HMN 2025: How New knowledge from ALICE could contribute to fixing the cosmic muon puzzle

Cosmic rays are high-energy particles from outer house that strike Earth’s environment, producing showers of secondary particles, reminiscent of muons, that may attain the planet’s floor. In current years, ground-based experiments have detected extra cosmic muons than present theoretical models predict, a discrepancy often known as the muon puzzle.

Underground experiments provide good circumstances for the detection of cosmic muons, as a result of the rock or soil above the experiments absorbs the opposite bathe parts. They might subsequently assist to resolve the muon puzzle. One instance is ALICE on the Large Hadron Collider (LHC).

Designed to review the merchandise of heavy-ion collisions, ALICE can be well-suited for detecting cosmic muons due to its location in a cavern 52 meters underground, shielded by 28 meters of overburden rock and an extra 1 meter of iron magnet yoke.

In a current article published within the Journal of Cosmology and Astroparticle Physics, the ALICE collaboration reviews the detection of round 165 million occasions containing a minimum of one cosmic muon, in addition to 15,702 occasions with greater than 4 cosmic muons.

This giant pattern was collected between 2015 and 2018 throughout pauses in LHC Run 2, when no particle beams had been circulating within the collider. The complete data-taking time amounted to 62.5 days—greater than double the length of the previous cosmic-ray campaign in LHC Run 1 (2010–2013), which recorded roughly 22.6 million occasions with a minimum of one muon.

By analyzing how the variety of occasions varies with rising muon multiplicity (the variety of muons per occasion), the ALICE collaboration noticed a easy, lowering development from a multiplicity of 5 to a multiplicity of fifty, past which the numbers of occasions are very small and topic to giant statistical uncertainties (see determine beneath).

The ALICE researchers in contrast this lowering muon multiplicity distribution with simulations based mostly on three models of secondary-particle manufacturing and assuming two excessive compositions of main cosmic rays—hydrogen nuclei (protons), representing the lightest doable composition, and iron nuclei, representing a really heavy composition.

These comparisons confirmed that the measured distribution corresponds to main cosmic rays with energies starting from 4 to 60 PeV, where 1 PeV is 10¹⁵ electronvolts. In this power vary, the composition of the first cosmic rays is predicted to be a mix of nuclear species, from protons to iron.

One of the three models reproduces the noticed distribution, however solely when assuming that the first cosmic rays are composed of iron. By distinction, the opposite two models underpredict the occasion depend even when assuming an iron composition.

While these outcomes counsel that heavy parts dominate the composition of the first cosmic rays, they fail to account for the anticipated combined composition and the rising fraction of heavy parts as multiplicity, and thus main cosmic-ray power, will increase.

Focusing on uncommon occasions with greater than 100 muons, the researchers discovered that these high-multiplicity occasions are effectively described by two of the models when assuming an iron composition. These findings are suitable with a median power of about 100 PeV for the first cosmic rays that probably produced these occasions.

The new ALICE outcomes affirm the discrepancy between ground-based knowledge and models that constitutes the muon puzzle. Improving the models by incorporating these outcomes from the LHC could assist to resolve the puzzle.