David Flay holds one of the probes that Muon g-2 scientists will use to map the magnetic field inside the experiment’s storage ring. Photo: Reidar Hahn
Muons are mysterious, and scientists are diving deep into the particle to get a handle on a property that might render it — and the universe — a little less mysterious.
Like electrons – muons’ lighter siblings – they are particles with a sort of natural internal magnet. They also have an angular momentum called spin, kind of like a spinning top. The combination of the spin and internal magnet of a particle is called the gyromagnetic ratio, dubbed “g,” but previous attempts at measuring it for muons have thrown up intriguing surprises.
The goal of the Muon g-2 experiment at Fermilab is to measure it more precisely than ever before.
To reach these remarkable levels of precision, scientists have to keep very careful tabs on a few parts of the experiment, one of which is how strong its magnetic field is. The team has been measuring and tweaking the magnetic field for months and is now very close to achieving a stable field before experiments can properly begin.
“We’re in the experiment’s commissioning period right now, where we’re basically learning how our systems behave and making sure everything works properly before we transition into stable running,” said David Flay, a University of Massachusetts scientist working on the calibration of the magnetic field for Muon g-2.
Muon mystery
Muon g-2 is following up on an intriguing result seen at Brookhaven National Laboratory in New York in the early 2000s, when the experiment made observations of muons that didn’t match with theoretical predictions. The experiment’s 15-meter-diameter circular magnet, called a storage ring, was shipped to Illinois across land and sea in 2013, and the measurement is now being conducted at Fermilab with four times the precision.
When Brookhaven carried out the experiment, the result was surprising: The muon value of g differed significantly from what calculations said it should be, and no one is quite sure why. It’s possible the experiment itself was flawed and the result was false, but it also opens the door to the possibility of exotic new particles and theories. With its four-fold increase in precision, Muon g-2 will shed more light on the situation.
To measure g, beams of muons circulating inside the experiment’s storage ring are subjected to an intense magnetic field – about 30,000 times the strength of Earth’s natural field. This causes the muons to rotate around the magnetic field, or precess, in a particular way. By measuring this precession, it is possible to precisely extract the value of g.
The strength of magnetic field to which the muons are exposed directly affects how they precess, so it’s absolutely crucial to make extremely precise measurements of the field strength and maintain its uniformity throughout the ring – not an easy task.
If Muon g-2 backs up Brookhaven’s result, it would be huge news. The Standard Model would need rethinking and it would open up a whole new chapter of particle physics.
A leading theory to explain the intriguing results are new kinds of virtual particles, quantum phenomena that flit in and out of existence, even in an otherwise empty vacuum. All known particles do this, but their total effect doesn’t quite account for Brookhaven’s results. Scientists are therefore predicting one or more new, undiscovered kinds, whose additional ephemeral presence could be providing the strange muon observations.
“The biggest challenge so far has been dealing with the unexpected,” said Joe Grange, scientist at Argonne National Laboratory working on Muon g-2’s magnetic field. “When a mystery pops up that needs to be solved relatively quickly, things can get hectic. But it’s also one of the more fun parts of our work.”
Probing the field
The magnetic field strength measurements are made using small, sensitive electronic devices called probes. Three types of probes – fixed, trolley and plunging – work together to build up a 3-D map of the magnetic field inside the experiment. The field can drift over time, and things like temperature changes in the experiment’s building can subtly affect the ring’s shape, so roughly 400 fixed probes are positioned just above and below the storage ring to keep a constant eye on the field inside. Because these probes are always watching, the scientists know when and by how much to tweak the field to keep it uniform.
For these measurements, and every few days when the experiments is paused and the muon beam is stopped, a 0.5-meter-long, curved cylindrical trolley on rails containing 17 probes is sent around the ring to take a precise field map in the region where the muons are stored. Each orbit takes a couple of hours. The trolley probes are themselves calibrated by a plunging probe, which can move in and out of its own chamber at a specific location in the ring when needed.
The fixed probes have been installed and working since fall 2016, while the 17 trolley probes have recently been removed, upgraded and reinstalled.
“The probes are inside the ring where we can’t see them,” Flay said. “So matching up their positions to get an accurate calibration between them is not an easy thing to do.”
The team developed some innovative solutions to tackle this problem, including a barcode-style system inside the ring, which the trolley scans to relay where it is as it moves around.
