Editor’s note: The following press release was issued by Brookhaven National Laboratory, one of 34 collaborating institutions in seven countries working on the new Muon g-2 experiment at Fermilab. The Muon g-2 experiment completed construction on schedule and under budget in January 2018. The collaboration is now taking physics data with Fermilab’s powerful beamline and will use it to study muons with unprecedented accuracy. By comparing real-world results to precise predictions, physicists can determine whether new physics is afoot. The results reported here are part of a global Muon g-2 Theory Initiative launched at Fermilab last June.
Latest calculation based on how subatomic muons interact with all known particles comes out just in time for precision measurements at new “Muon g-2” experiment
Theoretical physicists at Brookhaven National Laboratory recently published in Physical Review Letters a calculation related to the way muons interact with all other known particles through three of nature’s four fundamental forces, reducing the greatest source of uncertainty in the prediction. The result comes out just in time for precision measurements at Fermilab’s Muon g-2 experiment, pictured here. Photo: Reidar Hahn
UPTON, NY—Theoretical physicists at the U.S. Department of Energy’s Brookhaven National Laboratory and their collaborators have just released the most precise prediction of how subatomic particles called muons—heavy cousins of electrons—“wobble” off their path in a powerful magnetic field. The calculations take into account how muons interact with all other known particles through three of nature’s four fundamental forces (the strong nuclear force, the weak nuclear force and electromagnetism) while reducing the greatest source of uncertainty in the prediction. The results, published in Physical Review Letters, come just in time for the start of a new experiment measuring the wobble now under way at DOE’s Fermi National Accelerator Laboratory.
A version of this experiment, known as Muon g-2, ran at Brookhaven Lab in the late 1990s and early 2000s, producing a series of results indicating a discrepancy between the measurement and the prediction. Although not quite significant enough to declare a discovery, those results hinted that new, yet-to-be discovered particles might be affecting the muons’ behavior. The new experiment at Fermilab, combined with the higher-precision calculations, will provide a more stringent test of the Standard Model, the reigning theory of particle physics. If the discrepancy between experiment and theory still stands, it could point to the existence of new particles.
“If there’s another particle that pops into existence and interacts with the muon before it interacts with the magnetic field, that could explain the difference between the experimental measurement and our theoretical prediction,” said Christoph Lehner, one of the Brookhaven Lab theorists involved in the latest calculations. “That could be a particle we’ve never seen before, one not included in the Standard Model.”
Finding new particles beyond those already cataloged by the Standard Model has long been a quest for particle physicists. Spotting signs of a new particle affecting the behavior of muons could guide the design of experiments to search for direct evidence of such particles, said Taku Izubuchi, another leader of Brookhaven’s theoretical physics team.
“It would be a strong hint and would give us some information about what this unknown particle might be — something about what the new physics is, how this particle affects the muon and what to look for,” Izubuchi said.
The muon anomaly
The Muon g-2 experiment measures what happens as muons circulate through a 50-foot-diameter electromagnet storage ring. The muons, which have intrinsic magnetism and spin (sort of like spinning toy tops), start off with their spins aligned with their direction of motion. But as the particles go ’round and ’round the magnet racetrack, they interact with the storage ring’s magnetic field and also with a zoo of virtual particles that pop in and out of existence within the vacuum. This all happens in accordance with the rules of the Standard Model, which describes all the known particles and their interactions, so the mathematical calculations based on that theory can precisely predict how the muons’ alignment should precess, or “wobble,” away from their spin-aligned path. Sensors surrounding the magnet measure the precession with extreme precision so the physicists can test whether the theory-generated prediction is correct.
Both the experiments measuring this quantity and the theoretical predictions have become more and more precise, tracing a journey across the country with input from many famous physicists.
A race and collaboration for precision
“There is a race of sorts between experiment and theory,” Lehner said. “Getting a more precise experimental measurement allows you to test more and more details of the theory. And then you also need to control the theory calculation at higher and higher levels to match the precision of the experiment.”
With lingering hints of a new discovery from the Brookhaven experiment — but also the possibility that the discrepancy would disappear with higher-precision measurements — physicists pushed for the opportunity to continue the search using a higher-intensity muon beam at Fermilab. In the summer of 2013, the two labs teamed up to transport Brookhaven’s storage ring via an epic land-and-sea journey from Long Island to Illinois. After tuning up the magnet and making a slew of other adjustments, the team at Fermilab recently started taking new data.
Meanwhile, the theorists have been refining their calculations to match the precision of the new experiment.
“There have been many heroic physicists who have spent a huge part of their lives on this problem,” Izubuchi said. “What we are measuring is a tiny deviation from the expected behavior of these particles — like measuring a half a millimeter deviation in the flight distance between New York and Los Angeles! But everything about the fate of the laws of physics depends on that difference. So, it sounds small, but it’s really important. You have to understand everything to explain this deviation,” he said.
