CODALEMA is one of the pioneer experiments dedicated to the radio detection of ultra high energy cosmic rays (UHECR), located at the radio observatory of Nançay (France).

Figure: the experimentally observed core shift. The black dots represent the radio core positions relatively to the particle core positions of the 216 selected events detected by CODALEMA. The red lines are the contour levels of a 10 m Gaussian smoothed map. The average radio core position is shifted toward the east by 30 m. Figure: the experimentally observed core shift. The black dots represent the radio core positions relatively to the particle core positions of the 216 selected events detected by CODALEMA. The red lines are the contour levels of a 10 m Gaussian smoothed map. The average radio core position is shifted toward the east by 30 m.
The CODALEMA experiment uses both a particle detector array and a radio antenna array. Data from both detection systems have been used to determine the ground coordinates of the core of extensive air showers (EAS). We discuss the observed systematic shift of the core positions determined with these two detection techniques. We show that this shift is due to the charge-excess contribution to the total radio emission of air showers, using the simulation code SELFAS. The dependences of the radio core shift to the primary cosmic ray characteristics are studied in details. The observation of this systematic shift can be considered as an experimental signature of the charge excess contribution.

Published by A. Bellétoile, R. Dallier, A. Lecacheux, V. Marin, L. Martin, B. Revenu, D. Torres, in Astroparticle Physics 69, April 2015, P. 50–60 https://dx.doi.org/10.1016/j.astropartphys.2015.03.007