🔬 applied physics

Measurement of directional muon beams generated at the Berkeley Lab Laser Accelerator

Researchers at the Lawrence Berkeley National Laboratory successfully demonstrated the generation of high-flux, directional muon beams using a petawatt laser-driven electron beam and a high-Z converter, proving that such compact laser-plasma sources can produce muons in numbers far exceeding cosmic backgrounds and significantly enhancing muon imaging applications.

Original authors: Davide Terzani, Stanimir Kisyov, Stephen Greenberg, Luc Le Pottier, Maria Mironova, Alex Picksley, Joshua Stackhouse, Hai-En Tsai, Raymond Li, Ela Rockafellow, Bo Miao, Jaron Shrock, Timon Heim, Mauri

Published 2026-07-28

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Davide Terzani, Stanimir Kisyov, Stephen Greenberg, Luc Le Pottier, Maria Mironova, Alex Picksley, Joshua Stackhouse, Hai-En Tsai, Raymond Li, Ela Rockafellow, Bo Miao, Jaron Shrock, Timon Heim, Maurice Garcia-Sciveres, Carlo Benedetti, John Valentine, Howard Milchberg, Kei Nakamura, Anthony J. Gonsalves, Jeroen van Tilborg, Carl B. Schroeder, Eric Esarey, Cameron G. R. Geddes

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to take a picture of a giant, ancient castle, but you can't move the castle, and you don't have a flashlight strong enough to see through its thick stone walls. In the world of physics, scientists use a special kind of "ghost particle" called a muon to solve this puzzle. Muons are like cosmic messengers that rain down on Earth from space. They are heavy, fast, and incredibly tough, able to punch through mountains and steel without stopping. Because they are so penetrating, scientists use them to take pictures of things that are hidden, like the inside of a volcano or the secret chambers of a pyramid. This technique is called "muography."

However, there's a catch: the muons from space are like a gentle, steady drizzle. They arrive slowly and randomly, so to get a clear picture of a large object, you might have to wait for months or even years for enough of them to pass through and hit your detector. It's like trying to fill a bucket with a single drop of water every hour. To make this process faster and more practical, scientists have been looking for a way to create their own "muon rain" right in the lab, using powerful lasers to shoot out a concentrated beam of these particles on demand. This is the challenge tackled in a new study from the Lawrence Berkeley National Laboratory.

The Laser-Powered Muon Factory

In this exciting new work, a team of researchers at the Berkeley Lab Laser Accelerator (BELLA) facility successfully demonstrated how to create a directional beam of muons using a high-powered laser. Think of their setup as a particle slingshot. They fired a massive laser pulse, lasting only about 40 femtoseconds (that's 0.00000000000004 seconds!), into a jet of hydrogen gas. This interaction acted like a wake in the ocean, creating a "plasma wave" that grabbed electrons and accelerated them to incredible speeds, reaching energies of up to 9.2 GeV (gigaelectronvolts).

Once these super-fast electrons were ready, the team didn't just let them fly off into space. Instead, they steered the beam into a massive block of shielding material—essentially a giant wall made of lead, steel, and concrete. When the high-speed electrons slammed into this wall, they didn't just stop; they sparked a chain reaction. The collision created a shower of new particles, including the muons the scientists were hunting for.

The Great Muon Hunt: Finding the Ghosts

The team set up their "trap" in a room behind the thick concrete wall. They used special detectors called scintillators—plastic panels that glow when a particle hits them. But here's the tricky part: the room was also filled with a chaotic storm of other particles (like photons and electrons) that arrived instantly when the laser fired. If the scientists just looked for a flash of light, they would see a blinding mess and wouldn't know if a muon was there.

So, the researchers used a clever trick based on time. They knew that while most particles zoomed through the detectors instantly, some muons were slow enough to get stuck in the plastic. Once stuck, these muons would live for a tiny fraction of a second (about 2.2 microseconds) before decaying into an electron and a burst of light. By waiting for this "delayed signal," the team could filter out the noise and say with confidence, "Aha! We caught a muon!"