Global g-2
Muon g-2 is an international collaboration hosted by Fermilab. Together with scientists from Fermilab, Argonne, and Brookhaven, several universities across the U.S. work with international collaborators from countries as wide-ranging as South Korea, Italy and the UK. In total, around 30 institutions and 150 people work on the experiment.
“It’s the detailed efforts of the Argonne, University of Washington, University of Massachusetts and University of Michigan teams that have produced these reliable, quality tools that give us a complete picture of the magnetic field,” said Brendan Kiburg, Fermilab scientist working on Muon g-2. “It has taken years of meticulous work.”
The team is working to finish the main field strength measurement part of the commissioning process by early 2018, before going on to analyze exactly how the muons experience the generated field. The experiment is planned to begin in full in February 2018.
A Fermilab team built and tested the first new superconducting accelerator cryomodule for the LCLS-II project, which will be the nation’s only X-ray free-electron laser facility.
The first cryomodule for SLAC’s LCLS-II X-ray laser departed Fermilab on Jan. 16. Photo: Reidar Hahn
Earlier this week, scientists and engineers at the U.S. Department of Energy’s Fermilab in Illinois loaded one of the most advanced superconducting radio-frequency cryomodules ever created onto a truck and sent it heading west.
Today, that cryomodule arrived at the U.S. DOE’s SLAC National Accelerator Laboratory in California, where it will become the first of 37 powering a three-mile-long machine that will revolutionize atomic X-ray imaging. The modules are the product of many years of innovation in accelerator technology, and the first cryomodule Fermilab developed for this project set a world record in energy efficiency.
These modules, when lined up end to end, will make up the bulk of the accelerator that will power a massive upgrade to the capabilities of the Linac Coherent Light Source at SLAC, a unique X-ray microscope that will use the brightest X-ray pulses ever made to provide unprecedented details of the atomic world. Fermilab will provide 22 of the cryomodules, with the rest built and tested at the U.S. DOE’s Thomas Jefferson National Accelerator Facility in Virginia.
The quality factor achieved in these components is unprecedented for superconducting radio-frequency cryomodules. The higher the quality factor, the lower the cryogenic load, and the more efficiently the cavity imparts energy to the particle beam. Fermilab’s record-setting cryomodule doubled the quality factor compared to the previous state-of-the-art.
“LCLS-II represents an important technological step which demonstrates that we can build more efficient and more powerful accelerators,” said Fermilab Director Nigel Lockyer. “This is a major milestone for our accelerator program, for our productive collaboration with SLAC and Jefferson Lab and for the worldwide accelerator community.”
Today’s arrival is merely the first. From now into 2019, the teams at Fermilab and Jefferson Lab will build the remaining cryomodules, including spares, and scrutinize them from top to bottom, sending them to SLAC only after they pass the rigorous review.
“It’s safe to say that this is the most advanced machine of its type,” said Elvin Harms, a Fermilab accelerator physicist working on the project. “This upgrade will boost the power of LCLS, allowing it to deliver X-ray laser beams that are 10,000 times brighter than it can give us right now.”
With short, ultrabright pulses that will arrive up to a million times per second, LCLS-II will further sharpen our view of how nature works at the smallest scales and help advance transformative technologies of the future, including novel electronics, life-saving drugs and innovative energy solutions. Hundreds of scientists use LCLS each year to catch a glimpse of nature’s fundamental processes.
To meet the machine’s standards, each Fermilab-built cryomodule must be tested in nearly identical conditions as in the actual accelerator. Each large metal cylinder — up to 40 feet in length and 4 feet in diameter — contains accelerating cavities through which electrons zip at nearly the speed of light. But the cavities, made of superconducting metal, must be kept at a temperature of 2 Kelvin (minus 456 degrees Fahrenheit).
Thirty-seven cryomodules lined end to end — half from Fermilab and half from Jefferson Lab — will make up the bulk of the LCLS-II accelerator. Photo: Reidar Hahn
To achieve this, ultracold liquid helium flows through pipes in the cryomodule, and keeping that temperature steady is part of the testing process.
“The difference between room temperature and a few Kelvin creates a problem, one that manifests as vibrations in the cryomodule,” said Genfa Wu, a Fermilab scientist working on LCLS-II. “And vibrations are bad for linear accelerator operation.”
