Search for Top-squark Pairs Decaying Into Higgs Or Z Bosons in Pp Collisions at {u221A}s
Search for Top-squark Pairs Decaying Into Higgs Or Z Bosons in Pp Collisions at {u221A}s

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Published: 2014

Total Pages: 27

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Our 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= 8TeV collected in 2012 with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 19.5 fb-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 t̃/sup2 source decaying to a lighter top-squark mass eigenstate t̃/sup2 via either t̃/sup2 → Ht̃/sup1 or tsup̃/sup2 →Ztsup̃/sup1, followed in both cases by tsup̃/sup1→ t?sup0̃1, where ?sup0̃1 source is an undetected, stable, lightest supersymmetric particle. The interpretation is performed in the region where the mass difference between the tsup̃/sup1 and ?sup0̃1 states is approximately equal to the top-quark mass (mt1-msup?01≃mt), which is not probed by searches for direct tsup̃/sup1 squark pair production. This analysis excludes top squarks with mass mt1 575 GeVand msubt2

Observation of Top Quark Pairs Produced in Association with a Vector Boson in Pp Collisions at {u221A}(s)
Observation of Top Quark Pairs Produced in Association with a Vector Boson in Pp Collisions at {u221A}(s)

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Published: 2016

Total Pages: 52

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Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb-1, collected by the CMS experiment at the LHC. We found that final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from tt-W or tt-Z decays. The tt-W cross section is measured to be 382+117 -102 fb with a significance of 4.8 standard deviations from the background-only hypothesis. The tt-Z cross section is measured to be 242+65 -55 fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the tt-W and tt-Z cross sections.

Search for a CP-odd Higgs Boson Decaying to Zh in Pp Collisions at {u221A}s
Search for a CP-odd Higgs Boson Decaying to Zh in Pp Collisions at {u221A}s

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Published: 2015

Total Pages: 21

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A search for a heavy, CP-odd Higgs boson, A, decaying into a Z boson and a 125 GeV Higgs boson, h, with the ATLAS detector at the LHC is presented. The search uses proton–proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3fb-1. Decays of CP-even h bosons to ?? or bb pairs with the Z boson decaying to electron or muon pairs are considered, as well as h → bb decays with the Z boson decaying to neutrinos. No evidence for the production of an A boson in these channels is found and the 95% confidence level upper limits derived for ?(gg → A) × BR(A → Zh) × BR(h → f¯f) are 0.098–0.013pb for f = ? and 0.57–0.014 pb for f = b in a range of mA=220–1000 GeV. The results are combined and interpreted in the context of two-Higgs-doublet models.

Search for a CP-Odd Higgs Boson Decaying to Z H in Pp Collisions at [square Root]s
Search for a CP-Odd Higgs Boson Decaying to Z H in Pp Collisions at [square Root]s

Author: Hannah Arnold

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Published: 2018

Total Pages:

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Abstract: In this thesis a search for a CP-odd Higgs boson A decaying into a Z boson and a neutral CP-even Higgs boson h with a mass of 125 GeV is described. The search is based on proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in the years 2015 and 2016 corresponding to an integrated luminosity of 36/fb. Final states, where the Z boson decays into pairs of electrons, muons or neutrinos and the h boson into bottom-quark pairs, are considered. The presence of neutrinos is inferred by a momentum imbalance in the transverse plane, the bottom-quark pairs are reconstructed either as two small-radius or one large-radius calorimeter jet(s). The reconstructed invariant or transverse mass distributions of the Zh candidates are searched for the presence of localised excesses above the expectation from known Standard Model processes. No evidence for an A-boson signal is found. Upper limits on the A-boson production cross-section times the branching fractions for A-boson decays into Zh and h-boson decays to bottom-quark pairs are derived at the 95% confidence level for A bosons with masses between 220 GeV and 2 TeV; they range between 0.0019 pb and 0.81 pb. The results are interpreted in the context of Two-Higgs-Doublet Models. Furthermore, a novel method to calibrate the b-tagging efficiency for c-quark jets is described. It is demonstrated for the standard b-tagging algorithm of the ATLAS experiment in Run 1 using proton-proton collision data at a centre-of-mass energy of 7 TeV collected with the ATLAS detector in the year 2011. The calibration uses a sample of c jets obtained from events, where a W boson is produced in association with a single c quark; the resulting c jet is identified by a muon stemming from the semileptonic c-hadron decay. The measured c-jet tagging efficiencies are presented relative to the corresponding efficiencies in simulated W+c events as scale factors. The measured scale factors are extrapolated using a simulation-based and data-supported procedure to obtain scale factors valid for inclusive samples of c jets. Their uncertainties range between 5% and 13 %.

Search for Pair Production of a New B' Quark that Decays Into a Z Boson and a Bottom Quark with the ATLAS Detector at the LHC
Search for Pair Production of a New B' Quark that Decays Into a Z Boson and a Bottom Quark with the ATLAS Detector at the LHC

Author: Andre Milton Bach

Publisher:

Published: 2012

Total Pages: 260

ISBN-13:

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A search with the ATLAS detector for the pair production of a new b' quark in the Z boson plus b quark decay channel is reported. The pp collision data comprise 4.7 fb-1 of integrated luminosity at sqrt(s) = 7 TeV collected at the CERN Large Hadron Collider in 2011. This search complements recent searches for a b' decaying to a W boson and a top quark, and it is particularly important for new physics models containing vector-like quarks. From events containing both a Z boson reconstructed from electrons and a b tagged jet, any potential signal is enriched by selecting a subsample with large b' candidate pT. No evidence for a b' signal is found in this subsample; the invariant mass spectrum of the b' candidate agrees well with the Standard Model prediction. For the case of a b' that decays only to Z + b, masses mb'