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This Letter presents a search for the production of a long-lived neutralparticle ($Z_d$) decaying within the ATLAS hadronic calorimeter, in associationwith a Standard Model (SM) $Z$ boson produced via an intermediate scalar boson,where $Z\to l^+l^-$ ($l=e,\mu$). The data used were collected by the ATLASdetector during 2015 and 2016 $pp$ collisions with a center-of-mass energy of$\sqrt{s} = 13$ TeV at the Large Hadron Collider and corresponds to anintegrated luminosity of $36.1\pm0.8$ fb$^{-1}$. No significant excess ofevents is observed above the expected background. Limits on the productioncross section of the scalar boson times its decay branching fraction into thelong-lived neutral particle are derived as a function of the mass of theintermediate scalar boson, the mass of the long-lived neutral particle, and its$c\tau$ from a few centimeters to one hundred meters. In the case that theintermediate scalar boson is the SM Higgs boson, its decay branching fractionto a long-lived neutral particle with a $c\tau$ approximately between 0.1 m and7 m is excluded with a 95% confidence level up to 10% for $m_{Z_d}$ between 5and 15 GeV.

physical review letters

Search for the production of a long-lived neutral particle decaying  within the ATLAS hadronic calorimeter in association with a $Z$ boson from  $pp$ collisions at $\sqrt{s} = 13$ TeV