Thin Double-gap RPCs for the Phase-2 Upgrade of the CMS Muon System

High-sensitive double-gap phenolic Resistive Plate Chambers are studied for the Phase-2 upgrade of the CMS muon system at high pseudorapidity $\eta$. Whereas the present CMS RPCs have a gas gap thickness of 2 mm, we propose to use thinner gas gaps, which will improve the performance of these RPCs. To validate this proposal, we constructed double-gap RPCs with two different gap thicknesses of 1.2 and 1.4 mm using high-pressure laminated plates having a mean resistivity of about 5 $\times$ 10$^{10}$ $\Omega$-cm. This paper presents test results using cosmic muons and $^{137}$Cs gamma rays. The rate capabilities of these thin-gap RPCs measured with the gamma source exceed the maximum rate expected in the new high-$\eta$ endcap RPCs planned for future Phase-2 runs of LHC.