Gigahertz Identification of Giant Air Showers – GIGAS

Optimization and construction of GHz sensors: We have developed, constructed and installed a set of autonomous gigahertz radio sensors in order to equip part of the detectors constituting the surface array of the Pierre Auger Observatory in Argentina. This demonstrator, that we have realized with different technologies, operates in two frequency ranges, the L band between 1 and 2 GHz and the band C between 3.4 and 4.2 GHz. In total we have installed 75 antennas in 3 sub-networks covering an area just above 100 km2. GIGAS combines the operation of the microwave signal of the cascades and the detection and reconstruction capacities of a conventional particle detector array. Our team is the only one in the world to use such an approach where radio detection is associated at the fundamental detector level with particle detectors. The principle of the measurement consists in continuously digitizing the power of the radio signal received by the antenna with a temporal resolution of 25 nanoseconds. When the particles of the cascade reach the ground, the Auger particle detector triggers the recording of a radio time sequence of 20 microseconds, thus comprising almost a thousand points along the radio emission profile of the cascade.

We recorded a total of about ten events where the radio signal is clearly identifiable. These signals all come from cascades whose core position is less than 250 meters from the detector. This characteristic calls into question the origin of the signal which cannot be attributed unequivocally to MBR. Moreover, the weakness of the signal has led us to study in detail the MBR emission in the peculiar conditions of atmospheric cascades. These calculations are complex and suffer from approximations which are difficult to validate, apart from direct experimentations. However, they represent a significant improvement over existing estimates. The MBR of large atmospheric showers is much lower than what publications made before 2012 were advancing. Despite the high sensitivity of our sensors, no signal could be attributed unambiguously to this mechanism.

Publications:

• Molecular Bremsstrahlung Radiation at GHz Frequencies in Air, Imen Al Samarai, Corinne Bérat, Olivier Deligny, A. Letessier-Selvon, F. Montanet, Mariangela Settimo, Patrick Stassi, Phys.Rev. D93 (2016) no.5, 052004

• An Estimate of the Spectral Intensity Expected from the Molecular Bremsstrahlung Radiation in Extensive Air Showers, Imen Al Samarai, Olivier Deligny, D. Lebrun, A. Letessier-Selvon, F. Salamida , Astropart.Phys. 67 (2015) 26-32

• The Small Contribution of Molecular Bremsstrahlung Radiation to the Air-Fluorescence Yield of Cosmic Ray Shower Particles, Imen Al Samarai, Olivier Deligny, Jaime Rosado, Astropart.Phys. 83 (2016) 1-5

• GIGAS, a set of microwave detector arrays to measure molecular Bremsstrahlung radiation from extensive air shower, Romain Gaior, Imen Al Samarai, Corinne Bérat, Olivier Deligny, Antoine Letessier-Selvon, François Montanet, Mariangela Settimo, Patrick Stassi, Hervé Lebbolo, submited to Nucl. Instr. And Meth. A, september 2017

Although very promising, the EAS radio detection technique to explore the interactions of the highest energy cosmic rays is ultimately unsuitable for instrumenting surfaces of the order of several tens of thousands of square kilometres .The estimate of the emission intensity given by the SLAC T471 experiment proved, to say the least, fanciful with a difference of at least two orders of magnitude according to the finer estimates and experimental measurements which have followed.
It may be that an unambiguous detection of the EAS through their MBR signal can be done but it will require much larger infrastructure and resources than what GIGHAS could possibly provide. To improve the sensitivity of sensors by several orders of magnitude, it is essential to cool down the antennas in order to lower the system temperature to a few degrees (against several tens of degrees, 50 to 100 degrees for the current GIGAS sensors) and to increase the effective area of the antennas to several square meters (compared to a few tens of square centimeters). This type of infrastructure, similar to that of fluorescence detectors currently in use in Auger (less the cryogeny), could eventually be used to construct a network of tens of thousands of square kilometres but is certainly not cheap.

Completed PhDs:

• Extensive air shower identification using electron radiometer, Romain Gaior, 2013

• Different approaches to determine the composition of the ultra-high energy cosmic rays in the Pierre Auger Observatory, Miguel Blanco Otano, 2014

• Radiodétection des gerbes atmosphériques à l'observatoire Pierre Auger, Sandra Le Coz, 2014

Publications :
• Molecular Bremsstrahlung Radiation at GHz Frequencies in Air, Imen Al Samarai, Corinne Bérat, Olivier Deligny, A. Letessier-Selvon, F. Montanet, Mariangela Settimo, Patrick Stassi, Phys.Rev. D93 (2016) no.5, 052004

• An Estimate of the Spectral Intensity Expected from the Molecular Bremsstrahlung Radiation in Extensive Air Showers, Imen Al Samarai, Olivier Deligny, D. Lebrun, A. Letessier-Selvon, F. Salamida , Astropart.Phys. 67 (2015) 26-32

• The Small Contribution of Molecular Bremsstrahlung Radiation to the Air-Fluorescence Yield of Cosmic Ray Shower Particles, Imen Al Samarai, Olivier Deligny, Jaime Rosado, Astropart.Phys. 83 (2016) 1-5

• GIGAS, a set of microwave detector arrays to measure molecular Bremsstrahlung radiation from extensive air shower, Romain Gaior, Imen Al Samarai, Corinne Bérat, Olivier Deligny, Antoine Letessier-Selvon, François Montanet, Mariangela Settimo, Patrick Stassi, Hervé Lebbolo, submited to Nucl. Instr. And Meth. A, september 2017

Publications d’actes de conférences (invité):
• Studies of the microwave emission of extensive air showers with GIGAS and MIDAS at the Pierre Auger Observatory, Romain Gaïor, ICRC2017
• Recent results and developments of EASIER, Romain Gaïor, Rencontres de Moriond 2017
• Detection of Cosmic Rays Using Microwave Radiation at the Pierre Auger Observatory, Romain Gaïor, ICRC2013
• Intensity of Microwave Signals Expected from Molecular Bremsstrahlung Radiation in Extensive Air Showers, Imen Al Samarai ICRC2015
• An estimate of the spectral intensity expected from the molecular Bremsstrahlung radiation in extensive air showers, Imen Al Samarai ARENA2014
• Radio detection of Cosmic Rays in the GHz band at the Pierre Auger Observatory, Imen Al Samarai ARENA2014

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.