Scientists work to shed light on the Standard Model of particle physics

Typical magnetic field variations as mapped by the trolley at different positions in the storage ring of the Muon g-2 experiment, presented at the parts per million level. Credit: Argonne National Laboratory.

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

Argonne scientists have improved the measurement system, which uses an advanced communications system and new magnetic field probes and electronics to map the field in the 45-meter circumference ring in which the experience.

The experiment, which started in 2017 and continues today, could have big implications for the field of particle physics. As a follow-up to a past experiment at the DOE’s Brookhaven National Laboratory, it has the power to affirm or refute previous findings, which could shed light on the validity of parts of the reigning standard model of science. particle physics.

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

The standard model predicts very accurately the value of the g-factor of the muon. “Because the theory predicts this number so clearly, testing the g-factor by experiment is an effective way to test the theory,” said Simon Corrodi, a postdoctoral fellow appointed in the High Energy Physics (HEP) division of Argonne. . “There was a large discrepancy between the Brookhaven measurement and the theoretical prediction, and if we confirm this discrepancy, it will signal the existence of undiscovered particles.”

Just as the Earth’s axis of rotation precedes – meaning the poles move gradually in circles – the spin of the muon, a quantum version of angular momentum, precedes in the presence of a magnetic field. The strength of the magnetic field surrounding a muon influences the rate at which its spin precedes. Scientists can determine the g-factor of the muon by measuring the rate of spin precession and the strength of the magnetic field.

The more precise these initial measurements are, the more conclusive the final result will be. Scientists are on their way to make accurate field measurements at 70 parts per billion. This level of precision allows 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 value expected from the Standard Model, this 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, scientists designed a cart system to drive measurement probes around the ring and collect data.

Heidelberg University developed the cart system for the Brookhaven experiment, and Argonne scientists refurbished the equipment and replaced the electronics. In addition to the 378 probes mounted in the ring to continuously monitor field drifts, the cart contains 17 probes that periodically measure the field with higher resolution.

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

Scientists know the cart’s exact location within the ring thanks to a new barcode reader that registers marks at the bottom of the ring as it moves.

The ring is filled with a void to facilitate controlled muon decay. To preserve the vacuum inside the ring, a garage connected to the ring and the vacuum stores the trolley between measures. Automating the process of loading and unloading the cart 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 of the cart’s electronics to limit the heat introduced into the system, which would otherwise disrupt measurement accuracy in the field.

Scientists work to shed light on the Standard Model of particle physics

Fully assembled cart system with wheels for rolling on rails and the new external barcode reader for exact position measurement. The 50 cm long cylindrical shell houses the 17 NMR probes and the custom-made reading and control electronics. Credit: Argonne National Laboratory.

The scientists designed the trolley and the garage to operate in the strong magnetic field of the ring without influencing it. “We used a motor that operates in a strong magnetic field and with a minimal magnetic signature, and the motor moves the cart mechanically, using ropes,” Corrodi said. “This reduces noise in the field measurements introduced by the equipment.”

The system uses as little magnetic material as possible, and scientists tested the magnetic footprint of each component using test magnets at the University of Washington and Argonne to characterize the system’s overall magnetic signature. cart.

The power to communicate

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

To measure the field, scientists send a radio frequency through the cable to the cart’s 17 probes. The radio frequency rotates the spins of the molecules inside the probe in the magnetic field. The radio frequency is then cut off at the right moment, causing the spins of the water molecules to precess. This approach is called nuclear magnetic resonance (NMR).

The frequency at which the probes’ rotations precede depends on the magnetic field in the ring, and a digitizer on board the cart converts the analog radio frequency into multiple digital values ​​communicated via cable to a monitoring station. At the control station, scientists analyze the digital data to construct the spin precession frequency and, from this, a complete map of the magnetic field.

During the Brookhaven experiment, all signals were sent simultaneously over the cable. However, due to the conversion from analog to digital signal in the new experiment, a lot more data has to travel over the wire, and this increased speed could disrupt the very precise radio frequency needed for the probe measurement. To avoid this disruption, the scientists separated the signals in time, switching between the radio frequency signal and the data communication in the cable.

“We feed the probes a radio frequency via an analog signal,” Corrodi said, “and we use a digital signal to communicate 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 scientists achieve specifications not only for accuracy, but also for noise levels. An upgrade of the Brookhaven experiment, time division multiplexing allows for higher resolution mapping and new capabilities for analyzing magnetic field data.

Upcoming results

The field-mapping NMR system and its motion control were successfully commissioned at Fermilab and operated reliably for the first three data-taking periods of the experiment.

Scientists 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 set of data from 2018 and plan to publish the results by the end of 2020.

The scientists detailed the complex setup in a paper titled “Design and Performance of a Vacuum Magnetic Field Mapping System for the Muon g-2 Experiment,” published in the Instrumentation review.


Muons tell tales of undiscovered particles


More information:
S. Corrodi et al, Design and performance of a vacuum magnetic field mapping system for the Muon g-2 experiment, Instrumentation review (2020). DOI: 10.1088/1748-0221/15/11/P11008

Provided by Argonne National Laboratory

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