What They Found: Two Different Muon Streams

The experiment revealed something fascinating: the muons weren't all coming from the same place or behaving the same way. The team identified two distinct "streams" of muons, much like finding two different types of fish in a river.

The Directional Stream (The "Sniper"): Some muons were born right inside the heavy lead block of the beam dump. These were created through a process called "pair production," where high-energy photons turn directly into a muon and an anti-muon. These muons were like snipers: they were high-energy, traveled in a tight, straight line, and kept going in the same direction as the original electron beam. The simulations showed that these muons were confined within a narrow cone of about 100 milliradians.
The Scattered Stream (The "Crowd"): The other muons came from a different source. As the electron beam traveled toward the dump, it grazed other metal parts of the machine (like the magnetic spectrometer). These interactions created short-lived particles called "mesons," which then decayed into muons. These muons were like a scattered crowd; they didn't have a strong direction and spread out in all angles. They were also lower in energy and were mostly found away from the center of the beam.

The data confirmed this split. When the detectors were placed right in the path of the beam ("on-axis"), they caught very few muons (about 1.9% of the shots). But when they moved the detectors 1 meter to the side ("off-axis"), the count jumped significantly (about 14.4% of the shots). This proved that the "scattered crowd" of meson-decay muons was dominating the area away from the beam, while the "sniper" muons stayed tightly focused near the center.

Why This Matters

The researchers used powerful computer simulations (based on the Geant4 toolkit) to model exactly what was happening. These simulations matched their real-world measurements almost perfectly, giving them high confidence in their results. They found that their laser-driven source could produce muons at a rate that is orders of magnitude higher than the natural rain of cosmic muons.

While the current setup used the lab's existing shielding to generate the muons, the team suggests that in the future, we could design specialized targets to make this process even more efficient. If we can build laser systems that fire faster (like thousands of times a second instead of just once a second), we could generate enough muons to scan large objects—like a volcano or a nuclear reactor—in just a few minutes instead of months.

This paper doesn't just suggest that laser-driven muon sources are possible; it proves they work. It shows that we can create a compact, directional beam of these ghostly particles right in a laboratory, opening the door to a new era of fast, high-resolution imaging that doesn't rely on waiting for the universe to send us a gift. The authors conclude that with further improvements, such as staging multiple laser accelerators to reach even higher energies, this technology could revolutionize how we see the hidden world around us.

Technical Summary: Measurement of Directional Muon Beams Generated at the Berkeley Lab Laser Accelerator

Problem Statement
Muon beams are valuable for fundamental physics research and non-destructive imaging (muography) due to their high penetration power. However, conventional muon sources rely on large-scale proton accelerators colliding with fixed targets, making them unsuitable for on-site imaging of large or immobile objects. Currently, the only accessible source for such applications is cosmic ray muons. While ubiquitous, cosmic rays offer a low flux (approx. $70 \cos^2(\theta) \text{ m}^{-2}\text{s}^{-1}\text{sr}^{-1}$ ), necessitating exposure times of months to accumulate statistically significant signals. This limitation restricts muography to static targets. There is a need for a compact, directional, and high-yield muon source that can reduce exposure times by orders of magnitude.

Methodology
The authors utilized the BELLA (Berkeley Lab Laser Accelerator) PW laser facility at Lawrence Berkeley National Laboratory to generate muons. The experimental setup involved:

