NOvA turns its eyes to the skies

The NOvA experiment, best known for its measurements of neutrino oscillations using particle beams from Fermilab accelerators, has been turning its eyes to the skies, examining phenomena ranging from supernovae to magnetic monopoles. Thanks in large part to modern computing capabilities, researchers can collect and analyze data for these topics simultaneously, as well as for the primary neutrino program at the U.S. Department of Energy’s Fermilab, where it is based.

The most dramatic astrophysical phenomena that NOvA studies are supernovae. When a massive star collapses, it releases 99% of its energy in a burst of neutrinos. The other 1% becomes a visible supernova, bright enough to outshine an entire galaxy. While the neutrinos carry vastly more energy than do the particles of light, called photons, the elusive neutrinos are much more difficult to observe. Hundreds of visible-light supernovae are discovered each year, but only one since the dawn of the age of neutrino detectors has been near enough to have been seen through its neutrino signature: SN 1987A, in a satellite galaxy of our Milky Way.

Both of NOvA’s particle detectors — the near detector at Fermilab and the far detector in northern Minnesota — are capable of detecting neutrinos generated by supernovae. Each supernova-neutrino signature would appear much smaller than that from an accelerator-generated neutrino beam, but it would still be observable. If a supernova were to be born in our galaxy, NOvA’s 14,000-ton far detector would see thousands of these neutrinos in a few-second burst, and the 300-ton near detector dozens.

In a new paper to be published in the Journal of Cosmology and Astroparticle Physics, the NOvA collaboration describes the system that will be used to trigger on such a burst. Because of the rarity of nearby supernovae and the high value of the neutrino data, NOvA uses several redundant systems to ensure the collection of supernova data. Besides running a continuous real-time search for a burst of neutrinos in its own data, NOvA subscribes to the Supernova Early Warning System, or SNEWS, a network of neutrino experiments that alert each other when any two of them see supernova-like activity at the same time. NOvA also subscribes to alerts sent by the LIGO/Virgo collaboration when a gravitational-wave event is observed, treating each one as a potential source of interesting data. Since gravitational-wave astronomy is brand new, there is great potential for surprises.

The simplest model explaining the majority of gravitational-wave events — black holes merging in vacuum — does not predict particle emissions. But if the black holes merged within a gaseous medium, particles would be accelerated, possibly leading to an observable signal. Other more exotic alternative models explaining some gravitational-wave events could also yield a burst of particles visible to NOvA.

Another scenario that could trigger NOvA is a case of mistaken identity, one in which a supernova is misidentified as a black hole gravitational-wave event. The collaboration performed a search for any emissions visible to NOvA, ranging from supernova-like neutrinos up to high-energy particle showers large enough to light up the entire far detector. As yet, using two dozen gravitational wave events reported through mid-2019, NOvA has found no indication of a signal. This result appears in Physical Review D. NOvA will continue to examine events as they are reported. With the capabilities of gravitational-wave detectors set to rapidly improve over the next few years, there will be many more opportunities to participate in new discoveries.

Closer to home, the NOvA’s underground near detector has been used to examine the seasonal variation of cosmic-ray muons underground. Cosmic rays are particles from outer space that constantly rain down from the sky. They collide with particles in the upper atmosphere, producing muons. The number of muons is affected by atmospheric conditions, and the total number of muons reaching underground detectors is higher in the summer. Summer’s less dense atmosphere favors the production of muons, whereas the denser winter atmosphere tends to degrade the energy of the muons’ parent particles. NOvA is the second experiment, after its predecessor MINOS, to observe that this seasonal correlation is flipped when pairs of muons arriving simultaneously, instead of lone muons, are counted. These are more common in the winter for reasons not well understood.

NOvA also uses its large far detector to look for other exotic cosmic phenomena. In a new paper on the arXiv, the collaboration reports on a search for magnetic monopoles. These hypothetical particles carry a single magnetic charge — either a north or a south pole, but not both. Never observed, the existence of monopoles would help tie together fundamental theories in physics, as well as bring a satisfying symmetry to Maxwell’s equations describing electromagnetism. Magnetic monopoles may be a rare component of cosmic rays, and the NOvA far detector is a very capable cosmic-ray detector, able to observe detailed particle tracks. Unlike most previous neutrino detectors and many previous monopole detectors, it is not underground. This means that if monopoles turn out to be relatively slow and light particles, they would reach NOvA, unlike detectors used in previous searches. Using a small set of early data, NOvA researchers searched for monopoles in a mass range never before searched. They saw none, ruling out a large flux of lightweight monopoles. They will examine further data to tighten these limits or, just maybe, to discover the elusive particle.