In initial tests of the prototype cryomodule, scientists found vibration levels that were higher than specification. To diagnose the problem, they used geophones — the same kind of equipment that can detect earthquakes — to rule out external vibration sources. They determined that the cause was inside the cryomodule and made a number of changes, including adjusting the path of the flow of liquid helium. The changes worked, substantially reducing vibration levels — to a 10th of what they were originally — and have been successfully applied to subsequent cryomodules.
Fermilab scientists and engineers are also ensuring that unwanted magnetic fields in the cryomodule are kept to a minimum, since excessive magnetic fields reduce the operating efficiency.
“At Fermilab, we are building this machine from head to toe,” Lockyer said. “From nanoengineering the cavity surface to the integration of thousands of complex components, we have come a long way to the successful delivery of LCLS-II’s first cryomodule.”
Fermilab has tested seven cryomodules, plus one built and previously tested at Jefferson Lab, with great success. Each of those, along with the modules yet to be built and tested, will get its own cross-country trip in the months and years to come.
Read more about the LCLS-II project in SLAC’s press release.
This project is supported by DOE’s Office of Science. LCLS is a DOE Office of Science user facility.
The DOE 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 http://science.energy.gov.
In one sense, the two ProtoDUNE detectors are small. As prototypes of the much larger planned Deep Underground Neutrino Experiment, they are only representative slices, each measuring about 1 percent of the size of the final detector. But in all other ways, the ProtoDUNE detectors are simply massive.
Once they are complete later this year, these two test detectors will be larger than any detector ever built that uses liquid argon, its active material. The international project involves dozens of experimental groups coordinating around the world. And most critically, the ProtoDUNE detectors, which are being installed and tested at the European particle physics laboratory CERN, are the rehearsal spaces in which physicists, engineers and technicians will hammer out nearly every engineering problem confronting DUNE, the biggest international science project ever conducted in the United States. It is hosted by the Department of Energy’s Fermi National Accelerator Laboratory.
The ProtoDUNE detectors are being assembled at the European laboratory CERN. Photo: Maximilien Brice, CERN
Gigantic detector, tiny neutrino
DUNE’s mission, when it comes online in the mid-2020s, will be to pin down the nature of the neutrino, the most ubiquitous particle of matter in the universe. Despite neutrinos’ omnipresence — they fill the universe, and trillions of them stream through us every second — they are a pain in the neck to capture. Neutrinos are vanishingly small, fleeting particles that, unlike other members of the subatomic realm, are heedless of the matter through which they fly, never stopping to interact.
Well, almost never.
Once in a while, scientists can catch one. And when they do, it might tell them a bit about the origins of the universe and why matter predominates over antimatter — and thus how we came to be here at all.
A global community of more than 1,000 scientists from 31 countries are building DUNE, a megascience experiment hosted by Fermilab. The researchers’ plan is to observe neutrinos using two detectors separated by 1,300 kilometers — one at Fermilab outside Chicago and a second one a mile underground in South Dakota at the Sanford Underground Research Facility. Having one at each end enables scientists to see how neutrinos transform as they travel over a long distance.
The DUNE collaboration is going all-in on the bigger-is-better strategy; after all, the bigger the detector, the more likely scientists are to snag a neutrino. The detector located in South Dakota, called the DUNE far detector, will hold 70,000 metric tons (equivalent to about 525,000 bathtubs) of liquid argon to serve as the neutrino fishing net. It comprises four large modules. Each will stand four stories high and, not including the structures that house the utilities, occupy a footprint roughly equal to a soccer field.
In short, DUNE is giant.
The ProtoDUNE detectors are small only when compared to the giant DUNE detector. If each of the four DUNE modules is a 20-room building, then each ProtoDUNE detector is one room.
But one room large enough to envelop a small house.
As one repeatable unit of the ultimate detector, the ProtoDUNE detectors are necessarily big. Each is an enormous cube—about two stories high and about as wide — and contains about 800 metric tons of liquid argon.
Why two prototypes? Researchers are investigating two ways to use argon and so are constructing two slightly different but equally sized test beds. The single-phase ProtoDUNE uses only liquid argon, while the dual-phase ProtoDUNE uses argon as both a liquid and a gas.
“They’re the largest liquid-argon particle detectors that have ever been built,” said Ed Blucher, DUNE co-spokesperson and a physicist at the University of Chicago.