The path to reduced uncertainty
By “everything,” he means how all the known particles of the Standard Model affect muons via nature’s four fundamental forces — gravity, electromagnetism, the strong nuclear force and the electroweak force. Fortunately, the electroweak contributions are well understood, and gravity is thought to play a currently negligible role in the muon’s wobble. So the latest effort — led by the Brookhaven team with contributions from the RBC Collaboration (made up of physicists from the RIKEN BNL Research Center, Brookhaven Lab and Columbia University) and the UKQCD collaboration — focuses specifically on the combined effects of the strong force (described by a theory called quantum chromodynamics, or QCD) and electromagnetism.
“This has been the least understood part of the theory, and therefore the greatest source of uncertainty in the overall prediction. Our paper is the most successful attempt to reduce those uncertainties, the last piece at the so-called “precision frontier” — the one that improves the overall theory calculation,” Lehner said.
The mathematical calculations are extremely complex — from laying out all the possible particle interactions and understanding their individual contributions to calculating their combined effects. To tackle the challenge, the physicists used a method known as lattice QCD, originally developed at Brookhaven Lab, and powerful supercomputers. The largest was the Leadership Computing Facility at Argonne National Laboratory, a DOE Office of Science user facility, while smaller supercomputers hosted by Brookhaven’s Computational Sciences Initiative (CSI) — including one machine purchased with funds from RIKEN, CSI and Lehner’s DOE Early Career Research Award funding — were also essential to the final result.
“One of the reasons for our increased precision was our new methodology, which combined the most precise data from supercomputer simulations with related experimental measurements,” Lehner noted.
Other groups have also been working on this problem, he said, and the entire community of about 100 theoretical physicists will be discussing all of the results in a series of workshops over the next several months to come to agreement on the value they will use to compare with the Fermilab measurements.
“We’re really looking forward to Fermilab’s results,” Izubuchi said, echoing the anticipation of all the physicists who have come before him in this quest to understand the secrets of the universe.
The theoretical work at Brookhaven was funded by the DOE Office of Science, RIKEN and Lehner’s Early Career Research Award.
The Muon g-2 experiment at Fermilab is supported by DOE’s Office of Science and the National Science Foundation. The Muon g-2 collaboration has almost 200 scientists and engineers from 34 institutions in seven countries. Learn more about the new Muon g-2 experiment or take a virtual tour.
Brookhaven National Laboratory and Fermi National Accelerator Laboratory are both 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.
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From left: Brazilian Center for Research in Physics Director Ron Shellard, Fermilab user Mateus Carneiro of Oregon State University, Fermilab user Mônica Nunes of the University of Campinas in Brazil and FAPESP Advisor Roberto Marcondes Cesar chat at a recent LBNF/DUNE Resources Review Board meeting. Photo: Reidar Hahn
A longstanding partnership between the Brazilian scientific community and Fermilab is getting even stronger, thanks in part to programs funded by the São Paulo Research Foundation. The foundation, whose Portuguese acronym is FAPESP, provides funding to experts from universities and companies in the state of São Paulo to engage in a broad spectrum of research both at home and abroad, from the study of Shakespeare to the simulation of particles in neutrino detectors.
“We’ve been funding many initiatives regarding international collaboration, such as telescopes in Chile, the CMS and ALICE experiments at the LHC, and researchers at Fermilab,” said Advisor Roberto Marcondes Cesar, whose portfolio at FAPESP includes particle physics research projects such as the Fermilab-hosted Deep Underground Neutrino Experiment. “FAPESP is funding some important projects in São Paulo that are related to DUNE. This is nice because it’s an area that attracts a lot of the attention of young people. And part of our mission is to attract young people to the different fields of research in São Paulo.”
Professors, postdoctoral researchers and students funded by FAPESP are currently engaged in a range of activities connected to Fermilab neutrino experiments. Theoretical physicists collaborate with Fermilab, Northwestern University and other institutions to refine predictions and develop more accurate interpretations of experimental data. Scientists and students work on neutrino experiments including DUNE, NOvA and the R&D project LArIAT.
“Neutrinos will probably be part of the most important discoveries in physics for the next 10 or 20 years,” Cesar said. “We are speaking about the most basic and important questions regarding matter and the universe.”
One of the largest efforts by São Paulo researchers on DUNE is the light-detection system known as ARAPUCA. This innovative device, developed by three Brazilian universities and Fermilab and named for the Guaraní word for bird trap, will collect very low light signals coming from the DUNE detectors. This will help scientists get more information out of the massive detectors being built to record the interactions of neutrinos.
Ernesto Kemp is a professor at the University of Campinas. Photo: Reidar Hahn
“FAPESP support is a key element to insert Brazil in a proactive way into DUNE,” said Ernesto Kemp, professor at the University of Campinas. “Most of our work has always been in data analysis, and we have gradually begun to provide parts and calibration systems for detectors. But now we are starting to take on more important responsibilities to build whole systems for the big detectors like DUNE. FAPESP is an important funding agency because it is providing long-term support.”
