Investigating double bump air showers with the SKA-Low

TL;DR

This work tackles the identification and characterization of rare double bump air showers by leveraging radio footprints observed with SKA-Low. It introduces an Akaike Information Criterion–based approach to distinguish single- and double-peaked longitudinal profiles and employs skeleton plots to connect leading-particle dynamics to the double-peak signature. The study demonstrates radio-based reconstruction of both shower maxima, revealing interference patterns and proposing paths to estimate the associated secondary energies. The findings underscore SKA-Low's potential to constrain hadronic interaction models and cosmic-ray composition at ultra-high energies, with reconstruction accuracy within ~$30$ g/cm$^2$ for the two maxima across independent methods.

Abstract

Double-bump showers are a rare class of extensive air showers (EAS) predicted by Monte Carlo simulations. They occur when a high-energy secondary particle, the leading particle, travels significantly farther than the rest, creating a distinct double-peaked longitudinal profile. So far, no experiment has been able to directly detect these showers. The unique radio footprint of double-bump showers, characterized by multiple Cherenkov rings, provides a way to reconstruct longitudinal profiles from radio observations. With its dense antenna array and broad frequency range, the Square Kilometer Array Observatory (SKAO) will be the first experiment capable of detecting these features, offering a new opportunity to probe hadronic interactions and constrain particle cross sections at high energies. In our analysis, we simulate the EAS using CORSIKA with the CoREAS plugin for radio. We developed a new method based on the Akaike information criterion to identify double bump showers in simulations by analyzing their longitudinal profiles. Then we investigate the prevalence of these double bump showers across different cosmic ray primary particles and various hadronic interaction models. We create a skeleton of the EAS which consists of all the particles with at least $1\%$ of the primary energy, allowing us to confirm the leading particle hypothesis and track shower development following these particles. This will enable us to relate the attributes of the leading particle to measurable parameters. Depending on the exact shower properties, the radio footprint of a double bump shower can create a complex interference pattern, consisting of multiple rings. From this information, the longitudinal profiles can be extracted. SKA due to its dense antenna array and frequency range will be the first experiment able to observe these double bump showers in detail.

Figures (6)

• Figure 1: The same double bump EAS as it would be detected by SKA (left) or LOFAR (right) because individual showers will be observed with thousands of antennas distributed much more homogeneously. The complete radiation pattern immediately becomes apparent compared to LOFAR. Middle: example traces (in the shower-plane) of antennas at roughly the same position measured as by both SKA and LOFAR taking into account the different bandwidths, the double bump nature is clear from the unusual waveform shape in the trace signal of the SKA antenna.
• Figure 2: Fraction of anomalous shower profiles for different primaries for different hadronic interaction models, plotted against $\lg(E/\text{eV})$. Each data point represents the percentage of anomalous shower profiles with double bumps detected for different primary particles (proton, helium, carbon, silicon, iron).
• Figure 3: Longitudinal profile of a double bump shower with the path travelled by all particles which have more than $1\%$ of the primaries energy. Left: The sub bump is caused by a negatively neutron penetrating deeply in the atmosphere, Single leading particle case. Middle: The sub bump is caused by positive pion followed by a negative kaon, Ladder case. Right: The sub bump is caused by two neutrons travelling parallel, Branching case.
• Figure 4: The different type of causes for double bumps: Single leading particle (single particle penetrating deeply), Ladder (successive penetration of particles together traveling a significant distance) and Branching (independent deeply penetrating particles traveling significant distances). A clear linear correlation is observed between $\Delta X$ and $\Delta X_{\text{max}}$ for events within $3\sigma$, as expected. Outliers from this trend are likely cases where the AIC method misidentifies showers as double-bump events, despite the underlying particle content suggesting otherwise.
• Figure 5: Left: Time-domain electric field traces for two antennas at different lateral distances from the shower core (at 50 m and at 200 m). Right: Corresponding Fourier transforms of the electric field components. A frequency dip around $100$ MHz is clearly visible for the closer antenna.