Evidence for the charge-excess contribution in air shower radio emission observed by the CODALEMA experiment

•Observation of the charge-excess mechanism in the cosmic-ray air showers.•The shift in the core positions of the showers is due to the charge-excess.•We are using simulated showers with SELFAS to demonstrate the role of the charge excess.

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). 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.