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Muons are elementary particles that resemble electrons, but they are heavier and decay very rapidly (i.e., in just a few microseconds). Studying muons can help to test and refine the standard of particle physics, while also potentially unveiling new phenomena or effects.

So far, the generation of muons in experimental settings has been primarily achieved using proton accelerators, which are large and expensive instruments. Muons can also originate from cosmic rays, rays of high-energy particles originating from outer space that can collide with atoms in the Earth's atmosphere, producing muons and other secondary particles.

Researchers at the China Academy of Engineering Â鶹ÒùÔºics (CAEP), Guangdong Laboratory, the Chinese Academy of Sciences (CAS) and other institutes recently introduced a new method to produce muons in experimental settings, using an ultra-short high-intensity laser.

Using this method, outlined in published in Nature Â鶹ÒùÔºics, they attained a high-yield of muons, reaching up to 0.01 muon per incoming electron.

"Muons play an important role in both basic physical research and applied physical research," Yuqiu Gu, co-author of the paper, told Â鶹ÒùÔº.

"Generally, muons come from cosmic rays or proton accelerators. The former is limited by a very low flux (less than 1/cm²/min), and the latter is restricted by limited facilities and high operation costs. Benefiting from the rapid development of the Chirped Pulse Amplification (CPA) technology, currently based on the Laser Wakefield Acceleration (LWFA) technology, electrons can be accelerated to the GeV level within a few centimeters."

Leveraging recently developed laser amplification techniques, Gu and his colleagues set out to generate muons via the interaction of high-energy electrons with a conversion target. Their recent paper is the first to report the successful generation of muons in a laser laboratory.

"The interaction between high-energy electrons and the conversion target is a very complex process, involving many secondary radiation processes such as gamma rays, neutrons, electrons, and so on," explained Gu.

"Since the production cross-section of muons is not high, it is difficult to confirm the generation of muons by means of a magnetic spectrometer and other methods, and the detector is prone to be saturated by those secondary radiations."

To confirm that the particles generated by their ultra-short high-intensity laser are indeed muons, the researchers thus had to devise an alternative approach that does not rely on magnetic spectrometers. Ultimately, they were able to identify muons by measuring their rest decay lifetime (i.e., how long they lived at rest before decaying).

"On the one hand, because muons have a lifetime in the microsecond range, they can avoid the interference of prompt secondary radiations," said Gu. "On the other hand, the lifetime of muons (2.2 μs) is a unique physical signal, which can be easily distinguished from other accidental coincidence backgrounds."

The proof-of-principle experiment carried out by Gu and his colleagues attained very promising results.

With their proposed methods, they were able to detect the lifetime spectrum of the particles generated clearly and the spectrum observed is consistent with the known lifetime of muons, confirming that they did in fact produce muons.

"We generated a muon source for the first time on a new platform at a laser laboratory," said Gu.

"We reached a yield of 0.01 μ/e. Taking this experiment as an example, the muon yield can reach 10⁷ muons per shot. In the Supplementary information to the article, we have further estimated the yields of surface muons and decay muons under the current experimental conditions, and the yield is expected to reach 10³/s muons.

"This new type of muon source makes it possible for small laser laboratories to conduct muon-related research (such as high-energy muon radiography, μSR, MIXE, etc.), thus greatly reducing the threshold for muon application research."

The new approach devised by this team of researchers could soon enable the efficient generation of muons in smaller laboratories, relying on laser technology. In the future, it could thus open new exciting possibilities for muon-related research, which could potentially yield new results and achievements.

"Next, we will first conduct in-depth research on the energy spectrum distribution and angular distribution of this muon source," added Gu.

"We then plan to use this muon source to carry out studies such as muon point projection radiography, all-optical muon acceleration, and muon nuclear excitation et al."

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