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  • richardmitnick 1:49 pm on April 12, 2021 Permalink | Reply
    Tags: "Have Fermilab Scientists Broken Modern Physics?", , , Don Lincoln at FNAL, , Muon g-2 experiment,   

    From Forbes Magazine : “Have Fermilab Scientists Broken Modern Physics?” 

    From Forbes Magazine

    Apr 7, 2021
    Don Lincoln, FNAL

    1
    Researchers at DOE’s Fermi National Accelerator Laboratory(US) have made a measurement that could mean that scientists have to rethink their understanding of the rules that govern the subatomic world. Credit: Reidar Hahn/Fermilab.

    The past half century has been relatively uneventful for scientist’s understanding of the subatomic world. Theories developed in the 1960s and early 1970s have been combined into what is now called the Standard Model of Particle Physics.

    Standard Model of Particle Physics via Particle Fever movie.

    While there are a few unexplained phenomena (for example Dark Matter and Dark Energy), scientists have tested predictions of the standard model against measurements and the theory has passed with flying colors. Well, except for a few loose ends, including a decade-old disagreement between data and theory pertaining to the magnetic properties of a subatomic particle called the muon.

    Scientists have waited for two decades to see if this discrepancy is real. And today, the wait is over. A new measurement has been announced that goes a long way towards telling us if the venerable theory will need revising.

    Muons are ephemeral subatomic particles, much like the more familiar electron. Like their electron brethren, muons have electric charge and spin. They also decay in about a millionth of a second, which makes them challenging to study.

    Objects that are both electrically charged and spin are also magnets, and muons are no exception. Physicists call the magnetic strength of a magnet made in this way the “magnetic moment” of a particle. One can predict the magnetic moment of both electrons and muons using the conventional quantum mechanics of the 1930s. However, when the first measurement of the magnetic moment of the electron was accomplished in 1948, it was 0.1% too high. The cause of this tiny discrepancy was traced to some truly odd quantum behavior. At the very smallest size scales, space is not quiescent. Instead, it’s a writhing mess, with pairs of particles and antimatter particles appearing and disappearing in the blink of an eye.

    We can’t see this frenetic sea of objects appearing and disappearing, but if you accept that it is true and calculate its effect on the magnetic moment of both muon and electron, it is in exact agreement with the tiny, 0.1%, excess, first reported back in 1948.

    In the intervening 70 years, scientists have both predicted and measured the magnetic moment of the both the muon and electron to a staggering precision of twelve digits of accuracy. And measurement and prediction agree, digit for digit, for the first ten digits. But they disagree for the last two. Furthermore, the disagreement is larger than can be explained by the uncertainty on either the prediction or measurement. It appears that the two disagree.

    If data and theory disagree, one (or both) is wrong. It’s possible that the measurement was inaccurate in some way. It’s also possible that the calculation has an error, or the calculation doesn’t include all relevant effects. If that last option is true – overlooked effects – it means that the standard model of particle physics is incomplete. There is at least something new and unexpected.

    For the past two decades, the best measurement of the magnetic moment of the muon is one made by the Muon g-2 experiment at DOE’s Brookhaven National Laboratory (US), on Long Island, New York. (The experiment is pronounced “muon gee minus two.”) The “g-2” is historical and refers specifically only to the 0.1% excess over the prediction of standard quantum mechanics. Standard quantum mechanics predicts that the magnetic moment of the electron or muon is “g.”

    The discrepancy between theory and measurement was pretty large. If you divided the difference by the combined experimental and theoretical uncertainty, the result was 3.7σ. Scientists call that ratio “sigma,” and use sigma to rate how important a measurement is. If under 3σ, scientists say it is not interesting. If between 3σ and 5σ, scientists start to get interested and call that state of affairs to be “evidence of a discovery.” If above 5σ, scientists are confident that the discrepancy is real and meaningful. For sigmas above 5, scientists usually title their papers as “Observation of…” 5σ is a big deal.

    So, the Muon g-2 experiment at Brookhaven reported a 3.7σ, which is a big deal, but not big enough to be super excited. Another measurement was needed.

