No Need for Fractional Particles
At first glance, the scattering of a massless, charged, fermionic particle off a single magnetic monopole might seem a simple phenomenon to model. Yet when two theorists independently tackled that problem in 1982, they came to the same puzzling conclusion: In the framework of conventional field theory, the scattering resulted in fractional numbers of particles, called semitons [1]. Now Vazha Loladze of Oxford University and Takemichi Okui of Florida State University have taken a fresh look at the puzzle [2]. According to their calculations, the solutions that were understood to imply semitons in fact embody the free propagation of the fermion.
Loladze and Okui’s starting point was a 1984 theory that reduced the dimensions of the problem to a spatial one and a temporal one and that treated the magnetic monopole as a stationary “rotor” with a single internal degree of freedom whose state changed over time [3]. Despite its simplicity, the 1984 model preserved—and clarified—the complicated physics. It also retained semitons. Loladze and Okui modified this model by exciting the rotor and the particle in a way that accounted for the particle’s speed of approach and the rotor’s time-varying state. Doing so removed the semitons but left the rest of the 1982 and 1984 theories intact.
The scattering—and consequent splitting into semitons—of a proton’s quarks off a magnetic monopole has been proposed as a route for proton decay, which shows up unavoidably in certain grand unified field theories. No one has observed a proton decay, which implies that magnetic monopoles are rare. But if magnetic monopoles are no longer implicated in this process, they could be more abundant.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- V.A. Rubakov, “Adler-Bell-Jackiw anomaly and fermion-number breaking in the presence of a magnetic monopole,” Nucl. Phys. B 203, 311 (1982); C. G. Callan, “Disappearing dyons,” Phys. Rev. D 25, 2141 (1982).
- V. Loladze and T. Okui, “Monopole-fermion scattering and the solution to the semiton–unitarity puzzle,” Phys. Rev. Lett. 134, 051602 (2025).
- J. Polchinski, “Monopole catalysis: The fermion-rotor system,” Nucl. Phys. B 242, 345 (1984).