1,297 research outputs found
The many-body reciprocal theorem and swimmer hydrodynamics
We present a reinterpretation and extension of the reciprocal theorem for
swimmers, extending its application from the motion of a single swimmer in an
unbounded domain to the general setting, giving results for both swimmer
interactions and general hydrodynamics. We illustrate the method for a squirmer
near a planar surface, recovering standard literature results and extending
them to a general squirming set, to motion in the presence of a ciliated
surface, and expressions for the flow field throughout the domain. Finally, we
present exact results for the hydrodynamics in two dimensions which shed light
on the near-field behaviour.Comment: 6 pages, 6 figure
The Pinch Technique to All Orders
The generalization of the pinch technique to all orders in perturbation
theory is presented. The effective Green's functions constructed with this
procedure are singled out in a unique way through the full exploitation of the
underlying Becchi-Rouet-Stora-Tyutin symmetry. A simple all-order
correspondence between the pinch technique and the background field method in
the Feynman gauge is established.Comment: 10 pages, 4 figures; one reference added, typos corrected; final
version to match the pubblished on
Exact solutions for hydrodynamic interactions of two squirming spheres
We provide exact solutions of the Stokes equations for a squirming sphere
close to a no-slip surface, both planar and spherical, and for the interactions
between two squirmers, in three dimensions. These allow the hydrodynamic
interactions of swimming microscopic organisms with confining boundaries, or
each other, to be determined for arbitrary separation and, in particular, in
the close proximity regime where approximate methods based on point singularity
descriptions cease to be valid. We give a detailed description of the circular
motion of an arbitrary squirmer moving parallel to a no-slip spherical boundary
or flat free surface at close separation, finding that the circling generically
has opposite sense at free surfaces and at solid boundaries. While the
asymptotic interaction is symmetric under head-tail reversal of the swimmer, in
the near field microscopic structure can result in significant asymmetry. We
also find the translational velocity towards the surface for a simple model
with only the lowest two squirming modes. By comparing these to asymptotic
approximations of the interaction we find that the transition from near- to
far-field behaviour occurs at a separation of about two swimmer diameters.
These solutions are for the rotational velocity about the wall normal, or
common diameter of two spheres, and the translational speed along that same
direction, and are obtained using the Lorentz reciprocal theorem for Stokes
flows in conjunction with known solutions for the conjugate Stokes drag
problems, the derivations of which are demonstrated here for completeness. The
analogous motions in the perpendicular directions, i.e. parallel to the wall,
currently cannot be calculated exactly since the relevant Stokes drag solutions
needed for the reciprocal theorem are not available.Comment: 27 pages, 7 figure
Electroweak pinch technique to all orders
The generalization of the pinch technique to all orders in the electroweak
sector of the Standard Model within the class of the renormalizable 't Hooft
gauges, is presented. In particular, both the all-order PT gauge-boson-- and
scalar--fermions vertices, as well as the diagonal and mixed gauge-boson and
scalar self-energies are explicitly constructed. This is achieved through the
generalization to the Standard Model of the procedure recently applied to the
QCD case, which consist of two steps: (i) the identification of special Green's
functions, which serve as a common kernel to all self-energy and vertex
diagrams, and (ii) the study of the (on-shell) Slavnov-Taylor identities they
satisfy. It is then shown that the ghost, scalar and scalar--gauge-boson
Green's functions appearing in these identities capture precisely the result of
the pinching action at arbitrary order. It turns out that the aforementioned
Green's functions play a crucial role, their net effect being the non-trivial
modification of the ghost, scalar and scalar--gauge-boson diagrams of the
gauge-boson-- or scalar--fermions vertex we have started from, in such a way as
to dynamically generate the characteristic ghost and scalar sector of the
background field method. The pinch technique gauge-boson and scalar
self-energies are also explicitly constructed by resorting to the method of the
background-quantum identities.Comment: 48 pages, 8 figures; v2: typos correcte
The pinch technique at two-loops: The case of mass-less Yang-Mills theories
The generalization of the pinch technique beyond one loop is presented. It is
shown that the crucial physical principles of gauge-invariance, unitarity, and
gauge-fixing-parameter independence single out at two loops exactly the same
algorithm which has been used to define the pinch technique at one loop,
without any additional assumptions. The two-loop construction of the pinch
technique gluon self-energy, and quark-gluon vertex are carried out in detail
for the case of mass-less Yang-Mills theories, such as perturbative QCD. We
present two different but complementary derivations. First we carry out the
construction by directly rearranging two-loop diagrams. The analysis reveals
