MicroBooNE begins recording neutrinos

Scientists celebrate the arrival of the first

neutrino beam at the MicroBooNE detector.

October is a fitting month for ghost sightings. Halloween came early for scientists of the MicroBooNE collaboration, who recorded their first neutrinos, often called ghost particles, on Oct. 15.

"It's nine years since we proposed, designed, built, assembled and commissioned this experiment," said Bonnie Fleming, MicroBooNE co-spokesperson and a professor of physics at Yale University. "That kind of investment makes seeing first neutrinos incredible."

MicroBooNE is located at DOE’s Fermi National Accelerator Laboratory. Last month, the lab's accelerator complex began delivering neutrinos to the new experiment. MicroBooNE scientists immediately began to analyze the data recorded by their particle detector to find evidence of its first neutrino interactions.

"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's detector is a liquid-argon time projection chamber. It resembles a silo lying on its side, but instead of grain, it's filled with 170 tons of 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. Tracking these particles allows scientists to reveal the type and properties of the neutrino that produced them.

Neutrinos have recently received quite a bit of media attention. The 2015 Nobel Prize in physics was awarded for neutrino oscillations, a phenomenon that is of great importance to the field of elementary particle physics. Intense activity is under way worldwide to capture neutrinos and examine their behavior of transforming from one type to another.

MicroBooNE is an example of a new liquid-argon detector being developed to further probe this phenomenon while reconstructing the particle tracks emerging from neutrino collisions as finely detailed three-dimensional images. Its findings will be relevant for the forthcoming Deep Underground Neutrino Experiment, known as DUNE, which plans to examine neutrino transitions over longer distances and a much broader energy range. Scientists are also using MicroBooNE as an R&D platform for the large DUNE liquid-argon detectors.

"Future neutrino experiments will use this technology," said Sam Zeller, Fermilab physicist and MicroBooNE co-spokesperson. "We're learning a lot from this detector. It's important not just for us, but for the entire neutrino community."

In August, the experiment saw its first cosmic ray events, recording the tracks of cosmic ray muons. The first “ghost sightings” bring MicroBooNE researchers much closer to one of their scientific goals, determining whether the excess of low-energy events observed in a previous Fermilab experiment was the footprint of a sterile neutrino or a new type of background.

Before they can do that, however, MicroBooNE will have to collect data for several years.

During this time, MicroBooNE will also be the first liquid-argon detector to measure neutrino interactions from a beam of low energy. At less than 800 MeV (megaelectronvolts), the beam produces the lowest-energy neutrinos to be observed with a liquid-argon detector yet.

MicroBooNE is part of Fermilab's Short-Baseline Neutrino Program , and scientists will eventually add two more detectors (ICARUS and the Short-Baseline Near Detector) to its neutrino beamline. For now, it'll spot ghosts by itself. – Chris Patrick

Submitted by DOE’s Fermi National Accelerator Laboratory