World news – Field guide: Scientists support the proof of new physics in the Muon g-2 experiment

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April 7, 2021

from Argonne National Laboratory

Scientists are testing our basic understanding of the universe, and there is much more to discover.

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What do touchscreens, radiation therapy and shrink film have in common? They were all made possible by particle physics research. Discoveries of how the universe works on the smallest scale often lead to tremendous advances in the technology we use every day.

Scientists from the Argonne National Laboratory and the Fermi National Accelerator Laboratory of the US Department of Energy (DOE) bring them together conducted an experiment with staff from 46 other institutions and seven countries to test our current understanding of the universe. The first result indicates the existence of undiscovered particles or forces. These new physics could help explain longstanding scientific secrets, and the new findings add to a repository of information that scientists can use in modeling our universe and developing new technologies.

The experiment, muon g-2 (pronounced muon g minus 2), follows an experiment that began in the 1990s at the DOE’s Brookhaven National Laboratory in which scientists measured a magnetic property of a ground particle called a muon.

The Brookhaven experiment produced a result that differed from differed from the value predicted by the Standard Model, the best ever description of the composition and behavior of the universe by scientists. The new experiment is a replica of Brookhaven, built to challenge or confirm the discrepancy with greater precision.

The Standard Model predicts the muon’s g-factor very accurately – a value that scientists say as this particle behaves in a magnetic field. It is known that this g-factor is close to two, and the experiments measure its deviation from two, hence the name muon g-2.

The experiment in Brookhaven showed that g-2 is a few ppm differed from the theoretical prediction. This tiny difference pointed to unknown interactions between the muon and the magnetic field – interactions that might involve new particles or forces.

The first result of the new experiment strongly agrees with that of Brookhaven and confirms the evidence that it is new Physics to discover. The combined results from Fermilab and Brookhaven show a difference from the Standard Model with a significance of 4.2 sigma (or standard deviations), slightly less than the 5 sigma that scientists need to claim a discovery, but still compelling evidence for new ones Physics. The probability that the results are statistical fluctuations is around 1 in 40,000.

Particles beyond the Standard Model could help explain puzzling phenomena in physics, such as the nature of dark matter, a mysterious and ubiquitous substance that physicists know exists but has not yet been discovered.

« This is an incredibly exciting result, » said Ran Hong of Argonne, a postdoctoral fellow who worked on muon g-2 for four years. Experiment has worked. « These results could have a significant impact on future particle physics experiments and lead to a better understanding of how the universe works. »

The Argonne team of scientists contributed significantly to the success of the experiment. The original team, put together and led by physicist Peter Winter, consisted of Argonnes Hong and Simon Corrodi and Suvarna Ramachandran and Joe Grange, who have since left Argonne.

« This team has impressive and unique skills with a high level of expertise on hardware, operational planning and data analysis, « said Winter, who directs Argonne’s Muon g-2 contributions. « You made important contributions to the experiment, and we would not have been able to achieve these results without your work. »

To deduce the true g-2 of the muon, the Fermilab scientists produce muon beams that move in the presence of a strong magnetic field move in a circle through a large, hollow ring. This field keeps the muons in the ring and causes the direction of rotation of a muon to rotate. The rotation, which scientists call precession, is similar to the rotation of the earth’s axis, only much, much faster.

In order to calculate g-2 with the desired accuracy, scientists have to measure two values ​​with a very high degree of certainty. One is the speed of spin precession of the muon as it traverses the ring. The other is the strength of the magnetic field surrounding the muon, which affects its precession. This is where Argonne comes in.

Although the muons move through an impressively constant magnetic field, changes in ambient temperature and hardware effects of the experiment cause slight variations throughout the ring. Even these small shifts in field strength, if not taken into account, can have a significant impact on the accuracy of the g-2 calculation.

To correct the field fluctuations, the scientists continuously measure the drift field with hundreds of probes attached to the ring walls are appropriate. They also send a cart around the ring every three days to measure the field strength through which the muon beam actually passes. Probes are mounted on the carriage that map the magnetic field over the entire 45 meter circumference of the ring with incredibly high precision.

To achieve the final uncertainty target of less than 70 parts per billion (about 2.5 times better than the field measurement in the previous experiment), the Argonne scientists have redesigned the trolley system used in the Brookhaven experiment with advanced communication capabilities and new, high-precision magnetic field probes developed by the University of Washington.

