Magnetic field mapping for the Fermilab Muon g-2 experiment.
As scientists await the much-anticipated initial results of the US Department of Energy (DOE) National Farm Accelerator Laboratory, the collaborating scientists from the DOE Argonne National Laboratory continue to use and maintain the unique field design system magnetic in the experiment with unprecedented accuracy.
Argon scientists updated the measurement system, which uses an advanced communication scheme and new magnetic field probes and electronics to map the field across the 45-meter perimeter ring in which the experiment takes place.
“There was a large deviation between the Brookhaven measurement and the theoretical prediction, and if we confirm this discrepancy, it would signal the existence of undetected particles.” – Simon Corrodi, appointed after the doctorate in the HEP division of Argonne
The experiment, which began in 2017 and continues today, could have major implications in the field of particle physics. As a follow-up to a previous experiment at the DOE Brookhaven National Laboratory, it has the power to affirm or lower previous results, which may shed light on the validity of parts of the Standard Model of the reign of particle physics.
High-precision measurements of significant quantities in the experiment are essential for the production of meaningful results. The main amount of interest is the gi muon factor, a property that characterizes the magnetic and quantum mechanical attributes of the particle.
The Standard model predicts the value of the muon factor g very accurately. “Because 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 fellow in the Argonne High Energy Physics (HEP) division. “There was a large deviation between the Brookhaven measurement and the theoretical prediction, and if we confirm this discrepancy, it would signal the existence of undetected particles.”
Just as the Earth’s rotating axis prevents – that is, the poles gradually traveling in circles – the rotation of the muon, a quantum version of angular momentum, precedes the presence of a magnetic field. The strength of the magnetic field that surrounds a muon affects the speed at which its processes rotate. Scientists can determine the muon factor g using measurements of the degree of rotational precession and the strength of the magnetic field.
The more accurate these initial measurements are, the more convincing the final result will be. Scientists are on their way to achieving accurate field measurements at 70 parts per billion. This level of precision enables the final calculation of the factor g to be accurate with four times the accuracy of the Brookhaven experiment results. If the value measured experimentally differs significantly from the expected value of the Standard Model, it may indicate the existence of unknown particles, the presence of which disturbs the local magnetic field around the muon.
During data collection, a magnetic field causes a beam of muons to travel around a large, empty ring. To compute the magnetic field strength across the ring with high resolution and precision, the scientists created a trolley system to run measuring probes around the ring and collect data.
The University of Heidelberg developed the wheelchair system for the Brookhaven experiment, and Argonne scientists renewed equipment and replaced electronics. In addition to the 378 probes that are mounted inside the ring to continuously monitor terrain extensions, the cart holds 17 probes that periodically measure the field with the highest resolution.
“Every three days, the cart goes around the ring in both directions, taking about 9,000 measurements per probe and direction,” Corrodi said. “Then we take measurements to build slices of magnetic field and then a complete, 3D map of the ring.”
Scientists know the exact location of the cart in the ring from a young barcode reader who records signs at the bottom of the ring as it moves.
The ring is filled with a vacuum to facilitate controlled decay of muons. To maintain the vacuum inside the ring, a garage connected to the ring and vacuum stores the cart between measurements. Automating the process of loading and unloading the trolley into the ring reduces the risk of scientists compromising the vacuum and magnetic field by interacting with the system. They also minimized the power consumption in the wheelchair electronics in order to limit the heat introduced into the system, which would otherwise impair the accuracy of the field measurement.
Scientists created the cart and garage to act on the ring’s strong magnetic field without affecting it. “We used a motor that works in strong magnetic field and with minimal magnetic signature, and the motor moves the cart mechanically, using wire,” Corrodi said. “This reduces noise in field measurements introduced by equipment.”
The system uses the smallest amount of magnetic material possible, and the scientists tested the magnetic trace of every single component using test magnets in University of Washington and Argonne to characterize the overall magnetic signature of the cart system.
The power of communication
Of the two cables that pull the trolley around the ring, one of them also acts as the power and communication cable between the control station and the measuring probes.
To measure the field, the scientists send a radio frequency through the cable to the 17 probes of the cart. The radio frequency causes the rotations of the molecules inside the probe to rotate in the magnetic field. The radio frequency is then turned off at the right moment, causing the rotations of the water molecules to be predetermined. This approach is called nuclear magnetic resonance imaging (NMR).
The frequency at which the probes rotate in advance depends on the magnetic field in the ring and a digitizer in the cart converts the analog radio frequency into multiple digital values communicated via the cable to a control station. At the control station, scientists analyze digital data to construct the rotation precession frequency and, from this, a complete map of the magnetic field.
During the experiment in Brookhaven, all signals were sent via cable simultaneously. However, due to the conversion from analog to digital signal in the new experiment, much more data has to pass through the cable and this increased rate may disturb the very accurate radio frequency needed to measure the probe. To prevent this inconvenience, the scientists split the signals in time, switching between the radio frequency signal and the data communication on the cable.
“We provide probes with a radio frequency through an analog signal,” Corrodi said, “and we use a digital signal for data communication. The cable passes between these two modes every 35 milliseconds.”
The tactic of switching between signals traveling through the same cable is called “time division multiplexing” and helps scientists achieve specifications not only accuracy, but also noise levels. An update from the Brookhaven experiment, time-division multiplexing allows for higher-resolution mapping and new skills in magnetic field data analysis.
Both the NMR system for field mapping and its motion control were successfully commissioned at Fermilab and have been in reliable operation during the first three periods of receiving the experimental data.
Scientists have achieved an unprecedented precision for field measurements, as well as the 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 structure in a paper, published in Instrumentation Journal.
Reference: “Design and performance of an in-vacuum system, of the magnetic field map for the Muon g-2 experiment” by S. Corrodia, P. De Lurgioa, D. Flayb, J. Grangea, R. Honga, D. Kawallb , M. Oberlinga, S. Ramachandrana and P. Wintera, 4 November 2020, Instrumentation Journal.
DOI: 10.1088 / 1748-0221 / 15/11 / P11008
This research was funded by the DOE Office of Science, High Energy Physics (HEP). The Fermilab particle accelerator complex is a DOE Science Office.