Abstract
The result of a search for flavor changing neutral currents (FCNC) through single top quark production in association with a photon is presented. The study is based on proton-proton collisions at a center-of-mass energy of 8 TeV using data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8 inverse femtobarns. The search for t gamma events where t to Wb and W to mu nu is conducted in final states with a muon, a photon, at least one hadronic jet with at most one being consistent with originating from a bottom quark, and missing transverse momentum. No evidence of single top quark production in association with a photon through a FCNC is observed. Upper limits at the 95% confidence level are set on the tu gamma and tc gamma anomalous couplings and translated into upper limits on the branching fraction of the FCNC top quark decays: B(t to u gamma) < 1.3E-4 and B(t to c gamma) < 1.7E-3. Upper limits are also set on the cross section of associated t gamma production in a restricted phase-space region. These are the most stringent limits currently available.
Highlights
Background estimationThe main background contributions arise from Wγ+jets and W + jets events, where the W + jets background can mimic the signal when a jet is misidentified as a photon
A silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections are contained within the superconducting solenoid volume
The Wγ+jets and W + jets backgrounds are estimated from data, while estimates for the backgrounds from single top quark, t + γ, tt, tt +γ, Z+γ+jets, Drell-Yan, WWγ + jets, and diboson backgrounds are calculated from the numbers of simulated events passing the event selection, scaled to their theoretical cross sections
Summary
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. A silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections are contained within the superconducting solenoid volume. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. The first level of the trigger system, composed of custom hardware processors, is designed to select the most interesting events in less than 4 μs, using information from the calorimeters and muon detectors. The high-level trigger processor farm further decreases the event rate from about 100 kHz to less than 1 kHz, before data storage. A more detailed description of the CMS detector, together with a definition of the coordinate system and kinematic variables used in this analysis, can be found in ref. [13]
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