Nature’s cosmic accelerators continue providing interesting physics for the NOvA collaboration to study.

Matthew Strait is a University of Minnesota physicist on the NOvA neutrino experiment.

Fermilab 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, visit science.energy.gov.

This is a version of an article originally published by Argonne National Laboratory.

As scientists await the highly anticipated initial results of the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, collaborating scientists from DOE’s Argonne National Laboratory continue to employ and maintain the unique system that maps the magnetic field in the experiment with unprecedented precision.

Argonne scientists upgraded the measurement system, which uses an advanced communication scheme and new magnetic field probes and electronics to map the field throughout the 45-meter circumference ring in which the experiment takes place.

The experiment, which began in 2017 and continues today, could be of great consequence to the field of particle physics. As a follow-up to a past experiment at DOE’s Brookhaven National Laboratory, it has the power to affirm or discount the previous results, which could shed light on the validity of parts of the reigning Standard Model of particle physics.

High-precision measurements of important quantities in the experiment are crucial for producing meaningful results. The primary quantity of interest is the muon’s g-factor, a property that characterizes magnetic and quantum mechanical attributes of the particle.

The Standard Model predicts the value of the muon’s g-factor very precisely.

“Because the theory so clearly predicts this number, testing the g-factor through experiment is an effective way to test the theory,” said Simon Corrodi, a postdoctoral appointee in Argonne’s High Energy Physics Division. “There was a large deviation between Brookhaven’s measurement and the theoretical prediction, and if we confirm this discrepancy, it will signal the existence of undiscovered particles.”

Just as Earth’s rotational axis precesses — meaning the poles gradually travel in circles — the muon’s spin, a quantum version of angular momentum, precesses in the presence of a magnetic field. The strength of the magnetic field surrounding a muon influences the rate at which its spin precesses. Scientists can determine the muon’s g-factor using measurements of the spin precession rate and the magnetic field strength.

The more precise these initial measurements are, the more convincing the final result will be. The scientists are on their way to achieve field measurements accurate to 70 parts per billion. This level of precision enables the final calculation of the g-factor to be accurate to four times the precision of the results of the Brookhaven experiment. If the experimentally measured value differs significantly from the expected Standard Model value, it may indicate the existence of unknown particles whose presence disturbs the local magnetic field around the muon.

Trolley ride

During data collection, a magnetic field causes a beam of muons to travel around a large, hollow ring. To map the magnetic field strength throughout the ring with high resolution and precision, the scientists designed a trolley system to drive measurement probes around the ring and collect data.

The University of Heidelberg developed the trolley system for the Brookhaven experiment, and Argonne scientists refurbished the equipment and replaced the electronics. In addition to 378 probes that are mounted within the ring that constantly monitor field drifts, the trolley holds 17 probes that periodically measure the field with higher resolution.

“Every three days, the trolley goes around the ring in both directions, taking around 9,000 measurements per probe and direction,” Corrodi said. “Then we take the measurements to construct slices of the magnetic field and then a full, 3-D map of the ring.”

The scientists know the exact location of the trolley in the ring from a new barcode reader that records marks on the bottom of the ring as it moves around.

The ring is filled with a vacuum to facilitate controlled decay of the muons. To preserve the vacuum within the ring, a garage connected to the ring and vacuum stores the trolley between measurements. Automating the process of loading and unloading the trolley into the ring reduces the risk of the scientists compromising the vacuum and the magnetic field by interacting with the system. They also minimized the power consumption of the trolley’s electronics in order to limit the heat introduced to the system, which would otherwise disrupt the precision of the field measurement.

The scientists designed the trolley and garage to operate in the ring’s strong magnetic field without influencing it.

