Measurement of the W-boson helicity fractions in top-quark decays at CDF

We present a measurement of the fractions F_0 and F_+ 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-1. We select top-antitop 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 top-antotop candidates. We find F_0 = 0.59 +- 0.12 (stat) +0.07 -0.06 (syst) and F_+ = -0.03 +- 0.06 (stat) +0.04 -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_+ < 0.1 at the 95% confidence level.Comment: For the Proceedings of the 42nd Rencontres de Moriond 2007, Electroweak Interactions and Unified Theorie

Examining the "No-Choice" Option in Choice-Based Conjoint Analysis

Choice-Based Conjoint analysis (CBC) is a method used to determine how individual consumers value attributes of a product or service. The consumer selects the option he or she would be most likely to purchase among those presented, which includes a "no-choice" option. The "no-choice" option is an alternative within a CBC analysis that allows consumers to decide against purchasing any of the presented options. This no-choice option is necessary in order to correctly simulate real choices, but could be problematic if each consumer perceives this option differently (Allenby, et. al. 1995). The goal of this research is to provide a recommendation for how to improve the consistency of choices in CBC analysis. We employ the framework in General Evaluability Theory (Hsee & Zhang 2010) to justify potential improvements to CBC. We hypothesize that evaluability of the no-choice option can be improved by creating a consistent view of the frontier of products available to the consumer. We will test this hypothesis by sampling 1000 respondents randomly split into three different groups, with each group receiving a different amount of information about the products in the market. A control group will engage in standard CBC, the first treatment group will be shown the prices of products in the market, and a second treatment group will be shown the price of products in relation to multiple attributes of the product. The results show no significant differences in the propensity to select the no-choice option between the three experimental groups. Fitting a Hierarchical Multinomial Logit Model to the data, three attributes show significant differences in the upper level between the control group and treatment group one. This suggests that participants in the treatment group value products with those attributes more highly compared to participants in the control group. Further investigation looked at qualified versus non-qualified survey participants and showed that additional information about the market gave non-qualified participants a lower propensity to select the no-choice option. We conclude that using proper screening techniques allows standard Choice-Based Conjoint Analysis to be a robust tool for marketing researchers given the results from qualified survey respondents.No embargoAcademic Major: Financ

Angular position of nodes in the superconducting gap of YBCO

The thermal conductivity of a YBCO single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45 degrees in k-space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a four-fold symmetry such as sample shape or anisotropic pinning. Our results impose an upper limit of 10% on the weight of the s-wave component of the essentially d-wave superconducting order parameter of YBCO.Comment: 10 pages, 4 figure

Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity

Recent application of ultrafast pump/probe optical techniques to superconductors has renewed interest in nonequilibrium superconductivity and the predictions that would be available for novel superconductors, such as the high-Tc cuprates. We have reexamined two of the classical models which have been used in the past to interpret nonequilibrium experiments with some success: the mu* model of Owen and Scalapino and the T* model of Parker. Predictions depend on pairing symmetry. For instance, the gap suppression due to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave as opposed to n for s-wave. Finally, we consider these models in the context of S-I-N tunneling and optical excitation experiments. While we confirm that recent pump/probe experiments in YBCO, as presently interpreted, are in conflict with d-wave pairing, we refute the further claim that they agree with s-wave.Comment: 14 pages, 11 figure

Spatial Symmetry of Superconducting Gap in YBa2Cu3O7-\delta Obtained from Femtosecond Spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-\delta thin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.Comment: 8 pages, 4 figures. To be published in Physical Review B, Rapid Communication

Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV

A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7 TeV is presented. The data were collected at the LHC, with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. No significant excess is observed above the background expectation, and upper limits are set on the Higgs boson production cross section. The presence of the standard model Higgs boson with a mass in the 270-440 GeV range is excluded at 95% confidence level.Comment: Submitted to JHE