How blue-sky research shapes the future

In particle physics, the difference of a millimeter or two can make or break an experiment. In March, the LArIAT experiment began a proof-of-concept test to make sure the planned Deep Underground Neutrino Experiment (DUNE) will work well with that 2-millimeter difference.

Specifically, scientists are looking at what will happen when you increase the space between detection wires inside the future DUNE detectors.

DUNE will measure neutrinos, mysterious particles that are ubiquitous but elusive and may hold answers to questions about the origins of the universe.

Like the future DUNE detectors, LArIAT is filled with liquid argon. When a particle strikes an argon nucleus inside the detector, the interaction creates electrons that float through the argon until they’re captured by a wire, which registers a signal. Scientists measure the signal to learn about the particle interaction.

Unlike the DUNE detectors, LArIAT does not detect neutrinos. Rather, it uses the interactions of other particle types to make inferences about neutrino interactions. And very unlike DUNE, LArIAT is the size of a mini-fridge, a mere speck compared to DUNE’s detectors, which will hold about 22 Olympic-size swimming pools’ worth of liquid argon.

LArIAT scientists use a beam of charged particles provided by the Fermilab Test Beam Facility that are fired into the liquid argon. These particles interact with matter far more than neutrinos do, so the beam results in many more interactions than a similar beam of neutrinos, which would mostly pass through the argon. The higher level of interactions is what allows LArIAT to forgo the massive size of DUNE.

Results from LArIAT may help physicists better understand other liquid-argon neutrino detectors at the DOE Office of Science’s Fermilab such as MicroBooNE and SBND.

“The point of the LArIAT experiment is to measure how well we can identify the various types of particles that come out of neutrino interactions and how well we can reconstruct their energy,” said Jen Raaf, LArIAT spokesperson.

Although LArIAT doesn’t detect neutrinos, the charged-particle interactions can give scientists clues about how neutrinos interact with argon nuclei.

“Instead of sending a neutrino in and looking at what stuff comes out, you send the other stuff in and see what it does,” Raaf said.

Interactions in LArIAT are characterized primarily by a mesh of wires that detects the drift electrons. One key factor that affects the accuracy of drift-electron detection is the spacing between each wire.

“The closer together your wires are, the better spatial resolution you get,” Raaf said. But the more closely spaced the wires are, the more wires that are needed. More wires means more electronics to detect signals from the wires, which can become expensive in a giant detector such as DUNE.

To keep costs down, scientists are investigating whether DUNE will have a high enough resolution in its measurements of neutrino interactions with wires spaced 5 millimeters apart — larger than the 3-millimeter spacing in smaller Fermilab neutrino experiments such as MicroBooNE.

Simulations suggest that it should work, but it’s up to Raaf and her team to test whether or not 5-millimeter spacing will do the job.

LArIAT uses the Fermilab Test Beam Facility, which is an important part of the equation. The facility’s test beam originates from the lab’s accelerators and passes through a set of particle detection instruments before arriving at the LArIAT detector. Scientists can then compare the results from the first set of instruments with the LArIAT results.

“If you know that it was truly a pion going in to the detector, and then you run your algorithm on it and it says ‘Oh no that was an electron,’ you’re like ‘I know you’re wrong!’” Raaf said.  “So you just compare how often you’re wrong with 5 millimeters versus 3 millimeters.”

She and her team are optimistic, but committed to being thorough.

“It works in theory, but we always like to measure,” she said.

This research receives support from the Department of Energy Office of Science and the National Science Foundation.

My first year at Fermilab was 1968. I was a maintenance man when I first started, and we did all kinds of things in the early days: digging holes for trees, planting trees, shoveling snow, plowing snow, hauling garbage. We took care of furnaces, toilets, windows, window shades. We exchanged water bottles for drinking water, moved furniture, ran errands.

I worked on Robert Wilson’s car at times, which I enjoyed. We also did a lot of yard work out at Site 29. Wilson liked it clear, and we spent at least a good summer clearing the brush in the front of the house. There were lots of hawthorn trees in the front yard. The thorns in them would fall out, and our lawn tractor would get flat tires — constantly. So we made some steel wheels for the front of the tractor.

Our crew worked closely with Wilson. He was very charismatic, very friendly and outgoing. If he met you once, he remembered you by name. We’d have hot dog cookouts in our shop on Fridays, and our boss, George (I got called little George for forever), would always invite the directorate. They came over and had hot dogs with us, and we chatted.

I appreciated Wilson’s attitude toward the lab’s trees and landscaping. I was told you needed his permission to cut a tree down or even cut a limb from a tree. According to one story, Wilson moved the beam target areas because there was a group of trees at the targets’ planned location.

Wilson didn’t like things that reminded him of the war years. He didn’t like barbed wire. One of the early jobs I did was going around cutting barbed wire off the fences. He didn’t like fences, period, but particularly didn’t like barbed wire. In one case, right across from our office was a water tower with a fence around it and some barbed wire on top. I had to go cut that off. He wanted no part of it. Eventually, he took the fence and everything down. He also didn’t like trailers or Quonset huts – anything that was military-related. Word was he was just intense about that kind of thing.

As a director, he was completely geared to building an accelerator — very hands-on in that respect.

I vaguely remember the speech he gave in front of the old director’s complex. We were all standing out in the street. He said something to the effect that we had just received a few million dollars, and we were going to spend it all in the next couple of days so we could build our machine. That was his attitude.

He was building a laboratory, and we were going to build an accelerator — today.

George Davidson is the head of transportation services at Fermilab.

Plans for a new accelerator laboratory began in April 1963. Subsequent Aprils brought the completion of the Central Laboratory Building, the installation of the final Main Ring magnet and other important milestones. Read on.