The big questions: Mary Bishai on mining for neutrinos

The Mu2e experiment, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, reached an important milestone in November as the collaboration moved a key component of the experiment, called the tracker, across the Fermilab campus into Mu2e Hall.

“This is a major moment for Mu2e,” said Bob Bernstein, Mu2e co-spokesperson. “Soon, we’re going to be able to start looking at our first particle tracks from cosmic rays passing through all of our detectors.” 

Although the Standard Model of particle physics is scientists’ best explanation for how the universe works, it doesn’t account for phenomena like dark matter and dark energy. So, scientists are on the hunt for new physics. 

Subatomic particles called muons are in the same family as electrons: charged leptons. The Standard Model dictates that when a muon decays into an electron, two neutrinos are also produced. But physicists believe it’s possible that charged leptons convert directly into each other.  

Mu2e, as its name suggests, will be looking for the direct conversion of a muon into an electron without producing neutrinos. From previous experiments, scientists know that this hypothetical muon-to-electron conversion would be extremely rare. If the phenomenon occurs, it will happen less often than once every 1 trillion muon decays.

“And Mu2e is going to produce one muon for every grain of sand on Earth’s beaches, which is kind of an incomprehensible number of muons.”

“One of the keys to intensity frontier experiments is designing an experiment that enables us to look at a process very quickly and then repeat that billions, if not trillions of times,” said Brendan Kiburg, a project manager for the Mu2e tracker. “And Mu2e is going to produce one muon for every grain of sand on Earth’s beaches, which is kind of an incomprehensible number of muons.”

To handle so many muons, Mu2e has a unique, sinuous design. Three magnets make up the body of the experiment. The first magnet is called the production solenoid. This is where the muons are produced. The transport solenoid then delivers those muons into the detector solenoid that will hold Mu2e’s subdetectors. There, the muons encounter an aluminum target where they are stopped, captured and potentially undergo this hypothetical muon-to-electron conversion. The secondary particles produced will then encounter the first subdetector, called the tracker.

“The Mu2e tracker is an interesting construction because we didn’t want to have any mass on the inside of the tracker to interfere with the particles we’re trying to detect,” said Kiburg.

This design also helps the experiment weed out particles at the wrong energies.

As an electron enters the detector solenoid, the magnetic field will force the particle to spiral. If an electron has too low a momentum, the spiral is too tight, and it will never encounter the tracker, leaving the subdetector without ever interacting. But a higher momentum particle, which is more likely to be within the desired energy range, can spiral into the heart of the detector and produce a signal.

To complement the tracker, another subdetector, called the calorimeter, is installed downstream in the detector solenoid. While the tracker will have the precision to measure the momentum of the signal, the calorimeter will check its energy and timing, allowing the collaboration to definitively identify the particle as an electron. The calorimeter will also be an important step in cleaning up the particle tracks identified by the tracker.

The third subdetector is the cosmic ray veto that surrounds the detector solenoids and, as its name implies, helps identify cosmic ray tracks before they enter the tracker and calorimeter so that they can be easily eliminated.

“Thanks to this sophisticated detection system, we can beautifully reconstruct the candidate particle’s momentum,” said Stefano Miscetti, Mu2e co-spokesperson. “So, if we say this event is just an electron, there will be no doubt that we saw just an electron.”

Now that the tracker has joined the calorimeter and cosmic ray veto at the Mu2e Hall, the experiment can begin integrating the three subdetectors together to be read out by Mu2e’s data acquisition system.

At that point, the experiment will be able to start testing the ensemble of detectors with cosmic rays. This will enable the collaboration to calibrate the signals from the subdetectors before the beam is sent to the detector. “Mu2e is one of the most exciting experiments to start looking for new physics in the muon sector,” said Miscetti. “We have been working on this system for more than 10 years, and now we finally see the experiment taking shape.”

John Byrd has been named director of the U.S. Particle Accelerator School (USPAS). He succeeds Professor Steven Lund of Michigan State University, who has served as director since 2018.

USPAS is the nation’s premier training program in accelerator science and engineering. The school’s collaboration includes seven Department of Energy Office of Science laboratories, one DOE National Nuclear Security Administration laboratory and two universities.