As DUNE’s test bed, the ProtoDUNE detectors also have to offer researchers a realistic picture of how the liquid-argon detection technology will work in DUNE, so the instrumentation inside the detectors is also at full, giant scale.
“If you’re going to build a huge underground detector and invest all of this time and all of these resources into it, that prototype has to work properly and be well-understood,” said Bob Paulos, director of the University of Wisconsin–Madison Physical Sciences Lab and a DUNE engineer. “You need to understand all the engineering problems before you proceed to build literally hundreds of these components and try to transport them all underground.”
A crucial step for ProtoDUNE was welding together the cryostat, or cold vessel, that will house the detector components and liquid argon. Photo: CERN
Partners in ProtoDUNE
ProtoDUNE is a rehearsal for DUNE not only in its technical orchestration but also in the coordination of human activity.
When scientists were planning their next-generation neutrino experiment around 2013, they realized that it could succeed only by bringing the international scientific community together to build the project. They also saw that even the prototyping would require an effort of global proportions — both geographically and professionally. As a result, DUNE and ProtoDUNE actively invite students, early-career scientists and senior researchers from all around the world to contribute.
“The scale of ProtoDUNE, a global collaboration at CERN for a U.S.-based megaproject, is a paradigm change in the way neutrino science is done,” said Christos Touramanis, a physicist at the University of Liverpool and one of the co-coordinators of the single-phase detector. For both DUNE and ProtoDUNE, funding comes from partners around the world, including the Department of Energy’s Office of Science and CERN.
The successful execution of ProtoDUNE’s assembly and testing by international groups requires a unity of purpose from parties that could hardly be farther apart, geographically speaking.
Scientists say the effort is going smoothly.
“I’ve been doing neutrino physics and detector technology for the last 20 or 25 years. I’ve never seen such an effort go up so nicely and quickly. It’s astonishing,” said Fermilab scientist Flavio Cavanna, who co-coordinates the single-phase ProtoDUNE project. “We have a great collaboration, great atmosphere, great willingness to make it. Everybody is doing his or her best to contribute to the success of this big project. I used to say that ProtoDUNE was mission impossible, because—in the short time we were given to make the two detectors, it looked that way in the beginning. But looking at where we are now, and all the progress made so far, it starts turning out to be mission possible.”
The anode plane array is prepped for shipment at Daresbury Laboratory in the UK. Photo courtesy of Christos Touramanis
Inside the liquid-argon test bed
So how do neutrino liquid-argon detectors work? Most of the space inside serves as the arena of particle interaction, where neutrinos can smash into an argon atom and create secondary particles. Surrounding this interaction space is the instrumentation that records these rare collisions, like a camera committing the scene to film. DUNE collaborators are developing and constructing the recording instruments that will capture the evidence of these interactions.
One signal is ionization charge: A neutrino interaction generates other particles that propagate through the detector’s vast argon pool, kicking electrons — called ionization electrons — off atoms as they go. The second signal is light.
To record the signal, scientists will use something called an anode plane array, or APA. An APA is a screen created using 24 kilometers of precisely tensioned, closely spaced, continuously wound wire. This wire screen is positively charged, so it attracts the negatively charged electrons.
Much the way a wave front approaches the beach’s shore, the particle track — a string of the ionization electrons — will head toward the positively charged wires inside the ProtoDUNE detectors. The wires will send information about the track to computers, which will record its properties and thus information about the original neutrino interaction.
A group in the University of Wisconsin–Madison Physical Sciences Lab led by Paulos designed the single-phase ProtoDUNE wire arrays. The Wisconsin group, the Science and Technology Facilities Council’s Daresbury Laboratory in the UK and several UK universities are building APAs for the same detector. The first APA from Wisconsin arrived at CERN last year; the first from Daresbury Lab arrived earlier this week.
“These are complicated to build,” Paulos said, noting that it currently takes about three months to build just one. “Building these 6-meter-tall anode planes with continuously wound wire—that’s something that hasn’t been done before.”
The dual-phase detector will operate on the same principle but with a different configuration of wire arrays. A special layer of electronics near the cathode will allow for the amplification of faint electron tracks in a layer of gaseous argon. Groups at institutions in France, Germany and Switzerland are designing those instruments. Once complete, they will also send their arrays to be tested at CERN.