“We used a motor that works in the strong magnetic field and with minimal magnetic signature, and the motor moves the trolley mechanically, using strings,” Corrodi said. “This reduces noise in the field measurements introduced by the equipment.”

The system uses the least amount of magnetic material possible, and the scientists tested the magnetic footprint of every single component using test magnets at the University of Washington and Argonne to characterize the overall magnetic signature of the trolley system.

The power of communication

Of the two cables pulling the trolley around the ring, one of them also acts as the power and communication cable between the control station and the measurement probes.

To measure the field, the scientists send a radio frequency through the cable to the 17 trolley probes. The radio frequency causes the spins of the molecules inside the probe to rotate in the magnetic field. The radio frequency is then switched off at just the right moment, causing the water molecules’ spins to precess. This approach is called nuclear magnetic resonance, or NMR.

The frequency at which the probes’ spins precess depends on the magnetic field in the ring, and a digitizer on board the trolley converts the analog radio frequency into multiple digital values communicated through the cable to a control station. At the control station, the scientists analyze the digital data to construct the spin precession frequency and, from that, a complete magnetic field map.

During the Brookhaven experiment, all signals were sent through the cable simultaneously. However, due to the conversion from analog to digital signal in the new experiment, much more data has to travel over the cable, and this increased rate could disturb the very precise radio frequency needed for the probe measurement. To prevent this disturbance, the scientists separated the signals in time, switching between the radio frequency signal and data communication in the cable.

“We provide the probes with a radio frequency through an analog signal,” Corrodi said, “and we use a digital signal for communicating the data. The cable switches between these two modes every 35 milliseconds.”

The tactic of switching between signals traveling through the same cable is called “time-division multiplexing,” and it helps the scientists reach specifications for not only accuracy, but also noise levels. An upgrade from the Brookhaven experiment, time-division multiplexing allows for higher-resolution mapping and new capabilities in magnetic field data analysis.

Upcoming results

Both the field mapping NMR system and its motion control were successfully commissioned at Fermilab and have been in reliable operation during the first three data-taking periods of the experiment.

The scientists have achieved unprecedented precision for field measurements, as well as record uniformity of the ring’s magnetic field, in this Muon g-2 experiment. Scientists are currently analyzing the first round of data from 2018, and they expect to publish the results by the end of 2020.

The scientists detailed the complex setup in a paper titled “Design and performance of an in-vacuum, magnetic field mapping system for the Muon g-2 experiment,” published in the Journal of Instrumentation.

This research was funded by DOE’s Office of Science, High Energy Physics. The Fermilab particle accelerator complex is a DOE Office of Science User Facility.

On Oct. 21, the PIP-II Injector Test Facility at the U.S. Department of Energy’s Fermilab accelerated proton beam through its new superconducting section for the first time at nearly perfect transmission.

The facility, also known as PIP2IT, is a test bed for Fermilab’s upcoming PIP-II superconducting particle accelerator, whose proton beams will reach levels of power not seen before at the lab.

The milestone ushers in an unprecedented era at Fermilab of proton beam delivery using superconducting radio-frequency accelerators.

The PIP-II accelerator — the first major U.S. accelerator project with multinational contributions — will enable the production of intense neutrino beams to the international, Fermilab-hosted Deep Underground Neutrino Experiment. The versatile 800-million-electronvolt, 215-meter-long machine will also be capable of sending high-power proton beams of various patterns to other Fermilab experiments, bolstering the long-term future of the lab.

PIP-II’s power and versatility depend on its front section, that initial stretch through which protons are released from the gate and cranked up to about 20% the speed of light. As the locale of many of the new techniques that will make PIP-II the heart of the laboratory’s accelerator complex, the front end has a great deal riding on it.

That’s why the PIP-II collaboration established PIP2IT as a test bed for the front section of PIP-II. At this proving ground, scientists, engineers and technicians work to demonstrate concepts and technologies that will be deployed at PIP-II and reduce or remove the related technical risks.

“PIP2IT offers fantastic opportunities to further our knowledge and prepare us for surprises that surely lie ahead. It allows the team to test PIP-II critical technologies early, significantly reducing technical project risks, and gain commissioning experience that will be used later to commission PIP-II,” said Eduard Pozdeyev, PIP-II Project scientist and commissioning lead.

The first day the PIP-II team attempted to accelerate beam through the entire front section, the beam reached an energy of 7.5 million electronvolts at nearly 100% transmission efficiency — meaning that virtually none of the beam was lost along the way. It has since achieved an energy of 9.4 million electronvolts. Critical hardware components demonstrated solid performance and met design requirements. The achieved energy and transmission efficiency enable the project team to move ahead, working toward the ultimate PIP2IT test goals, including a beam energy of roughly 20 million electronvolts.

PIP2IT consists of two major sections. The first is based on room-temperature technology and includes a radio-frequency quadrupole, a device that focuses and accelerates the beam, designed and built by DOE’s Lawrence Berkeley National Laboratory.

The second section is based on superconducting radio-frequency technology, or SRF, and consists of two cryomodules which are large housing vessels for the cold, superconducting structures that accelerate the beam, known as cavities. DOE’s Argonne National Laboratory designed and built the first cryomodule, known as HWR. The second cryomodule, a type known as SSR1, was designed and built by Fermilab and houses an accelerator cavity provided by India’s Bhabha Atomic Research Center. (The complete PIP-II accelerator will have 23 cryomodules of five different types.)

The acceleration of particles through this superconducting section marks the start of SRF proton beam operation at Fermilab and sets the stage for the first large scale SRF accelerator of the Fermilab complex.

“First accelerated beam through PIP-II cryomodules at PIP2IT is a major achievement and marks the start of critical tests for one of the most ambitious aspects of the PIP-II project, the front end of the accelerator,” said Fermilab PIP-II Project Director Lia Merminga. “These cryomodules worked exactly as intended the very first time we sent beam through them. We rarely see a nearly perfect performance like this on the first try. It’s a solid accomplishment, and a credit to our team’s technical excellence and rigorous quality control.”

Not only is PIP2IT a high-performing accelerator, it also uses next-generation technology to create beautiful beams.

For example, Fermilab’s PIP-II team plans to employ “strong-back” technology on four of PIP-II’s five cryomodule types. The strong-back technology connects each accelerator cavity to a common frame rather than to its neighbor, as is typical. This structure keeps the cavities cold yet anchored to a room-temperature frame for easier alignment, less vibration and easy assembly.

PIP2IT’s SSR1 cryomodule is built on a strong back, and ongoing tests are validating the concept. This will be only the second time the technology is being applied on a superconducting accelerator, so rigorous validation is essential.

As the heart of the Fermilab accelerator chain, the PIP-II accelerator is designed to route beams of different patterns of particles to different experiments. For example, it could send every third packet of protons to experiment A while sending every first and fourth packet to experiment B. To deliver the required beam patterns to multiple users, the PIP-II front end includes a system called a chopper, which can remove or let pass through packets of protons according to a preprogrammed pattern. Incredibly, this fast chopper can remove one packet without affecting its neighbor — only six nanoseconds apart. That minuscule time interval is about 10 million times shorter than the flicker of time between movie frames.

The PIP-II team plans to implement machine learning to facilitate this flexible beam delivery. With the adaptation of computer code used at another accelerator — SLAC National Accelerator Laboratory’s LCLS-II X-ray laser — the PIP-II’s complicated sort-and-ship process will take about a 10^th of the time it would take to set up the accelerator and route beam manually. The PIP2IT algorithm tests are a first step in minimizing human intervention in PIP-II operation.

PIP2IT will also be used to validate a battery of lower-risk systems, including those that power the accelerator, protect the machine and diagnose the beam. Many of these components are contributions from international partners, such as India, who also lend their valuable expertise to the accelerator project.

“It was wonderful to learn that PIP2IT has reached 7.5 million electronvolts by accelerating beam through the first SSR1 cavity, which is powered by an amplifier developed by Bhabha Atomic Research Center of the Indian Department of Atomic Energy,” said Srinivas Krishnagopal, technical coordinator of the Indian Institutions and Fermilab Collaboration at the Bhabha Atomic Research Center. BARC is contributing nine amplifiers to the SSR1 cryomodule. “We look forward to the time when all of our amplifiers are installed and powered up and beam is accelerated through all the SSR1 cavities — including the one that was made in India. The smooth commissioning is testimony to the strength and depth of the Indian Institutions and Fermilab Collaboration.”

