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Demonstrating Agreement between Radio and Fluorescence Measurements of the Depth of Maximum of Extensive Air Showers

January 2024  Understanding the origin and nature of ultra-high energy cosmic rays has been one of the driving forces of the field of astroparticle physics. A key piece of information is what type of particles cosmic rays consist of. This study focused on measuring where the extensive air showers produced by cosmic rays reach their maximum size in the atmosphere. This depth of shower maximum (Xmax) depends on the masses of cosmic rays.

In this work, we have used 7 years of Auger Engineering Radio Array (AERA) data to investigate Xmax at energies where the cosmic-ray origin is expected to transition from Galactic to extragalactic sources. AERA consists of about 150 antennas in a grid that can measure the short pulses of radio emission that are produced in the air shower from the movement of electrons and positrons. This emission closely follows the location of the particles in the shower and so provides an independent way to study the shower.

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Figure 1: Mean (left) and width (right) of the distribution of Xmax values measured by AERA (black) as a function of energy. For comparison, the results of the fluorescence telescopes at the Pierre Auger Observatory are shown (gray) as well as various models for mass composition (lines).per limits on the fluence (energy per area) of ultra-high energy photons from the selected merger events for the long (a) and the short (b) time window.


We show our Xmax results to be in agreement with the results of the fluorescence telescopes (FD) at the Pierre Auger Observatory. Figure 1 shows the mean and spread in Xmax as a function of shower energy for both measurements (black and gray). The measurements can be interpreted in terms of particle masses with air shower simulation (red and blue lines). The figure shows this for various models for how the particle interactions work at these high energies.

In addition, the compatibility of the radio and fluorescence methods to measure Xmax is also demonstrated on an event-by-event level with simultaneous radio and fluorescence measurements of the same air showers. Figure 2 shows these measurements for 53 air showers that had a high-quality measurements with both methods. The mean difference is determined to be Xmax(AERA)-Xmax(FD)=-3.9+-11.2 g/cm2, demonstrating there to be no significant difference. Despite very different physical processes leading to the fluorescence and radio signals and very different methods to reconstruct Xmax both measurements agree, strongly suggesting that these
processes are now well-understood.

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Figure 2: Comparison of Xmax measured by AERA and FD.


With our method, we are able to achieve competitive high-resolution Xmax reconstructions, reaching resolutions near 15 g/cm2 at the highest energies. Figure 3 shows how this depends on the shower energy and how it compares to the fluorescence technique at the Pierre Auger Observatory. With this, we demonstrate that the reconstruction of Xmax with AERA is also competitive with established methods.

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Figure 3: Resolution of the Xmax reconstruction as a function of shower energy for AERA (green) and FD (gray).

In this work, we have been able to show that the radio-Xmax method is a viable, competitive, and independent method to probe the mass composition of cosmic rays.

 

Related papers:

Demonstrating Agreement between Radio and Fluorescence Measurements of the Depth of Maximum of Extensive Air Showers at the Pierre Auger Observatory
The Pierre Auger Collaboration, Phys. Rev. Lett. 132 (2024) 021001 (sibling of the PRD)
[arxiv.org/abs/2310.19963 ] [doi: 10.1103/PhysRevLett.132.021001 ]

Radio Measurements of the Depth of Air-Shower Maximum at the Pierre Auger Observatory
The Pierre Auger Collaboration, Phys. Rev. D 109 (2024) 022002 (sibling of a PRL)
[arxiv.org/abs/2310.19966] [doi: 10.1103/PhysRevD.109.022002 ]

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