MicroBooNE detects accelerator-born neutrinos

The MicroBooNE collaboration announced on November 2 that its researchers had recorded neutrinos for the first time. The sighting marks the beginning of studies detailing neutrino links to dark matter, matter’s dominance over antimatter in the universe and the evolution of the entire cosmos since the Big Bang. It also brings researchers closer to one of the project’s scientific goals, determining whether an excess of low energy events observed in a previous Fermilab experiment was the footprint of a new, sterile neutrino. “This was a big team effort,” said postdoc Anne Schukraft of Fermilab, one of 28 institutions contributing to the experiment, including five DOE national labs (Brookhaven, Fermilab, Los Alamos, Pacific Northwest and SLAC). “More than 100 people have been working very hard to make this happen. It’s exciting to see the first neutrinos.”

MicroBooNE is located at DOE’s Fermi National Accelerator Laboratory (CSA CSM) and is part of the lab’s Short-Baseline Neutrino program. Scientists will eventually add two more detectors (ICARUS and the Short-Baseline Near Detector) to its neutrino beamline. On October 15, the Fermi National Laboratory Booster team began delivering protons to MicroBooNE. After the beam was turned on scientists analyzed the data recorded by the particle detector to find evidence of its first neutrino interactions. At less than 800 MeV (megaelectronvolts), the beam produces the lowest energy neutrinos yet to be observed with a liquid-argon detector.

Neutrinos recently garnered attention when the 2015 Nobel Prize in physics was awarded to scientists studying neutrino oscillations, a phenomenon that is of great importance to the field of elementary particle physics. Over the next three years MicroBooNE researchers will search for additional sources of neutrino oscillation beyond those already known. MicroBooNE will also serve as a test bed for the 500-times-more-sensitive Deep Underground Neutrino Experiment (DUNE) that will study the metamorphoses of neutrinos during an 800-mile journey through the Earth from Fermilab in Illinois to the DUNE detector in South Dakota.

“Future neutrino experiments will use this technology,” said Sam Zeller, Fermilab physicist and MicroBooNE spokesman. “We’re learning a lot from this detector. It’s important not just for us, but for the entire neutrino community.”

Scientists, for example, are using MicroBooNE as an R&D platform for DUNE’s large liquid-argon detectors. At the heart of MicroBooNE lies a time projection chamber, a detector the size of a city bus and enclosed within a cryostat that holds 170 tons of frigid liquid argon. Liquid argon is 40 percent denser than water, and hence neutrinos are more likely to interact with it. When an accelerator-born neutrino hits the nucleus of an argon atom in the detector its collision creates a spray of subatomic particle debris. These particles, caught in the strong electric field within the time projection chamber, drift towards the detector’s wire electrodes, producing tiny electric currents. By mapping the pattern of currents, scientists can reconstruct a three-dimensional picture of the particle trajectories and use these trajectories to deduce information about the neutrinos that created them.

The combination of the MicroBooNE detector’s extreme temperature and size posed substantial technological and design challenges for the scientists who built it. Signal processing in smaller detectors, for example, can occur outside of the cryogenic chamber, but in large detectors like MicroBooNE, the long cables required to carry the signals from the electrodes to the electronics outside the chamber introduce excess “noise”—distortion created by excess charge stored in the wire—that can drown out tiny signals. Charged particles in large detectors must also also travel a longer distance between the interaction point and the electrodes that lie along a wall in the detector. This distance increases the likelihood that particles will recombine with other atoms in the detector.

An engineering team from Brookhaven National Laboratory solved the signal processing problem for MicroBooNE with cold electronics, nestling 50 circuit boards packed with microelectronics into the frigid cryostat. The sensing wires are arranged in three planes with different orientations to give three different views of the particle tracks. “MicroBooNE is the first instrument in which signal processing is achieved with cold electronics,” said Mary Bishai, a Brookhaven scientist specializing in neutrino physics. “This is a necessary first step to get to detectors that are tens of kilotons larger.”

Engineers developing kiloton detectors, such as those proposed for DUNE, will also face the challenge of designing containers both large and robust enough to hold tons of liquid argon. Most cryostats have a double-wall design similar to that of an insulated thermos. Liquid argon is placed inside an inner container isolated from an outer container by a vacuum layer. MicroBooNE engineers, faced with cooling a city bus sized detector made of stainless steel and fiberglass, opted for a different, less expensive design. “We constructed a single walled cryostat and then built foam insulation around it to minimize the heat leak,” said Hucheng Chen, a Brookhaven physicist who coordinated the design and construction of the detector read-out system in MicroBooNE. The design is also a first of its kind.

Now that the detector is operational, researchers hope to soon shift gears from design to interpretation. Sifting through vast amounts of MicroBooNE data, however, will be a challenge for both man and machine. For each 500 pulses of the neutrino beam, only one neutrino interaction might occur while approximately 24 cosmic muons will flow through the detector during the time it takes for each trigger. Different types of particles and interactions also follow unique trajectories. High-energy and relatively heavy cosmic muons, for example, usually follow long, relatively straight paths through the detector, while an interaction between an electron neutrino and an argon atom will generate relatively light electrons that can bounce around and produce more electrons and positrons, resulting in a big shower of particles.

Xin Quin, a Brookhaven physicist, is leading the effort to develop a computer system capable of interpreting one real neutrino track from more than 12,000 potential trajectories. “When you have an image, humans can actually process it pretty well, but to get a computer to achieve this level of pattern recognition is very difficult,” said Qian. “This is a very hard problem to solve, but once we figure it out, we can start to do the real physics research MicroBooNE was designed for.”