Fast detector simulation studies for the HIBEAM experiment at ESS

Abstract

The observed imbalance of matter and antimatter in the universe cannot be fully explained by the Standard Model of particle physics. One proposed mechanism to account for this discrepancy is the violation of baryon number B, a process that has yet to be experimentally observed. The HIBEAM/NNBAR project, a proposed two-stage experiment at the European Spallation Source, aims to find evidence of baryon number violation via neutron oscillations.

The goal of this thesis is to improve the existing parametrized simulation framework for the proposed WASA detector at ESS. This will be achieved by studies of a neutral pion reconstruction algorithm used in the simulations, as well as by implementation of a parametrized detector response to muons.

The studies of neutral pions found how reconstruction performance is affected by kinetic energy, stochastic term coefficient, and decay location. A parametrized muon response was successfully implemented and shows agreement with expected physical behavior.

Popular Abstract

Imagine the world’s most powerful microscope, able to observe surfaces on the scale of atoms. One of these are currently being built at the European spallation source (ESS for short) just outside of Lund, Sweden. The ESS is a particle accelerator that uses neutrons, one of the fundamental particles that is present in a lot of the atoms around (and inside of) you.

Most of these neutrons will be used in so called scattering experiments, where scientists from all over the world observe different surface structures useful to their experiments. Here, they use the neutrons as a really precise microscope. But instead of using the neutrons as an instrument to observe something else, the neutrons themselves can be studied! This is going to be done at the HIBEAM/NNBAR experiments at the ESS.

There is a set of rules that all particles in the universe seem to follow, and these are called the Standard Model of particle physics. However, there are signs that indicate that some of these rules can be broken. One example of this is the very fact that you exist to read this text. In the big bang, matter (that you are made of) and antimatter are believed to have been created in equal amounts. This means that all particles were created with a corresponding antimatter friend. One example of this is the neutron, which has an antimatter equivalent called the antineutron. When matter and antimatter meet, they annihilate, leaving only energy behind. Thus, all matter and antimatter should have been destroyed soon after its creation in the big bang, unless something tipped the scale and converted some of the antimatter into normal matter, via breaking of some of the rules in the Standard Model. Scientists have a theory that can explain this, namely that neutrons which are not bound to an atomic nuclei can spontaneously turn into an antineutron, and vice versa. This is referred to as neutron oscillations.

These neutron oscillations will be searched for at the ESS. Unbound neutrons will be fired at a target made of carbon, which contains a lot of neutrons and protons. If one of the fired neutrons happen to turn in to an antineutron, it will annihilate with one of the neutrons or protons in the target. This will in turn give rise to a distinct signal, sort of like a fingerprint, that can be read out by a detector. The problem is that these neutron oscillations, if they exist, are extremely rare. And in order to maximize our chances of even observing this once, we need a lot of neutrons. These will all leave their own fingerprint in the detector. Thus, it is very important that the detector is fast and precise, so that it can distinguish the important signal from all the unimportant noise. A detector that we think is suitable for this exists, and is called the WASA detector. A good way to asses if the WASA detector will be good enough for this purpose, is via programming simulations. The WASA detector is recreated in the computer via coding, and neutrons are then fired at it just as it will be in the real experiment.

This project will consist of expanding the already existing simulations for the WASA detector. How the detector responds to a type of particles called pions will be examined in more depth than previous experiments, and a detector response to a particle called a muon will be implemented. This will hopefully (in many years time) allow neutron oscillations to be discovered, which in turn would mean we have a whole new type of physics to explore!

• author: Gustafsson Holmlund, Nellie
• supervisor: Bernhard Meirose
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9199327
• date added to LUP: 2025-06-16 15:04:43
• date last changed: 2025-06-16 15:04:43