The Indian Institutions and Fermilab Collaboration is a partnership focused on high-power superconducting proton accelerator technologies.

“This first accelerated beam is a testament to the seamless integration of partner deliverables and the power of international partnership and collaboration,” Merminga said.

The PIP-II accelerator beam operation is planned to start in the late 2020s.

Fermilab 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, visit science.energy.gov.

In February and March, three batches of copper plates arrived at Fermilab and were rushed into storage 100 meters underground. The copper had been mined in Finland, rolled into plates in Germany and shipped across land and sea to the lab — all within 120 days. In the quest to detect dark matter, the mysterious substance making up 85% of the matter in the universe, every day that the copper spent above ground mattered.

“At the surface of the Earth, we’re in a shower of cosmic rays,” said Fermilab scientist Dan Bauer.

When these high-energy particles originating from space strike a copper atom, they can knock out protons and neutrons to produce another atom called cobalt-60. Cobalt-60 is radioactive, meaning that it is unstable and spontaneously decays into other particles. The minuscule number of copper atoms converted into cobalt has no impact on everyday uses for copper. But Bauer and others working on the Super Cryogenic Dark Matter Search must take drastic steps to ensure the copper they use is as pure as possible.

The latest in a lineage of similar experiments, SuperCDMS will search for dark matter at SNOLAB, an underground laboratory near Sudbury, Ontario, Canada. The copper plates will eventually take the shape of six oversized soda cans arranged like nesting dolls. The innermost can will house germanium and silicon devices designed to detect hypothesized weakly interacting massive particles, or WIMPs, especially those with less than 10 times the mass of a proton. The vacuum-sealed outermost can will measure a little over a meter in diameter. The whole contraption, dubbed the SNOBOX, will be linked via a set of copper stems to a special refrigerator that will cool the detectors to a tiny fraction of a degree above absolute zero.

At such frigid temperatures, thermal vibrations are so small that a WIMP could leave a detectable signal upon colliding with an atom.

But “you’re looking for a needle in a haystack with dark matter,” Bauer said. “The best you’re going to get is maybe a few events per year.”

Meanwhile, ordinary matter particles flying through the SuperCDMS detectors could produce extraneous signatures, known as background, that would drown the signals from dark matter interactions.

Burying SuperCDMS two kilometers underground and encasing the SNOBOX in layers of lead, plastic and water will screen out almost all the unwanted particles in the environment. But nothing stands between the copper cans and the detectors. And while copper’s superior ability to transport heat makes it ideal for cooling the detectors, any radioactive impurities in the metal would emit background particles.

That brings us back to cobalt-60.

“The bottom line is that the longer the copper sits around on the surface being exposed to cosmic rays, the more cobalt-60 is created,” explained Fermilab’s Matthew Hollister, the manager of the SuperCDMS cryogenics system. “So part of the background budget for the experiment includes a time limit for surface exposure.”

Cobalt-60 is not the only impurity to worry about. Radioactive isotopes of uranium, thorium and potassium occur naturally in Earth’s crust, so the SuperCDMS team had to buy copper sourced from a mine with as little of these metals as possible. Nonradioactive impurities matter, too — they can decrease the copper’s ability to conduct heat, thus making it harder to keep the detectors cold. In total, the copper for SuperCDMS must be over 99.99% pure with fewer than 0.1 parts per billion of radioactive impurities.

Between intrinsic impurities and those introduced through cutting, rolling and transporting the copper, the plates now sitting underground at Fermilab are not quite pristine.

“A lot of the process is not something that we have direct control over,” Hollister said. “Some of it really is a shot in the dark as to what we’re going to end up with at the end of the day.”

After receiving the plates, the researchers sent samples to the U.S. Department of Energy’s Pacific Northwest National Laboratory for detailed testing to quantify the remaining impurities. Soon, the plates will leave Fermilab for fabrication, and the cobalt clock will be ticking once again until the cans reach their home at SNOLAB.

“The last step before we take them underground will be to spray them with an acid etch that will take off some tens of microns of the surface,” Bauer said.

