26.5.2023: Neutral pions from particle collisions tell us about the structure of the nucleus (Rytkönen)

In her thesis work, Heidi Rytkönen has investigated the physics performance of a new particle detector of the ALICE experiment (A Large Ion Collider Experiment) in CERN. The main topic of this thesis is to answer how precisely the detector, called FoCal, is able to measure neutral pions and their relation to each other. Simulations show that the background in this measurement is convoluted, but the work introduces a way to deal with such contributions.
Heidi Rytkönen
Published
26.5.2023

Can we observe gluon saturation?

Why should we be interested in measuring neutral pions? Measuring particles emerging from the collisions between protons and heavier nuclei we can get information about their building blocks, quarks and gluons. Gluons are the mediators of strong interaction, one of the four main forces of nature, and keep the nuclei intact. A major question is what happens when the gluons in question have a really small momentum.

The gluon number has been observed to grow when reaching smaller and smaller gluon momentum. However, at some point it is expected that the growth stops due to the finite space. The gluons start to merge with each other. The stagnation of the number of gluons is called gluon saturation. Gluon saturation has been investigated theoretically via Color Glass Condensate models, but experimental evidence is lacking.

One of the main physics goals of FoCal is to make observations in the saturation region, to understand the phenomenon better. One prominent signal for gluon saturation is the correlations between neutral pions. The measurement is challenging due to the background occurring in the correlations. In the research Heidi has developed a method to remove the background, which has then been demonstrated in this work via simulations. It should be noted that the background is prominent at these collision energies, and it can not be ignored: a method like the one presented here is needed for measuring correct values.

Still some way to go

FoCal can not be yet included in the ALICE experiment. The data taking with FoCal is scheduled to 2029. Before that all the components of the calorimeter need to be tested carefully.

In 2021 a series of test beams were started at CERN. Testing the calorimeter components in particle beam allows to see if the electronics work as intended and the sensors pick up the signal correctly. In this thesis the analysis results regarding FoCal silicon sensors are presented from the first two test beams. 

This thesis is part of research of Helsinki Institute of Physics (HIP) ALICE project and Center of Excellence in Quark Matter in Ģֱ. The work has been funded by Ģֱ funding for doctoral training, Vilho, Yrjö ja Kalle Väisälä fund, ALICE project in HIP, and Center of Excellence in Quark Matter.

M.Sc. Heidi Rytkönen defends her doctoral dissertation ”Simulated π0–π0 Correlation and Test Beam Performance of the Future Forward Calorimeter in the ALICE experiment”&Բ;Dz 26.5.2023 at 12:00 in lecture hall FYS1 of the Department of Physics. Opponent is Dr. Gabor David (Brookhaven National Laboratory, USA) and custos is Senior Lecturer Sami Räsänen (Ģֱ). The language of the dissertation is English.

Publication details

The thesis “Simulated pi0–pi0 correlation and test beam performance of the future forward calorimeter in the ALICE experiment” is available in the JYX repository: