Newswise — The U.S. Department of Energy’s (DOE) Argonne National Laboratory made a major contribution to a high-profile experiment seeking to discover new physics. Hosted at DOE’s Fermi National Accelerator Laboratory (Fermilab), the Mu2e (muon-to-electron conversion) experiment aims to observe an extremely rare process in particle physics. The experiment is a multiyear collaboration among more than 30 institutions and 200 scientists from around the world.

An Argonne team of high energy physics scientists designed and — most recently — commissioned a crucial Mu2e subsystem called the Cosmic Ray Veto (CRV) detector. This subsystem filters out the biggest source of background noise in Mu2e. Background noise refers to signals that mimic the rare process that Mu2e seeks to detect. The commissioning tests demonstrated that the CRV detector is working properly and collecting background data.

The CRV’s development and deployment was a collaborative effort among Argonne, Fermilab, Kansas State University, University of South Alabama, University of Virginia, Northern Illinois University and University of Michigan.

“The CRV detector will enable Mu2e to more reliably and accurately detect an event expected to be vanishingly rare according to current particle physics theory,” said Yuri Oksuzian, an Argonne physicist who has played a key role in Mu2e and the CRV’s development. ​“The CRV is essential because it screens out background noise that could mimic this event. Observing even a few cases of the event would be compelling evidence of new physics.”

Searching for a muon-to-electron conversion

Since the 1970s, the dominant theory in particle physics has been the Standard Model. Widely considered a robust theory, the Standard Model describes the interactions among the fundamental particles and forces in the universe. But it leaves many big questions unanswered. For example, it cannot explain gravity or dark matter, a mysterious type of matter that cannot be observed directly.

As a result, particle physicists are searching for new theories, particles and forces beyond the Standard Model. The aim is to provide a more comprehensive understanding of the universe.

Mu2e seeks to observe a muon changing to an electron without any other particles being produced. A muon is a fundamental particle that is a heavier version of an electron. The Standard Model expects this transition to be so rare that observing it at Mu2e would be a major discovery and strong evidence of new physics.

“If Mu2e detects a muon-to-electron conversion, it would indicate that there’s a new particle or force involved in this process,” said Oksuzian. ​“The discovery would fundamentally change our understanding of how the universe works.”

Screening out cosmic-ray muons

The Mu2e apparatus directs a high-intensity beam of muons to a thin aluminum foil target. Detectors probe the target for conversion events, indicated by the presence of electrons with a precise amount of energy and momentum. Remarkably, Mu2e is expected to be 10,000 times more sensitive to conversions than previous similar experiments.

The apparatus has critical subsystems that measure background — in other words, signals that look like conversions but aren’t. A major background source is cosmic rays. These are high-energy particles from space that collide with atoms in the Earth’s atmosphere. The collisions produce showers of particles, including muons, that reach the ground.

Muons can penetrate the Mu2e apparatus and knock electrons from the aluminum foil. These free electrons can potentially have the exact energy and momentum of an electron from a conversion event. If that happens, Mu2e’s detectors will mistakenly register them as the real signal.

The CRV detector, engineered by Argonne, detects background events caused by cosmic-ray muons. It is essentially a giant cage that covers key parts of the Mu2e apparatus. It consists of 83 modules, which together weigh about 60 tons. The modules are made of thousands of plastic strips that produce light photons when muons pass through them.

Special fibers inside the strips carry the photons to sensors called silicon photomultipliers. The sensors measure the photons, indicating the exact time of the muon’s passage. If the CRV detects a cosmic muon just before an electron appears in the Mu2e apparatus, that electron is rejected from the data.

“We had to carefully design the CRV structure so that there are no gaps between the modules,” said Oksuzian. ​“The objective was to ensure that the detector would not miss any cosmic muons.”

Without the CRV, cosmic-ray muons would produce thousands of ​“fake” conversion events over Mu2e’s three-to-five-year run time. Because muon-to-electron conversions are so rare, even a small number of fake events would compromise Mu2e’s accuracy. As a result, the CRV must detect and reject 99.99% of cosmic-ray muons passing through. Argonne recently evaluated the CRV’s performance over a two-year period and found that it can meet this strict design requirement.

Commissioning the CRV at Mu2e

Recently, the Argonne team transported the CRV components to the Mu2e building at Fermilab in Batavia, Illinois, and commissioned them to detect cosmic-ray muons. The successful test enabled the CRV to fulfill a key DOE technical milestone and performance objective, helping to advance Mu2e’s commissioning. Other Mu2e subsystems will be commissioned and tested over the next year.

The experiment is expected to begin in 2027. Argonne scientists will have important roles in Mu2e operations, including the CRV, data acquisition system and analysis of datasets.

Besides Oksuzian, Argonne’s CRV team also includes Simon Corrodi, Sam Grant, Peter Winter and Lei Xia.

DOE’s Office of Science is a key supporter of Mu2e and Argonne’s CRV research and oversees Mu2e’s implementation.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.