Search for the associated production of the Higgs boson with a top quark pair in multilepton final states with the ATLAS detector

ST/N000420/1;We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; RGC, Hong Kong SAR, China; ISF, MINERVA, GIF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES and FCT, Portugal; MNE/IFA, Romania; MES of Russia and NRC KI, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; We acknowledge the support of ANPCyT , Argentina; YerPhI , Armenia; ARC , Australia; BMWFW and FWF , Austria; ANAS , Azerbaijan; SSTC , Belarus; CNPq and FAPESP , Brazil; NSERC , NRC and CFI , Canada; CERN ; CONICYT , Chile; CAS , MOST and NSFC , China; COLCIENCIAS , Colombia; MSMT CR , MPO CR and VSC CR , Czech Republic; DNRF , DNSRC and Lundbeck Foundation , Denmark; EPLANET , ERC and NSRF , European Union; IN2P3-CNRS , CEA-DSM/IRFU , France; GNSF , Georgia; BMBF , DFG , HGF , MPG and AvH Foundation , Germany; GSRT and NSRF , Greece; RGC , Hong Kong SAR , China; ISF , MINERVA , GIF , I-CORE and Benoziyo Center , Israel; INFN , Italy; MEXT and JSPS , Japan; CNRST , Morocco; FOM and NWO , Netherlands; BRF and RCN , Norway; MNiSW and NCN , Poland; GRICES and FCT , Portugal; MNE/IFA , Romania; MES of Russia and NRC KI , Russian Federation; JINR ; MSTD , Serbia; MSSR , Slovakia; ARRS and MIZŠ , Slovenia; DST/NRF , South Africa; MINECO , Spain; SRC and Wallenberg Foundation , Sweden; SER
A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb−1 of proton–proton collision data recorded by the ATLAS experiment at s=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW⁎, ττ, and ZZ⁎, are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the tt¯H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1−1.2 +1.4 times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p-value for compatibility with the background-only hypothesis is 1.8σ; the expectation in the presence of a Standard Model signal is 0.9σ.