    However, the accelerator facility at Brookhaven had done all it could do. A more powerful source of muons was needed. Enter Fermilab, America’s flagship particle physics laboratory, located just west of Chicago. Fermilab could make more muons than Brookhaven could.

    So, researchers bundled up the g-2 apparatus and sent it to Fermilab.

    DOE’s Fermi National Accelerator Laboratory(US) G-2 magnet from DOE’s Brookhaven National Laboratory(US) finds a new home in the FNAL Muon G-2 experiment. The move by barge and truck.

    Fermi National Accelerator Laboratory(US) Muon g-2 studio. As muons race around a ring at the , their spin axes twirl, reflecting the influence of unseen particles.

    Because the g-2 apparatus is shaped like a plate, but 50’ across and 6’ thick, it couldn’t easily be shipped on roads. So, the equipment was put on a barge that went down the east coast of the U.S., up the Mississippi and some of its tributaries, until it was at a debarkation point near Fermilab in northeast Illinois. Then the equipment was put on a flatbed truck and driven in the dead of night to Fermilab. It took two nights, but on July 26, 2013, the g-2 experiment was located at Fermilab.

    Scientists then set to work, building the buildings, accelerator, and infrastructure necessary to perform an improved measurement. In the spring of 2018, the scientists began taking data. Each year, the experiment operates for many months, collecting data. Each year is called a “run” and the Fermilab Muon g-2 experiment is expected to make five runs, including a few in the future.

    The measurement is incredibly precise. They are measuring something with twelve digits of accuracy. That is like measuring the distance around the Earth to a precision a little smaller than the thickness of a sheet of computer printer paper.

    This recent measurement using the g-2 equipment at Fermilab confirmed the earlier measurement at Brookhaven. When the data from the two laboratories are combined, the discrepancy between data and theory is now 4.2σ, tantalizingly close to the desired “Observation of” standard, but not quite there.

    On the other hand, the measurement reported today is based on a single run. Given improvements to the accelerator and facilities, researchers expect to record sixteen times more data than has been reported so far. If the measurement involving all of the data is consistent with the measurement reported today, and the precision of the measurement improves as expected, it is very likely that the g-2 experiment will definitively prove that the standard model is not a complete theory. That conclusion is premature, but it is looking likely.

    So, what does this mean? The most robust conclusion one can draw is that if future measurements tell the same story, the standard model needs modification. It appears that there is something going on in the subatomic realm that is giving the muon a different magnetic moment than the standard model predicts.

    What could that new physics be? Well, it is unlikely that the standard model will need to be completely discarded. It simply works too well on other measurements that aren’t quite as precise. What is more likely is that there exists an unknown class of subatomic particles that have not yet been discovered. One possibility is that an extension of the standard model, called supersymmetry, is true.

    Standard Model of Supersymmetry

    If supersymmetry is real, it predicts twice as many subatomic particles as the standard model. In a pure supersymmetric theory, these new particles would have the same mass as the known ones, but this is ruled out by many measurements. However, there could be a modified version of supersymmetry, which makes the undiscovered cousin particles heavier than the known ones. If true, it would modify the prediction of the magnetic moment of the muon in just the right way to make data and theory agree.

    3
    Particle physics supersymmetry. Conceptual illustration showing the standard model particles with their heavier superpartners introduced by the supersymmetry (SUSY) principle. In supersymmetry force and matter are treated identically. Using supersymmetry, physicists may find solutions for problems such as the weakness of gravity, the low mass of the Higgs boson and the unification of forces or even dark matter. Credit: Getty.

    But supersymmetry is just one possible explanation. The simple fact is that there could be many different kinds of subatomic particles that haven’t been discovered. Perhaps some new theory that explains dark matter might be relevant. Or something entirely unimagined by anyone at this point. We just don’t know.

    But not knowing isn’t bad. It just means that there are new things to learn, problems to solve. Theoretical physicists are already thinking through what might be the implications of the new measurement and what sorts of theories might explain it. The important thing is to accept that a venerable and long-accepted theory is incomplete, and that we need to rethink things. That’s how science is done.

    But I’m getting ahead of myself. The researchers need to analyze the other runs and verify that the more precise results validate today’s measurement. But things are definitely beginning to look interesting.

    See the full article here .

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  • richardmitnick 10:53 am on April 7, 2021 Permalink | Reply
    Tags: "First results from Fermilab’s Muon g-2 experiment strengthen evidence of new physics", , , , Muon g-2 experiment, ,   

    From Symmetry: “First results from Fermilab’s Muon g-2 experiment strengthen evidence of new physics” 

    Symmetry Mag

    04/07/21

    1
    FNAL Muon g-2 experiment at DOE’s Fermi National Accelerator Laboratory (US). Photo by Reidar Hahn, Fermilab.

    The long-awaited first results from the Muon g-2 experiment at the US Department of Energy’s Fermi National Accelerator Laboratory show fundamental particles called muons behaving in a way that is not predicted by scientists’ best theory, the Standard Model of particle physics. This landmark result, made with unprecedented precision, confirms a discrepancy that has been gnawing at researchers for decades.

    The strong evidence that muons deviate from the Standard Model calculation might hint at exciting new physics. Muons act as a window into the subatomic world and could be interacting with yet undiscovered particles or forces.

    “Today is an extraordinary day, long awaited not only by us but by the whole international physics community,” says Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist at the Italian National Institute for Nuclear Physics. “A large amount of credit goes to our young researchers who, with their talent, ideas and enthusiasm, have allowed us to achieve this incredible result.”

    A muon is about 200 times as massive as its cousin, the electron. Muons occur naturally when cosmic rays strike Earth’s atmosphere, and particle accelerators at Fermilab can produce them in large numbers. Like electrons, muons act as if they have a tiny internal magnet. In a strong magnetic field, the direction of the muon’s magnet precesses, or wobbles, much like the axis of a spinning top or gyroscope. The strength of the internal magnet determines the rate that the muon precesses in an external magnetic field and is described by a number that physicists call the g-factor. This number can be calculated with ultra-high precision.

    As the muons circulate in the Muon g-2 magnet, they also interact with a quantum foam of subatomic particles popping in and out of existence. Interactions with these short-lived particles affect the value of the g-factor, causing the muons’ precession to speed up or slow down very slightly. The Standard Model predicts this so-called anomalous magnetic moment extremely precisely. But if the quantum foam contains additional forces or particles not accounted for by the Standard Model, that would tweak the muon g-factor further.

    “This quantity we measure reflects the interactions of the muon with everything else in the universe. But when the theorists calculate the same quantity, using all of the known forces and particles in the Standard Model, we don’t get the same answer,” says Renee Fatemi, a physicist at the University of Kentucky and the simulations manager for the Muon g-2 experiment. “This is strong evidence that the muon is sensitive to something that is not in our best theory.”

    The predecessor experiment at DOE’s Brookhaven National Laboratory, which concluded in 2001, offered hints that the muon’s behavior disagreed with the Standard Model. The new measurement from the Muon g-2 experiment at Fermilab strongly agrees with the value found at Brookhaven and diverges from theory with the most precise measurement to date.

    The accepted theoretical values for the muon are:

    g-factor: 2.00233183620(86) [uncertainty in parentheses]

    anomalous magnetic moment: 0.00116591810(43)

    The new experimental world-average results announced by the Muon g-2 collaboration today are:

    g-factor: 2.00233184122(82)

    anomalous magnetic moment: 0.00116592061(41)

    2
    The first result of the Muon g-2 experiment at Fermilab confirms the result from the experiment performed at DOE’s Brookhaven National Laboratory(US) two decades ago. Together, the two results show strong evidence that muons diverge from the Standard Model prediction. Credit: Ryan Postel, Fermilab/Muon g-2 collaboration.

    The combined results from Fermilab and Brookhaven show a difference with theory at a significance of 4.2 sigma, a little shy of the 5 sigma (or standard deviations) that scientists require to claim a discovery but still compelling evidence of new physics. The chance that the results are a statistical fluctuation is about 1 in 40,000.

    The Fermilab experiment reuses the main component from the Brookhaven experiment, a 50-foot-diameter superconducting magnetic storage ring.

    DOE’s Fermi National Accelerator Laboratory(US) G-2 magnet from DOE’s Brookhaven National Laboratory(US) finds a new home in the FNAL Muon G-2 experiment. The move by barge and truck.

    In 2013, it was transported 3200 miles by land and sea from Long Island to the Chicago suburbs, where scientists could take advantage of Fermilab’s particle accelerator and produce the most intense beam of muons in the United States. Over the next four years, researchers assembled the experiment; tuned and calibrated an incredibly uniform magnetic field; developed new techniques, instrumentation, and simulations; and thoroughly tested the entire system.

    The Muon g-2 experiment sends a beam of muons into the storage ring, where they circulate thousands of times at nearly the speed of light. Detectors lining the ring allow scientists to determine how fast the muons are precessing.

    In its first year of operation, in 2018, the Fermilab experiment collected more data than all prior muon g-factor experiments combined. With more than 200 scientists from 35 institutions in seven countries, the Muon g-2 collaboration has now finished analyzing the motion of more than 8 billion muons from that first run.

    “After the 20 years that have passed since the Brookhaven experiment ended, it is so gratifying to finally be resolving this mystery,” says Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a lead graduate student on the Brookhaven experiment.

    Data analysis on the second and third runs of the experiment is under way, the fourth run is ongoing, and a fifth run is planned. Combining the results from all five runs will give scientists an even more precise measurement of the muon’s wobble, revealing with greater certainty whether new physics is hiding within the quantum foam.

    “So far we have analyzed less than 6% of the data that the experiment will eventually collect. Although these first results are telling us that there is an intriguing difference with the Standard Model, we will learn much more in the next couple of years,” Polly says.

    “Pinning down the subtle behavior of muons is a remarkable achievement that will guide the search for physics beyond the Standard Model for years to come,” says Fermilab Deputy Director of Research Joe Lykken. “This is an exciting time for particle physics research, and Fermilab is at the forefront.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:15 pm on March 30, 2021 Permalink | Reply
    Tags: "The mystery of the muon’s magnetism", , , , Muon g-2 experiment, , , ,   

    From Symmetry: “The mystery of the muon’s magnetism” 

    Symmetry Mag
    From Symmetry

    03/30/21
    Brianna Barbu

    A super-precise experiment at DOE’s Fermi National Accelerator Laboratory(US) is carefully analyzing every detail of the muon’s magnetic moment.

    1

    Modern physics is full of the sort of twisty, puzzle-within-a-puzzle plots you’d find in a classic detective story: Both physicists and detectives must carefully separate important clues from unrelated information. Both physicists and detectives must sometimes push beyond the obvious explanation to fully reveal what’s going on.

    And for both physicists and detectives, momentous discoveries can hinge upon Sherlock Holmes-level deductions based on evidence that is easy to overlook. Case in point: the Muon g-2 experiment currently underway at the US Department of Energy’s Fermi National Accelerator Laboratory.

    The current Muon g-2 (pronounced g minus two) experiment is actually a sequel, an experiment designed to reexamine a slight discrepancy between theory and the results from an earlier experiment at DOE’s Brookhaven National Laboratory(US), which was also called Muon g-2.

    DOE’s Fermi National Accelerator Laboratory(US) G-2 magnet from DOE’s Brookhaven National Laboratory(US) finds a new home in the FNAL Muon G-2 experiment. The move by barge and truck.

    Fermi National Accelerator Laboratory(US) Muon g-2 studio. As muons race around a ring at the , their spin axes twirl, reflecting the influence of unseen particles.

    The discrepancy could be a sign that new physics is afoot. Scientists want to know whether the measurement holds up… or if it’s nothing but a red herring.

    The Fermilab Muon g-2 collaboration has announced it will present its first result on April 7. Until then, let’s unpack the facts of the case.

    The mysterious magnetic moment

    All spinning, charged objects—including muons and their better-known particle siblings, electrons—generate their own magnetic fields. The strength of a particle’s magnetic field is referred to as its “magnetic moment” or its “g-factor.” (That’s what the “g” part of “g-2” refers to.)

    To understand the “-2” part of “g-2,” we have to travel a bit back in time.

    Spectroscopy experiments in the 1920s (before the discovery of muons in 1936) revealed that the electron has an intrinsic spin and a magnetic moment. The value of that magnetic moment, g, was found experimentally to be 2. As for why that was the value—that mystery was soon solved using the new but fast-growing field of quantum mechanics.

    In 1928, physicist Paul Dirac—building upon the work of Llewelyn Thomas and others—produced a now-famous equation that combined quantum mechanics and special relativity to accurately describe the motion and electromagnetic interactions of electrons and all other particles with the same spin quantum number. The Dirac equation, which incorporated spin as a fundamental part of the theory, predicted that g should be equal to 2, exactly what scientists had measured at the time.

    The Dirac equation in the form originally proposed by Dirac is

    4

    But as experiments became more precise in the 1940s, new evidence came to light that reopened the case and led to surprising new insights about the quantum realm.

    3
    Credit: Sandbox Studio, Chicago with Steve Shanabruch.

    A conspiracy of particles

    The electron, it turned out, had a little bit of extra magnetism that Dirac’s equation didn’t account for. That extra magnetism, mathematically expressed as “g-2” (or the amount that g differs from Dirac’s prediction), is known as the “anomalous magnetic moment.” For a while, scientists didn’t know what caused it.

    If this were a murder mystery, the anomalous magnetic moment would be sort of like an extra fingerprint of unknown provenance on a knife used to stab a victim—a small but suspicious detail that warrants further investigation and could unveil a whole new dimension of the story.

    Physicist Julian Schwinger explained the anomaly in 1947 by theorizing that the electron could emit and then reabsorb a “virtual photon.” The fleeting interaction would slightly boost the electron’s internal magnetism by a tenth of a percent, the amount needed to bring the predicted value into line with the experimental evidence. But the photon isn’t the only accomplice.

    Over time, researchers discovered that there was an extensive network of “virtual particles” constantly popping in and out of existence from the quantum vacuum. That’s what had been messing with the electron’s little spinning magnet.

    The anomalous magnetic moment represents the simultaneous combined influence of every possible effect of those ephemeral quantum conspirators on the electron. Some interactions are more likely to occur, or are more strongly felt than others, and they therefore make a larger contribution. But every particle and force in the Standard Model takes part.

    The theoretical models that describe these virtual interactions have been quite successful in describing the magnetism of electrons. For the electron’s g-2, theoretical calculations are now in such close agreement with the experimental value that it’s like measuring the circumference of the Earth with an accuracy smaller than the width of a single human hair.

    All of the evidence points to quantum mischief perpetrated by known particles causing any magnetic anomalies. Case closed, right?

    Not quite. It’s now time to hear the muon’s side of the story.

    Not a hair out of place—or is there?

    Early measurements of the muon’s anomalous magnetic moment at Columbia University (US) in the 1950s and at the European physics laboratory CERN [European Organization for Nuclear Research (Organisation européenne pour la recherche nucléaire)(EU)] in the 1960s and 1970s agreed well with theoretical predictions. The measurement’s uncertainty shrank from 2% in 1961 to 0.0007% in 1979. It looked as if the same conspiracy of particles that affected the electron’s g-2 were responsible for the magnetic moment of the muon as well.

    But then, in 2001, the Brookhaven Muon g-2 experiment turned up something strange. The experiment was designed to increase the precision from the CERN measurements and look at the weak interaction’s contribution to the anomaly. It succeeded in shrinking the error bars to half a part per million. But it also showed a tiny discrepancy—less than 3 parts per million—between the new measurement and the theoretical value. This time, theorists couldn’t come up with a way to recalculate their models to explain it. Nothing in the Standard Model could account for the difference.

    It was the physics mystery equivalent of a single hair found at a crime scene with DNA that didn’t seem to match anyone connected to the case. The question was—and still is—whether the presence of the hair is just a coincidence, or whether it is actually an important clue.

    Physicists are now re-examining this “hair” at Fermilab, with support from the DOE Office of Science (US), the National Science Foundation (US) and several international agencies in Italy, the UK, the EU, China, Korea and Germany.

    In the new Muon g-2 experiment, a beam of muons—their spins all pointing the same direction—are shot into a type of accelerator called a storage ring. The ring’s strong magnetic field keeps the muons on a well-defined circular path. If g were exactly 2, then the muons’ spins would follow their momentum exactly. But, because of the anomalous magnetic moment, the muons have a slight additional wobble in the rotation of their spins.

