Scientists say: muon

On Dec. 18, the physics laboratory CERN in Geneva, Switzerland, celebrated the arrival of a very large parcel from the United States. Inside was a 13-meter-long assembly comprising two magnets with 4.2-meter-long coils. These are the first U.S.-built magnets for the high-luminosity upgrade to the Large Hadron Collider. Over the next few years, another nine assemblies will follow, thus completing a two-decades’ effort by a consortium of U.S. Department of Energy national laboratories—Fermilab, Brookhaven and Berkeley—to design and build new accelerator focusing magnets. These magnets, along with those from CERN, will be installed around two of the LHC’s collision points in two years’ time.

“In the realm of large scientific endeavors like the HL-LHC, global collaboration and expertise play pivotal roles. The delivery of the first cryo-assembly housing fully validated niobium–tin series magnets is a tangible testament to the success of the U.S. Accelerator Upgrade Project,” said Mike Lamont, CERN director for accelerators and technology. “This event not only marks a crucial milestone in our collaboration with our U.S. partners, but also celebrates the outstanding contributions shaping the future landscape of particle physics at CERN.”  

The Large Hadron Collider smashes protons and other atomic nuclei together at close to the speed of light, recreating conditions that existed shortly after the Big Bang. Scientists study these collisions to learn about subatomic particles and the fundamental laws of physics. They look for answers to some of the biggest questions in physics. What is the nature of dark matter? What happened to all the antimatter? How did particles acquire mass during the early universe?

The magnets built by the U.S. HL-LHC Accelerator Upgrade Project will tightly squeeze the two proton beams traveling in opposite directions around the LHC just before they collide. This tight squeeze will contribute to increasing the LHC’s collision rate by a factor of 5 over the original design value. Together with other upgrades, it will allow scientists to collect more data much faster than ever before. With these huge data sets, scientists will be able to study extremely rare subatomic events with high precision and explore phenomena beyond what the current LHC capabilities allow.

Upgrading the collision rate is no easy feat. Scientists needed to create magnets that are strong enough to focus and squeeze the proton beams of the LHC into much tighter bunches as they collide head on. So how are they doing it?

The answer is a rarely used and fussy superconducting material called niobium-3-tin, or Nb₃Sn. When cooled with liquid helium to minus 271.25 degrees Celsius, Nb₃Sn is an excellent superconductor and transports electricity without resistance. It is a type of superconductor that can reach higher magnetic fields than the standard niobium-titanium currently used in LHC superconducting magnets. The new magnets produce a maximum magnetic field of 12 tesla, roughly 50% more than the strength of the niobium-titanium focusing magnets currently in the Large Hadron Collider.

Scientists had never used Nb₃Sn in a large-scale accelerator project because it is extremely difficult to work with.

“It’s brittle like glass,” said Giorgio Apollinari, the head of the HL-LHC accelerator upgrade project at the U.S. Department of Energy’s Fermi National Accelerator Laboratory.  “When properly handled, glass can last for centuries, like in a cathedral window, but then one day you knock it, and it breaks. We spent a lot of time figuring out how to treat and handle this material and address its brittleness.”

In 2003, the U.S. Department of Energy established the LHC Accelerator Research Program to explore the properties of Nb₃Sn and figure out how to incorporate it into accelerator magnets. By 2015, scientists at participating labs were ready to move to production. The first HL-LHC cryo-assembly was completed in 2022 and successfully passed testing at Fermilab in 2023.

“Designing, building and successfully testing these advanced Nb₃Sn magnets with accelerator-quality features is a first for humanity and represents a major progress in accelerator technology,” said Apollinari. “But this initial success is only part of the story.”

Shipping magnets and their components among the US laboratories has been a backbone of the upgrade project. But according to Apollinari, safely sending a 25-ton cryo-assembly around the world is a challenge. “You can’t just go to the store and buy Styrofoam or bubble wrap to package it for shipment,” he said.

The magnet needed to arrive at CERN in mint-condition and incur no damage during the transit. So the magnet team started as any good scientific team would: with a dummy-magnet made from 25 tons of concrete and iron, and loaded with sensors.

“We drove a block of concrete around the U.S. and then sent it by ship to CERN,” Apollinari said. “The sensors were so good that we could tell when the truck was going over railroad tracks.”

From this data, they were able to design shipping equipment—their special version of “bubble wrap”—and a shipment plan that took into account how much force and acceleration the magnet could tolerate from the turbulence the magnet might experience during its journey.

After one month on land and at sea, the real magnet finally arrived at CERN in November 2023, where it is currently being evaluated. Eventually, CERN personnel will install this magnet—along with 7 more from the United States and 8 from CERN—100 meters underground in the LHC tunnel to focus the particle beams approaching the collision points of the two largest LHC experiments: ATLAS and CMS. The HL-LHC installation will start in 2025, with the plan to start colliding protons with the upgraded machine in 2029.

