3,655 research outputs found
Jungen - Pädagogik. Praxis und Theorie von Genderpädagogik
No full textGraff U, Chwalek D-T, Diaz M, Fegter S, eds. Jungen - Pädagogik. Praxis und Theorie von Genderpädagogik. Kinder, Kindheiten und Kindheitsforschung. Springer VS; 2012
Sonderangebot oder Notlösung? Zum Status geschlechtshomogener Pädagogik in der „reflexiven Koedukation“
No full textGraff U. Sonderangebot oder Notlösung? Zum Status geschlechtshomogener Pädagogik in der „reflexiven Koedukation“. In: Chwalek D-T, Diaz M, Fegter S, Graff U, eds. Jungen - Pädagogik. Praxis und Theorie von Genderpädagogik. Kinder, Kindheiten, Kindheitsforschung. Vol 6. Wiesbaden: Springer VS; 2012: 91-99
Measurement of the W boson helicity in top-antitop quark events with the CDF II experiment
No full textIn 1995 the top quark was discovered at the Tevatron proton-antiproton collider at Fermilab by the CDF and D0 collaborations [1, 2]. It is the most massive known elementary particle and its mass is currently measured with a precision of about 1.3% [3, 4]. However, the measurements of several other top quark properties are still statistically limited, so the question remains whether the Standard Model of elementary particle physics successfully predicts these properties. This thesis addresses one interesting aspect of top quark decay, the helicity of the produced W boson. Until the start of the Large Hadron Collider (LHC) at CERN, the Tevatron with a center-of-mass energy of {radical}s = 1.96 TeV is the only collider, where top quarks can be produced. In the Standard Model the top quark decays predominantly into a W boson and a b quark, with a branching ratio close to 100%. The V-A structure of the weak interaction of the Standard Model predicts that the W{sup +} bosons from the top quark decay t {yields} W{sup +}b are dominantly either longitudinally polarized or left handed, while right handed W bosons are heavily suppressed and even forbidden in the limit of a massless b quark. Under the assumption of a massless b quark, for a top quark mass of 173 GeV/c{sup 2} the Standard Model predicts the fraction F0 of longitudinally polarized W bosons to be 0.7 and 0.3 for the fraction F{_} of left handed W bosons, while the fraction F{sub +} of right handed W bosons is predicted to be zero. Since next-to-leading order corrections change these fractions only slightly, a significant deviation from the predicted value for F{sub 0} or a nonzero value for F{sub +} could indicate new physics. Left-right symmetric models [5], for example, lead to a significant right handed fraction of W bosons in top decays. Such a right handed component (V+A coupling) would lead to a smaller left handed fraction, while F{sub 0} would remain unchanged. Since the decay rate to longitudinal W bosons depends on the Yukawa coupling of the top quarks, the measurement of F{sub 0} is sensitive to the mechanism of electroweak symmetry breaking. Alternative models can lead to an altered F{sub 0} fraction. In this analysis the W helicity fractions are measured in a selected sample rich in t{bar B} events where one lepton, at least four jets, and missing transverse energy are required. All kinematic quantities describing the t{bar t} decay are determined. As a sensitive observable, we use the cosine of the decay angle {theta}*, which is defined as the angle between the momentum of the charged lepton in the W boson rest frame and the W boson momentum in the top quark rest frame. The data used in this analysis were taken with the Collider Detector at Fermilab (CDF II) in the years 2002-2006 and correspond to an integrated luminosity of about 955 pb{sup -1}. Previous CDF measurements of the W boson helicity fractions in top quark decays used either the square of the invariant mass of the charged lepton and the b quark jet, M{sub {ell}b}{sup 2}, or the lepton p{sub T} distribution as a discriminant. The D0 collaboration used a matrix-element method to extract a value of F{sub 0}; in a second analysis the reconstructed distribution of cos {theta}* was utilized to measure F{sub +}. CDF gives the latest value of F{sub 0} = 0.74{sub -0.34}{sup +0.22}, while D measured F{sub 0} = 0.56 {+-} 0.31. The CDF collaboration also gives the current upper limit of F{sub +} < 0.09
Measurement of the top-quark mass in all-jets events in pp collisions at = 7 TeV
No full textThe mass of the top quark is measured using a sample of candidate events with at least six jets in the final state. The sample is selected from data collected with the CMS detector in pp collisions at = 7 TeV in 2011 and corresponds to an integrated luminosity of 3.54 . The mass is reconstructed for each event employing a kinematic fit of the jets to a hypothesis. The top-quark mass is measured to be 173.49 0.69(stat.) 1.21(syst.) GeV. A combination with previously published measurements in other decay modes by CMS yields a mass of 173.54 0.33(stat.) 0.96(syst.) GeV