that, quite interestingly, the well-known one-loop correspondence between the
pinch technique and the background field method in the Feynman gauge persists
also at two-loops. The renormalization is discussed in detail, and is shown to
respect the aforementioned correspondence. Second, we present an absorptive
derivation, exploiting the unitarity of the -matrix and the underlying BRS
symmetry; at this stage we deal only with tree-level and one-loop physical
amplitudes. The gauge-invariant sub-amplitudes defined by means of this
absorptive construction correspond precisely to the imaginary parts of the
-point functions defined in the full two-loop derivation, thus furnishing a
highly non-trivial self-consistency check for the entire method. Various future
applications are briefly discussed.Comment: 29 pages, uses Revtex, 22 Figures in a separate ps fil
On the observability of the neutrino charge radius
It is shown that the probe-independent charge radius of the neutrino is a
physical observable; as such, it may be extracted from experiment, at least in
principle. This is accomplished by expressing a set of experimental
neutrino-electron cross-sections in terms of the finite charge radius and two
additional gauge- and renormalization-group-invariant quantities, corresponding
to the electroweak effective charge and mixing angle.Comment: 10 pages, 1 figure; a typo in Eq.1 corrected, some comments adde
The Two-Loop Pinch Technique in the Electroweak Sector
The generalization of the two-loop Pinch Technique to the Electroweak Sector
of the Standard Model is presented. We restrict ourselves to the case of
conserved external currents, and provide a detailed analysis of both the
charged and neutral sectors. The crucial ingredient for this construction is
the identification of the parts discarded during the pinching procedure with
well-defined contributions to the Slavnov-Taylor identity satisfied by the
off-shell one-loop gauge-boson vertices; the latter are nested inside the
conventional two-loop self-energies. It is shown by resorting to a set of
powerful identities that the two-loop effective Pinch Technique self-energies
coincide with the corresponding ones computed in the Background Feynman gauge.
The aforementioned identities are derived in the context of the
Batalin-Vilkovisky formalism, a fact which enables the individual treatment of
the self-energies of the photon and the -boson. Some possible
phenomenological applications are briefly discussed.Comment: 50 pages, uses axodra
Low-Energy Constraints on New Physics Revisited
It is possible to place constraints on non-Standard-Model gauge-boson
self-couplings and other new physics by studying their one-loop contributions
to precisely measured observables. We extend previous analyses which constrain
such nonstandard couplings, and we present the results in a compact and
transparent form. Particular attention is given to comparing results for the
light-Higgs scenario, where nonstandard effects are parameterized by an
effective Lagrangian with a linear realization of the electroweak symmetry
breaking sector, and the heavy-Higgs/strongly interacting scenario, described
by the electroweak chiral Lagrangian. The constraints on nonstandard
gauge-boson self-couplings which are obtained from a global analysis of
low-energy data and LEP/SLC measurements on the Z pole are updated and improved
from previous studies. Replaced version: tables and figures of Section VIb
recalculated. There were roundoff problems, especially in Fig. 8. Text
unchanged.Comment: \documentstyle[preprint,aps,floats,psfig]{revtex}, 10 figures,
postscript version available from ftp://ftp.kek.jp/kek/preprints/TH/TH-51
Gauge-Independent Off-Shell Fermion Self-Energies at Two Loops: The Cases of QED and QCD
We use the pinch technique formalism to construct the gauge-independent
off-shell two-loop fermion self-energy, both for Abelian (QED) and non-Abelian
(QCD) gauge theories. The new key observation is that all contributions
originating from the longitudinal parts of gauge boson propagators, by virtue
of the elementary tree-level Ward identities they trigger, give rise to
effective vertices, which do not exist in the original Lagrangian; all such
vertices cancel diagrammatically inside physical quantities, such as current
correlation functions or S-matrix elements. We present two different, but
complementary derivations: First, we explicitly track down the aforementioned
cancellations inside two-loop diagrams, resorting to nothing more than basic
algebraic manipulations. Second, we present an absorptive derivation,
exploiting the unitarity of the S-matrix, and the Ward identities imposed on
tree-level and one-loop physical amplitudes by gauge invariance, in the case of
QED, or by the underlying Becchi-Rouet-Stora symmetry, in the case of QCD. The
propagator-like sub-amplitude defined by means of this latter construction
corresponds precisely to the imaginary parts of the effective self-energy
obtained in the former case; the real part may be obtained from a (twice
subtracted) dispersion relation. As in the one-loop case, the final two-loop
fermion self-energy constructed using either method coincides with the
conventional fermion self-energy computed in the Feynman gauge.Comment: 30 pages; uses axodraw (axodraw.sty included in the src); final
version to appear in Phys. Rev.
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