The cart travels in both directions around the ring and takes around 9,000 measurements per probe and direction. The scientists use the measurements to reconstruct layers of the magnetic field and then derive a full 3D map of the field in the ring. Field values ​​at points on the map are included in the g-2 calculation for muons that pass these locations. The better the field measurements, the more telling the end result.

The scientists also converted some of the analog signals used in the old experiment to digital signals to increase the amount of data they could get from the probes. This required complex engineering of the car’s communication system to minimize interference with the sensitive inspection mechanisms.

« It was quite difficult to keep the car running smoothly and safely. The control system had to handle routine operations, but also identify emergencies and make them appropriate « said Hong, whose background in both scientific research and engineering was vital to the design of the cart, work with limited interruption to the experiment.

The team plans to update the cart system for the next data collection period update to further improve the measurements by reducing the uncertainty little by little.

In precision experiments like muon g-2, the main goal is to reduce systematic uncertainties or errors that could affect the measurements.

« Measuring the raw numbers is relatively easy – find out Finding how well we know the numbers is the real challenge, « said Corrodi, a postdoctoral fellow in the Argonne High Energy Physics (HEP) department.

To ensure the accuracy of the magnetic field measurements, the scientists calibrated the probes with the 4th grade -Tesla magnet system from Argonne, which contains a magnet from an earlier MRI (magnetic resonance imaging) scanner. The magnet generates an even and stable magnetic field with over 400 times the strength of a refrigerator magnet.

The Argonne scientists calibrated the probes in the car using the readings from a probe that was designed and tested in the solenoid valve. This process ensures that the probes read the same measurement in the same magnetic field and allows scientists to make accurate corrections. The test facility enabled the scientists to take field measurements up to several parts per billion – like measuring the volume of water in a swimming pool to the point of dropping.

« In addition to calibrating the probes, we have improved the field measurements by changing the operational settings on the fly Operation have adjusted, « said Corrodi. « During the data analysis, we noticed some effects that we did not expect. »

When Corrodi and his team saw glitches in the data, they examined the system to determine the cause. For example, certain devices in the ring focus the muon beam to keep it centered. However, these devices disturb the magnetic field in the ring slightly. The scientists developed a way to measure this effect in order to remove it from the analysis.

The path of the magnetic field data from the probe to the computer is complex. Corrodi, Hong, and others configured the hardware and software so that the data from the field probes was read with the correct time and location stamps. They also had to understand the data starting in binary to fit it into the common analytical framework for the experiment. « We had to convert the raw data into something to work with, » said Hong, « and us were responsible for data quality control and determined which erroneous data should be discarded in the final g-2 analysis. « 

Corrodi will lead the magnetic field analysis team, resolve conflicts with the equipment and ensure that the various teams converge on the next result in the experiment, said Winter. « You really have to understand all of the field analysis to achieve our scientific goals. »

« So far, the accuracy of the final g-2 measurement is comparable to that of the Brookhaven experiment, but that is dominated by the fact that the data so far are limited, « said Corrodi. « We only analyzed 6% of the data that we want to use for the entire experiment. This added data will significantly reduce the uncertainty. »

The first result is also encouraging for scientists who are carrying out other current and planned muon experiments, including a future g-2 experiment that will be carried out in Japan and the next muon experiment in Fermilab – the Mu2e experiment. These projects are already using Argonne’s Solonoid Facility to calibrate their magnetic field probes with Fermilab’s.

« There could be a renewed effort to search for muons at the Large Hadron Collider for possible clues to the new physics behind to look for the g-2 value, « said Carlos Wagner, a theoretical physicist in Argonne’s HEP who is trying to explain these phenomena. « There could also be renewed interest in constructing a muon collider that provides a direct way to test this new physics. »

Once scientists get a grip on this new physics, it may be able to inform cosmological and quantum mechanical models or even help scientists invent new technologies – maybe the next shrink wrap.

The collaboration published an article on the result in Physical Review Letters entitled « Measuring the anomalous magnetic moment of the positive muon to 0.46 ppm ». An article on magnetic field measurement was also published in Physical Review A entitled « Magnetic field measurement and analysis for the Muon g-2 experiment in Fermilab ».

T. Albahri et al. Magnetic field measurement and analysis for the Muon g – 2 experiment in Fermilab, Physical Review A (2021). DOI: 10.1103 / PhysRevA.103.042208

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