Recommended from our members
Measurement of the W-boson helicity fractions in top-quark decays at CDF
No full textWe present a measurement of the fractions F{sub 0} and F{sub +} of longitudinally polarized and right-handed W bosons in top-quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 955 pb{sup -1}. We select t{bar t} candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle {theta}*, which is defined as the angle between the momentum of the charged lepton in the W boson rest-frame and the W momentum in the top-quark rest-frame. The cos{theta}* distribution in the data is determined by full kinematic reconstruction of the t{bar t} candidates. We find F{sub 0}= 0.59 {+-} 0.12(stat){sup +0.07}{sub -0.06}(syst) and F{sub +}=-0.03 {+-} 0.06(stat){sup +0.04}{sub -0.03}(syst), which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed W bosons of F{sub +} {le} 0.10 at the 95% confidence level
Selection of Top-Like Events in the Lepton+Jets Channels in Early 7 TeV LHC-Data with CMS
No full textThe top quark is the heaviest known fermion in the standard model. Due to its large production cross section, pairs of top and antitop quarks will be copiously produced at the Large Hadron Collider (LHC). The event selection deemed for the selection of events is applied to an early data set of proton-proton collisions at a center-of-mass energy of corresponding to an integrated luminosity of . Since this data set is obviously too small to make a statement on the top quark cross section at the LHC, the focus is on the comparison of the predicted background yields and the overall yield of events collected. In addition data-driven techniques to estimate the background contribution from QCD multijet events are evaluated in sideband regions
Measurement of -Boson Helicity-Fractions in Top-Quark Decays with the CDF II Experiment and Prospects for an Early Cross-Section Measurement with the CMS Experiment
No full textSubpicosecond time-resolved spectroscopy of high-transition-temperature superconductors.
No full textA better understanding of the electronic properties of high-transition-temperature superconductors is needed to fully exploit their current applications as well as promote the further development of new materials and devices. The goal of this work was to exploit the unique offerings of ultrafast, laser-based, time-domain techniques to study the carriers, lattice, and carrier-lattice coupling in various high-T\sb{c} systems. This was accomplished in two unique ways. Through the use of short optical pulses (100-fs) combined with the photoconductive effect, the first technique utilized the ability to generate bursts of coherent radiation. This generated radiation acted as a coherent probe of the carriers in the terahertz regime. The second technique was an all optical one, and used the short optical pulses directly, either to create and probe nonequilibrium carrier distributions and their subsequent relaxation in various high-T\sb{c} systems or to impulsively excite and probe lattice vibrations, directly in the time domain. This research produced a number of significant contributions, some of which are listed below. An extension of the coherent spectroscopy technique to the study of thin-film superconductors was made. With this system, the complex conductivity of a YBa\sb2Cu\sb3O\sb7 thin-film was directly extracted without the need for a Kramers-Kronig analysis. The measured conductivity, exhibited a qualitatively similar behavior as that predicted for weak coupling superconductors outside the extreme clean limit. However, the reasons behind this behavior may be quite different from the coherence effects responsible for the behavior in conventional superconductors. An investigation into the nonequilibrium carrier dynamics of two cuprate superconductors (YBa \sb2 Cu \sb3 O \sb{7-x} and Bi \sb2 Sr \sb2 Ca \sb2 Cu \sb3 O \sb{10} ) was made using femtosecond optical pulses. From these experiments it was determined that the coupling parameter was too low for the observed T\sb{c} in these materials (within the Eliashberg framework). With this same technique, the first observation of impulsively generated, coherent phonons in a high-T\sb{c} cuprate compound was made. From temperature dependent studies, the first direct observation of an enhanced quasiparticle lifetime in high-T\sb{c} materials was made. This is believed to be a direct result of relaxation of the order parameter.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105674/1/9208519.pdfDescription of 9208519.pdf : Restricted to UM users only
- …
