8,636 research outputs found
Longitudinal top polarisation measurement and anomalous coupling
Kinematical distributions of decay products of the top quark carry
information on the polarisation of the top as well as on any possible new
physics in the decay of the top quark. We construct observables in the form of
asymmetries in the kinematical distributions to probe their effects.
Charged-lepton angular distributions in the decay are insensitive to anomalous
couplings to leading order. Hence these can be a robust probe of top
polarisation. However, these are difficult to measure in the case of highly
boosted top quarks as compared to energy distributions of decay products. These
are then sensitive, in general, to both top polarisation and top anomalous
couplings. We compare various asymmetries for their sensitivities to the
longitudinal polarisation of the top quark as well as to possible new physics
in the vertex, paying special attention to the case of highly boosted top
quarks. We perform a - analysis to determine the regions in the
longitudinal polarisation of the top quark and the couplings of the
vertex constrained by different combinations of the asymmetries. Moreover, we
find that use of observables sensitive to the longitudinal top polarisation can
add to the sensitivity to which the vertex can be probed.Comment: significantly revised version, clarifications on the term
'polarisation' added, new references added, the title modified, 41 figures.
Accepted for publication in EPJ
Evidence for two spin-glass transitions with magnetoelastic and magnetoelectric couplings in the multiferroic (BiBa)(FeTi)O system
For disordered Heisenberg systems with small single ion anisotropy, two spin
glass transitions below the long range ordered phase transition temperature has
been predicted theoretically for compositions close to the percolation
threshold. Experimental verification of these predictions is still
controversial for conventional spin glasses. We show that multiferroic spin
glass systems can provide a unique platform for verifying these theoretical
predictions via a study of change in magnetoelastic and magnetoelectric
couplings, obtained from an analysis of diffraction data, at the spin glass
transition temperatures. Results of macroscopic and microscopic (x-ray and
neutron scattering) measurements are presented on disordered BiFeO3, a
canonical Heisenberg system with small single ion anisotropy, which reveal
appearance of two spin glass phases SG1 and SG2 in coexistence with the LRO
phase below the A-T and G-T lines. It is shown that the temperature dependence
of the integrated intensity of the antiferromagnetic peak shows dips with
respect to the Brillouin function behaviour around the SG1 and SG2 transition
temperatures. The ferroelectric polarisation changes significantly at the two
spin glass transition temperatures. These results, obtained using microscopic
techniques, clearly demonstrate that the SG1 and SG2 transitions occur on the
same magnetic sublattice and are intrinsic to the system. We also construct a
phase diagram showing all the magnetic phases in BF-xBT system. While our
results on the two spin glass transitions support the theoretical predictions,
it also raises several open questions which need to be addressed by revisiting
the existing theories of spin glass transitions by taking into account the
effect of magnetoelastic and magnetoelectric couplings as well as
electromagnons.Comment: 59 pages 21 figure
Neel to staggered dimer order transition in a generalized honeycomb lattice Heisenberg model
We study a generalized honeycomb lattice spin-1/2 Heisenberg model with
nearest-neighbor antiferromagnetic 2-spin exchange, and competing 4-spin
interactions which serve to stabilize a staggered dimer state which breaks
lattice rotational symmetry. Using a combination of quantum Monte Carlo
numerics, spin wave theory, and bond operator theory, we show that this model
undergoes a strong first-order transition between a Neel state and a staggered
dimer state upon increasing the strength of the 4-spin interactions. We
attribute the strong first order character of this transition to the spinless
nature of the core of point-like Z(3) vortices obtained in the staggered dimer
state. Unlike in the case of a columnar dimer state, disordering such vortices
in the staggered dimer state does not naturally lead to magnetic order,
suggesting that, in this model, the dimer and Neel order parameters should be
thought of as independent fields as in conventional Landau theory.Comment: 13 pages, 10 fig
Singular value decomposition in parametrised tests of post-Newtonian theory
Various coefficients of the 3.5 post-Newtonian (PN) phasing formula of
non-spinning compact binaries moving in circular orbits is fully characterized
by the two component masses. If two of these coefficients are independently
measured, the masses can be estimated. Future gravitational wave observations
could measure many of the 8 independent PN coefficients calculated to date.
These additional measurements can be used to test the PN predictions of the
underlying theory of gravity. Since all of these parameters are functions of
the two component masses, there is strong correlation between the parameters
when treated independently. Using Singular Value Decomposition of the Fisher
information matrix, we remove this correlations and obtain a new set of
parameters which are linear combinations of the original phasing coefficients.
We show that the new set of parameters can be estimated with significantly
improved accuracies which has implications for the ongoing efforts to implement
parametrised tests of PN theory in the data analysis pipelines.Comment: 17 pages, 6 figures, Accepted for publication in Classical and
Quantum Gravity (Matches with the published version
Factors Affecting the Adoption of Genetically Modified Crops by Young and Beginning U.S. Farmers and Ranchers
The comprehensive set of programs in the 2008 Farm Bill designed to support Young and Beginning Farmers and Ranchers (YBFR), combined with a substantial amount of resources allocated to each of these programs, can be viewed as an investment in ensuring the future sustainability of the U.S. agriculture system. Understanding the factors that influence YBFR to adopt technology will become increasingly important if YBFR are to succeed. Of particular interest is why YBFR adopt Bt corn, Bt cotton, and HT soybeans. Results conform to a majority of our a priori expectations; YBFRs are more likely to adopt GM crops if they are not a full owner of the farm operation, as sales of the farm operation grow, if the crop is important to their region, and as they become more risk averse.Agricultural and Food Policy,
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