572 research outputs found
Leptoquarks decaying to a top quark and a charged lepton at hadron colliders
We study the sensitivity of the Tevatron and the 7 TeV LHC to a leptoquark S
coupling to a top quark and a charged lepton L (= e, mu, or tau). For the
Tevatron, we focus on the case m_S < m_t, where the leptoquark pair production
cross section is large, and the decay is three-body: S --> W b L^{\pm}. We
argue that existing Tevatron observations could exclude m_S \lsim 160 GeV. For
m_S > m_t, we show that the LHC experiments with low integrated luminosity
could be sensitive to such leptoquarks decaying to tl^{\pm} with l= mu or tau.Comment: 13 pages, 6 figures, minor changes (typos
Non-sequential triple ionization in strong fields
We consider the final stage of triple ionization of atoms in a strong
linearly polarized laser field. We propose that for intensities below the
saturation value for triple ionization the process is dominated by the
simultaneous escape of three electrons from a highly excited intermediate
complex. We identify within a classical model two pathways to triple
ionization, one with a triangular configuration of electrons and one with a
more linear one. Both are saddles in phase space. A stability analysis
indicates that the triangular configuration has the larger cross sections and
should be the dominant one. Trajectory simulations within the dominant symmetry
subspace reproduce the experimentally observed distribution of ion momenta
parallel to the polarization axis.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Localization of solitons: linear response of the mean-field ground state to weak external potentials
Two aspects of bright matter-wave solitons in weak external potentials are
discussed. First, we briefly review recent results on the Anderson localization
of an entire soliton in disordered potentials [Sacha et al. PRL 103, 210402
(2009)], as a paradigmatic showcase of genuine quantum dynamics beyond simple
perturbation theory. Second, we calculate the linear response of the mean-field
soliton shape to a weak, but otherwise arbitrary external potential, with a
detailed application to lattice potentials.Comment: Selected paper presented at the 2010 Spring Meeting of the Quantum
Optics and Photonics Section of the German Physical Society. V2: minor
changes, published versio
Routes towards Anderson-Like localization of Bose-Einstein condensates in disordered optical lattices
We investigate, both experimentally and theoretically, possible routes
towards Anderson-like localization of Bose-Einstein condensates in disordered
potentials. The dependence of this quantum interference effect on the nonlinear
interactions and the shape of the disorder potential is investigated.
Experiments with an optical lattice and a superimposed disordered potential
reveal the lack of Anderson localization. A theoretical analysis shows that
this absence is due to the large length scale of the disorder potential as well
as its screening by the nonlinear interactions. Further analysis shows that
incommensurable superlattices should allow for the observation of the
cross-over from the nonlinear screening regime to the Anderson localized case
within realistic experimental parameters.Comment: 4 pages to appear in Phys. Rev. Let
Attosecond electron thermalization by laser-driven electron recollision in atoms
Nonsequential multiple ionization of atoms in intense laser fields is
initiated by a recollision between an electron, freed by tunneling, and its
parent ion. Following recollision, the initial electron shares its energy with
several bound electrons. We use a classical model based on rapid electron
thermalization to interpret recent experiments. For neon, good agreement with
the available data is obtained with an upper bound of 460 attoseconds for the
thermalization time.Comment: 5 pages revtex and 4 figures (eps files
Stochastic ionization through noble tori: Renormalization results
We find that chaos in the stochastic ionization problem develops through the
break-up of a sequence of noble tori. In addition to being very accurate, our
method of choice, the renormalization map, is ideally suited for analyzing
properties at criticality. Our computations of chaos thresholds agree closely
with the widely used empirical Chirikov criterion
Changes of the topological charge of vortices
We consider changes of the topological charge of vortices in quantum
mechanics by investigating analytical examples where the creation or
annihilation of vortices occurs. In classical hydrodynamics of non-viscous
fluids the Helmholtz-Kelvin theorem ensures that the velocity field circulation
is conserved. We discuss applicability of the theorem in the hydrodynamical
formulation of quantum mechanics showing that the assumptions of the theorem
may be broken in quantum evolution of the wavefunction leading to a change of
the topological charge.Comment: 5 pages, 2 figures, version accepted for publication in J. Phys.
Pathways to double ionization of atoms in strong fields
We discuss the final stages of double ionization of atoms in a strong
linearly polarized laser field within a classical model. We propose that all
trajectories leading to non-sequential double ionization pass close to a saddle
in phase space which we identify and characterize. The saddle lies in a two
degree of freedom subspace of symmetrically escaping electrons. The
distribution of longitudinal momenta of ions as calculated within the subspace
shows the double hump structure observed in experiments. Including a symmetric
bending mode of the electrons allows us to reproduce the transverse ion
momenta. We discuss also a path to sequential ionization and show that it does
not lead to the observed momentum distributions.Comment: 10 pages, 10 figures; fig.6 and 7 exchanged in the final version
accepted for publication in Phys. Rev.
A method for collective excitation of Bose-Einstein condensate
It is shown that by an appropriate modification of the trapping potential one
may create collective excitation in cold atom Bose-Einstein condensate. The
proposed method is complementary to earlier suggestions. It seems to be
feasible experimentally --- it requires only a proper change in time of the
potential in atomic traps, as realized in laboratories already.Comment: 4 pages, 4 figures; major revision, several references added,
interacting particles case adde
Analysis of Localization Phenomena in Weakly Interacting Disordered Lattice Gases
Disorder plays a crucial role in many systems particularly in solid state
physics. However, the disorder in a particular system can usually not be chosen
or controlled. We show that the unique control available for ultracold atomic
gases may be used for the production and observation of disordered quantum
degenerate gases. A detailed analysis of localization effects for two possible
realizations of a disordered potential is presented. In a theoretical analysis
clear localization effects are observed when a superlattice is used to provide
a quasiperiodic disorder. The effects of localization are analyzed by
investigating the superfluid fraction and the localization length within the
system. The theoretical analysis in this paper paves a clear path for the
future observation of Anderson-like localization in disordered quantum gases.Comment: 9 pages, 13 figure
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