Study of the Background Magnetic Field Distribution for Neutron Transition Magnetic Moment Searches

Abstract

At Oak Ridge National Laboratory (ORNL), where the High Flux Isotope Reactor (HFIR) GP-SANS beamline is located, an experiment searching for the neutron Transition Magnetic Moment (nTMM) was conducted. This experiment is dependent on the magnetic field homogeneity and requires a magnetic field scanning precision of at least 4 μT. A previous experiment performed at the same facility used powerful magnets, which magnetised parts of the steel support structure of the beamline. As a result, the background magnetic field in the experimental region was modified. The aim of this thesis was to determine the magnitude and homogeneity of the magnetic field inside the beamline, and to assess whether the variation of the field within the region of interest is larger or smaller than the required scanning precision. The magnetic field components were measured in the vicinity of the beamline. These data were used to reconstruct the magnetic scalar potential by fitting the measured field with spherical-harmonic gradients. The resulting expression in spherical harmonics was implemented as a boundary condition in the COMSOL simulation. The reconstructed field indicates that the variation in field magnitude across the region of interest is of the same order as the required scanning precision. This indicates that the residual background field may be relevant for the magnetic-field scan and should be accounted for when interpreting the experimental results.

Popular Abstract

In Search of Dark Matter

Modern physics can explain incredible things. Scientists have developed theories describing atoms, the nucleons that make up atomic nuclei, and even the quarks that make up the nucleons themselves. The theory that successfully describes the interactions of all known particles is called the Standard Model. Despite this, many mysteries still remain unsolved. One of these mysteries is dark matter. It cannot be seen, and we do not interact with it in everyday life. But how did scientists come to the idea that it exists? For that, we have astrophysicists to thank! Generations of scientists who studied the motion of matter in different galaxies throughout the 20th century came to the conclusion that the visibly observable mass in galaxies is not enough to explain how matter is distributed or how fast it moves. Based on the well-known law of gravity, we would expect the speed of matter in galaxies to decrease as we move farther from the center. In practice, however, the speed remains almost constant. This is where the idea of the dark matter originally came from. Dark matter interacts with ordinary matter through gravity, but it remains invisible and untouchable because it does not interact electromagnetically with ordinary matter. If dark matter exists, then there must also be laws of physics describing the connection between it and ordinary matter. Theoretical physicists are working on models that could describe such particles. At the same time, experimental physicists are trying to find ways to confirm the existence of dark matter in a laboratory environment. Among the theories that attempt to explain dark matter is the theory of mirror matter. It suggests that there exist mirror particles that are complete copies of the particles from the Standard Model. The idea is that ordinary particles do not interact with mirror particles in any common way except through gravity, but under certain conditions they may transform into one another with a very small probability. Such a model can explain both the behavior of galaxies in space and the fact that we do not encounter dark matter in everyday life. In addition, it opens the possibility of searching for conditions under which ordinary matter could transform into mirror matter. The transformation of neutrons into mirror neutrons is a process that attracts special interest. And here is why. All types of particles have a lifetime — the average time after which they decay. There are two types of experiments that measure the neutron lifetime are called ”bottle” and ”beam” measurements. In a bottle experiment, neutrons are trapped in a special container, and after some time scientists check how many remain. In a beam experiment, scientists create a stream of neutrons and observe how many of them decay by detecting the outgoing protons and electrons. Both experiments should give the same value for the neutron lifetime, but in reality there is a small difference between them. Surprising! It is as extraordinary as if measuring your height at home and on a bus gave different results. Perhaps this might be happening due to some neutrons partially transform into mirror neutrons. It is still not clear under exactly which conditions such a transformation could occur, but in theory this process may strongly depend on the magnetic field surrounding the particles. Therefore, experiments require strict control of magnetic fields. This is exactly what my bachelor thesis is about. In the experiments at Oak Ridge National Laboratory where scientists were looking for field conditions at which neutrons can convert into their mirror doubles, a neutron beam was placed in a controlled environment. The pipe through which the neutrons traveled was surrounded by an electromagnet, and around it a shield was built to protect the experiment from external magnetic fields. In other nearby experiments, strong electromagnets were used, which magnetized the roof and the support structures on which the experiment stood. Because of the design of the setup, it is impossible to measure the magnetic field directly inside of the beam tubes. My work is focused on reconstructing the magnetic field inside the installation using external measurements and computer simulations.

• author: Luzin, German
• supervisor: Valentina Santoro; Linus Persson; Yuri Kamyshkov
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: ESS, European Spallation Source, magnetic field reconstruction, neutron transition magnetic moment, sterile neutron searches, magnetic shielding, spherical harmonics, scalar magnetic potential, COMSOL Multiphysics, finite element method, HFIR GP-SANS, HIBEAM/NNBAR
• language: English
• id: 9242028
• date added to LUP: 2026-06-28 11:10:41
• date last changed: 2026-06-28 11:10:41