GIGAS

Ultra High Energy Cosmic Rays (UHECR) are a unique mean to study and test fundamental physics and astrophysics at energies (> 1 Joule) well above those available to current terrestrial accelerators. However, these studies are extremely challenging. The particles “beam” is unknown, the flux is very low (less than one per km² and per year) and we cannot observe directly the primary interaction but only the cascade of secondary particles it produces (the Extensive Air Shower or EAS). To overcome these limitations, one measures the EAS components in as much detail as possible and in particular the evolution of their electromagnetic component. Modern observatories, such as the Pierre Auger Observatory, have been designed as hybrid detectors: they include a surface particle detector array (sampling the electromagnetic and muonic components at ground) and fluorescence telescopes (observing the longitudinal development of the electromagnetic component). However, hybrid measurements are only available 10% of the time due to the duty cycle limitations of the fluorescence technique. The goal of the GIGAS proposal is to demonstrate the possibility to measure the time evolution of the electromagnetic cascade with 100% duty cycle for all EAS energies above 1 J. To do so, we will measure the microwave emission produced by the electrons and positrons of the cascade, connecting microwave sensors to each Auger surface detector. Such a measurement will provide a calorimetric energy estimate and an excellent identification of the primary particle on an event-by-event basis. Individual identification of UHECR primaries is in itself a major progress since primary separation is today only possible on a statistical basis. Our approach is both innovative and conservative. It exploits the microwave signal of EAS conjugated with the detection and reconstruction capabilities of a “traditional” array. We are the only groups in the world working on this symbiotic approach. Furthermore, a pre-prototype of 7 sensors deployed on the Auger array to test our integration method observed for the first time in the world a signal from an EAS in the 3.4 to 4.2 GHz band. Of course, a lot still needs to be done to understand all the characteristics of this signal but this first success promise solid results. The GIGAS proposal covers the optimization of the microwave sensors for the particular case of Auger, as well as a medium-scale (300 km2 or 10% of Auger surface) demonstrator that we will operate in symbiosis with the Auger detectors. This project carries ambitious scientific expectations. It will allow for the measurement of hadronic cross sections above 100 TeV center-of-mass and it will also help to model hadronic interactions, to test fundamental physics laws, to identify cosmic ray sources and to provide constraints on the Galactic and extra-galactic magnetic fields. Moreover adding profile information about the electromagnetic component of the shower will significantly improve measurements of ultra-high energy gamma and neutrino induced air showers. While ambitious in terms of science, this project is well rooted from the instrumental point of view. The use in an existing facility, the Auger Observatory, which we contributed to design and build, and which we know very well, will allow a timely, efficient and cost effective realization.