What Are UHECRs Made Of?

When a cosmic ray strikes Earth's atmosphere, we see only the shower it produces — not the particle itself. Determining whether that original particle was a lone proton or an iron nucleus with 56 nucleons is one of the field's greatest challenges. The answer determines almost everything about source identification.

Why Composition Matters

The composition of ultra-high-energy cosmic rays affects every aspect of the field:

Magnetic Deflection

Charged particles curve in magnetic fields proportionally to their charge. An iron nucleus (Z=26) bends 26 times more than a proton of the same energy. If UHECRs are protons, we might trace them back to sources. If they're iron, arrival directions tell us almost nothing about origins.

Propagation Distance

Protons lose energy through pion production on the CMB (GZK effect). Heavy nuclei instead undergo photodisintegration — the CMB photons knock off individual nucleons. The horizons are different, affecting which sources can contribute.

Source Requirements

Accelerating an iron nucleus to 10²⁰ eV is "easier" than accelerating a proton to the same energy — the Hillas criterion scales with charge Z. A source that can barely reach 10²⁰ eV for protons could reach 26 × 10²⁰ eV for iron. Different compositions imply different source populations.

The Measurement Challenge

At ultra-high energies, we can't catch cosmic rays directly. We observe air showers — cascades of billions of secondary particles. From this shower, we must infer what started it.

The fundamental problem: shower development has large statistical fluctuations. A proton shower reaching deep in the atmosphere might look like an iron shower reaching shallow. Any single event is ambiguous.

Composition studies therefore rely on statistical analysis of many events, comparing distributions to model predictions.

The Primary Observable: Xmax

The most powerful composition diagnostic is Xmax — the atmospheric depth at which the shower reaches maximum particle number. It's measured in g/cm² of atmospheric matter.

Physical Basis

A heavy nucleus can be thought of as many independent nucleons. An iron nucleus of energy E behaves roughly like 56 nucleons each with energy E/56. These "sub-showers" begin developing immediately after the first interaction.

Result: heavy nuclei reach shower maximum earlier (at smaller Xmax). The average Xmax differs by about 100 g/cm² between protons and iron at ultra-high energies.

Fluctuations

Equally important: the width of the Xmax distribution. Proton showers show large fluctuations (σ ~ 60 g/cm²) because the first interaction depth varies and a single nucleon drives development. Iron showers fluctuate less (σ ~ 20 g/cm²) because 56 independent sub-showers average out.

Measuring both ⟨Xmax⟩ and σ(Xmax) provides two constraints on composition.

The Elongation Rate

As energy increases, Xmax grows logarithmically — the "elongation rate." For pure compositions, this rate is about 50-60 g/cm² per decade of energy. Changes in the elongation rate indicate composition evolution with energy.

Current Results

Both Auger and Telescope Array have measured Xmax distributions, reaching somewhat different conclusions:

Pierre Auger Results

Auger's fluorescence detectors have accumulated the world's largest Xmax dataset. Key findings:

• At 10¹⁸ eV: composition consistent with light nuclei (protons and helium)
• Above 10¹⁸·⁵ eV: gradual shift toward heavier composition
• At 10¹⁹·⁵ eV: composition intermediate (CNO-like or mixed)
• Fluctuations decrease with energy, supporting the heavy trend

Telescope Array Results

TA's Xmax measurements suggest lighter composition than Auger across most energies — closer to proton-dominated. Whether this reflects:

• Different sky regions (TA sees north, Auger sees south)
• Systematic differences in analysis
• Hadronic model dependencies

remains under active investigation through joint working groups.

Secondary Diagnostics

Muon Number

Heavy nuclei produce more muons than protons at the same energy. Auger measures that showers contain more muons than simulations predict — the "muon puzzle." This could indicate:

• Composition heavier than Xmax suggests
• New physics in hadronic interactions at ultra-high energies
• Issues with hadronic models

The muon excess persists regardless of which interaction model is used, suggesting a real phenomenon rather than model-dependent artifact.

Risetime

The signal risetime in surface detectors depends on shower development — earlier development (heavy composition) gives different risetimes. This provides a surface-detector-based composition proxy with 100% duty cycle.

AugerPrime: The Future

The AugerPrime upgrade adds scintillator detectors atop the water tanks. By comparing the two signals event-by-event, the electromagnetic and muonic components can be separated. This will dramatically improve composition sensitivity for the highest-energy events.

Model Dependence

All composition interpretations require hadronic interaction models to translate shower observables into primary particle type. The main models (EPOS-LHC, QGSJet-II, SIBYLL) give somewhat different composition interpretations for the same data.

LHC data has helped constrain these models in the forward region, but extrapolation to UHECR energies (100× higher) remains necessary. The muon puzzle suggests this extrapolation may be imperfect.

Implications

If Composition Is Heavy

A heavy composition at the highest energies would mean:

• Sources must accelerate heavy nuclei (not all can)
• Source environments must preserve nuclei (not photodisintegrate them)
• Magnetic deflection is severe — arrival directions are nearly random
• The GZK suppression might be photodisintegration, not pion production
• Charged particle astronomy is very difficult

If Composition Is Light

A proton-dominated composition would mean:

• Sources can be proton accelerators (broader range of candidates)
• Magnetic deflection is moderate — some directional information survives
• The GZK suppression is classic pion production
• Cosmogenic neutrinos should be detectable (they're not — tension?)

The Mixed Reality

Most likely, composition is mixed and evolves with energy. This is scientifically interesting but observationally challenging — each energy bin may have a different mix, complicating interpretation.

Photon and Neutrino Primaries

Searches for photon and neutrino primaries provide complementary constraints:

Photons

Ultra-high-energy photons would produce distinctive showers: purely electromagnetic, developing very deep, with essentially no muons. No photons have been definitively identified; upper limits constrain exotic "top-down" source models.

Neutrinos

Neutrinos rarely interact, but when they do (deep in the atmosphere or in the Earth), they produce detectable showers. Searches look for nearly horizontal or Earth-skimming events. No detections yet; limits constrain cosmogenic neutrino models.

The Path Forward

Resolving the composition question requires:

• More statistics: Especially at the highest energies where events are rare
• Better measurements: AugerPrime's event-by-event composition sensitivity
• Improved models: LHC forward data, possibly future higher-energy colliders
• TA-Auger agreement: Understanding systematic differences

Composition is the key that unlocks cosmic ray origins. Until we know what these particles are, we can't confidently say where they come from.