38,664 research outputs found
Casimir energy density in closed hyperbolic universes
The original Casimir effect results from the difference in the vacuum
energies of the electromagnetic field, between that in a region of space with
boundary conditions and that in the same region without boundary conditions. In
this paper we develop the theory of a similar situation, involving a scalar
field in spacetimes with compact spatial sections of negative spatial
curvature.Comment: 10 pages. Contribution to the "Fifth Alexander Friedmann
International Seminar on Gravitation and Cosmology," Joao Pessoa, Brazil,
2002. Revised version, with altered Abstract and one new referenc
Symmetries and Triplet Dispersion in a Modified Shastry-Sutherland Model for SrCu_2(BO_3)_2
We investigate the one-triplet dispersion in a modified Shastry-Sutherland
Model for SrCu_2(BO_3)_2 by means of a series expansion about the limit of
strong dimerization. Our perturbative method is based on a continuous unitary
transformation that maps the original Hamiltonian to an effective, energy
quanta conserving block diagonal Hamiltonian H_{eff}. The dispersion splits
into two branches which are nearly degenerated. We analyse the symmetries of
the model and show that space group operations are necessary to explain the
degeneracy of the dispersion at k=0 and at the border of the magnetic Brillouin
zone. Moreover, we investigate the behaviour of the dispersion for small |k|
and compare our results to INS data.Comment: 9 pages, 8 figures accepted by J. Phys.: Condens. Matte
Observation of quantum spin noise in a 1D light-atoms quantum interface
We observe collective quantum spin states of an ensemble of atoms in a
one-dimensional light-atom interface. Strings of hundreds of cesium atoms
trapped in the evanescent fiel of a tapered nanofiber are prepared in a
coherent spin state, a superposition of the two clock states. A weak quantum
nondemolition measurement of one projection of the collective spin is performed
using a detuned probe dispersively coupled to the collective atomic observable,
followed by a strong destructive measurement of the same spin projection. For
the coherent spin state we achieve the value of the quantum projection noise 40
dB above the detection noise, well above the 3 dB required for reconstruction
of the negative Wigner function of nonclassical states. We analyze the effects
of strong spatial inhomogeneity inherent to atoms trapped and probed by the
evanescent waves. We furthermore study temporal dynamics of quantum
fluctuations relevant for measurement-induced spin squeezing and assess the
impact of thermal atomic motion. This work paves the road towards observation
of spin squeezed and entangled states and many-body interactions in 1D spin
ensembles
Anomalous Viscosity of an Expanding Quark-Gluon Plasma
We argue that an expanding quark-gluon plasma has an anomalous viscosity,
which arises from interactions with dynamically generated color fields. We
derive an expression for the anomalous viscosity in the turbulent plasma domain
and apply it to the hydrodynamic expansion phase, when the quark-gluon plasma
is near equilibrium. The anomalous viscosity dominates over the collisional
viscosity for weak coupling and not too late times. This effect may provide an
explanation for the apparent ``nearly perfect'' liquidity of the matter
produced in nuclear collisions at the Relativistic Heavy Ion Collider without
the assumption that it is a strongly coupled state.Comment: Final version accepted for publicatio
Kilohertz laser ablation for doping helium nanodroplets
A new setup for doping helium nanodroplets by means of laser ablation at
kilohertz repetition rate is presented. The doping process is characterized and
two distinct regimes of laser ablation are identified. The setup is shown to be
efficient and stable enough to be used for spectroscopy, as demonstrated on
beam-depletion spectra of lithium atoms attached to helium nanodroplets. For
the first time, helium droplets are doped with high temperature refractory
materials such as titanium and tantalum. Doping with the non-volatile DNA basis
Guanine is found to be efficient and a number of oligomers are detected
Simulations of Electron Capture and Low-Mass Iron Core Supernovae
The evolutionary pathways of core-collapse supernova progenitors at the
low-mass end of the spectrum are beset with major uncertainties. In recent
years, a variety of evolutionary channels has been discovered in addition to
the classical electron capture supernova channel of super-AGB stars. The few
available progenitor models at the low-mass end have been studied with great
success in supernova simulations as the peculiar density structure makes for
robust neutrino-driven explosions in this mass range. Detailed nucleosynthesis
calculations have been conducted both for models of electron capture supernovae
and low-mass iron core supernovae and revealed an interesting production of the
lighter trans-iron elements (such as Zn, Sr, Y, Zr) as well as rare isotopes
like Ca-48 and Fe-60. We stress the need to explore the low-mass end of the
supernova spectrum further and link various observables to understand the
diversity of explosions in this regime.Comment: 7 page, 3 figures, proceedings of the conference "The AGB-Supernova
Mass Transition", to appear in Memorie della Societ\`a Astronomica Italian
Correlated Emission of Hadrons from Recombination of Correlated Partons
We discuss different sources of hadron correlations in relativistic heavy ion
collisions. We show that correlations among partons in a quasi-thermal medium
can lead to the correlated emission of hadrons by quark recombination and argue
that this mechanism offers a plausible explanation for the dihadron
correlations in the few GeV/c momentum range observed in Au+Au collisions at
RHIC.Comment: 4 pages, 2 figures; v2: typo on p.4 correcte
Properties of quark matter produced in heavy ion collision
We describe the hadronization of quark matter assuming that quarks creating
hadrons coalesce from a continuous mass distribution. The pion and antiproton
spectrum as well as the momentum dependence of the antiproton to pion ratio are
calculated. This model reproduces fairly well the experimental data at RHIC
energies.Comment: 9 pages, 6 Postscript figures, typos are correcte
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