STACEX: a RPC-based detector for a multi-messenger Southern observatory in the GeV-PeV range

Recent findings by the LHAASO experiment are opening a new window, that of the PeV sky, to the observation of the electromagnetic spectrum. Several astronomical objects emitting gamma-rays at energies well above 100 TeV have been observed with the LHAASO-KM2 array of scintillators and muon detectors, clearly demonstrating the feasibility of gamma-ray astronomy up to PeV energies. An all-sky gamma-ray detector in the Southern Hemisphere, operating in the GeV-PeV range, could complement LHAASO observations, monitor the Inner Galaxy and the Galactic Center looking for PeVatrons. As shown by LHAASO, a water-Cherenkov based detector is not well suited to measure the energy spectrum up to the PeV range, nor to reach the advisable 100 GeV threshold. The ARGO-YBJ experiment, operated for many years at 4300 m a.s.l. with an energy threshold of about 300 GeV, demonstrated, on the contrary, the capability of a carpet of Resistive Plate Chambers (RPCs) to fully reconstruct showers starting from the GeV range up to about 10 PeV. In this contribution we propose a hybrid detector made of a layer of RPCs on top of a water Cherenkov facility devoted to the detection of muons for the selection of gamma-induced showers by the muon-poor technique. We present the layout and discuss the expected performance.