Muonic Helium SEOP

Introduction: The Polarization Challenge

The study of muonic helium, an exotic atom where a negative muon replaces an electron, offers a unique pathway to test fundamental physics, such as CPT invariance, by precisely measuring its hyperfine structure (HFS). However, these measurements face a formidable obstacle. The very process of forming a muonic helium atom is violent, causing the negative muon to lose almost all of its initial spin polarization, a property that is essential for the HFS measurement. The residual polarization is only about 5%, resulting in a very weak experimental signal and severely limiting the achievable precision. To overcome this, a method to restore the lost polarization is required.

The Proposed Solution: Spin-Exchange Optical Pumping (SEOP)

The proposed solution to this problem is a clever technique known as Spin-Exchange Optical Pumping (SEOP). The concept was first demonstrated by Barton et al. and is now being adapted and modernized by the MuSEUM collaboration at J-PARC. The principle is to turn the experimental target itself into a source of spin polarization. The method involves filling a glass cell with helium gas and a small amount of alkali metal vapor, such as Rubidium (Rb) and Potassium (K). A high-power laser is then used to optically pump the alkali atoms, aligning their electron spins to achieve a very high degree of polarization. These polarized alkali atoms then act as a “spin reservoir.” When the unpolarized muonic helium atoms collide with the polarized alkali atoms, they exchange spins, effectively “repolarizing” the muons within the muonic helium atoms. This process is expected to dramatically boost the experimental signal.

The collaboration has adopted an advanced version of this technique known as Alkali-Hybrid SEOP (AH-SEOP), which uses a mixture of Rb and K. In this scheme, only the Rb atoms are directly polarized by the laser, but their polarization is rapidly transferred to the K atoms through collisions. This hybrid approach is significantly more efficient, as it minimizes the wasted laser power lost to spin-destruction collisions, allowing for a higher density of polarized atoms to be achieved.

Development of the Polarized Target

To implement this technique, significant research and development were undertaken. A prototype sealed Pyrex glass cell was fabricated, containing a mixture of Rb and K metals, pressurized helium gas, and a small amount of nitrogen gas to aid the optical pumping process. Initial offline tests were highly successful. The cell was heated to 200°C to vaporize the alkali metals, and a 75 W diode laser was used for optical pumping. Using Electron Paramagnetic Resonance (EPR) spectroscopy, the team confirmed that the alkali metal vapor could be polarized to nearly 100%. These tests also revealed a materials science challenge: the Pyrex glass showed signs of degradation after prolonged operation at high temperatures, likely due to a reaction with the alkali metals. This led to the conclusion that future experiments must use cells made from a more robust, alkali-resistant aluminosilicate glass.

The First Successful Demonstration at J-PARC

The culmination of this development was a milestone experiment conducted at the J-PARC MLF D-Line. For the first time, the repolarization of muonic helium atoms using the SEOP method was successfully demonstrated. In this experiment, a negative muon beam was stopped in the target cell containing the polarized alkali vapor. The key observation was the time evolution of the decay electron asymmetry, which is the direct signal of the muon’s polarization.

When the polarizing laser was on, the asymmetry signal was observed to increase over time, growing by a factor of 1.5 to 2. This time-dependent growth is the unmistakable signature of successful spin exchange; it is direct evidence that the spins were being transferred from the polarized alkali atoms to the muons in the muonic helium atoms, restoring their lost polarization.

Conclusion and Future Impact

The successful demonstration of SEOP repolarization is a critical breakthrough for the muonic helium HFS program. It validates a key technology that directly addresses the experiment’s primary bottleneck, paving the way for a new generation of high-precision measurements. The combination of this repolarization technique with the high-intensity muon beam at the future H-Line facility will dramatically enhance the statistical power of the experiment. This will enable an unprecedented test of CPT symmetry in the second-generation lepton sector and a world-leading, precise determination of the fundamental properties of the negative muon.