7,347 research outputs found
Compositeness Effects in the Anomalous Weak-Magnetic Moment of Leptons
We investigate the effects induced by excited leptons, at the one-loop level,
in the anomalous magnetic and weak-magnetic form factors of the leptons. Using
a general effective Lagrangian approach to describe the couplings of the
excited leptons, we compute their contributions to the weak-magnetic moment of
the lepton, which can be measured on the peak, and we compare it
with the contributions to , measured at low energies.Comment: Latex File using Rev Tex. 16 pages 5 .eps figure
Comb entanglement in quantum spin chains
Bipartite entanglement in the ground state of a chain of quantum spins
can be quantified either by computing pairwise concurrence or by dividing the
chain into two complementary subsystems. In the latter case the smaller
subsystem is usually a single spin or a block of adjacent spins and the
entanglement differentiates between critical and non-critical regimes. Here we
extend this approach by considering a more general setting: our smaller
subsystem consists of a {\it comb} of spins, spaced sites apart.
Our results are thus not restricted to a simple `area law', but contain
non-local information, parameterized by the spacing . For the XX model we
calculate the von-Neumann entropy analytically when and
investigate its dependence on and . We find that an external magnetic
field induces an unexpected length scale for entanglement in this case.Comment: 6 pages, 4 figure
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are
computed from first-principles. The theory is based on a strong-coupling
T-matrix approach that evaluates the decay of a spin excitation due to
electron-hole pair creation. Using a previously developed theory [Phys. Rev.
Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we
compute the excitation rates by a tunneling current for all the Mn spin states.
A rate equation approach permits us to simulate the experimental results by
Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling
currents, taking into account the finite population of excited states. Our
simulations give us insight into the spin dynamics, in particular in the way
polarized electrons can reveal the existence of an excited state population. In
addition, it reveals that the excitation process occurs in a way very different
from the deexcitation one. Indeed, while excitation by tunnelling electrons
proceeds via the s and p electrons of the adsorbate, deexcitation mainly
involves the d electrons
Standard Model: An Introduction
We present a primer on the Standard Model of the electroweak interaction.
Emphasis is given to the historical aspects of the theory's formulation. The
radiative corrections to the Standard Model are presented and its predictions
for the electroweak parameters are compared with the precise experimental data
obtained at the Z pole. Finally, we make some remarks on the perspectives for
the discovery of the Higgs boson, the most important challenge of the Standard
Model.Comment: 101 pages, 14 figures, To be published in "Particle and Fields",
Proceedings of the X J. A. Swieca Summer School (World Scientific, Singapore,
2000
A new correlator in quantum spin chains
We propose a new correlator in one-dimensional quantum spin chains, the
Emptiness Formation Probability (EFP). This is a natural generalization
of the Emptiness Formation Probability (EFP), which is the probability that the
first spins of the chain are all aligned downwards. In the EFP we let
the spins in question be separated by sites. The usual EFP corresponds to
the special case when , and taking allows us to quantify non-local
correlations. We express the EFP for the anisotropic XY model in a
transverse magnetic field, a system with both critical and non-critical
regimes, in terms of a Toeplitz determinant. For the isotropic XY model we find
that the magnetic field induces an interesting length scale.Comment: 6 pages, 1 figur
Fermion Helicity Flip in Weak Gravitational Fields
The helicity flip of a spin- Dirac particle
interacting gravitationally with a scalar field is analyzed in the context of
linearized quantum gravity. It is shown that massive fermions may have their
helicity flipped by gravity, in opposition to massless fermions which preserve
their helicity.Comment: RevTeX 3.0, 8 pages, 3 figures (available upon request), Preprint
IFT-P.013/9
Three Jet Events and New Strong Couplings at LEP and NLC
We study the effects of new dimension--6 operators, resulting from a general
invariant effective Lagrangian, on
three jet production at LEP and at the Next Linear Collider. Contributions to
the total event rate and to some event shape variables are analysed in order to
establish bounds on these operators.Comment: 5 pages, LaTeX, 1 Figur
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