LHAASO measures the cosmic-ray knee one element at a time

Cosmic rays, the fast particles that rain on the Earth from space, do not arrive with a label saying where they were made. Astronomers infer their origin indirectly, and the single most important clue is a feature called the knee: at an energy around three to four petaelectronvolts, the number of particles arriving begins to fall much more steeply than the smooth power law that holds at lower energies. Something in the Galaxy stops accelerating cosmic rays, or starts losing them, at that scale, and for eighty years the debate has been over what that something is. A paper in the 26 March issue of Physical Review Letters reports a measurement from the LHAASO observatory that attacks the question from a new angle: instead of measuring all particles together, it measures the helium spectrum alone, and the helium spectrum shows structure that the all-particle curve had hidden.

The knee is thought to be connected to the maximum energy at which a galactic accelerator, most plausibly a supernova remnant, can hold and boost a charged particle. The acceleration limit depends on charge: a nucleus with atomic number Z can be boosted Z times further than a proton, so the lightest nuclei should run out of energy first. If that picture is right, the knee is not a single wall but a staircase, with each element’s spectrum bending at its own energy, and the composition of cosmic rays should change across the knee region, becoming heavier as energy rises. Measuring that predicted change has been difficult because ground-based observatories reconstruct cosmic rays from the air showers they create, and shower properties depend on the primary mass in ways that are easy to model and hard to constrain.

LHAASO, the Large High Altitude Air Shower Observatory on the Tibetan plateau, was designed precisely to sharpen this measurement. The new result reports the helium flux with a precision that allows the team to see the shape of the helium spectrum in detail across the knee region, and what they see is not a simple bend but a more complex evolution: features in the helium spectrum that exceed the expectations of the standard single-bend picture, together with a new pattern in how the abundance of helium relative to other elements changes with energy. The composition story that emerges is richer than the simple staircase, and it carries information about the sources and the propagation of galactic cosmic rays that the all-particle spectrum cannot provide.

The finding matters because the knee sits at the intersection of several open questions in high-energy astrophysics. Whether the acceleration limit is set by supernova remnants, whether the escape of particles from the Galaxy shapes the spectrum, whether there is a contribution from sources beyond the standard candidates, all of these predictions differ in how the individual element spectra behave, and all of them can now be tested against precise single-component measurements. The helium channel is particularly valuable because helium is the second most abundant cosmic-ray nucleus and the one whose behaviour most cleanly separates source physics from propagation effects.

The collaboration, led by the Institute of High Energy Physics of the Chinese Academy of Sciences with international partners including the University of Rome, describes the result as one step in a programme rather than a conclusion. The knee remains a problem with more than one candidate explanation, and a single element’s spectrum, however precise, will not settle it alone. But the measurement converts the old debate into a quantitative one: the predictions now have to match the shape of the helium knee, and the shape is not what anyone expected. That is exactly what a good measurement is supposed to do.