HL LHC perspectives for the ATLAS RPC system

The architecture of the present muon trigger detector was conceived in the early nineties having in mind a fast, robust and simple device. According to the Atlas requirements the ageing qualification were done taking a reference luminosity of 1034 cm-2 s-1included a safety factor of 5 with respect to the simulated background rates. The experience made so far has shown that the average rate measured in 2012 LHC run is very close to the predicted one, however it is unevenly distributed in the barrel: the chambers at the barrel edge (i.e. higher eta values) show rates of about a factor of 2.5 higher than the average thus absorbing part of the safety factor. In the next decade, ATLAS will run with an increasing luminosity and beam energy resulting in much higher background rate, up to almost one order of magnitude with respect to the present condition. This will affect both the detector rate capability and robustness against the ageing effects and the demand of trigger performance to increase the muon momentum selectivity and coverage. This situation will make the present RPC muon trigger to run in a critical condition, imposing to design an appropriate upgrade plan involving both the detector and the trigger and readout electronics. We present here a comprehensive upgrade scheme for the ATLAS RPC system, which will allow withstanding the HL LHC conditions and the very long future operation scheduled, of about 20 years. This upgrade pivots on an additional layer of next generation RPC chambers to increase the redundancy and the coverage, as it was foreseen originally in the ATLAS RPC proposal. The present RPC system performance will be enhanced with a new readout electronics necessary also to cope with the high luminosity trigger scheme; a tight maintenance and tuning program is also necessary improve and preserve the actual chambers performance. A non-negligible side effect of this upgrade scheme is providing ATLAS with good TOF capabilities with a few hundreds of ps resolution with 10 m lever arm with four independent measurements, extending the ATLAS potential for new physics discoveries.