USPAS is the nation’s premier training program in accelerator science and engineering. The school’s collaboration includes seven Department of Energy Office of Science laboratories, one DOE National Nuclear Security Administration laboratory and two universities.

“Being selected as the USPAS director is a true ‘circle-of-life’ moment for me,” said Byrd. “My career in accelerator science began in 1988, inspired by a USPAS course taught by Don Edwards and Mike Syphers, and it has been a privilege to witness and contribute to the field’s extraordinary development over the past three decades. It is both humbling and profoundly meaningful to now have the opportunity to give back to the community and institution that helped shape my professional life.”

After earning his Ph.D. in physics from Cornell University in 1991, Byrd spent 26 years at Lawrence Berkeley National Laboratory (LBNL), contributing to major accelerator facilities, including PEP-II at SLAC, the Advanced Light Source at LBNL, the Linac Coherent Light Source at SLAC, the Large Hadron Collider at CERN and the Free Electron Laser Radiation for Multidisciplinary Investigations at Elettra.

n 2017, he joined Argonne National Laboratory as division director for the accelerator systems division at the Advanced Photon Source (APS), leading efforts for the operation of the APS and contributing to the design, construction and commissioning of the APS upgrade. After retiring from Argonne in 2024, he worked on several industrial accelerator projects before accepting the position of USPAS director.

“The U.S. Particle Accelerator School has educated next-generation accelerator physicists and engineers for decades and through that, made sure that we can build ever better accelerators and develop new important technologies,” said Norbert Holtkamp, director of Fermi National Accelerator Laboratory.

Founded in 1981, USPAS convenes twice each year to offer a broad range of graduate-level courses through an intensive school format. USPAS offers a continually updated curriculum ranging from foundational accelerator science to advanced physics and engineering topics. In addition to meeting the workforce needs of national laboratories, the school educates students and professionals on the many applications of particle accelerators in areas such as science, medicine, security and industry. The training and documentation produced through USPAS sessions are broadly recognized for excellence and have had a profound and lasting impact on the accelerator community.

The USPAS office at Fermilab and the USPAS director are responsible for the operation and management of the school, coordinating across the entire accelerator community through the USPAS Institutional Board and the USPAS Director’s Advisory Council.

“The U.S. Particle Accelerator School has educated next-generation accelerator physicists and engineers for decades and through that, made sure that we can build ever better accelerators and develop new important technologies,”

“I am very pleased to hear this news,” said Cameron Geddes, director of the Accelerator Technology & Applied Physics Division at Lawrence Berkeley National Laboratory and a member of the USPAS Institutional Board. “John’s distinguished research perspective will advance the vital role of the school in providing accelerator physics education not available at most universities and help him leverage the contributions of lecturers from around the community.”

“Over the past 45 years, USPAS has become a cornerstone of education and training for national laboratories that rely on accelerator-based research,” said Zhirong Huang of SLAC National Accelerator Laboratory and Stanford University, and also chair of the USPAS Curriculum Committee and the Director Search Committee. “On behalf of the national laboratory and university communities, we extend our sincere thanks to the outgoing director, Steve Lund, for his eight years of dedicated leadership at USPAS. We have also valued our work with John Byrd through his many years of teaching USPAS classes, his years of service on the USPAS Director’s Advisory Council and Curriculum Committee, and we look forward to continuing our work with him as he steps into the role of the USPAS director, furthering this vital mission.”

Particle accelerators are a critical driver of both discovery science and industry that broadly enable the mission of the U.S. Department of Energy Office of Science. As some of the most complex, large-scale systems in the world, DOE’s accelerators play a key role in DOE’s new Genesis Mission, which aims to revolutionize the nation’s science and technology output by leveraging and extending state-of-the-art artificial intelligence. Genesis will open new capabilities and horizons in accelerator science and technology. The world-class training that USPAS provides will ensure that students, scientists and industry professionals in the accelerator field are ready for that future.

