Limits on top FCNC decay t$\rightarrow$cH and t$\rightarrow$c$\gamma$ from CLIC at 380 GeV

Abstract

FCNC top decays are very strongly suppressed in the Standard Model and the observation of any such decay would be a direct signature of physics beyond SM. Many "new physics" scenarios predict contributions to FCNC processes and the largest enhancement in many models is for t$\rightarrow$cH decay. Enhancements for the decay channel t$\rightarrow$c$\gamma$ are more modest, but the decay still has a clearly identifiable kinematic signature. Prospects for measuring these decays at CLIC running at 380 GeV were studied with full detector simulation, taking the luminosity distribution, beam polarization and beam induced background into account. Top pair production events with t$\rightarrow$cH decays can be identified based on the kinematic constraints and flavour tagging information. The analysis was divided into three steps: classification of top pair candidate events, event quality determination and kinematic reconstruction based on signal or background hypotheses, and final separation of signal from background. To obtain optimal results, selection criteria based on the dedicated Boosted Decision Trees (BDT) were used at each step. The expected limit on BR(t$\rightarrow$cH)$\times$BR(H$\rightarrow b\bar{b}$) from a combined analysis of hadronic and semi-leptonic top pair samples, as well as the limit on BR(t$\rightarrow$c$\gamma$) from hadronic top pair decays are presented.