Stumbling Through the Dark

Abstract

This project aims to study wide angle scattering events at LDMX through simulation. LDMX is a fixed target experiment searching for missing momentum of the beam electrons to identify dark matter signals. It consists of a tracking system around the thin tungsten target placed in a 1.5 T magnetic field, to measure the momentum of electrons before and after the target, and calorimeters (electromagnetic and hadronic) to obtain the energy of the electrons. In wide angle scattering events, the beam electron undergoes hard scattering in the target and misses the ECal so that it only produces a track in the recoil tracker and a signal in the side HCal. It is important to correctly identify this type of event to distinguish them from possible DM signals.

The project work includes the development of a configurable generator to produce wide angle scattering events, as well as the modification and expansion of the existing reconstruction method for the side HCal, which uncovered an existing issue that could be resolved. Furthermore, an energy weighting of reconstructed hits remains to be implemented to account for energy deposition in the absorber material.

The particle flow algorithm, matching signals in different detector components, is extended such that a track can be matched to an HCal cluster and a sample of 10,000 events is analysed. This analysis confirms the successful development of the generation, reconstruction and particle flow algorithms. It also reveals that the clustering algorithm needs to be optimised and a satellite clustering algorithm should be implemented to correctly identify wide angle scattering events. The configuration parameters of these two algorithms need to be adjusted to ensure their reliable performance on events with simultaneous electron signals (pileup events).

Popular Abstract

What is holding our universe together? Well, the simple answer to this is the gravitational force which lets the moon orbit the earth and keeps our solar system as well as our galaxy, the Milky Way from falling apart. However, the studies of these large scale objects have shown that the force binding them isn’t sufficient. So why does the moon then not fly off? The answer to this could be dark matter.

But what exactly is it? Imagine holding a bucket of water in your hand and spinning around. If you do not hold it tight enough, it will just glide out of your hand and fly off. Similarly, the moon would fly off from its orbit of the earth, the planets wouldn’t be in stable orbits in the solar system anymore and our whole galaxy would fly apart simply because the force holding them together is not sufficiently strong. Instead, if
you would hold the bucket firmly in your hand, it wouldn’t fly off and this extra strength is what we think dark matter is giving us.

Okay, but how do we prove that it exists? This is what the Light Dark Matter eXperiment (LDMX) will be built for. It hopes to find dark matter by the missing momentum of a created dark matter particle like a dark photon. Wait, missing momentum, dark photon? What are these things? We can describe the working principle through an analogy to billiard balls.

Imagine shooting the white ball towards the centre of the end of the billiard table. At the middle, the white billiard ball suddenly ejects out a black ball such that they both reach the end of the table at different positions. At the end, you will try to determine the position and energy of the billiard balls to obtain information about their momentum. However, you are only able to detect the deflected white ball at the end and
not the black one. How do you then know that the black one even exists, despite knowing its momentum? Well, if you know how the white ball was moving before and after ejecting the black ball, you will be able to infer information about the black one and will be able to say if it existed or not. LDMX will search for dark matter in a similar way. Instead of a white billiard ball, electrons are fired at a tungsten target in
the hope to emit dark matter particles (black billiard ball) and the momentum of the electron before and after the collision will be determined. This is done firstly, through a tracker before and behind the target which records its track. Afterwards, the electron will deposit its energy in a calorimeter and we can infer if something is missing. However, there are some cases, where the electron is deflected through interaction with the nuclei in the target such that it will miss the central part of our calorimeter, where it usually deposits its energy and instead it will end up in the side hadronic calorimeter. We refer to this as wide angle scattering events since the beam electron is scattered by a wide angle. Such events will look very similar to
the one where a dark matter signal (black billiard ball) is created as the initial beam electron is deflected in both events. In order to distinguish them, we need to measure the outgoing momentum and energy accurately.

The subject of the project work is to improve the abilities of LDMX to identify wide angle scattering events. Specifically, we first implement a simulator to create such events and we revise the reconstruction algorithm so that we can mimic how the energy deposition of the beam electron in the side HCal looks like. Furthermore, we develop a procedure to match the tracks of the beam electrons to the signals in the side HCal and
analyse how well it performs to find possible improvements. So overall, if we go back to our billiard analogy, we basically try to match the path of our white billiard ball with the information we gain from the end of the table so that we can later tell if a dark matter particle (black billiard ball) caused the deflection or if it was caused by a scattering process in the target.

• author: Rüber, Till
• supervisor: Lene Bryngemark; Ruth Pöttgen
• organization: Department of Physics; Particle and nuclear physics
• alternative title: Wide Angle Scattering Events at LDMX
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: LDMX, wide angle scattering event, dark matter, bachelor degree project, report
• language: English
• id: 9195974
• date added to LUP: 2025-06-10 12:11:57
• date last changed: 2025-06-10 12:11:57