The comparatively large mass of the muon also makes it a sensitive probe for fundamental physics, and the particles underpin a range of experiments in high energy and astroparticle physics. One long standing limitation, however, is that muons are unstable and possess a half life of only about one microsecond, restricting the time window over which they can be exploited in practical measurements and precision tests.
Researchers at the University of Plymouth have now outlined a theoretical method for slowing muon decay using intense laser pulses, with calculations indicating that the technique could at least double the effective lifetime of the particles. They argue that extending muon lifetimes in this way could benefit applications across geology, materials science and particle physics, where muon based methods are already in use or under active development.
The work also has implications for the design of future large scale research facilities, including proposals for next generation accelerators that would use muons instead of electrons as the primary particle species. Muons have been suggested as attractive candidates for such machines because their greater mass can reduce certain beam related limitations and improve sensitivity for exploring new physics, provided their short lifetimes can be mitigated.
The new theory is presented in Physical Review Letters by Associate Professor of Theoretical Physics Ben King and Postdoctoral Research Fellow Di Liu from the University of Plymouth's School of Engineering, Computing and Mathematics. Describing the motivation behind the study, Dr King said that he has long regarded high power lasers as having strong potential for investigations of fundamental physics and that recent advances justified revisiting long held assumptions about muon instability.
"I've always believed high power lasers have great potential to study fundamental physics," said Dr King. "Although it was long thought to be effectively impossible to modify the natural instability of muons, we decided to revisit the question in the light of developments in experiment and theory. Ultimately, we were able to find a new route to influencing the muon's lifetime that circumvented the established difficulties."
Muons are generated both in man made environments and in nature. In laboratory settings they appear in particle collisions in accelerator facilities, while in the atmosphere they are created when high energy cosmic rays from space strike molecules in the upper air. Experiments have also recently shown that firing an intense laser at a thin target can generate several exotic particle species, including muons, opening another potential route for controlled production.
Because muons begin to decay almost as soon as they are created, the Plymouth project, supported by funding from the Leverhulme Trust, set out to determine whether a strong laser field could alter their decay properties. The researchers developed a theory that exploits a quantum mechanical principle known as quantum interference, which states that different pathways leading to the same outcome can behave like waves and either reinforce or cancel one another when combined.
In this case, the outcome of interest is muon decay, and the various decay routes can interfere in a way that changes the overall probability for the process to occur. The theory predicts that this interference pattern should manifest itself in the spatial distribution of the decay products detected in an experiment, providing a measurable signature of the modified decay dynamics in the presence of the laser field.
Dr King noted that the proposed effect lies within reach of existing experimental capabilities rather than requiring unattainable laser strengths or exotic facilities. "This is a process that can be investigated with technology we have at our disposal today. We are working with others in the field to overcome any remaining hurdles before experiments can be performed, such as excluding background processes and ensuring a good overlap of the muons with the laser," he said.
The researchers stress that their method represents a general strategy for influencing the decay of charged particles using electromagnetic fields, even when straightforward estimates suggest that the required field strengths would far exceed those accessible in laboratories. By exploiting interference and carefully chosen configurations, they argue, it should be possible to achieve significant modifications to decay rates without exceeding realistic technological limits.
If confirmed experimentally, the approach could open a new front in the control of unstable particles, adding a tunable parameter to muon based imaging and measurement techniques across science and engineering. It could also feed into the conceptual design of future muon collider facilities and other advanced infrastructure, where extending particle lifetimes and understanding their decay in complex electromagnetic environments are central challenges for both theory and experiment.
Research Report:Vacuum Muon Decay and Interaction with Laser Pulses
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