3,250 research outputs found

### A quantum jump description for the non-Markovian dynamics of the spin-boson model

We derive a time-convolutionless master equation for the spin-boson model in
the weak coupling limit. The temporarily negative decay rates in the master
equation indicate short time memory effects in the dynamics which is explicitly
revealed when the dynamics is studied using the non-Markovian jump description.
The approach gives new insight into the memory effects influencing the spin
dynamics and demonstrates, how for the spin-boson model the the co-operative
action of different channels complicates the detection of memory effects in the
dynamics.Comment: 9 pages, 6 figures, submitted to Proceedings of CEWQO200

### Thermodynamics of the QCD plasma and the large-N limit

The equilibrium thermodynamic properties of the SU(N) plasma at finite
temperature are studied non-perturbatively in the large-N limit, via lattice
simulations. We present high-precision numerical results for the pressure,
trace of the energy-momentum tensor, energy density and entropy density of
SU(N) Yang-Mills theories with N=3, 4, 5, 6 and 8 colors, in a temperature
range from 0.8T_c to 3.4T_c (where T_c denotes the critical deconfinement
temperature). The results, normalized according to the number of gluons, show a
very mild dependence on N, supporting the idea that the dynamics of the
strongly-interacting QCD plasma could admit a description based on large-N
models. We compare our numerical data with general expectations about the
thermal behavior of the deconfined gluon plasma and with various theoretical
descriptions, including, in particular, the improved holographic QCD model
recently proposed by Kiritsis and collaborators. We also comment on the
relevance of an AdS/CFT description for the QCD plasma in a phenomenologically
interesting temperature range where the system, while still strongly-coupled,
approaches a `quasi-conformal' regime characterized by approximate scale
invariance. Finally, we perform an extrapolation of our results to the N to
$\infty$ limit.Comment: 1+38 pages, 13 eps figures; v2: added reference

### Determination of the $\Delta S = 1$ weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions

A new method to determine the low-energy couplings of the $\Delta S=1$ weak
Hamiltonian is presented. It relies on a matching of the topological poles in
$1/m^2$ of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the $\epsilon$-regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure

### Weak low-energy couplings from topological zero-mode wavefunctions

We discuss a new method to determine the low-energy couplings of the $\Delta
S=1$ weak Hamiltonian in the $\epsilon$-regime. It relies on a matching of the
topological poles in $1/m^2$ of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200

### Helical Magnetic Fields from Inflation

We analyze the generation of seed magnetic fields during de Sitter inflation
considering a non-invariant conformal term in the electromagnetic Lagrangian of
the form $-\frac14 I(\phi) F_{\mu \nu} \widetilde{F}^{\mu \nu}$, where
$I(\phi)$ is a pseudoscalar function of a non-trivial background field $\phi$.
In particular, we consider a toy model, that could be realized owing to the
coupling between the photon and either a (tachyonic) massive pseudoscalar field
and a massless pseudoscalar field non-minimally coupled to gravity, where $I$
follows a simple power-law behavior $I(k,\eta) = g/(-k\eta)^{\beta}$ during
inflation, while it is negligibly small subsequently. Here, $g$ is a positive
dimensionless constant, $k$ the wavenumber, $\eta$ the conformal time, and
$\beta$ a real positive number. We find that only when $\beta = 1$ and $0.1
\lesssim g \lesssim 2$ astrophysically interesting fields can be produced as
excitation of the vacuum, and that they are maximally helical.Comment: 17 pages, 1 figure, subsection IIc and references added; accepted for
publication in IJMP

### Real-time static potential in hot QCD

We derive a static potential for a heavy quark-antiquark pair propagating in
Minkowski time at finite temperature, by defining a suitable gauge-invariant
Green's function and computing it to first non-trivial order in Hard Thermal
Loop resummed perturbation theory. The resulting Debye-screened potential could
be used in models that attempt to describe the ``melting'' of heavy quarkonium
at high temperatures. We show, in particular, that the potential develops an
imaginary part, implying that thermal effects generate a finite width for the
quarkonium peak in the dilepton production rate. For quarkonium with a very
heavy constituent mass M, the width can be ignored for T \lsim g^2 M/12\pi,
where g^2 is the strong gauge coupling; for a physical case like bottomonium,
it could become important at temperatures as low as 250 MeV. Finally, we point
out that the physics related to the finite width originates from the
Landau-damping of low-frequency gauge fields, and could be studied
non-perturbatively by making use of the classical approximation.Comment: 20 pages. v2: a number of clarifications and a few references added;
published versio

### Remote polarization entanglement generation by local dephasing and frequency upconversion

We introduce a scheme for remote entanglement generation for the photon
polarization. The technique is based on transferring the initial frequency
correlations to specific polarization-frequency correlations by local dephasing
and their subsequent removal by frequency up-conversion. On fundamental level,
our theoretical results show how to create and transfer entanglement, to
particles which never interact, by means of local operations. This possibility
stems from the multi-path interference and its control in frequency space. For
applications, the developed techniques and results allow for the remote
generation of entanglement with distant parties without Bell state measurements
and opens the perspective to probe frequency-frequency entanglement by
measuring the polarization state of the photons.Comment: 8 page

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