Background Suppression for a Top Quark Mass Measurement in the Lepton+jets Tt̄ Decay Channel and Alignment of the ATLAS Silicon Detectors with Cosmic Rays PDF Download

Author: Tobias Göttfert
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Languages : en
Pages : 123

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Author: Roland Härtel
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Category :
Languages : en
Pages : 123

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Author: Stefanie Adomeit
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Languages : en
Pages : 0

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Languages : en
Pages : 6

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Since the discovery of the top quark in 1995 at the Fermliab Tevatron Collider, top quark properties have been measured with ever higher precision. In this article, recent measurements of the top quark mass and its width using up to 3.6 fb−1 of D0 data are summarized. Different techniques and final states have been examined and no deviations within these measurements have been observed. In addition to the direct measurements, a measurement of the top quark mass from its production cross section and a measurement of the top-antitop quark mass difference are discussed. With a mass of 173.3 ± 1.1 GeV, the top quark is the heaviest of all known fundamental particles. Due to the high mass, its Yukawa coupling is close to unity suggesting that it may play a special role in electroweak symmetry breaking. Precise measurements of both, the W boson and the top quark mass, constrain the mass of the yet unobserved Higgs boson and allow to restrict certain extensions of the Standard Model. At the Tevatron collider with a center-of-mass energy of 1.96 TeV, 85% of the top quark pairs are produced in quark-antiquark annihilation; 15% originate from gluon fusion. Top quarks are predicted to decay almost exclusively to a W boson and a bottom quark. According to the number of hadronic W decays, top events are classified into all-jets, lepton+jets and dilepton events. The lepton+jets channel is characterized by four jets, one isolated, energetic charged lepton and missing transverse energy. With 30%, the branching fraction of the lepton+jets channel is about seven times larger than the one of the dilepton channel whereas the signal to background ratio is about three times smaller. The main background in this final state comes from W +jets events. Instrumental background arises from events in which a jet is misidentified as an electron and events with heavy hadrons that decay into leptons which pass the isolation requirements. The topology of the dilepton channel is described by two jets, two isolated, energetic charged leptons and significant missing transverse energy from the undetected neutrinos. The main background are Z + jets and diboson events (WW/WZ/ZZ+jets) as well as instrumental background as characterized above. At the D0 experiment, different techniques are used to measure the top quark mass. They are summarized in the following sections together with the first measurement of the top anti-top quark mass difference and the first precise determination of the top quark width.

Author: Stefan Kasselmann
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ISBN:
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Languages : en
Pages : 130

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Author: Ford Garberson
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ISBN: 9781109483871
Category :
Languages : en
Pages : 376

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I perform my measurements of the top quark mass in the lepton plus jets channel with approximately 1.9 fb-1 of integrated luminosity collected with the CDF II detector using quantities with minimal dependence on the jet energies. One measurement exploits the transverse decay length of b-tagged jets to determine a top quark mass of 166.9+9.5-8.5 (stat) +/- 2.9 (syst) GeV/c2, and another the transverse momentum of electrons and muons from Wdecays to determine a top quark mass of 173.5+8.8-8.9 (stat) +/- 3.8 (syst) GeV/c2. I combine these quantities in a third, simultaneous mass measurement to determine a top quark mass of 170.7 +/- 6.3 (stat) +/- 2.6 (syst) GeV/ c2.

Author: Philipp Stolte-Cord to Krax
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Languages : en
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The top quark t plays an essential role in the field of elementary particle physics - in particular due to its exceptional properties comprising a large mass, which approximately equals the mass of a tungsten atom, and an enormously short lifetime. This thesis is devoted to the study of a fundamental property of the top quark - its decay width. Its value is predicted by the established Standard Model of particle physics and deviations may hint at yet unknown physics beyond this model. A direct measurement of the decay width of the top quark is presented. The analysis is based on tt events i...

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Languages : en
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We present a measurement of the top-quark mass with tt events using a data sample corresponding to an integrated luminosity of 5.7 fb -1 of pp collisions at the Fermilab Tevatron with √s = 1.96 TeV and collected by the CDF II Detector. We select events having no identified charged leptons, large missing transverse energy, and four, five, or six jets with at least one jet containing a secondary vertex consistent with the decay of a b quark. This analysis considers events from the semileptonic tt decay channel, including events that contain tau leptons, which are usually not included in the top-quark mass measurements. The measurement uses as kinematic variables the invariant mass of two jets consistent with the mass of the W boson, and the invariant masses of two different three-jet combinations. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass of Mtop = 172.3 ± 2.4 (stat) ± 1.0 (syst) GeV/c2.

Author: Kevin Matthew Black
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ISBN:
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Languages : en
Pages : 226

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This dissertation describes the measurement of the top quark mass using events recorded during a {approx} 230 pb{sup -1} exposure of the D0 detector to proton-anti-proton (p{bar p}) collisions at a center of mass energy of 1.96 TeV. The Standard Model of particle physics predicts that the top quark will decay into a bottom quark and a W boson close to 100% of the time. The bottom quark will hadronize (bind with another quark) and produce a jet of hadronic particles. The W bosons can decay either into a charged lepton and a neutrino or a pair of quarks. this dissertation focuses on the top quark (t{bar t}) events in which one W decays hadronically and the other decays leptonically. Two methods of identifying t{bar t} events from the large number of events produced are used. The first is based on the unique topology of the final state particles of a heavy particle. By using the topological information of the event, the t{bar t} events can be efficiently extracted from the background. The second method relies on the identification of the remnants of the long lived bottom quarks that are expected to be produced in the decay of almost every top quark. Because the largest background processes do not contain bottom quarks, this is an extremely efficient way to select the events retaining about 60% of the t{bar t} events and removing almost 90% of the background. A kinematic fit to the top quark mass is performed on the t{bar t} candidate events using the final state particles that are seen in the detector. A likelihood technique is then used to extract the most likely value of the top quark mass, m{sub t}, and signal fraction. The result for the topological selection is m{sub t} = 169.9 {+-} 5.8(statistical){sub -7.8}{sup +8.0}(systematic) GeV while the results on the sample selected from identification of a b quark in the event is m{sub t} = 170.6 {+-} 4.2(statistical){sub -6.8}{sup +6.3}(systematic) GeV.

Author: Koji Sato
Publisher:
ISBN:
Category :
Languages : en
Pages : 164

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We present a measurement of the top quark mass using tt pair creation events decaying into the lepton+jets channel in pp collisions at √s = 1.96 TeV. The data sample used in this analysis was collected with the Collider Detector at Fermilab (CDF) in Tevatron Run II during the period from March 2002 through August 2003.