    When a muon decays into an electron and two neutrinos, the electron tends to shoot off in the direction that the muon’s spin was pointing. Detectors on the inside of the ring pick up a portion of the electrons flung by muons experiencing the wobble. Recording the numbers and energies of electrons they detect over time will tell researchers how much the muon spin has rotated.

    Using the same magnet from the Brookhaven experiment with significantly better instrumentation, plus a more intense beam of muons produced by Fermilab’s accelerator complex, researchers are collecting 21 times more data to achieve four times greater precision.

    The experiment may confirm the existence of the discrepancy; it may find no discrepancy at all, pointing to a problem with the Brookhaven result; or it may find something in between, leaving the case unsolved.

    Seeking the quantum underworld

    There’s reason to believe something is going on that the Standard Model hasn’t told us about.

    The Standard Model is a remarkably consistent explanation for pretty much everything that goes on in the subatomic world.

    Standard Model of Particle Physics from “Particle Fever” via Symmetry Magazine

    But there are still a number of unsolved mysteries in physics that it doesn’t address.

    Dark matter, for instance, makes up about 27% of the universe. And yet, scientists still have no idea what it’s made of. None of the known particles seem to fit the bill. The Standard Model also can’t explain the mass of the Higgs boson, which is surprisingly small. If the Fermilab Muon g-2 experiment determines that something beyond the Standard Model—for example an unknown particle—is measurably messing with the muon’s magnetic moment, it may point researchers in the right direction to close another one of these open files.

    A confirmed discrepancy won’t actually provide DNA-level details about what particle or force is making its presence known, but it will help narrow down the ranges of mass and interaction strength in which future experiments are most likely to find something new. Even if the discrepancy fades, the data will still be useful for deciding where to look.

    It might be that a shadowy quantum figure lurking beyond the Standard Model is too well hidden for current technology to detect. But if it’s not, physicists will leave no stone unturned and no speck of evidence un-analyzed until they crack the case.

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 5:22 pm on November 5, 2020 Permalink | Reply
    Tags: , , Muon g-2 experiment   

    From DOE’s Argonne National Laboratory: “Scientists work to shed light on Standard Model of particle physics” 

    Argonne Lab
    News from From DOE’s Argonne National Laboratory

    November 4, 2020
    Savannah Mitchem

    Mapping the magnetic field for Fermilab’s Muon g-2 experiment

    FNAL Muon G-2 studio.

    As scientists await the highly anticipated initial results of the Muon g-2 experiment at the U.S. Department of Energy’s (DOE) 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.

    Brookhaven Muon g-2 ring.

    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 (HEP) 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.”

    Standard Model of Particle Physics via http://www.plus.maths.org .

    Just as the 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.

    2
    Fully assembled trolley system with wheels for riding on rails and the new external barcode reader for an exact position measurement. The 50 cm long cylindrical shell encloses the 17 NMR probes and custom-built readout and control electronics. (Image by Argonne National Laboratory.)

    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 Ruprecht-Karls-Universität Heidelberg (DE) 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 to 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,” said Corrodi. ​“Then we take the measurements to construct slices of the magnetic field and then a full, 3D 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,” said Corrodi. ​“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 (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,” said Corrodi, ​“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 the Journal of Instrumentation.

    See the full article here .

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    About the Advanced Photon Source

    The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.

    This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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  • richardmitnick 2:36 pm on June 11, 2020 Permalink | Reply
    Tags: , , Muon g-2 experiment, , Scientists studying the muon have been puzzled by a strange pattern in the way muons rotate in magnetic fields., This week an international team of more than 170 physicists published the most reliable prediction so far for the theoretical value of the muon’s anomalous magnetic moment.   

    From Fermi National Accelerator Lab: “Physicists publish worldwide consensus of muon magnetic moment calculation” 

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

    June 11, 2020
    Jerald Pinson

    For decades, scientists studying the muon have been puzzled by a strange pattern in the way muons rotate in magnetic fields, one that left physicists wondering if it can be explained by the Standard Model — the best tool physicists have to understand the universe.