Electron Beam Generation: A 21 J, $\sim$ 40 fs laser pulse drove a 30 cm hydrogen gas jet. Using ionization injection with a nitrogen dopant, the system produced quasi-monoenergetic electron bunches with peak energies up to 9.2 GeV and charges averaging 80 pC (integrated above 2 GeV).
Muon Production: The electron beam was deflected by a 1.08 T dipole magnet and directed into a massive electron beam dump composed of lead, steel, and concrete (totaling $\sim$ $\sim$ 142 radiation lengths). The interaction of the high-energy electrons with the high-Z materials in the dump and along the beam path (including laser diagnostics and the spectrometer) generated muons via two primary mechanisms:
- Bethe-Heitler Pair Production: High-energy Bremsstrahlung photons interacting with nuclei to produce $\mu^+\mu^-$ pairs.
- Meson Decay: Photoproduction of pions and kaons which subsequently decay into muons.
Detection: Scintillator panels were placed in a measurement room behind a 90 cm concrete wall. To distinguish muons from the prompt background of the electron beam interaction, the team utilized the delayed decay signature of stopped muons. Muons with energies $\lesssim 20$ MeV stopped in the plastic scintillators and decayed ( $\mu \to e + \nu + \bar{\nu}$ ) with a characteristic lifetime of $\tau_\mu \approx 2.2 \mu$ s. The detectors recorded the time delay between the initial beam trigger ( $t=0$ ) and the decay signal.
Simulation: A custom Geant4-based simulation code was developed to model the entire experimental geometry, including the beam dump, shielding, and detector response. The simulation incorporated the measured electron beam spectra and pointing fluctuations to reproduce the experimental muon flux and spatial distribution.

Key Results

Muon Identification: The experiment successfully identified muons by analyzing the time-delay distribution of signals in the scintillators. Fitting the decay curve yielded a lifetime of $\tau = 2.24 \pm 0.50 \mu$ s, consistent with the known muon lifetime.
Spatial Separation of Sources: The study demonstrated a clear spatial distinction between two muon populations:
- Directional Source: Muons generated via pair production in the beam dump are highly collimated (within $\sim$ 100 mrad) and high-energy. These were detected primarily in the "on-axis" configuration.
- Quasi-Isotropic Source: Muons generated via meson decay in upstream high-Z elements (before the beam dump) are not filtered by the dump's shielding. They form a quasi-isotropic background, detected significantly in the "off-axis" configuration (1 m from the beam axis).
Quantitative Yields:
- The experiment recorded 126 $\pm$ 12 stopped muons over a two-hour period (760 shots).
- The probability of detecting a stopped muon per shot was $(1.9 \pm 0.5)\%$ on-axis and $(14.4 \pm 1.4)\%$ off-axis.
- Simulations indicated a total muon flux behind the wall of $F \gtrsim 10 \text{ m}^{-2}\text{shot}^{-1}$ for the experimental beam parameters.
Comparison to Cosmic Rays: For a setup parallel to the Earth's surface with a 100 mrad aperture, the LPA source produces $>40$ times more muons per second than cosmic rays (assuming 1 Hz operation and 100 pC beams). The directional yield is orders of magnitude higher than the isotropic cosmic background.

Significance and Claims
The paper claims to demonstrate the feasibility of using Laser-Plasma Accelerators (LPAs) as a compact source for directional, high-energy muon beams. The authors assert that LPA-based sources offer unique advantages over cosmic rays, specifically:

Compactness and Deployability: The source is generated in a facility of tens of centimeters, unlike the multi-kilometer scales of proton-based facilities.
Enhanced Imaging Speed: The high yield and directionality of LPA-generated muons can reduce imaging exposure times from months to minutes or hours.
Scalability: The authors note that future improvements, such as staged acceleration to reach $>10$ GeV electron energies and kHz repetition rate lasers, could increase muon yields by orders of magnitude.

The work establishes that LPA-generated electron beams possess the necessary characteristics (high energy, low divergence) to produce muons suitable for muography. While the current experiment used the laboratory's shielding configuration as a converter, the authors suggest that optimized converter targets could further maximize flux and minimize energy losses. The study concludes that LPA technology is ready for application development, particularly for imaging large objects where cosmic ray flux is insufficient.

The Laser-Powered Muon Factory

The Great Muon Hunt: Finding the Ghosts

What They Found: Two Different Muon Streams

Why This Matters

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