46,442 research outputs found
Small-Recoil Approximation
In this review we discuss a technique to compute and to sum a class of
Feynman diagrams, and some of its applications. These are diagrams containing
one or more energetic particles that suffer very little recoil in their
interactions. When recoil is completely neglected, a decomposition formula can
be proven. This formula is a generalization of the well-known eikonal formula,
to non-abelian interactions. It expresses the amplitude as a sum of products of
irreducible amplitudes, with each irreducible amplitude being the amplitude to
emit one, or several mutually interacting, quasi-particles. For abelian
interaction a quasi-particle is nothing but the original boson, so this
decomposition formula reduces to the eikonal formula. In non-abelian situations
each quasi-particle can be made up of many bosons, though always with a total
quantum number identical to that of a single boson. This decomposition enables
certain amplitudes of all orders to be summed up into an exponential form, and
it allows subleading contributions of a certain kind, which is difficult to
reach in the usual way, to be computed. For bosonic emissions from a heavy
source with many constituents, a quasi-particle amplitude turns out to be an
amplitude in which all bosons are emitted from the same constituent. For
high-energy parton-parton scattering in the near-forward direction, the
quasi-particle turns out to be the Reggeon, and this formalism shows clearly
why gluons reggeize but photons do not. The ablility to compute subleading
terms in this formalism allows the BFKL-Pomeron amplitude to be extrapolated to
asymptotic energies, in a unitary way preserving the Froissart bound. We also
consider recoil corrections for abelian interactions in order to accommodate
the Landau-Pomeranchuk-Migdal effect.Comment: 21 pages with 4 figure
Unitarity Cuts with Massive Propagators and Algebraic Expressions for Coefficients
In the first part of this paper, we extend the d-dimensional unitarity cut
method of hep-ph/0609191 to cases with massive propagators. We present formulas
for integral reduction with which one can obtain coefficients of all pentagon,
box, triangle and massive bubble integrals. In the second part of this paper,
we present a detailed study of the phase space integration for unitarity cuts.
We carry out spinor integration in generality and give algebraic expressions
for coefficients, intended for automated evaluation.Comment: 33 pages. v2: notation modified. v3: typos fixe
Achieving precise mechanical control in intrinsically noisy systems
How can precise control be realized in intrinsically noisy systems? Here, we develop a general theoretical framework that provides a way of achieving precise control in signal-dependent noisy environments. When the control signal has Poisson or supra-Poisson noise, precise control is not possible. If, however, the control signal has sub-Poisson noise, then precise control is possible. For this case, the precise control solution is not a function, but a rapidly varying random process that must be averaged with respect to a governing probability density functional. Our theoretical approach is applied to the control of straight-trajectory arm movement. Sub-Poisson noise in the control signal is shown to be capable of leading to precise control. Intriguingly, the control signal for this system has a natural counterpart, namely the bursting pulses of neurons-trains of Dirac-delta functions-in biological systems to achieve precise control performance
Gravitino and Axino SuperWIMPs
Gravitinos and axinos produced in the late decays of other supersymmetric
particles are well-motivated dark matter (DM) candidates, whose experimental
evidences are very distinctive and different from other standard candidates, as
thermal produced neutralinos in similar supersymmetric models. In particular,
charged sleptons could appear stable because of the length of its lifetime. The
direct production of such particles at both the Large Hadron Collider (LHC) and
a future International Linear Collider (ILC) can give not only a clear
signature of supersymmetry but also the first non-gravitational evidence of
dark matter.Comment: 4 pages, LaTeX, 1 figure, Updated references. To appear in
Proceedings of SUSY06, the 14th International Conference on Supersymmetry and
the Unification of Fundamental Interactions, UC Irvine, California, 12-17
June 200
Entanglement of separate nitrogen-vacancy centers coupled to a whispering-gallery mode cavity
We present a quantum electrodynamical model involving nitrogen-vacancy
centers coupled to a whispering-gallery mode cavity. Two schemes are considered
to create W state and Bell state, respectively. One of the schemes makes use of
the Raman transition with the cavity field virtually excited; The other enables
the Bell state preparation and quantum information transfer by virtue of dark
state evolution and adiabatic passage, which is tolerant to ambient noise and
experimental parameter fluctuations. We justify our schemes by considering the
experimental feasibility and challenge using currently available technology.Comment: 8 pages and 5 figure
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