Search for top-squark pairs decaying into Higgs or Z bosons in pp collisions at √s = 8 TeV

A search for supersymmetry through the direct pair production of top squarks, with Higgs (H) or Z bosons in the decay chain, is performed using a data sample of proton–proton collisions at √s = 8 TeV collected in 2012 with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 19.5fb⁻¹. The search is performed using a selection of events containing leptons and bottom-quark jets. No evidence for a significant excess of events over the standard model background prediction is observed. The results are interpreted in the context of simplified supersymmetric models with pair production of a heavier top-squark mass eigenstate t˜₂ decaying to a lighter top-squark mass eigenstate t˜₁ via either t˜₂ → Ht˜₁ or t˜₂ → Zt˜₁, followed in both cases by t˜₁ → t_χ˜⁰1, where χ˜⁰₁ is an undetected, stable, lightest supersymmetric particle. The interpretation is performed in the region where the mass difference between the t˜₁ and χ˜⁰₁ states is approximately equal to the top-quark mass (m_t˜1 − m_χ˜⁰1 ≅ mt), which is not probed by searches for direct t˜₁ squark pair production. The analysis excludes top squarks with masses m_t˜2 < 575 GeV and m_t˜1 < 400 GeV at a 95% confidence level.