first generation consists of either two gluons or a quark and an anti-quark. However, if the charm is created from a single gluon splitting into a charm and anticharm, which is hopefully not too relatable to anyone’s actual family tree, then it is said to have been created indirectly.

The fraction of charm quarks that are created directly versus indirectly can then be determined by optimizing the data created from a simulation of the event to match data from an experiment at a particle accelerator. This fraction is crucial as it enables researchers to use simulations with greater accuracy. Hopefully, that will correspond to a deeper knowledge in the field, uncovering more about not only the start of the Universe but also its evolution as a whole.

• author: O'Riordan, Marcus
• supervisor: Peter Christiansen
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Correlation function, charm, anticharm, PYTHIA, pT spectrum, direct, indirect
• language: English
• id: 9198579
• date added to LUP: 2025-06-13 12:32:22
• date last changed: 2025-06-13 12:32:22