First results from the ARGO-YBJ experiment

The central array of the ARGO detector at the YangBaJing Cosmic Ray Laboratory (4300 m a.s.l., Tibet, P.R. China) has been put into operation for physics runs. It is made of 130 identical sub-units of 12 RPCs each (”cluster”) covering a surface of about 5800 m 2 with 92% active area. Signals are picked-up by external electrodes of small size, thus allowing the sampling of EAS with high space-time granularity. Shower events are detected at a trigger rate of about 4 kHz. Events with a few particles detected by a single cluster are counted in scaler mode on a time base of 500 milliseconds. Preliminary results from the analysis of events collected in a few months of data taking are presented