657 research outputs found
-mixing matrix elements from lattice QCD for the Standard Model and beyond
We calculate---for the first time in three-flavor lattice QCD---the hadronic
matrix elements of all five local operators that contribute to neutral -
and -meson mixing in and beyond the Standard Model. We present a complete
error budget for each matrix element and also provide the full set of
correlations among the matrix elements. We also present the corresponding bag
parameters and their correlations, as well as specific combinations of the
mixing matrix elements that enter the expression for the neutral -meson
width difference. We obtain the most precise determination to date of the
SU(3)-breaking ratio , where the second error stems from
the omission of charm sea quarks, while the first encompasses all other
uncertainties. The threefold reduction in total uncertainty, relative to the
2013 Flavor Lattice Averaging Group results, tightens the constraint from
mixing on the Cabibbo-Kobayashi-Maskawa (CKM) unitarity triangle. Our
calculation employs gauge-field ensembles generated by the MILC Collaboration
with four lattice spacings and pion masses close to the physical value. We use
the asqtad-improved staggered action for the light valence quarks, and the
Fermilab method for the bottom quark. We use heavy-light meson chiral
perturbation theory modified to include lattice-spacing effects to extrapolate
the five matrix elements to the physical point. We combine our results with
experimental measurements of the neutral -meson oscillation frequencies to
determine the CKM matrix elements ,
, and , which differ from CKM-unitarity expectations by about
2. These results and others from flavor-changing-neutral currents point
towards an emerging tension between weak processes that are mediated at the
loop and tree levels.Comment: 75 pp, 17 figs. Ver 2 fixes typos; corrects mistakes resulting in
slight changes to results, correlation matrices; updates decay constants to
agree with recent PDG update; corrects uncertainties for tree-level CKM
matrix elements used in comparison, slightly reducing tensions; includes
additional analyses that support mostly-nonperturbative matching; expands
discussion of isospin-breaking effect
Yukawa Unification and the Superpartner Mass Scale
Naturalness in supersymmetry (SUSY) is under siege by increasingly stringent
LHC constraints, but natural electroweak symmetry breaking still remains the
most powerful motivation for superpartner masses within experimental reach. If
naturalness is the wrong criterion then what determines the mass scale of the
superpartners? We motivate supersymmetry by (1) gauge coupling unification, (2)
dark matter, and (3) precision b-tau Yukawa unification. We show that for an
LSP that is a bino-Higgsino admixture, these three requirements lead to an
upper-bound on the stop and sbottom masses in the several TeV regime because
the threshold correction to the bottom mass at the superpartner scale is
required to have a particular size. For tan beta about 50, which is needed for
t-b-tau unification, the stops must be lighter than 2.8 TeV when A_t has the
opposite sign of the gluino mass, as is favored by renormalization group
scaling. For lower values of tan beta, the top and bottom squarks must be even
lighter. Yukawa unification plus dark matter implies that superpartners are
likely in reach of the LHC, after the upgrade to 14 (or 13) TeV, independent of
any considerations of naturalness. We present a model-independent, bottom-up
analysis of the SUSY parameter space that is simultaneously consistent with
Yukawa unification and the hint for m_h = 125 GeV. We study the flavor and dark
matter phenomenology that accompanies this Yukawa unification. A large portion
of the parameter space predicts that the branching fraction for B_s to mu^+
mu^- will be observed to be significantly lower than the SM value.Comment: 34 pages plus appendices, 20 figure
Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites:TL-LUE Parameterization and Validation
Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (εmsh) was 2.63 to 4.59 times that of sunlit leaves (εmsu). Generally, the relationships of εmsh and εmsu with εmax were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR
Measurement of the Bottom-Strange Meson Mixing Phase in the Full CDF Data Set
We report a measurement of the bottom-strange meson mixing phase \beta_s
using the time evolution of B0_s -> J/\psi (->\mu+\mu-) \phi (-> K+ K-) decays
in which the quark-flavor content of the bottom-strange meson is identified at
production. This measurement uses the full data set of proton-antiproton
collisions at sqrt(s)= 1.96 TeV collected by the Collider Detector experiment
at the Fermilab Tevatron, corresponding to 9.6 fb-1 of integrated luminosity.
We report confidence regions in the two-dimensional space of \beta_s and the
B0_s decay-width difference \Delta\Gamma_s, and measure \beta_s in [-\pi/2,
-1.51] U [-0.06, 0.30] U [1.26, \pi/2] at the 68% confidence level, in
agreement with the standard model expectation. Assuming the standard model
value of \beta_s, we also determine \Delta\Gamma_s = 0.068 +- 0.026 (stat) +-
0.009 (syst) ps-1 and the mean B0_s lifetime, \tau_s = 1.528 +- 0.019 (stat) +-
0.009 (syst) ps, which are consistent and competitive with determinations by
other experiments.Comment: 8 pages, 2 figures, Phys. Rev. Lett 109, 171802 (2012
SLDV technology for measurement of mistuned bladed disc vibration
Bladed discs are very sensitive structures and the amplitude vibration of each
blade can vary significantly from blade to blade due to a series of factors such as
geometrical inhomogeneity between blades or material properties. These factors
lead to bladed disks mistuned thus the forced response amplitudes can be much
higher than the level predicted for a tuned assembly.
Designed models need to be “validate” to predict the response of a real bladed
disc within the tolerances set by the manufactures and this process is very
expensive as well as difficult. The validation process needs “reference data” as
fundamental input against what all predictions can be compared and validated.
Data that can be provided both under stationary conditions and under rotating
conditions and the latter is the most difficult to achieve, especially for bladed
disc assemblies which are very sensitive to any structural modification as it
could be attaching a transducer to measure vibrations. There are contact-less
measurement techniques available which, however, provide limited information
because they can measure only limited areas of the vibrating structures.
The aim of this study is to design measurement methods, using a standard
Scanning Laser Doppler Vibrometer (SLDV) and to integrate it into a software
platform which will be able to handle a series of measurement tasks both under
stationary and rotating conditions. The main contribution of this thesis is to
extend the use of Continuous Scanning LDV (CSLDV) to the rotating structures,
such as bladed discs, thus to perform synchronous measurements. Hence, a
bladed disc is needed to be designed to perform vibration predictions and
measurements and a mathematical model of the measurement test to control,
critically, all possible sources of errors involved in measurement under rotating conditions; all these to produce a robust measurement method. While the
primary focus is the measurement method, the study also extends to evaluation
of the sensitivity properties of the bladed disk test pieces that are the object of
the measurement tool
Evidence for the associated production of a W boson and a top quark in ATLAS at √s = 7 TeV
This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb −1 of pp collision data at √ s = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the crosssection. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σWt = 16.8 ±2.9 (stat)±4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb| = 1.03 +0.16 −0.19 is derived assuming that the Wt production through |Vts | and |Vtd| is smal
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