This Letter presents a search for the production of a long-lived neutral particle ($Z_d$) decaying within the ATLAS hadronic calorimeter, in association with a standard model (SM) $Z$ boson produced via an intermediate scalar boson, where $Z\to {\ell }^{+}{\ell }^{-}$ ($\ell =e$, $\mu$). The data used were collected by the ATLAS detector during 2015 and 2016 $pp$ collisions with a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ at the Large Hadron Collider and correspond to an integrated luminosity of $36.1\pm 0.8{\mathrm{fb}}^{-1}$. No significant excess of events is observed above the expected background. Limits on the production cross section of the scalar boson times its decay branching fraction into the long-lived neutral particle are derived as a function of the mass of the intermediate 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 the intermediate scalar boson is the SM Higgs boson, its decay branching fraction to a long-lived neutral particle with a $c\tau$ approximately between 0.1 and 7 m is excluded with a 95% confidence level up to 10% for $m_{Z_d}$ between 5 and 15 GeV.

• Received 7 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.151801

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