CERN-LHC. A search is presented for long-lived neutral particles using 33 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data, collected during 2016 by the ATLAS detector at the LHC. This search focuses on a topology in which one long-lived particle decays in the ATLAS inner detector and the other decays in the muon spectrometer. Special techniques are required in order to reconstruct the displaced tracks and vertices in the inner detector and in the muon spectrometer. One event is observed that passes the full event selection, which is consistent with the background estimation. Limits are placed on scalar boson propagators of masses from 125 GeV to 1000 GeV decaying to pairs of hidden sector scalars of masses from 8 GeV to 400 GeV. The limits placed on several low mass scalars extend previous exclusion limits in the range of proper lifetimes from 5 cm to 1 m.

Comparison of the IDVx reconstruction and selection efficiency for a $\Phi$ with a mass of 1000 GeV decaying to an LLP with a mass of 150 GeV, using only standard tracking versus using standard and large radius tracking for all ID vertices in the signal MC sample.