16,248 research outputs found
Near-horizon modes and self-adjoint extensions of the Schroedinger operator
We investigate the dynamics of scalar fields in the near-horizon exterior
region of a Schwarzschild black hole. We show that low-energy modes are
typically long-living and might be considered as being confined near the black
hole horizon. Such dynamics are effectively governed by a Schroedinger operator
with infinitely many self-adjoint extensions parameterized by , a
situation closely resembling the case of an ordinary free particle moving on a
semiaxis. Even though these different self-adjoint extensions lead to
equivalent scattering and thermal processes, a comparison with a simplified
model suggests a physical prescription to chose the pertinent self-adjoint
extensions. However, since all extensions are in principle physically
equivalent, they might be considered in equal footing for statistical analyses
of near-horizon modes around black holes. Analogous results hold for any
non-extremal, spherically symmetric, asymptotically flat black hole.Comment: 10 pages, 1 fig, contribution submitted to the volume "Classical and
Quantum Physics: Geometry, Dynamics and Control. (60 Years Alberto Ibort
Fest)" Springer (2018
Spin and pseudospin symmetries of the Dirac equation with confining central potentials
We derive the node structure of the radial functions which are solutions of
the Dirac equation with scalar and vector confining central potentials,
in the conditions of exact spin or pseudospin symmetry, i.e., when one has
, where is a constant. We show that the node structure for exact
spin symmetry is the same as the one for central potentials which go to zero at
infinity but for exact pseudospin symmetry the structure is reversed. We obtain
the important result that it is possible to have positive energy bound
solutions in exact pseudospin symmetry conditions for confining potentials of
any shape, including naturally those used in hadron physics, from nuclear to
quark models. Since this does not happen for potentials going to zero at large
distances, used in nuclear relativistic mean-field potentials or in the atomic
nucleus, this shows the decisive importance of the asymptotic behavior of the
scalar and vector central potentials on the onset of pseudospin symmetry and on
the node structure of the radial functions. Finally, we show that these results
are still valid for negative energy bound solutions for anti-fermions.Comment: 7 pages, uses revtex macro
Frictional dynamics of viscoelastic solids driven on a rough surface
We study the effect of viscoelastic dynamics on the frictional properties of
a (mean field) spring-block system pulled on a rough surface by an external
drive. When the drive moves at constant velocity V, two dynamical regimes are
observed: at fast driving, above a critical threshold Vc, the system slides at
the drive velocity and displays a friction force with velocity weakening. Below
Vc the steady sliding becomes unstable and a stick-slip regime sets in. In the
slide-hold-slide driving protocol, a peak of the friction force appears after
the hold time and its amplitude increases with the hold duration. These
observations are consistent with the frictional force encoded
phenomenologically in the rate-and-state equations. Our model gives a
microscopical basis for such macroscopic description.Comment: 10 figures, 7 pages, +4 pages of appendi
Higgs-Boson Production and Decay Close to Thresholds
At one loop in the conventional on-mass-shell renormalization scheme, the
production and decay rates of the Higgs boson H exhibit singularities
proportional to (2 M_V - M)^{-1/2} as the Higgs-boson mass M approaches from
below the pair-production threshold of a vector boson V with mass M_V. This
problem is of phenomenological interest because the values 2 M_W and 2 M_Z,
corresponding to the W- and Z-boson thresholds, lie within the M range
presently favoured by electroweak precision data. We demonstrate how these
threshold singularities are eliminated when the definitions of mass and total
decay width of the Higgs boson are based on the complex-valued pole of its
propagator. We illustrate the phenomenological implications of this
modification for the partial width of the H -> W^+ W^- decay.Comment: 18 pages (Latex), 4 figures (Postscript); two references added; to
appear in Nuclear Physics
Role of the Coulomb and the vector-isovector potentials in the isospin asymmetry of nuclear pseudospin
We investigate the role of the Coulomb and the vector-isovector
potentials in the asymmetry of the neutron and proton pseudospin splittings in
nuclei. To this end, we solve the Dirac equation for the nucleons using central
vector and scalar potentials with Woods-Saxon shape and and dependent
Coulomb and potentials added to the vector potential. We study the
effect of these potentials on the energy splittings of proton and neutron
pseudospin partners along a Sn isotopic chain. We use an energy decomposition
proposed in a previous work to assess the effect of a pseudospin-orbit
potential on those splittings. We conclude that the effect of the Coulomb
potential is quite small and the potential gives the main contribution
to the observed isospin asymmetry of the pseudospin splittings. This isospin
asymmetry results from a cancellation of the various energy terms and cannot be
attributed only to the pseudospin-orbit term, confirming the dynamical
character of this symmetry pointed out in previous works.Comment: 9 pages, 11 figures, uses revtex4; title was changed and several
small corrections were made throughout the tex
One-loop Effective Action of the Holographic Antisymmetric Wilson Loop
We systematically study the spectrum of excitations and the one-loop
determinant of holographic Wilson loop operators in antisymmetric
representations of supersymmetric Yang-Mills theory.
Holographically, these operators are described by D5-branes carrying electric
flux and wrapping an in the bulk
background. We derive the dynamics of both bosonic and fermionic excitations
for such D5-branes. A particularly important configuration in this class is the
D5-brane with worldvolume and units of electric flux,
which is dual to the circular Wilson loop in the totally antisymmetric
representation of rank . For this Wilson loop, we obtain the spectrum, show
explicitly that it is supersymmetric and calculate the one-loop effective
action using heat kernel techniques.Comment: 42 pages, one tabl
On the relativistic L-S coupling
The fact that the Dirac equation is linear in the space and time derivatives
leads to the coupling of spin and orbital angular momenta that is of a pure
relativistic nature. We illustrate this fact by computing the solutions of the
Dirac equation in an infinite spherical well, which allows to go from the
relativistic to the non-relativistic limit by just varying the radius of the
well.Comment: LateX2e, 12 pages, 1 figure, accepted in Eur. J. Phy
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