HCal bar calibration with cosmics at LDMX

Abstract

This thesis aims at understanding, through simulation, how the cosmic muon flux will look like in LDMX’s HCal and to use the characteristics of minimally ionizing muons to set up a method for scintillator bar calibration. LDMX’s HCal is a sampling calorimeter with scintillator bars as the active material. Cosmic muons enter the HCal from different angles, and thereby deposit different amounts of energy in the detector. This thesis implements a method for taking this directional dependence into account. A tracking algorithm is constructed that finds the straight path of a minimally ionizing particle; muons in this case. The algorithm is then used to estimate the energy deposited in a single bar, and is compared to the actual energy deposition. The ratio between estimated and actual energy in a bar is found to be 0.724 (0.233 MeV/mm vs 0.170 MeV/mm). This discrepancy requires further investigation. Based on the geometry of the back-HCal and a known muon flux, the back-HCal is found to be hit by 330,167 muons/min. With a requirement of every bar in the back-HCal having to register 1,000 hits each for calibration, and trigger selections for MIP-like events, it would take a runtime of 6,058 minutes, or 101 hours to collect the required dataset.

Popular Abstract

Since the dawn of time, from religion to science, mankind has always been trying to explain the unexplainable. Scientists have at multiple points throughout history said that everything that can be known is known. However, nature has always proved them wrong. During the first half of the 20th century, scientists were observing stars orbiting galaxies. Some stars orbited galaxies at higher velocities than what should have been possible. That the stars weren’t escaping the gravitational pull of the galaxy lead to the conclusion that there was some unknown gravitational influence. This would come to be called dark matter. In recent years, dark matter searches have been performed at the leading high-energy particle physics institutes, with little advancement. These searches have mostly been looking for a dark matter particle in the mass range of GeV- TeV. The Light Dark Matter eXperiment (LDMX) plans to search for dark matter in a lower mass range of MeVGeV, thereby the name light dark matter. LDMX uses a theorised dark matter interaction called dark bremsstrahlung. Regular bremsstrahlung occurs when an electron interacts with the nucleus of an atom, and consequently emits a photon. The theory is that this process occurs for dark matter as well, with a resulting dark photon. The dark photon then decays into two dark matter particles, which we can’t observe in our detectors. What can be done, is to look for what is missing. If we know everything that goes into our detector, then we know what we should expect to leave our detector. By measuring a difference in momentum, it is possible to claim that it must have gone into a dark photon. LDMX’s detector consists of multiple components for observing particles. One such is the hadronic calorimeter (HCal) that detects heavy particles, such as the neutron. While it is not the HCal’s primary objective, it can detect a particle called the muon. By measuring where the particle interacts with the HCal, and deposits energy, one can look for subsequent interactions and build a track reconstruction. Muons are very important for LDMX, even though they don’t tell us much directly about dark matter, they can be used as a primary source for the calibration of the HCal. The very best thing about muons is that we can get them for free, without even having to run the experiment. Cosmic muons are muons that come from outer space; they come from decay of cosmic radiation such as pions or kaons, and would hit our detector about 300,000 times per minute. Myprojects looks at what muons would look like in LDMX’s detector, through simulation. Muons enter the HCal from many different angles, and thereby deposit different amounts of energy in the detector. I looked at this angular dependence. Furthermore, a tracking algorithm was constructed that finds the straight path of a muon. This algorithm was then used to estimate the energy deposited in a single bar, a necessary step in calibration of the detector. With this information, I also looked at how long time it would take to calibrate the detector.

• author: Pettersson, Andreas
• supervisor: Ruth Pöttgen; Lene Bryngemark
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: LDMX, calibration, muons, cosmics, HCal, "dark matter"
• language: English
• id: 9195143
• date added to LUP: 2025-06-09 12:14:42
• date last changed: 2025-06-09 12:14:42