Cosmic rays are charged particles that constantly bombard Earth from deep space. When one of these particles enters the atmosphere, it collides with air nuclei and spawns a whole cascade of secondary particles: electrons, muons, photons. This cascade is called a wide atmospheric shower, and it can stretch for kilometers. By studying the shower, scientists try to figure out what particle arrived and how much energy it carried, but it's not easy. The shower looks like a smeared-out trail, not a crisp footprint.
At the LHAASO observatory (Large High Altitude Air Shower Observatory) in China, researchers have devised a new way to reconstruct the energy and mass of the primary particle. Instead of collecting the entire shower, they measure the particle density at a fixed distance from the shower axis, like focusing on the clarity of a single point in a footprint rather than the whole print. This approach turned out to be more accurate: energy uncertainty ranges from 5 to 15 percent, and for heavy nuclei like iron, the mass can be pinned down to better than 25 percent. The method works uniformly for all particle types and is almost independent of which interaction model physicists use — a common headache in such measurements.
Why does this matter? Cosmic rays with energies above about 1 PeV (peta-electronvolt — a thousand times more than the Large Hadron Collider can produce) carry information about the most powerful accelerators in the universe: supernova explosions, pulsars, active galactic nuclei. The cosmic-ray spectrum has a break — the so-called "knee"—where the number of particles drops sharply. This might mark the limit of our galaxy's ability to contain particles or the ceiling of their acceleration. LHAASO's new method will help clarify which particles and energies actually reach Earth, and thus where and how they were born.
This approach is like going from a blurry photo to a sharp snapshot: less noise, more detail. And it opens the door to a more precise map of cosmic accelerators.
The most energetic cosmic particle ever recorded had an energy of about 300 EeV: that's like the impact of a tennis ball moving at 100 km/h, but packed into a single proton.
In Stanislaw Lem's novel "His Master's Voice," the protagonists try to decipher a signal from space but find their instruments distorting the data — a similar problem faces physicists reconstructing cosmic-ray properties from atmospheric showers.