Neutrinos are elusive subatomic particles that may have played a key role in the evolution of the universe. Scientists study them by capturing their interactions in giant particle detectors. The particles that emerge from these interactions can leave tracks in the detectors. The science of tracing those tracks is as subtle as neutrinos themselves. Scientists developed algorithms that clarify the signal that neutrinos leave behind in this school-bus-sized detector.
If you want sharp images from a camera using film, it’s important to get the chemistry right. If you want clear images from neutrino detectors, it’s vital to get the algorithms right. Here, researchers conquered significant challenges in tracking neutrinos in the detector. Their work provides cleaner images of how particles move. It also offers a solid foundation for using the technology for precision physics measurements that can offer insights into the nature of the universe.
The central piece of the MicroBooNE detector is a liquid-argon time projection chamber (LArTPC) filled with argon in liquid form at minus 303 degrees Fahrenheit and lined with electronics designed to operate in that extremely cold environment. This assembly acts like a powerful 3D digital camera to capture the trajectories of particles generated when neutrinos interact with argon atoms in the tank.
By tracking the timing and locations of these signals with complex algorithms, the detector can construct images of the electrons’ trajectories to reveal information about the energy and flavor of the neutrino that triggered each chain of events. Just as it’s important to get the chemistry right when processing film, neutrino-tracking scientists face challenges in developing their algorithms. Scientists at Brookhaven National Laboratory, part of the MicroBooNE Collaboration, have conquered two significant challenges in tracking neutrinos in their detector.
For one thing, the signals are relatively small. It is particularly difficult to discriminate the tiny signals from background “noise." By designing some of the electronics so that they can operate inside the cold liquid-argon chamber, the team helped to minimize noise by reducing the distance signals have to travel before being read out. These low-noise “cold electronics” are a crucial technology for large LArTPCs.
A second challenge is that drifting electrons can cancel key signals. These cancellations depend on the distribution of ionization electrons, leading to highly complex signals. To address this challenge, the team developed an algorithm to extract the distribution of electrons from the measured induced current on the wires.
The team also developed significantly improved simulations of expected LArTPC signals and noise. Comparing the simulations with results extracted from real data produced consistent results, which is a crucial step toward using the detector for future physics studies.
Brookhaven National Laboratory
This work was supported by the Department of Energy, Office of Science, Offices of High Energy Physics and Office of Nuclear Physics; the National Science Foundation; the Swiss National Science Foundation; the Science and Technology Facilities Council of the United Kingdom; and The Royal Society (United Kingdom). Additional support for the laser calibration system and cosmic ray tagger was provided by the Albert Einstein Center for Fundamental Physics, Bern, Switzerland.
MicroBooNE Collaboration, “Ionization electron signal processing in single phase LArTPCs. Part I. Algorithm description and quantitative evaluation with MicroBooNE simulation.” Journal of Instrumentation 13, P07006 (2018). [DOI: 10.1088/1748-0221/13/07/P07006]
MicroBooNE Collaboration, “Ionization electron signal processing in single phase LArTPCs. Part II. Data/simulation comparison and performance in MicroBooNE.” Journal of Instrumentation 13, P07007 (2018). [DOI: 10.1088/1748-0221/13/07/P07007]
Brookhaven National Laboratory feature article: Extracting Signals of Elusive Particles from Giant Chambers Filled with Liquefied Argon