Reduction of coherent betatron oscillations in a muon g − 2 storage ring experiment using RF fields

Abstract

The muon g − 2 experiments store a beam of polarized muons in a weak focusing storage ring. As the ensemble of muons goes around the ring, their spin precesses, and when they decay through the weak interaction: µ + → e + νeνµ the decay positrons are detected by electromagnetic calorimeters. In addition to the expected exponential decay in the positron time spectrum, the weak decay asymmetry causes a modulation in the number of positrons in a selected energy range at the difference frequency between the spin and cyclotron frequencies, ωa. This frequency is directly proportional to the magnetic anomaly a = (g − 2)/2, where g is the gyromagnetic ratio of the muon, which is slightly greater than 2. The detector acceptance depends on the radial position of the muon decay, so the coherent betatron oscillations of the muon bunch following injection into the storage ring amplitude modulates the measured muon signal with the frequency ωCBO. Applying radio frequency (RF) electric fields has the potential to significantly decrease the beam CBO, and additionally to scrape the beam to reduce muon losses from the storage ring during the measurement period. Preliminary tests showed significant CBO amplitude reduction.

Measurement of the muon anomalous precession frequency $\omega_a$ in the Fermilab Muon $g-2$ experiment

“…Looking ahead, the Muon 𝑔 − 2 Collaboration has collected three additional years of data, having surpassed its design goal of 21 times the Brookhaven experiment [5]. In addition, improved running conditions through the implementation of a RF signal [17] to the focusing electrostatic quadrupole system [16] reduces the CBO oscillation. With more data and improved running conditions, we are on target to reach and improve upon our uncertainty goal.…”

Status of the Muon g − 2 experiment

“…After the first data taking period, a few key upgrades were performed that will help to reduce some of the systematic uncertainties compared to the Run-1 result. These upgrades include i) new qaudrupole resistors replacing the broken ones which will significantly reduce C pa , ii) modifications of the kicker system to increase the voltage and center the beam radially, iii) thermal magnet insulation and better hall cooling to reduce the field drift, iv) improved quadrupole transient measurements significantly reducing the largest systematic uncertainty in Run-1, and v) a new RF system [68] which suppresses the coherent betatron oscillation amplitude and its associated systematic uncertainty. A final result for the Run-4/5/6 datasets is anticipated about 2-3 years from now.…”

Status of the Fermilab Muon $g-2$ Experiment

“…The kicker strength has been improved [10], and the electrostatic quadrupole resistors have been upgraded. New beam scraping techniques and quadrupole radio-frequency pulses have been put in place to increase the storage efficiency and minimize the betatron oscillations [11]. On the analysis side, independent efforts from the ω a groups try to increase pileup separation and decrease gain related systematics.…”