    This week, an international team of more than 170 physicists published the most reliable prediction so far for the theoretical value of the muon’s anomalous magnetic moment, which would account for its particular rotation, or precession. The magnetic moment of subatomic particles is generally expressed in terms of the dimensionless Landé factor, called g. While a number of international groups have worked separately on the calculation, this publication marks the first time the global theoretical physics community has come together to publish a consensus value for the muon’s magnetic moment [ https://arxiv.org/abs/2006.04822 ].

    The result differs from the most recent experimental measurement, which was performed at Brookhaven National Laboratory in 2004, but not significantly enough to unambiguously answer this question.

    Now the world awaits the result from Fermilab’s current Muon g-2 experiment. In the upcoming months, physicists working on the experiment will unveil their preliminary measurement for the value. Depending on how much the Standard Model theoretical calculation differs from the upcoming experimental measurement, physicists may be one step closer to determining whether the muon’s magnetic interactions are hinting at particles or forces that have yet to be discovered.

    1
    Today’s publication by the Muon g-2 Theory Initiative marks the first time the global theoretical physics community has come together to publish a consensus value for the muon’s magnetic moment. Now the world awaits the result from Fermilab’s current Muon g-2 experiment, whose magnetic storage ring is pictured here. Photo: Reidar Hahn, Fermilab.

    In the late 1960s at CERN laboratory, scientists began using a large circular magnetic ring to test the theory that described how muons should “wobble” when moving through a magnetic field. Since then, experimenters have continued to quantify that wobble, making more and more precise measurements of the muon’s anomalous magnetic moment.

    The decades-long effort eventually led to an experiment at Brookhaven National Laboratory and its successor at Fermilab, as well as plans for a new experiment in Japan. At the same time, theorists worked to improve the precision of their calculations and fine-tune their predictions.

    The theoretical value of the anomalous magnetic moment of the muon, published today, is:

    a = (g-2)/2 (muon, theory) = 116 591 810(43) x 10-12

    The most precise experimental result available so far is:

    a = (g-2)/2 (muon, expmt) = 116 592 089(63) x 10-12

    Again, the slight discrepancy between the experimental measurements and the predicted value has persisted, and again it is just beneath the threshold to make a definitive statement.

    This theoretical value, published in the arXiv [above], is the result of over three years of work by 130 physicists from 78 institutions in 21 countries.

    “We’ve not had a theory effort like this before in which all the different evaluations are combined into a single Standard Model prediction,” said Aida El-Khadra, a physicist at the University of Illinois and co-chair of the Steering Committee for the Muon g-2 Theory Initiative, the name of the group of scientists who worked on the calculation.

    Their work builds on a single equation published in 1928 that simultaneously started the field of quantum electrodynamics and laid the foundations for the Muon g-2 experiment.

    An elegant theory

    If you were to ask physicists what they considered the most accurate and successful equation in their field, chances are more than a few would say it’s Dirac’s equation, which describes the relativistic quantum theory of the electron. Published in 1928, Dirac described the spin motion of electrons, and his equation bridged the gap between Einstein’s theory of relativity and the theory of quantum mechanics, and unintentionally predicted the existence of antimatter with only a single equation.

    Dirac was also able to calculate something called the magnetic moment of the electron, which he described as being “an unexpected bonus.”

    Electrons can be thought of as tiny spinning tops that rotate on their axis, an intrinsic property that makes each electron act like a tiny magnet. When placed in a magnetic field, such as the ones generated in particle accelerators, electrons will precess — or wobble on their axis — in a specific and predictable pattern. This wobble is an effect of the particle’s magnetic moment, and it applies to more than electrons. Every electrically charged particle with ½ spin (spin is quantified in half units) behaves in the same way, including particles called muons, which have the same properties as electrons but are more than 200 times as massive.

    Dirac’s equation, which did not take into account the effects of quantum fluctuations, predicted that g would equal 2. Experiment has shown that the actual value differs from that simple expectation — hence the name “muon g-2.”

    Physicists now have a much better understanding of what those quantum fluctuations are and how they behave at subatomic scales, but precisely calculating how they affect the muon’s path is no easy task.

    “Calculating the effects of these quantum fluctuations at the precision level demanded by modern experiment isn’t something that one brilliant person can do alone,” El-Khadra said. “It really takes the whole village.”