Kemp and his students have been the recipient of several FAPESP grants and scholarships, including one that enabled him to spend a 2017 year-long sabbatical at Fermilab working on R&D for ARAPUCA. During his sabbatical he conducted experiments in Fermilab facilities and coordinated the construction of two ARAPUCA models for the ProtoDUNE detectors about to begin operating at CERN.
One of Kemp’s colleagues at the University of Campinas, Ettore Segreto, is funded by the FAPESP Young Investigators program for his work on DUNE. The Young Investigators program is designed to attract early-career scientists from around the world to São Paulo institutions.
“Talent is rare, and we have to compete to bring the best researchers to São Paulo,” Cesar said. “We want to bring people from abroad to do research here. We also want our researchers to make connections abroad to that they can cooperate, exchange students and participate in international collaborations.”
The Young Investigators program provides research funding to highly qualified early-career researchers who intend to create new research groups in emerging institutions or new research areas in traditional institutions. The program has been in place since the 1980s and accepts applications year-round for research in many areas, including particle physics in collaboration with Fermilab.
For more on the relationship between FAPESP and DUNE view this video interview with Roberto Marcondes Cesar.
For more on the current state and history of Brazilian scientific collaboration with Fermilab, read the articles “Mobilizing Brazilian scientists for DUNE” and “Brazil in Batavia: How a timely invitation sparked 30 years of partnership.”
Summer means hot days, pool parties and Fermilab’s annual accelerator shutdown, when crews of technicians enter the tunnels to upgrade and maintain the complex machines.
For many months Fermilab’s accelerator complex has been delivering quality beam to experiments, with record up times and delivery of protons. This means more data for Fermilab’s hungry physics experiments. The proton beam delivered by the accelerator complex first reached a milestone of 700 kilowatts in 2016 and has been regularly running at this impressive power since around Christmas.
“It’s been a very good year for beam delivery,” said Fermilab’s Duane Newhart, deputy head of accelerator operations. “We’ve broken just about every record every machine has set this past year.”
The last two shutdowns were focused on upgrades to improve the accelerator complex, part of the Proton Improvement Plan.
“This shutdown is more maintenance-driven,” said Fermilab physicist Cons Gattuso, who coordinates the installation and maintenance activities during the shutdown. “While we have some of the best particle accelerators in the world, some of the equipment we operate is 40 years old. And we ask it to do more and more.”
The accelerator performance should see some gains when the machines come back online around mid-September. And there are still major projects in the works.
Technicians will finish the second half of an upgrade to the linear accelerator, or Linac. The required components, called Marx modulators, provide high voltage for the amplifiers and were designed and built at Fermilab. As modern replacements for old vacuum tubes that are hard to repair or replace, the new pieces of tech will improve the Linac’s reliability. One particular kind of vacuum tube is known as the 1123.
“There are about 90 left in the world, and Fermilab owns all of them,” Newhart said.
The Fermilab Linac will undergo upgrades to improve its reliability during the accelerator shutdown. Photo: Reidar Hahn
Other work includes changing out a component known as a target, which helps produce particles for study — in this case, neutrinos. Accelerator teams will also add components to improve beamline diagnostics, modify vacuum systems, and replace and repair magnets. One particular magnet in the Main Injector accelerator will need some attention.
“We haven’t had to change out a Lambertson magnet in the Main Injector accelerator since things were installed, some 25 years ago,” said Gattuso, referring to the lab’s flagship accelerator. “This is a testament to the quality and resilience of the components.”
The Muon g-2 experiment takes advantage of the lab’s powerful accelerator complex. Fermilab scientists have already collected twice the amount of total data gathered over its four years at Brookhaven National Laboratory, where the experiment ran prior to coming to Fermilab. Photo: Reidar Hahn
This will be the first shutdown since Fermilab’s Muon Campus came online last year. Technicians will work on the vacuum system for the upcoming muon-to-electron conversion experiment, known as Mu2e, completing about half the beamline that connects the experiment to the Muon Campus Delivery Ring by the end of the shutdown. They’ll also add devices that help reduce the spread of the particle beam for the Muon g-2 experiment.
Muon g-2 reuses a giant, 50-foot-diameter particle storage ring from a similar experiment that studied properties of muons at Brookhaven National Laboratory from 1997 to 2001. The ring was transported to Fermilab in 2013 to take advantage of the lab’s powerful accelerator complex and officially started up in 2018. Scientists have already collected twice the amount of total data gathered over four years at Brookhaven.
“We have a plan in place to double the muon flux this summer with a number of upgrades,” said Fermilab scientist and Muon g-2 co-spokesperson Chris Polly. “Hopefully we emerge from the shutdown taking the equivalent of one full Brookhaven data set every month.”