A solution of hydrogen peroxide and diluted hydrochloric acid will remove any surface impurities that have accumulated in the manufacturing process. And a weak citric acid solution will preserve copper’s high thermal conductivity by protecting it from oxidizing over the course of the experiment.

The SuperCDMS collaboration plans to begin collecting data in 2022. All in all, this iteration of the experiment is aiming for background levels 100 times lower than its predecessor, thanks in large part to the purity of the copper. With the increased sensitivity, researchers hope to spot any low-mass WIMPs that might be in the neighborhood.

“This program’s been quite a long time in development, so it’s good to see it starting to come together,” Hollister said. “The SNOBOX is really the last major piece, so we’re looking forward to getting this thing installed and getting it operational as soon as we can.”

SuperCDMS research on dark matter is supported by DOE’s Office of Science and the National Science Foundation, as well as the Canada Foundation for Innovation and SNOLAB.

Fermilab 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, visit science.energy.gov.

This is a version of an article originally published by Argonne National Laboratory.

A limiting factor in modern physics experiments is the precision at which scientists can measure important values, such as the magnetic field within a detector. Scientists at the U.S. Department of Energy’s Argonne National Laboratory and their collaborators have developed a unique facility to calibrate field measurement devices and test their limits inside powerful magnetic fields.

The facility features a solenoid magnet from a former magnetic resonance imaging scanner originally housed at a San Francisco hospital. The magnet produces a maximum field of four teslas — over 400 times the strength of a refrigerator magnet. Its large opening, originally intended to hold a patient during an MRI scan, gives scientists ample space to position devices and machinery inside the magnetic field. The field produced by the magnet is also exceptionally uniform and stable, a requirement for calibrating measurement devices to the ultrahigh precision necessary for many particle and nuclear physics experiments.

“We have worked with several researchers, at Argonne and from other institutions, that need a strong magnetic field and large bore to test their research,” said Peter Winter, physicist and group leader in Argonne’s High Energy Physics Division. “Scientists bring their devices and electronics, and we provide our magnet, expertise and infrastructure to help automate the processes and ensure the success of the tests.”

The team is seeking new users to continue to broaden the facility’s application portfolio.

Calibration station

A primary application of Argonne’s solenoid test facility is calibrating and cross-calibrating measurement probes to achieve high precision and to add layers of consistency between similar experiments across the world.

Originally, Argonne scientists acquired the magnet to test and calibrate several probes developed by the University of Massachusetts for measuring the magnetic field in the Muon g-2 experiment currently taking place at DOE’s Fermi National Accelerator Laboratory. The test facility allowed the scientists to achieve precise field measurements down to several parts per billion – like measuring the circumference of the Earth down to about two inches.

Precise measurement of the field in the experiment is crucial because the magnetic field strength is a major player in the ultimate determination of “g,” a property of the muon whose ultimate determination will either confirm present theories of particle physics or point to the existence of undiscovered particles.

“This facility has enabled the magnetic field team on Muon g-2 to meet strict goals on the experiment by reducing uncertainties and improving the robustness of our measurements,” said David Kawall, a physicist and professor from the University of Massachusetts. “To the best of my knowledge, there are no peer facilities in the world, and having access to these tools at Argonne has been essential to the success of the magnetic field effort on Muon g-2.”

Future g-2 experiments will be conducted in Japan at the Japan Proton Accelerator Complex of the High Energy Accelerator Research Organization, or KEK. The Japanese collaborators, led by Ken-ichi Sasaki, are using the facility to cross-calibrate their magnetic field probes with the ones used at Fermilab.

“By ensuring our probes all read the same values in the same magnetic field, we are adding certainty to the measurements coming from both g-2 experiments,” said Sasaki, who is a professor at KEK and subsection leader of the cryogenic section in J-PARC.

Another muon experiment, the Muonium Spectroscopy Experiment Using Microwaves, or MuSEUM, will contribute to the Japanese g-2 experiment by precisely measuring the mass ratio of the muon to the electron, a value also included in the g-2 determination.