    Meeting of the minds

    With so many physicists working on the latest developments to the theory around the world, El-Khadra and her colleagues at Fermilab knew the best way to facilitate interactions between the groups was to bring them all together. So, starting in 2016, El-Khadra and her colleagues in the Fermilab Theory Group, together with Brookhaven National Laboratory scientist Christoph Lehner, Theory Initiative co-chair, and several other international collaborators reached out to the leaders in the global community of physicists working on this problem to put together a new initiative, the Muon g-2 Theory Initiative. The initiative, led by a nine-person Steering Committee that includes leaders of all the major efforts in both theory and experiment, organized a series of workshops around the world, including in the U.S., Japan and Germany, the first of which was hosted at Fermilab in 2017.

    “We had some very intense discussions,” El-Khadra said, “That led to more detailed comparisons and a better understanding of the pros and cons of the various approaches.”

    The establishment of the Muon g-2 Theory Initiative was the first coherent international effort to bring together all of the parties working on the Standard Model value of the muon’s anomalous magnetic moment.

    “Before this initiative began, there were a number of evaluations in the literature of the Standard Model value, each of which differed slightly from the others,” said Boston University scientist Lee Roberts, co-founder of the Fermilab experiment and a member of the initiative’s Steering Committee. “The remarkable thing is that this worldwide community was able to come together and to agree on the ‘best’ value for each of the contributions to the value of the muon’s magnetic moment.”

    2
    Standard Model theory: The chart on the left shows the contributions to the value of the anomalous magnetic moment from the Standard Model of particles and interactions. About 99.994% comes from contributions due to the electromagnetic force while the hadronic contributions account for only 0.006% (note the blue sliver). The right chart shows the contributions to the total uncertainty in the theoretical prediction. About 99.95% of the total error in the theoretical prediction is due the uncertainties in the hadronic corrections, while, at about 0.05% of the total error, the uncertainties in the electromagnetic and electroweak contributions are negligibly small. (QED – quantum electrodynamic forces; EW – electroweak forces; HVP – hadronic vacuum polarization; HLbL – hadronic light-by-light). Image: Muon g-2 Theory Initiative.

    Quantum calculations

    “Muons and other spin-½ particles are never really alone in the universe,” said Fermilab scientist Chris Polly, who is one of Muon g-2’s spokespersons, along with University of Manchester physicist Mark Lancaster. “They interact with a whole entourage of particles that are constantly popping into and out of existence.”

    The two main sources of uncertainty are hadronic vacuum polarization and light-by-light scattering — in which a muon emits and reabsorbs photons after they have traveled through a bubble of quarks and gluons. Both of these factors combine to make up less than 0.01% of the effect on the muon’s wobble yet make up the main source of uncertainty in the theory calculation.

    Calculating the light-by-light scattering part of the hadronic contribution has proven to be especially difficult, and before the start of the Muon g-2 Theory Initiative, physicists had not yet produced reliable estimates of its effects. The best they could manage were rough approximations that led some to wonder whether these evaluations of the light-by-light scattering might be the source of the difference between the muon’s calculated anomalous magnetic moment and the experimentally measured value.

    But theorists are now confident that they can lay these doubts to rest. Thanks to heroic efforts in recent years within the theory community, not just one, but two independent evaluations are now available, each with reliably estimated uncertainties, which are included in the total error of the Standard Model prediction listed above.

    “We’ve now quantified the light-by-light scattering contribution to the extent that it can no longer be used as an explanation to save the Standard Model if the experimental value turns out to differ significantly from the theoretical prediction,” said Brookhaven National Laboratory physicist Christoph Lehner, Theory Initiative co-chair.

    And with so much riding on the line, El-Khadra and other members of the Theory Initiative have left nothing to chance.

    “We have strongly emphasized the importance of including evaluations based on several different methods in our construction of the Standard Model prediction of the anomalous magnetic moment of the muon,” El-Khadra said. “Because if we find that the Fermilab experiment’s measurement is inconsistent with the Standard Model, we want to be sure.”

    The Fermilab Muon g-2 experiment is supported by the DOE Office of Science.

    See the full here.


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    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
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