Nuclear physicists have developed a non-invasive way to measure the “spin tune” of polarized protons. Proton spin describes the direction a proton “turns” on its axis. The spin tune is the number of times during a rotation that a proton’s axis shifts. The protons in the Relativistic Heavy Ion Collider (RHIC)’s beam all need to spin in the same direction. Measuring the spin tune helps scientists maintain these particles’ alignment. The measurement technique is similar to how magnetic resonance imaging (MRI) manipulates proton spin to “see” structures inside the body. Like an MRI, scientists use the technique as a “diagnostic” tool.
To understand how the building blocks of protons—quarks and gluons—contribute to spin, nuclear physicists collide beams of protons. The protons’ spin directions are “polarized” or aligned as much as possible in the beam. External forces and some methods to measure deviations can make the protons in the beam less spin polarized. The new technique noninvasively measures how often and much the protons’ axes deviate from a perfectly aligned path as the accelerator operates. This measurement gives physicists information they can use to make adjustments that keep the protons aligned.
With polarized beams in RHIC—or any circular accelerator—the average spin direction of each proton bunch aligns with the accelerator’s magnetic field. But like a spinning top that starts to wobble, a proton’s axis sometimes starts to rotate around a circular path that deviates from perfect alignment. If some outside source, such as small imperfections in the magnetic field, syncs up with the frequency of this deviation, called precession, it can amplify the “wobble” of the protons and cause the beam to become depolarized. There are ways to measure the precession frequency. However, current techniques cause the depolarization such measurements would seek to avoid. Now nuclear physicsts at RHIC have found a new way to measure the frequency of the precession without depolarizing the beam so they can make corrections to keep the protons aligned (or even flip their spin direction when desired).
The technique operates similar to the way MRI scans work: First, a powerful magnetic field aligns all the protons’ spins, then scientists apply a variable frequency external electromagnetic field, searching for the frequency at which the protons’ axes start to tip away from stable—like tuning an old fashioned radio knob to search for a station. The key is to get close to the tipping frequency without triggering the destabilization. In MRI’s, the signals generated by protons’ precession give information about the inner structures of the body. At RHIC they give information about how to adjust the accelerator’s magnets to maintain beam polarization. The technique will lead to more stable and optimized operation at RHIC and could also be used at a planned polarized Electron-Ion Collider to be located in the United States.
Collider-Accelerator Department, Brookhaven National Laboratory
Work was supported by Brookhaven Science Associates, LLC by the Nuclear Physics program in the Office of Science of the U.S. Department of Energy.
H. Huang, et. al., “Measurement of the spin tune using the coherent spin motion of polarized protons in a storage ring.” Physical Review Letters 122, 204803 (2019). [DOI: 10.1103/PhysRevLett.122.204803]