The experiment at KEK in Japan uses very similar nuclear magnetic resonance calibration probes as the g-2 experiment. The development of the probe for MuSEUM has been led by Toya Tanaka, a graduate student at the University of Tokyo who uses the solenoid facility to calibrate the experiment’s probes. The collaboration between Japan and U.S. scientists will ensure that both g-2 experiments and the MuSEUM experiment have a consistent field measurement.

Helium and Hall probe development

Through a partnership with Fermilab’s Thomas Strauss, another Japanese group, led by Norihito Ohuchi and Yasushi Arimoto from KEK, is using the facility to calibrate their own probe — called a Hall probe — for the upcoming SuperKEKB experiment.

Although less precise than the NMR probes used in the current g-2 experiments, Hall probes can measure not only the magnitude of a magnetic field with the field gradient, but also its direction.

SuperKEKB, a recently upgraded, three-kilometer electron-positron collider, accelerates particles called electrons and positrons very close to the speed of light. The scientists will use the measurements from particles created in collisions to investigate a potential explanation of the matter-antimatter asymmetry in the universe.

The SuperKEKB experiment involves five superconducting solenoid magnets in the beam colliding region. The solenoid fields have a heavy influence on the efficiency of the collisions. To elevate the beam colliding efficiency, the team will use the calibrated data of Hall probes to make more precise solenoid field profiles.

“Using Argonne’s test facility, we believe we can improve the accuracy of the Hall probes by one order of magnitude,” said Ohuchi, who is a professor at KEK and leader of the superconducting magnet group in the Accelerator Laboratory. “This will enable us to map the complex magnetic fields produced by the SuperKEKB magnets and improve the quality of the beams.”

Another upcoming experiment at Fermilab, called Mu2e, will also employ Hall probes for field mapping. The experiment uses a solenoid magnet like Argonne’s, but bigger, to measure muon interactions. The reigning Standard Model of particle physics allows muons to decay in a specific way, but for this experiment, scientists will search for a “forbidden” interaction whose occurrence would violate the Standard Model and point to new physics.

The ability of Hall probes to measure the direction of a field makes it the preferred probe for the Mu2e experiment, but the added capability necessitates even more quality control. Argonne scientists have taken the responsibility for field mapping in the Mu2e experiment, and they are using the test facility to calibrate the probes.

“If you have a slight misalignment between the direction from which the probe reads its measurement and where the field is actually pointing, the measurement can veer away from the true value very quickly,” said Bob Wagner, leader of the field mapping team at Argonne. “Our magnet allows us to align the axes of the probes with the field and with each other.”

As Hall probes become more accurate and precise with the help of Argonne’s test facility, a new probe — one that uses helium — is making its debut. A group of researchers from the University of Michigan, led by Professor Tim Chupp and Midhat Farooq, developed the new calibration probe to act as an additional check for measuring fields

The helium isotope in the probe, helium-3, is an inert gas that behaves differently from the water used in traditional probes and has the potential for greater accuracy.

“We used the Argonne test magnet to cross-calibrate our probe with two water probes, including one with the same design as the UMass probe, and found agreement with high precision, confirming that any effects we had not considered are pretty small,” Chupp said. “Our next step is cross calibration of the UMass probe with an improved helium-3 probe that will be even more precise.”

Farooq and team published a paper in Physical Review Letters in June 2020 on the success of their helium probe.

A growing list of applications

Since accepting its first group of external users — scientists from Stony Brook University that tested a magnetic cloak to shield electronics in experiments — the facility’s applications and user base have grown significantly.

In addition to probe calibration, the magnet has also aided in testing and development of a variety of experimental equipment. Argonne’s Junqi Xie, a scientist in the lab’s Physics division, uses the magnet to develop detectors that operate in high magnetic fields for photosensing applications. The detectors will have future applications in the Electron-Ion Collider to be built at DOE’s Brookhaven National Laboratory.

Fermilab recently used the magnet to test their laser metrology systems that they use to measure distances and align equipment in experiments. They tested the ability of several laser trackers, which can measure distances at the submillimeter level, to remain accurate in the presence of high magnetic fields.

“The facility has also been helpful for training the next generation of scientists,” Kawall said, “and the international collaborations formed will be of enduring benefit.”

For inquiries about using the magnet for research and development, contact Peter Winter at winterp@anl.gov.