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A large-scale radio detector for the Pierre Auger cosmic-ray Observatory – precision measurements of ultra-high-energy cosmic rays

Periodic Reporting for period 2 - Auger-Horizon (A large-scale radio detector for the Pierre Auger cosmic-ray Observatory – precision measurements of ultra-high-energy cosmic rays)

Reporting period: 2020-04-01 to 2021-09-30

The project is an experimental activity in the field of astroparticle physics with the goal to understand the physics and the sources of the highest-energy particles in the Universe.

Cosmic Rays (ionized atomic nuclei) are the only matter from beyond our solar system or even from extragalactic space, that we can directly investigate. Up to energies of 10^17 eV they most likely originate in our Galaxy. The highest-energy cosmic rays (>10^18 eV) cannot be magnetically bound any more to the Galaxy and are most likely of extragalactic origin.

The pure existence of these particles raises the question about their origin – how and where are they accelerated? How do they propagate through the universe and interact? How can we directly probe extragalactic matter and how can we locate its origin?
A key to understand the origin of cosmic rays is to measure the particle species (atomic mass). A precise mass measurement will allow discriminating astrophysical models and will clarify the reason for the observed suppression of the cosmic-ray flux at the highest energies, namely the maximum energy of the accelerators or the energy losses during propagation.

We address these questions by employing a new technique to precisely measure the cosmic-ray mass composition, which my group pioneered, the radio detection of air showers (induced by high-energy cosmic rays in the atmosphere) on very large scales, detecting horizontal air showers with zenith angles from 60° to 90°.
The new set-up will be the world-largest radio array, operated together with the well-established Auger surface and fluorescence detectors, forming a unique set-up to measure the properties of cosmic rays with unprecedented precision for energies above 10^17.5 eV. In particular, the radio technique is a cost-effective and robust method to measure the cosmic-ray energy and mass, complementary to established techniques. The energy scale of the radio measurements is established from first principles. The proposed detectors will also enhance the detection capabilities for high-energy neutrinos and the search for new physics through precision measurements of the electromagnetic and muonic shower components.
We have developed new detection systems for the radio emission of extensive air showers. These systems are comprised of a radio antenna, a low-noise pre-amplifier, and a digitizer. The latter contains an amplifier and a bandwidth filter as well as an analogue to digital converter to record the radio emission from air showers in the frequency band from 30 to 80 MHz. The units will be installed at the Surface Detector array of the Pierre Auger Observatory in Argentina. A completely new and complementary layer of detectors will be added to the observatory, forming a 3000 km2 radio array for cosmic rays, the largest of its kind.

In November 2019 an engineering array has been installed at the observatory, comprised of 10 detection units. The units are used routinely to record radio signals from air showers. First air showers have been successfully recorded with the prototype systems and we are working on algorithms to reconstruct the properties of cosmic rays from the recorded signals. As a calibration reference we are using the radio emission from our Milky Way. For this purpose, firmware has been developed which runs on board the digitizer and analyses the recorded data in real time. Parameters are extracted from the data which allow us to assign an absolute scale for the magnitude of the recorded signals. The results from the prototype detector array at the Auger observatory indicate that the developed detectors are working according to our expectations.
The next important steps of the project are to start the mass production of the almost 2000 detection units and install them at the Pierre Auger Observatory in the Argentinian Pampas. This will be a major improvement for the biggest detector for cosmic rays on Earth.
This is then followed by the exploitation of the recorded data to increase our knowledge about the physics and origin of the highest-energy particles in Nature. We expect to gain detailed insights in the mass composition of cosmic rays at the highest energies. I.e. we will clarify what particles (protons, ionized nuclei, neutrinos, gamma rays) nature is sending to us at extreme energies.
Prototype detectors installed at the Pierre Auger observatory in Malargüe, Argentina.
The PI working at a prototype station at the Pierre Auger Observatory in Argentina
Impression of an upgraded detector station