ATLAS Transition Region Upgrade at Phase-1

This report presents the L1 Muon trigger transition region (1.0<|ƞ|<1.3) upgrade of ATLAS Detector at phase-1. The high fake trigger rate in the Endcap region 1.0<|ƞ|<2.4 would become a serious problem for the ATLAS L1 Muon trigger system at high luminosity. For the region 1.3<|ƞ|<2.4, covered by the Small Wheel, ATLAS is enhancing the present muon trigger by adding local fake rejection and track angle measurement capabilities. To reduce the rate in the remaining ƞ interval it has been proposed a similar enhancement by adding at the edge of the inner barrel a structure of 3-layers RPCs of a new generation. These RPCs will be based on a thinner gas gap and electrodes with respect to the ATLAS standards, a new high performance Front End, integrating fast TDC capabilities, and a new low profile and light mechanical structure allowing the installation in the tiny space available.This design effectively suppresses fake triggers by making the coincidence with both end-cap and interaction point. A preliminary study based on 2012 Run data demonstrates that the new system could reject more than 90% of the fake triggers and provides high efficiency. Finally this upgrade will contribute to keep the L1 muon trigger rate at a manageable level also improving L1 muon trigger selectivity, thanks to its granularity, by discriminating the low transverse momentum muons thus allowing to keep a low momentum threshold, matching the requirement of both Phase-1 and Phase-2 LCH runs.