Then there’s the business of observing light.
The flash of light is the result of a release of energy from the electron in the process of getting bumped from an argon atom. The appearance of light is like the signal to start a stopwatch; it marks the moment the neutrino interaction in a detector takes place. This enables scientists to reconstruct in three dimensions the picture of the interaction and resulting particles.
On the other side of the equator, a group at the University of Campinas in Brazil is coordinating the installation of instruments that will capture the flashes of light resulting from particle interactions in the single-phase ProtoDUNE detector.
Two of the designs for the single-phase prototype — one by Indiana University, the other by Fermilab and MIT — are of a type called guiding bars. These long, narrow strips work like fiber optic cables: they capture the light, convert it into light in the visible spectrum and finally guide it to an external sensor.
A third design, called ARAPUCA, was developed by three Brazilian universities and Fermilab and is being partially produced at Colorado State University. Named for the Guaraní word for a bird trap, the efficient ARAPUCA design will be able to “trap” even very low light signals and transmit them to its sensors.
The ARAPUCA array, designed by three Brazilian universities and Fermilab, was partially produced at Colorado State University. Photo: D. Warner, Colorado State University
“The ARAPUCA technology is totally new,” said University of Campinas scientist Ettore Segreto, who is co-coordinating the installation of the light detection systems in the single-phase prototype. “We might be able to get more information from the light detection — for example, greater energy resolution.”
Groups from France, Spain and the Swiss Federal Institute of Technology are developing the light detection system for the dual-phase prototype, which will comprise 36 photomultiplier tubes, or PMTs, situated near the cathode plane. A PMT works by picking up the light from the particle interaction and converting it into electrons, multiplying their number and so amplifying the signal’s strength as the electrons travel down the tube.
With two tricked-out detectors, the DUNE collaboration can test their picture-taking capabilities and prepare DUNE to capture in exquisite detail the fleeting interactions of neutrinos.
Bringing instruments into harmony
But even if they’re instrumented to the nines inside, two isolated prototypes do not a proper test bed make. Both ProtoDUNE detectors must be hooked up to computing systems so particle interaction signals can be converted into data. Each detector must be contained in a cryostat, which functions like a thermos, for the argon to be cold enough to maintain a liquid state. And the detectors must be fed particles in the first place.
CERN is addressing these key areas by providing particle beam, innovative cryogenics and computing infrastructures, and connecting the prototype detectors with the DUNE experimental environment.
DUNE’s neutrinos will be provided by the Long-Baseline Neutrino Facility, or LBNF, which held a groundbreaking for the start of its construction in July. LBNF, led by Fermilab, will provide the construction, beamline and cryogenics for the mammoth DUNE detector, as well as Fermilab’s chain of particle accelerators, which will provide the world’s most intense neutrino beam to the experiment.
CERN is helping simulate that environment as closely as possible with the scaled-down ProtoDUNE detectors, furnishing them with particle beams so researchers can characterize how the detectors respond. Under the leadership of scientist Marzio Nessi, last year the CERN group built a new facility for the test beds, where CERN is now constructing two new particle beamlines that extend the lab’s existing network.
The recently arrived anode plane array (hanging on the left) is moved by a crane to its new home in the ProtoDUNE cryostat. Photo: CERN
In addition, CERN built the ProtoDUNE cryostats — the largest ever constructed for a particle physics experiment — which also will serve as prototypes for those used in DUNE. Scientists will be able to gather and interpret the data generated from the detectors with a CERN computing farm and software and hardware from several UK universities.
“The very process of building these prototype detectors provides a stress test for building them in DUNE,” Blucher said.
CERN’s beam schedule sets the schedule for testing. In December, the European laboratory will temporarily shut off beam to its experiments for upgrades to the Large Hadron Collider. DUNE scientists aim to position the ProtoDUNE detectors in the CERN beam before then, testing the new technologies pioneered as part of the experiment.
“ProtoDUNE is a necessary and fundamental step towards LBNF/DUNE,” Nessi said. “Most of the engineering will be defined there and it is the place to learn and solve problems. The success of the LBNF/DUNE project depends on it.”
This article also appears in symmetry magazine.
Fermilab is America’s premier national laboratory for particle physics research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at http://www.fnal.gov and follow us on Twitter @Fermilab.
The DOE 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 http://science.energy.gov.