Diagram showing the layout of the Mu2e detector

Mu2e an (almost) impossible experiment


Physicists at the University of Minnesota are building  an experiment that they readily admit is almost impossible. The Mu2e collaboration is a detector that will study a very rare process in particle physics, which Professor Ken Heller says is “like finding a single special grain of sand in a pile of sand covering the East Bank campus 2600 feet high.”

The Mu2e experiment watches muons that it traps in atomic orbits.  Muons are unstable elementary particles that usually decay into an electron and two neutrinos. A muon in orbit can decay into a single electron without any neutrinos only if there exist certain types of forces or particles beyond our knowledge. “We know there is physics beyond our current knowledge. We just don’t know what it is. We know there are dark matter and dark energy. We know neutrinos have mass. There’s a whole bunch of things that don’t fit into our current physics, but we’re trying to narrow down the directions to study the new physics,” Heller says.

“There are theories predicting that for every 1016 times that a muon decays into an electron into two neutrinos, we will get one muon into just an electron,” Heller says. “To see that one decay, we need to capture a whole lot of muons and watch them” which sounds simple enough but it involves building the world’s most intense muon beam at Fermilab, building a detector precise enough to actually tell that decay from the normal decays and creating software that can help physicists tackle the mountain of data produced.

Heller’s group is principally involved in building the detector though Kate Ciampa, a graduate student in the group, is also creating machine learning algorithms to analyze the data. Heller previously worked on MINOS and NOvA, two large neutrino experiments, which were composed of detectors the size of buildings made up of modules that were also built on Campus. Though these earlier experiments were looking for different particles, many of the principals are the same. “What we’re building is sort of like NOVA, but much smaller in scale but more precise. Unlike NOvA we don’t have to rent a giant warehouse because these modules are only 2 meters long.”

Students play vital part in MU2E

The MU2E modules are assembled in the Physics and Nanotechnology Building by graduate and undergraduate student research assistants working with students who have recently graduated. Currently there are 40 undergraduate students working on the experiment.

Isabelle Van Hattan, young woman wearing a lab coat, mask and shower cap standing next to a detector module

The modules are assembled in a warren of laboratory rooms, including extra space to accommodate CDC COVID requirements and PPE. Heller says the pandemic has cost both  extra time and money, that is being supplied by the U.S. Department of Energy. Each of the 250 modules begins as a pile of components and is assembled on large frames with wheels in a production line. “This is a similar manufacturing approach  to what we did on NOvA, but smaller in scale.”  Each roughly six foot long module looks like a high tech harp, something that might have been played by Lieutenant Uhura on Star Trek.  At every stage in assembly, I had the chance to chat with students who are working in pairs, one at each end of the modules (which are handily appropriate social distancing spacing) who work to position and solder the wires into place. It’s delicate work, and Isabelle Van Hatten, a junior majoring in Aerospace Engineering, said that, though she’d had some experience soldering before, she had never done anything so intricate and technical. “You mostly learn on the job, but you have to work with these very small details. You need a steady hand and good eyesight.”

The components arrive at the lab from a variety of sources, some parts come from local manufacturers such as Sheldahl or ProtoLabs, from Duke or City University of New York, and some from Fermi National Accelerator Laboratory. There is a drinking straw manufacturer in Ohio that made  the detector tubes which are called “straws”. The walls of these straws are about the thickness of a human cell, with an aluminum coating on one side and a coating of gold on the other. One of the jobs in the lab is to inspect the incoming straws for various possible failures, examples of which are posted on a wall in the lab called “The Straw Wall of Shame.”

Dr. Dan Ambrose, researcher in the group says that there is a big focus on quality control because of a limited ability to fix equipment once installed:“You need to make sure it gets done right the first time.” Dr. Ben Messerly, the other post doctoral researcher in the group, works closely with Dan to assure the detector quality.  The 22,000 straws that make up the detector are a concern since the membranes are so thin. “It’s a very delicate detector, but they are surprisingly sturdy once inflated,” Ambrose says. The tubes are threaded are thinner than a hair and inflated in order to subject them to a variety of leak tests.  As the detectors are built, they come under the capable eye of Kaitlin Boedigheimer,  a recent graduate from the School, who decided to delay her plans to attend graduate school, because of the pandemic and to work on the detector. To troubleshoot and guide the undergraduates she works with other recent graduates of the School, Farhan Abid, Mitch Frand, and Griffen Rizzo as well as Dick Wildberger who helped manage the NOvA detector construction.

The module “factory” is scheduled to run until the end of 2022. “When we get close to finishing the modules, we’ll send some of our people to Fermilab to finish the assembly,” as we produce it, just like we did with NOVA. “The schedule is to have it up and running by 2024. COVID is messing up everyone’s schedules, but that is the goal for now.” The group plans to get the last pieces of the experiment to Fermilab by 2023, test it for a year, and take data in 2024.