“The Office of High Energy Physics has proudly supported the U.S. Particle Accelerator School since the early 1980s, recognizing its critical role in cultivating the next generation of accelerator scientists and engineers,” said Regina Rameika, associate director for the Office of High Energy Physics within DOE’s Office of Science. “Our continued investment, notably through HEP’s General Accelerator Research Development program and the Accelerator Traineeship program, ensures USPAS remains an invaluable resource for developing the future accelerator workforce. We are particularly pleased with the appointment of John Byrd, whose three decades of dedicated involvement in accelerator research, leadership and workforce development make him an exceptional choice to guide USPAS into its next chapter.”

More information on the USPAS and the announcement can be found on the USPAS website.

Leaders with the U.S. Department of Energy’s Fermi National Accelerator Laboratory and the University of Campinas in Brazil, known as UNICAMP, met at Fermilab in November to sign an addition to their Inter-Agency Cooperative Research and Development Agreement, strengthening collaboration on key technologies for the Long-Baseline Neutrino Facility far detector cryogenics for the Deep Underground Neutrino Experiment.

A new project called Annex C, added to the existing agreement, brought together Fermilab’s then-Interim Director Young-Kee Kim, UNICAMP Rector Paulo Cesar Montagner and UNICAMP Vice-Rector Fernando Antonio Santos for the signing ceremony, as researchers and engineers from both institutions gathered to watch the event.

Annex C marks the beginning of the next phase of the collaboration between Fermilab and UNICAMP. Building on earlier successes, this phase focuses on engineering design, manufacturing and testing of cryogenic subsystems for DUNE’s vertical and horizontal drift far detector modules. The systems will be installed a mile underground at the Sanford Underground Research Facility in Lead, South Dakota — the site where DUNE will detect neutrinos sent 800 miles through the Earth from the Fermilab Accelerator Complex in Batavia, Illinois.

Phase one of the collaboration, signed in March 2020 in Brazil, focused on R&D for liquid-argon purification, a process critical for maintaining the extreme purity of the argon used in DUNE’s detectors.

Annex C now transitions that work into production. Under the agreement, UNICAMP’s industrial partner in Brazil will lead the construction of both liquid and gaseous argon purification and regeneration systems. Each separate system is referred to as a “skid,” and the gaseous purification skid will be used during initial filling of the massive detector modules.

“The international LBNF/DUNE project is an engineering marvel which will enable experimental studies in previously unavailable regimes of high energy physics,” said LBNF/DUNE-US Project Director Jim Kerby. “Building on a strong foundation, the past year has seen continued development of the technical bond between not just our institutions but also associated industrial partners. I look forward to our continued partnership and ultimately the incorporation of these key contributions to the experiment.”

LBNF/DUNE represents a truly global scientific effort, with more than 200 institutions from over 35 countries contributing expertise and hardware. UNICAMP’s role includes both photon detection and cryogenic purification technology, making it a key partner in realizing DUNE’s ambitious goals.

“Our research partners at UNICAMP are highly valued and reliable, and they are great collaborators,” said Fermilab Senior Engineer Roza Doubnik. “Their work has provided precise calculations and excellent 3D modeling, and they are open to suggestions during the collaboration process. We value and appreciate their professionalism and their approach to providing high-quality deliverables that advance the project.”

The signing of Annex C continues a partnership rooted in scientific curiosity, technological excellence and international collaboration. Together, Fermilab and UNICAMP are making strides to advance the frontiers of particle physics by preparing infrastructure that can help us to understand why there is more matter than antimatter in our universe.

The Dark Energy Survey (DES) collaboration is releasing results that, for the first time, combine all six years of data from weak lensing and galaxy clustering probes. In the paper, which represents a summary of 18 supporting papers, they also present their first results found by combining all four probes — baryon acoustic oscillations (BAO), type-Ia supernovae, galaxy clusters, and weak gravitational lensing — as proposed at the inception of DES 25 years ago.

“The methodologies that our team developed form the bedrock of next-generation surveys.”

“DES really showcases how we can use multiple different measurements from the same sky images. I think that’s very powerful,” said Martin Crocce, research associate professor at the Institute for Space Science in Barcelona and co-coordinator of the analysis. “This is the only time it has been done in the current generation of dark energy experiments.”

The analysis yielded new, tighter constraints that narrow down the possible models for how the universe behaves. These constraints are more than twice as strong as those from past DES analyses, while remaining consistent with previous DES results.

