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Performance of the WASA detector for the HIBEAM experiment

Åstrand, Lucas LU (2026) FYSM64 20261
Department of Physics
Particle and nuclear physics
Abstract
The observed matter-antimatter asymmetry in the Universe suggests the existence of baryon number violating processes beyond what is present in the Standard Model of Particle Physics. The HIBEAM programme is a proposed experiment at the European Spallation Source (ESS) that investigates baryon-number violation through neutron-antineutron oscillations. Of specific interest to this work is the process $n \to \Bar{n}$, resulting in a violation of baryon number of two units, $\Delta \mathcal{B} = 2$. This thesis investigates the performance of the chosen detector setup, the WASA detector, with particular emphasis on the reconstruction of neutral and charged states. A \textsc{Geant4}-based simulation and a ROOT-based reconstruction framework are... (More)
The observed matter-antimatter asymmetry in the Universe suggests the existence of baryon number violating processes beyond what is present in the Standard Model of Particle Physics. The HIBEAM programme is a proposed experiment at the European Spallation Source (ESS) that investigates baryon-number violation through neutron-antineutron oscillations. Of specific interest to this work is the process $n \to \Bar{n}$, resulting in a violation of baryon number of two units, $\Delta \mathcal{B} = 2$. This thesis investigates the performance of the chosen detector setup, the WASA detector, with particular emphasis on the reconstruction of neutral and charged states. A \textsc{Geant4}-based simulation and a ROOT-based reconstruction framework are employed. The results show that the proposed detector system achieves high reconstruction efficiency for both charged and neutral states, with acceptance consistent with geometrical expectations. The employed particle identification scheme has been shown to provide high separation power among different charged particles. Finally, the developed event selection strategy is shown to achieve strong background suppression while maintaining high signal efficiency. (Less)
Popular Abstract
Following the events of the Big Bang, matter and antimatter were expected to be produced in equal amounts. However, observations reveal that the present-day Universe is almost entirely composed of matter, with no clear explanation for what happened to the missing antimatter. Formally, this is known as the matter-antimatter asymmetry, and represents one of the main mysteries of modern day particle physics. Many experiments and theoretical models aim to explain this imbalance, but one thing is clear, there needs to be a mechanism which allows particles to convert from matter to antimatter.

The HIBEAM collaboration at the European Spallation Source is one such experiment, looking for neutron oscillations, a process in which neutrons... (More)
Following the events of the Big Bang, matter and antimatter were expected to be produced in equal amounts. However, observations reveal that the present-day Universe is almost entirely composed of matter, with no clear explanation for what happened to the missing antimatter. Formally, this is known as the matter-antimatter asymmetry, and represents one of the main mysteries of modern day particle physics. Many experiments and theoretical models aim to explain this imbalance, but one thing is clear, there needs to be a mechanism which allows particles to convert from matter to antimatter.

The HIBEAM collaboration at the European Spallation Source is one such experiment, looking for neutron oscillations, a process in which neutrons transform to antineutrons. In the HIBEAM experiment, antineutrons are detected through their annihilation in a target volume, producing a well-defined and understood set of particles. For this reason, an efficient and reliable detector system is of invaluable importance.

This thesis investigates the performance of the proposed detector system for the HIBEAM experiment, using detailed computer simulations and complex reconstruction strategies. The results reveal that the detector system is capable of reconstructing these events with high efficiency, supporting its use for the planned study of neutron-antineutron oscillations. (Less)
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author
Åstrand, Lucas LU
supervisor
organization
course
FYSM64 20261
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Particle Physics, Detectors, Simulation, Geant4, ROOT, Clustering, Event Selection, Reconstruction, Wasa, Calorimeter
language
English
id
9239420
date added to LUP
2026-06-17 13:49:00
date last changed
2026-06-17 13:49:00
@misc{9239420,
  abstract     = {{The observed matter-antimatter asymmetry in the Universe suggests the existence of baryon number violating processes beyond what is present in the Standard Model of Particle Physics. The HIBEAM programme is a proposed experiment at the European Spallation Source (ESS) that investigates baryon-number violation through neutron-antineutron oscillations. Of specific interest to this work is the process $n \to \Bar{n}$, resulting in a violation of baryon number of two units, $\Delta \mathcal{B} = 2$. This thesis investigates the performance of the chosen detector setup, the WASA detector, with particular emphasis on the reconstruction of neutral and charged states. A \textsc{Geant4}-based simulation and a ROOT-based reconstruction framework are employed. The results show that the proposed detector system achieves high reconstruction efficiency for both charged and neutral states, with acceptance consistent with geometrical expectations. The employed particle identification scheme has been shown to provide high separation power among different charged particles. Finally, the developed event selection strategy is shown to achieve strong background suppression while maintaining high signal efficiency.}},
  author       = {{Åstrand, Lucas}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Performance of the WASA detector for the HIBEAM experiment}},
  year         = {{2026}},
}