LDMX - An Alternative Geometry for the Trigger Scintillator

Abstract

The Light Dark Matter eXperiment (LDMX) is an electron beam fixed-target experiment designed to probe dark matter candidates with masses below the GeV scale, which remains largely unexplored. The trigger scintillator is the detector subsystem mainly responsible for estimating the number of electrons per bunch in the beamline. The current geometry cannot distinguish electrons nearly aligned along the horizontal axis due to its bar configuration. The new geometry proposal intends to fix this by using a new set of vertical bars. Using the LDMX software simulation environment, a comparison of the electron counting and resolution along the horizontal and vertical axes was made. The new geometry improves the resolution along the horizontal axis while only slightly reducing the precision along the vertical axis. The electron counting was similar for the 1 electron/event case, but it led to considerably worse results for the 2 electron case. However, with further improvements to the algorithm, there is the potential to achieve similar results.

Popular Abstract

Most of the universe is invisible. Everything we can see like stars, planets, galaxies, and even ourselves, accounts for only a small fraction of what actually exists. The rest is believed to be made of something called dark matter, a mysterious form of matter that does not emit or absorb light, yet reveals itself through its gravitational effects. Despite decades of research, its true nature remains one of the biggest unsolved problems in modern physics.

Over the years, scientists have built powerful experiments to try to detect dark matter directly. Many of these searches have focused on relatively heavy particles, but so far, no clear signal has been found. This has led researchers to explore new possibilities, including the idea that dark matter might instead consist of much lighter particles. This is the focus of the Light Dark Matter eXperiment (LDMX), which aims to explore a largely uncharted region of this problem.

In LDMX, a beam of high-energy electrons is directed onto a thin target. When these electrons interact with the material, new particles can be produced. Most of these interactions are well understood, but if dark matter is created, it would pass through the detector without leaving a direct signal. Instead, its presence must be inferred indirectly. By carefully measuring the energy and motion of the particles before and after the collision, physicists can look for “missing” energy, an indication that something invisible may have been produced.

To make this kind of measurement reliable, the experiment must know precisely how many electrons enter the collision and where they are located. This is where a key component of the detector, called the Trigger Scintillator, becomes essential. Its role is to detect incoming electrons and estimate both their number and their position just before they hit the target. This information allows scientists to establish a baseline: how much energy is entering the system. After the interaction, the remaining energy is measured again, and any discrepancy could point to the production of dark matter.

However, achieving this level of precision is not trivial. The current detector design can measure particle positions accurately in one direction but has limited resolution in the other. This makes it harder to reconstruct the exact paths of incoming electrons, which in turn reduces the experiment’s sensitivity. If electrons are miscounted or their positions are not well determined, ordinary processes can mimic the signal of dark matter, making it more difficult to distinguish real discoveries from background noise.

This work focuses on improving this aspect of the detector. Specifically, it studies how modifying its internal structure can enhance its ability to track particles. By introducing an additional layer that measures positions in a complementary perpendicular direction, the detector can achieve a more complete picture of each electron’s trajectory. This improvement can not only increase the accuracy of electron counting but also provide more detailed information for reconstructing the events that occur after the collision.

These seemingly small design changes can have a significant impact. A more precise detector reduces uncertainty, improves the reliability of measurements, and increases the chances of identifying rare events. In experiments like LDMX, where scientists are searching for extremely subtle signals, even modest improvements in performance can make the difference between seeing nothing and uncovering new physics.

Ultimately, this work contributes to a much larger goal: understanding what the universe is made of. By refining the tools used to search for dark matter, we take another step toward answering a fundamental question that has puzzled scientists for nearly a century. If successful, experiments like LDMX could open a new window into the invisible majority of the universe and reshape our understanding of its most basic components.

• author: Roque Fernandes, Ricardo
• supervisor: Lene Bryngemark; Clay Barton
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: LDMX, Trigger Scintillator, Light Dark Matter eXperiment, Alternative Geometry
• language: English
• id: 9241777
• date added to LUP: 2026-06-28 10:47:58
• date last changed: 2026-06-28 10:47:58