Said HL-LHC project leader Oliver Brüning, CERN: “The arrival of this first cold-mass assembly marks the start of a new phase of the CERN-US collaboration: the delivery of final cryo-assemblies ready for the installation in the LHC.”

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The 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, please visit science.energy.gov

Editor’s note: The following press release was issued on Dec. 8, 2023 by the American Physical Society announcing the 2023 Particle Physics Project Prioritization Panel report.

From Fermilab Director, Lia Merminga:
“Fermilab strongly and enthusiastically supports the P5 report in its entirety. The 2023 P5 report provides an ambitious, bold and balanced vision and roadmap for U.S. high-energy physics. It strongly endorses the ongoing physics program, and prioritizes the timely completion and scientific operations of ongoing construction projects; reaffirms the science of DUNE and recommends early implementation of the definitive long-baseline neutrino oscillation experiment; encourages us to collaborate with our international partners towards an offshore Higgs factory; and supports the HEP community’s aspirations to host a muon collider at Fermilab.

“Thank you to the High Energy Physics Advisory Panel and the Particle Physics Project Prioritization Panel members for all of their hard work and time in creating this document that will guide us for the next decade and ensure continued U.S. leadership in particle physics.”

The High Energy Physics Advisory Panel (HEPAP) to the High Energy Physics program of the Office of Science of the U.S. Department of Energy and the National Science Foundation’s Division of Physics has released a new Particle Physics Project Prioritization Panel (P5) report, which outlines particle physicists’ recommendations for research priorities in a field whose projects — such as building new accelerator facilities — can take years or decades, contributions from thousands of scientists, and billions of dollars.

The 2023 P5 report represents the major activity in the field of particle physics that delivers recommendations to U.S. funding agencies. This year’s report builds on the output of the 2021 Snowmass planning exercise — a process organized by the American Physical Society (APS)’s Division of Particles and Fields that convened particle physicists and cosmologists from around the world to outline research priorities. This membership division constitutes the only independent body in the United States that represents particle physics as a whole.

“The P5 report will lay the foundation for a very bright future in the field,” said R. Sekhar Chivukula, 2023 chair of the APS Division of Particles and Fields and a Distinguished Professor of Physics at the University of California, San Diego. “There are extraordinarily important scientific questions remaining in particle physics, which the U.S. particle physics community has both the capability and opportunity to help address, within our own facilities and as a member of the global high energy physics community.”

The report includes a range of budget-conscious recommendations for federal investments in research programs, the U.S. technical workforce, and the technology and infrastructure needed to realize the next generation of transformative discoveries related to fundamental physics and the origin of the universe. For example, the report recommends continued support for the Deep Underground Neutrino Experiment (DUNE), based out of Fermilab in Illinois, for CMB-S4, a network of ground-based telescopes designed to observe the cosmic microwave background, and for the planned expansion of the South Pole’s neutrino observatory, an international collaboration known as IceCube-Gen2, in a facility operated by the University of Wisconsin–Madison.

“In the P5 exercise, it’s really important that we take this broad look at where the field of particle physics is headed, to deliver a report that amounts to a strategic plan for the U.S. community with a 10-year budgetary timeline and a 20-year context. The panel thought about where the next big discoveries might lie and how we could maximize impact within budget, to support future discoveries and the next generation of researchers and technical workers who will be needed to achieve them,” said Karsten Heeger, P5 panel deputy chair and Eugene Higgins Professor and chair of physics at Yale University.

New knowledge, and new technologies, set the stage for the most recent Snowmass and P5 convenings. “The Higgs boson had just been discovered before the previous P5 process, and now our continued study of the particle has greatly informed what we think may lie beyond the standard model of particle physics,” said Hitoshi Murayama, P5 panel chair and the MacAdams Professor of physics at the University of California, Berkeley. “Our thinking about what dark matter might be has also changed, forcing the community to look elsewhere — to the cosmos. And in 2015, the discovery of gravitational waves was reported. Accelerator technology is changing too, which has shifted the discussion to the technology R&D needed to build the next-generation particle collider.”

The United States participates in several major international scientific collaborations in high energy physics and cosmology, including the European Council for Nuclear Research (CERN), which operates the Large Hadron Collider, where the Higgs boson was discovered in 2012. The P5 report recommends that the United States support a significant in-kind contribution to a new international facility, the ‘Higgs factory,’ to further our understanding of the Higgs boson. It also recommends that the United States study the possibility of hosting the next most-advanced particle collider facility, to reinforce the country’s leading role in international high energy physics for decades to come.

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The American Physical Society is a nonprofit membership organization working to advance and diffuse the knowledge of physics through its outstanding research journals, scientific meetings, and education, outreach, advocacy, and international activities. APS represents more than 50,000 members, including physicists in academia, national laboratories, and industry in the United States and throughout the world.

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