Test setup for LDMX HCal readout

Abstract

It is evident from cosmological observations such as the Cosmic Microwave Background, and the motion of galaxies, that the universe consists of approximately 5 times more mass than visible matter can account for. The Light Dark Matter eXperiment (LDMX) will take a unique approach in its hunt for dark matter; it will look for missing momentum signals in electron collisions with a Tungsten target, and will be looking in the sub GeV energy range for evidence of dark matter interactions. To achieve this, a sensitive hadronic calorimeter system will be required to detect the resulting particles from events that need to be ignored, or vetoed. The basic physical processes that need to be considered for hadronic calorimetry in general, as well as for the hadronic calorimeter in LDMX experiment specifically, are discussed. The experimental setup is presented, consisting of a Styron 665 W Polystyrene based plastic scintillator bar[1], with Wavelength Shifting Fibre (WLS) at the centre, and a Photomultiplier Tube
(PMT) at each end. Light output was optimized by grinding the ends of the fibres down, and by increasing the fibre volume inside the bar. Light-tightness was maximised by implementing a simple plastic coupling insert for the fibre, made using a 3D printer. After beta particles failed to produce signals, minimally ionizing cosmic ray muons were used as events to measure the scintillation light in the bar. To achieve this, an area of ≈ 1dm2 was covered with overlapping scintillator paddles, which then triggered an oscilloscope to measure the output at the fibre ends. The measured amplitudes were compared to those of apparent single photo-electrons measured by the PMTs.
A basic Python simulation was constructed to attempt to identify parameters that could be optimized for photon collection by the fibre, in slightly different configurations, based on the amount of light that was obtained from cosmic events.
Though there was not sufficient light yield for resolving energies, there was some measurable time resolution. A rough estimate of the event location along the bar axis could be made, which indicated the effective mid point was closer to one end of the bar at ≈ 650mm. Finally, an outlook on what the next objectives could be is discussed.

Popular Abstract

Light Dark Matter eXperiment (LDMX): Hadronic Calorimeter Test Setup
Before the first beam tests for LDMX, it is necessary to have an accurate characterization of the equipment utilized for detection. It is important to know what to expect with regards to performance, and precisely what signals to expect from the detection systems when searching for dark matter. Aside from human curiosity and the advancement of fundamental knowledge, applications of experiments like LDMX are many. From energy considerations and propulsion technology to computing ability; without fundamental research we would not have reactors, GPS, or LASERS. Even the internet as we know it is a by product of fundamental particle physics research.
BACKGROUND
High energy physics research has turned its attention increasingly beyond the known framework of the in-teractions of ordinary matter; the standard model. One glaring unanswered question is that of dark matter. Dark matter manifests itself, for example, by drastically contributing to the the mass of observed galaxies; that is, galaxies have a gravitational potential approximately six times larger than their visible mass accounts for. Many theories have been explored, but one largely unexplored mass range is that below the GeV scale. It is possible that dark matter particles belong to a ’hidden sector’ of fundamental physics, which could indicate a fifth fundamental force. LDMX supposes that the particles that mediate this force will have mass, which means that if they are radiated by a standard model particle, they will carry off some momentum. This missing mo-mentum will show up as a signal in the LDMX detectors, indicating that a dark matter interaction has occurred.
METHOD
LDMX will leverage equipment that has been developed for another experiment searching beyond the standard model: Mu2e. They both require equipment which can detect particles interacting minimally by ionization. Such particles are produced in LDMX as a result of unwanted interaction events, which should therefore be ignored in the detection system. This avoids less certain results and potential false positives, and increases the fraction of desirable events. A test setup has been constructed consisting of one bar of scintillating plastic, one metre in length, containing a plastic fibre. Ionizing particles create flashes of light in the bar material as they deposit their energy when traveling through the bar. The emitted photons then enter the fibre, which transmit them to each end of the bar, where a detector creates a signal pulse proportional to the energy deposited. The presence of such signals could then be used to veto events in the LDMX experiment, narrowing down the search for likely dark matter interactions.
RESULTS AND CONSEQUENCES
Cosmic ray muons are created when high energy particles called ’cosmic rays’ collide in the upper atmosphere, and decay. They were used as test particles because they are naturally occurring charged particles, have very high energy, and are plentiful. The setup was able to consistently register coincident signals from each end of the bar, in line with what was predicted by a simple computer simulation. At a first approximation, some res-olution of the position of events along the bar was measured. With higher precision amplification and photon detection equipment, more information and sensitivity will certainly be achievable.

• author: Greaves, Joshua
• supervisor: Ruth Pöttgen
• organization: Particle and nuclear physics; Department of Physics
• course: FYSK02 20192
• year: 2020
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Dark matter, LDMX, dark photon, hadronic calorimeter, photomultiplier tube
• language: English
• id: 9004323
• date added to LUP: 2020-02-11 14:59:58
• date last changed: 2020-02-11 14:59:58