“There’s something very exciting about pulling the different cosmological probes together,” said Chihway Chang, associate professor at the University of Chicago and co-chair of the DES science committee. “It’s quite unique to DES that we have the expertise to do this.”

How to measure dark energy

About a century ago, astronomers noticed that distant galaxies appeared to be moving away from us. In fact, the farther away a galaxy is, the faster it recedes. This provided the first key evidence that the universe is expanding. But since the universe is permeated by gravity, a force that pulls matter together, astronomers expected the expansion would slow down over time.

Then, in 1998, two independent teams of cosmologists used distant supernovae to discover that the universe’s expansion is accelerating rather than slowing. To explain these observations, they proposed a new kind of energy that is responsible for driving the universe’s accelerated expansion: dark energy. Astrophysicists now believe dark energy makes up about 70% of the mass-energy density of the universe. Yet, we still know very little about it.

In the following years, scientists began devising experiments to study dark energy, including the Dark Energy Survey. Today, DES is an international collaboration of over 400 astrophysicists and scientists from 35 institutions in seven countries. Led by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, the DES collaboration also includes scientists from U.S. universities, NSF NOIRLab and DOE national laboratories Argonne, Lawrence Berkeley and SLAC.

To study dark energy, the DES collaboration carried out a deep, wide-area survey of the sky from 2013 to 2019. Fermilab built an extremely sensitive 570-megapixel digital camera, DECam, and installed it on the U.S. National Science Foundation Víctor M. Blanco 4-meter telescope at NSF Cerro Tololo Inter-American Observatory, a Program of NSF NOIRLab, in the Chilean Andes. For 758 nights over six years, the DES collaboration recorded information from 669 million galaxies that are billions of light-years from Earth, covering an eighth of the sky.

For the latest results, DES scientists greatly advanced methods using weak lensing to robustly reconstruct the distribution of matter in the universe. They did this by measuring the probability of two galaxies being a certain distance apart and the probability that they are also distorted similarly by weak lensing. By reconstructing the matter distribution over 6 billion years of cosmic history, these measurements of weak lensing and galaxy distribution tell scientists how much dark energy and dark matter there is at each moment.

“One of the most exciting parts of the final DES analysis is the advancement in calibrating the data,” said Alexandra Amon, co-lead of the DES weak lensing working group and assistant professor of astrophysics at Princeton University. “The methodologies that our team developed form the bedrock of next-generation surveys.”

In this analysis, DES tested their data against two models of the universe: the currently accepted standard model of cosmology — Lambda cold dark matter (ΛCDM) — in which the dark energy density is constant, and an extended model in which the dark energy density evolves over time — wCDM.

DES found that their data mostly aligned with the standard model of cosmology. Their data also fit the evolving dark energy model, but no better than they fit the standard model.

However, one parameter is still off. Based on measurements of the early universe, both the standard and evolving dark energy models predict how matter in the universe clusters at later times — times probed by surveys like DES. In previous analyses, galaxy clustering was found to be different from what was predicted. When DES added the most recent data, that gap widened, but not yet to the point of certainty that the standard model of cosmology is incorrect. The difference persisted even when DES combined their data with those of other experiments.

“What we are finding is that both the standard model and evolving dark energy model fit the early and late universe observations well, but not perfectly,” said Judit Prat, co-lead of the DES weak lensing working group and the Nordita Fellow at the Stockholm University and the KTH Royal Institute of Technology in Sweden.

Paving the way

Next, DES will combine this work with the most recent constraints from other dark energy experiments to investigate alternative gravity and dark energy models. This analysis is also important because it paves the way for the new NSF-DOE Vera C. Rubin Observatory, funded by the U.S. National Science Foundation and the U.S. Department of Energy’s Office of Science, to do similar work with its Legacy Survey of Space and Time (LSST).

“The measurements will get tighter and tighter in only a few years,” said Anna Porredon, co-lead of the DES Large Scale Structure working group and senior fellow at the Center for Energy, Environmental and Technological Research (CIEMAT) in Madrid. “We have added a significant step in precision, but all these measurements are going to improve much more with new observations from Rubin Observatory and other telescopes. It’s exciting that we will probably have some of the answers about dark energy in the next 10 years.”

More information on the DES collaboration and the funding for this project can be found on the DES website.