Every microsecond, billions of muons cascade through the cosmos from outer space, passing harmlessly through your body at near light-speed. These ghostly particles, two hundred times heavier than electrons, barely interact with ordinary matter at all. That’s precisely what makes them so useful. Like cosmic X-rays, they probe the interior of mountains and dams, revealing secrets hidden under tonnes of rock. They’re sensitive enough to discern fundamental physics that electrons can only hint at.
There’s a catch, though. A microsecond is barely long enough for a muon to travel three hundred metres, then they decay, vanishing in a burst of subatomic fragments. It’s been one of physics’ enduring frustrations: these particles are extraordinarily useful, yet gone before you can really use them. Physicists working at high-energy facilities have resigned themselves to this constraint, designing experiments around muons’ brief existence like architects building around an awkward plot boundary.
Now, researchers at the University of Plymouth think they’ve found a way around that problem. Using intense laser pulses, they believe they can stretch a muon’s lifetime by a factor of two, doubling the window for measurement and experiment. Better still, the lasers needed aren’t extraordinary. The technology already sits in laboratories around the world.
“I’ve always believed high power lasers have great potential to study fundamental physics,” says Dr Ben King, who led the theoretical work with colleague Di Liu. “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.” The results, published in Physical Review Letters this month, describe a mechanism so clever it bypasses decades of failed attempts to control muon decay.
What makes this breakthrough so surprising is that everyone assumed it couldn’t be done. For decades, physicists tried using extraordinarily powerful electromagnetic fields—so intense they’d tear apart atoms—to alter muon decay. The calculations always came to the same conclusion: you’d need fields far stronger than anything humans could create in the laboratory. Scientists moved on to other problems.
But King and Liu looked at the mathematics differently. Most previous work had assumed muons decayed inside an infinitely extended laser field. That’s a reasonable approximation if you’re working with theoretical abstractions, but it’s not what happens in practice. Real laser pulses have edges. They switch on and off in time. And that subtle difference, King realised, changes everything.
Here’s where quantum mechanics makes things wonderfully strange. When a muon passes through a laser pulse, something happens to its quantum state. The laser doesn’t knock the muon around classically—it’s too weak for that—but it does impart a quantum phase, like adding a particular tint to a wave of light. The crucial insight is that the muon can decay in multiple ways: before hitting the laser, after passing through it, or within it. These different “histories” of decay, in quantum mechanics, don’t stay separate; they interfere with each other, like sound waves crossing in air and either amplifying or cancelling depending on their phase relationship.
“What we showed is that this quantum interference between the muon decaying with or without interacting with the pulse creates fringes in the electron momentum spectra,” King explains. When the laser imparts exactly the right quantum phase, these interference patterns suppress the muon’s decay rate—essentially slowing down how quickly it falls apart. In theory, the suppression can be dramatic: the muon lifetime could increase by up to a factor of two.
The mechanism is genuinely elegant. “Our method suggests a general way to influence the decay of charged particles, even when one might naively expect the required electromagnetic strength to be much higher than anything we could ever hope to achieve in the lab,” says King. This sounds impossible on its face: how can a weak laser do what a trillion-tesla field cannot? The answer rests on something beautifully subtle: the controlling parameter doesn’t depend on how intense the laser field is. It depends on how far the muon gets displaced by that field—a classical effect, even though the overall mechanism is purely quantum.
The implications ripple outward. Muons created in high-energy collisions already have applications beyond fundamental physics. They can probe the interior of Egyptian pyramids (physicists have done this) and monitor the structural integrity of dams and bridges. By doubling their usable lifetime, researchers gain precious seconds—in muon time, an eternity—to extract data. The next generation of particle accelerators, expected to cost tens of billions of pounds, might use muons instead of electrons as their primary tool, because muons are more sensitive to certain kinds of new physics. Extending their lifetime makes such accelerators more practical.
But there’s a practical hurdle still standing. The theory is sound. King and Liu have shown mathematically that the effect should work with lasers available today. Yet no one has actually seen it happen. “This is a process that can be investigated with technology we have at our disposal today,” King says. “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.”
The first experiments, when they come, won’t be simple. Muons need to be created in the laboratory (usually by smashing high-energy particles into targets), then carefully aligned with a laser pulse. The laser itself must be timed with extraordinary precision—nanoseconds matter. Stray particles from the collision process could confuse measurements. The overlap between muon beam and laser focus must be near-perfect.
Yet none of this is fundamentally new. Physicists already create muons in labs using lasers in a different way. They just haven’t tried using a second laser to manipulate decay. The first team to pull this off will be attempting something theoretically predicted but experimentally virgin—the kind of work that attracts experimental physicists like nothing else.
King doesn’t expect the discovery overnight. Science, he’s careful to note, moves at its own pace. But the fact that the calculation works, that existing laser technology suffices, that the underlying physics is sound—these things have shifted what seemed impossible into the realm of plausible.
Muons, those ephemeral particles hurtling through the universe, might be about to live longer. And in the peculiar world of particle physics, where microseconds feel like hours, that matters far more than it should.
Study link: https://link.aps.org/doi/10.1103/823w-2g4b
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