Somewhere above our heads, every second, a proton accelerated to energies millions of times greater than those at the Large Hadron Collider slams into the atmosphere. It strikes an air molecule, and a cascade is born — a wide atmospheric shower of thousands of particles. Physicists catch these showers with ground-based detectors, but until now they couldn't say exactly what happened in the very first instant, at the point of the first impact.
That's because the subsequent development of the shower is like a snow avalanche: no matter how different the first snowflakes are, the same kind of flow rolls downhill. Same here — after the first collision, the particle cascade evolves according to nearly universal laws, and the details of the initial interaction are erased.
But the authors of the new work have found two numbers that together retain the memory of the first impact. The first is the depth at which the shower reaches its maximum size. The second is the number of muons, heavy relatives of the electron, that reach the ground. It turns out that if you look at how these two quantities fluctuate from shower to shower, their combined fluctuations reveal the imprint of how the primary proton shared its energy among secondary particles. It's as if you could reconstruct which exact rock started an avalanche by the shape of the avalanche itself.
The new method does not require knowing all the details of complex nuclear interactions. It relies on the fact that the second part of the shower is nearly universal, and all the uncertainty is concentrated in the first impact. The authors showed that the errors from this simplification are smaller than the spread among modern models and comparable to the precision of current experiments. So by measuring these two quantities together, we can directly test how particles behave at energies unattainable on Earth.
So now physicists have a new tool — cosmic billiards, where the scatter of balls reveals the first cue strike. And the more showers we catch, the more precisely we'll learn what the fastest particles in the universe are made of.
A single ultra-high-energy proton carries as much energy as a well-served tennis ball, but concentrated in an object the size of an atomic nucleus.
In Greg Egan's novel Schild's Ladder, physicists use ultra-high-energy cosmic rays to test theories beyond the Standard Model — the new method is precisely a step toward such tests.