27,179 research outputs found
Constraints on Electroweak Effective Operators at One Loop
We derive bounds on nine dimension-six operators involving electroweak gauge
bosons and the Higgs boson from precision electroweak data. Four of these
operators contribute at tree level, and five contribute only at one loop. Using
the full power of effective field theory, we show that the bounds on the five
loop-level operators are much weaker than previously claimed, and thus much
weaker than bounds from tree-level processes at high-energy colliders.Comment: 15 page
Optimal rates of convergence for covariance matrix estimation
Covariance matrix plays a central role in multivariate statistical analysis.
Significant advances have been made recently on developing both theory and
methodology for estimating large covariance matrices. However, a minimax theory
has yet been developed. In this paper we establish the optimal rates of
convergence for estimating the covariance matrix under both the operator norm
and Frobenius norm. It is shown that optimal procedures under the two norms are
different and consequently matrix estimation under the operator norm is
fundamentally different from vector estimation. The minimax upper bound is
obtained by constructing a special class of tapering estimators and by studying
their risk properties. A key step in obtaining the optimal rate of convergence
is the derivation of the minimax lower bound. The technical analysis requires
new ideas that are quite different from those used in the more conventional
function/sequence estimation problems.Comment: Published in at http://dx.doi.org/10.1214/09-AOS752 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Fabrication and characterization of smart fabric using energy storage fibres
Fibre supercapacitors were designed and manufactured using a dip-coating method. Their electrochemical properties were characterized using a VersaSTAT 3 workstation. Chinese ink with a fine dispersion of carbon and binder was coated as the electrode material. The specific capacitance per unit length of a copper fibre supercapacitor with the length of 41 cm reached 34.5 mF/cm. When this fibre supercapacitor was bent on rods with a diameter of 10.5 cm, the specific capacitance per length was 93% of the original value (without bending). It showed that these fibre supercapacitors have good flexibility and energy storage capacity. Furthermore, the fibre supercapacitor in the fabric showed the same capacitance before and after weaving.The European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement no. [281063]
A statistical study of the post-impulsive-phase acceleration of flare-associated coronal mass ejections
It is now generally accepted that the impulsive acceleration of a coronal
mass ejection (CME) in the inner corona is closely correlated in time with the
main energy release of the associated solar flare. In this paper, we examine in
detail the post-impulsive-phase acceleration of a CME in the outer corona,
which is the phase of evolution immediately following the main impulsive
acceleration of the CME; this phase is believed to correspond to the decay
phase of the associated flare. This observational study is based on a
statistical sample of 247 CMEs that are associated with M- and X-class GOES
soft X-ray flares from 1996 to 2006. We find that, from many examples of
events, the CMEs associated with flares with long-decay time (or so-called
long-duration flares) tend to have positive post-impulsive-phase acceleration,
even though some of them have already obtained a high speed at the end of the
impulsive acceleration but do not show a deceleration expected from the
aerodynamic dragging of the background solar wind. On the other hand, the CMEs
associated with flares of short-decay time tend to have significant
deceleration. In the scattering plot of all events, there is a weak correlation
between CME post-impulsive-phase acceleration and flare decay time. The CMEs
deviated from the general trend are mostly slow or weak ones associated with
flares of short-decay time; the deviation is caused by the relatively stronger
solar wind dragging force for these events. The implications of our results on
CME dynamics and CME-flare relations are discussed.Comment: 32 pages, 9 figures, accepted for publication in Ap
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