Design of an optics system for the short-HIBEAM beamline at the European Spallation Source

Abstract

Dark matter remains a largely elusive phenomenon, particularly its composition. Several candidate particles have been proposed, including axions and axion-like particles (ALPs). Detection of axions and ALPs have been attempted but none so far have yielded positive results. The High Intensity Baryon Extraction and Measurement instrument (HIBEAM) at the European Spallation Source (ESS) will be used to attempt ALP detection using Ramsey interferometry with increased sensitivity by 2-3 orders of magnitude compared to current performance of other experiments. In order to maximize the sensitivity of the search, HIBEAM’s neutron guide must be optimized. The optimization of this guide is the subject of this thesis. The work was carried out using McStas, a neutron ray-tracing simulation framework that employs Monte Carlo sampling to model neutron transport through instrument components. The optimal guide specifications were determined to be a supermirror reflectivity of m = 4 , together with a selective collimation of the high-intensity region of the beam placed 15 m from the source to match the 10 cm diameter constraint of the polarizer at the end of the neutron guide section. These specifications were determined based on the maximization of a simplified figure of merit NT^2, where N is the neutron intensity and T is the time of flight in the neutron guide. By maximizing this quantity, the uncertainty in the Ramsey fringes is minimized.

Popular Abstract

Dark matter has been a prominent concept in physics for the past few decades, but what is dark matter really? To this day, it remains one of physics’ most mysterious phenomena. Although dark matter cannot be observed directly, we know that it exists due to how it affects classical matter and light. For example, large astronomical bodies can bend light, due to their gravitational pull. However, in some cases, the bending of light is much greater than what is expected from the amount of regular matter. This has led scientists to form the theory of dark matter, which is matter that we cannot observe but that clearly has a gravitational pull. Since dark matter can interact with classical matter, this means that we can leverage this property to learn more about what dark matter is made of.

Classical matter is made up different types of sub-particles, which is described by the Standard Model of physics. For instance, a proton is made up of two Up quarks and one Down quark. Scientists believe that dark matter most likely is also made up of some kind of subparticle and seek to establish a model for dark matter analogous to the Standard Model. Dark matter sub-particles have not yet been observed, but scientists have formulated different types of theoretical particles they believe might exist. One of these particles is the Axion-Like Particle (ALP), which will be investigated by the HIBEAM project at ESS. The HIBEAM project will use neutrons to determine if ALPs exist, using a method called Ramsey interferometry.


Neutrons behave like tiny magnets. When placed in a magnetic field, their spins precess at a well-defined frequency, similarly to how a spinning top rotates around the vertical direction under gravity. Ramsey interferometry is a technique that measures this precession frequency with extremely high precision. If an axion-like particle interacts with the neutrons, it can slightly modify this frequency, producing a measurable shift in the interference pattern known as the Ramsey fringes. This difference between the original frequency and the new one is called detuning. Of course, there are many factors which could affect the frequency, which is why we are interested in recording a range of different frequencies.

By plotting the different amounts of detuning against their respective probabilities, we obtain a graph showing what are known as Ramsey fringes. The central fringe (which is also the most probable one) should be located exactly at the neutron’s natural frequency. If an ALP has interacted with a neutron, the entire Ramsey fringe graph would be shifted, which would allow us to determine that an ALP had interacted with the neutron. This technique is incredibly important as it is on this principle that modern atomic clocks are based, which are used for setting the standard units of time.


This work requires extremely precise measurements to be able to determine whether there was a shift in the Ramsey fringes. This thesis work aimed to increase the sensitivity and precision of the experiment by optimizing the beamline through which the neutrons are guided from the neutron source to the experimental detector. This optimization task was carried out by using computer simulations and testing out various configurations of the beamline to find the best design (within the scope of this thesis).

ESS is set to be the brightest neutron source in the world and HIBEAM is expected to improve sensitivity for this experiment anywhere from 10 to 100 times compared to previous experiments. Coupled with the optimized beamline, the chances of detecting an ALP are increasingly high! If the HIBEAM project successfully detects an ALP, this would reshape our understanding of particle physics and cosmology and potentially usher in a new golden era of physics.

• author: Kuhl, Marianne Pauline
• supervisor: Valentina Santoro; Matthias Holl; Linus Persson
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: ess, hibeam, axions, dark matter, mcstas, neutron, ramsey interferometry
• language: English
• id: 9241726
• date added to LUP: 2026-06-28 11:05:24
• date last changed: 2026-06-28 11:05:24