1,221 research outputs found
Growinâ good corn: Rocket science or common sense?
For 70 years, beginning in 1866, national corn grain yields in the U.S. were essentially flat (Fig. 1) and averaged only 26 bpa (bushels per acre) during that entire 70-year time period. The absence of noticeable yield improvement throughout all those years is remarkable given that farmers of the day were essentially also plant breeders practicing a recognized form of plant breeding (mass selection) as they saved the best ears from each yearâs crop for planting the next
Ag on Ge(111): 2D X-ray structure analysis of the (Wurzel)3 x (Wurzel)3 superstructure
We have studied the Ag/Ge(111)(Wurzel)3 x (Wurzel)3 superstructure by grazing-incidence X-ray diffraction. In our structural analysis we find striking similarities to the geometry of Au on Si(111). The Ag atoms form trimer clusters with an Ag-Ag distance of 2.94+-0.04°A with the centers of the trimers being located at the origins of the (Wurzel)3 x (Wurzel)3 lattice. The Ag layer is incomplete and at least one substrate layer is distorted
Non-negative Wigner functions in prime dimensions
According to a classical result due to Hudson, the Wigner function of a pure,
continuous variable quantum state is non-negative if and only if the state is
Gaussian. We have proven an analogous statement for finite-dimensional quantum
systems. In this context, the role of Gaussian states is taken on by stabilizer
states. The general results have been published in [D. Gross, J. Math. Phys.
47, 122107 (2006)]. For the case of systems of odd prime dimension, a greatly
simplified proof can be employed which still exhibits the main ideas. The
present paper gives a self-contained account of these methods.Comment: 5 pages. Special case of a result proved in quant-ph/0602001. The
proof is greatly simplified, making the general case more accessible. To
appear in Appl. Phys. B as part of the proceedings of the 2006 DPG Spring
Meeting (Quantum Optics and Photonics section
Quantum Error Correction via Convex Optimization
We show that the problem of designing a quantum information error correcting
procedure can be cast as a bi-convex optimization problem, iterating between
encoding and recovery, each being a semidefinite program. For a given encoding
operator the problem is convex in the recovery operator. For a given method of
recovery, the problem is convex in the encoding scheme. This allows us to
derive new codes that are locally optimal. We present examples of such codes
that can handle errors which are too strong for codes derived by analogy to
classical error correction techniques.Comment: 16 page
A QCD Analysis of the Mass Structure of the Nucleon
{}From the deep-inelastic momentum sum rule and the trace anomaly of the
energy-momentum tensor, I derive a separation of the nucleon mass into the
contributions of the quark and gluon kinetic and potential energies, the quark
masses, and the trace anomaly.Comment: 9 pages, MIT-CTP #2368, revtex with 1 tabl
Fermionic massive modes along cosmic strings
The influence on cosmic string dynamics of fermionic massive bound states
propagating in the vortex, and getting their mass only from coupling to the
string forming Higgs field, is studied. Such massive fermionic currents are
numerically found to exist for a wide range of model parameters and seen to
modify drastically the usual string dynamics coming from the zero mode currents
alone. In particular, by means of a quantization procedure, a new equation of
state describing cosmic strings with any kind of fermionic current, massive or
massless, is derived and found to involve, at least, one state parameter per
trapped fermion species. This equation of state exhibits transitions from
subsonic to supersonic regimes while the massive modes are filled.Comment: 27 pages, 15 figures, uses ReVTeX. Shortened version, accepted for
publication in Phys. Rev.
Progress in the determination of the cross section
Improving previous calculations, we compute the cross section using QCD sum rules. Our sum rules for the , , and hadronic
matrix elements are constructed by using vaccum-pion correlation functions, and
we work up to twist-4 in the soft-pion limit. Our results suggest that, using
meson exchange models is perfectly acceptable, provided that they include form
factors and that they respect chiral symmetry. After doing a thermal average we
get mb at T=150\MeV.Comment: 22 pages, RevTeX4 including 7 figures in ps file
Equation of state of cosmic strings with fermionic current-carriers
The relevant characteristic features, including energy per unit length and
tension, of a cosmic string carrying massless fermionic currents in the
framework of the Witten model in the neutral limit are derived through
quantization of the spinor fields along the string. The construction of a Fock
space is performed by means of a separation between longitudinal modes and the
so-called transverse zero energy solutions of the Dirac equation in the vortex.
As a result, quantization leads to a set of naturally defined state parameters
which are the number densities of particles and anti-particles trapped in the
cosmic string. It is seen that the usual one-parameter formalism for describing
the macroscopic dynamics of current-carrying vortices is not sufficient in the
case of fermionic carriers.Comment: 30 pages, 15 figures, uses ReVTeX, equation of state corrected,
comments and references added. Accepted for publication in Phys. Rev.
decays into and mesons
We consider the nonleptonic and semileptonic decays of -mesons into
and mesons. QCD sum rules are used to calculate the form
factors associated with these decays, and the correspondig decay rates. On the
basis of data on , which goes dominantly via the
transition , we conclude that there is space for a
sizeable light quark component on .Comment: 14 pages, RevTeX4 including 5 figures in ps file
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