3,096 research outputs found
The Traveling Salesman Problem: Low-Dimensionality Implies a Polynomial Time Approximation Scheme
The Traveling Salesman Problem (TSP) is among the most famous NP-hard
optimization problems. We design for this problem a randomized polynomial-time
algorithm that computes a (1+eps)-approximation to the optimal tour, for any
fixed eps>0, in TSP instances that form an arbitrary metric space with bounded
intrinsic dimension.
The celebrated results of Arora (A-98) and Mitchell (M-99) prove that the
above result holds in the special case of TSP in a fixed-dimensional Euclidean
space. Thus, our algorithm demonstrates that the algorithmic tractability of
metric TSP depends on the dimensionality of the space and not on its specific
geometry. This result resolves a problem that has been open since the
quasi-polynomial time algorithm of Talwar (T-04)
Effects of Chemical Potential on Hadron Masses in the Phase Transition Region
We study the response of hadron masses with respect to chemical potential at
. Our preliminary results of the pion channel show that in the confinement phase is significantly larger than that in
the deconfinement phase, which is consistent with the chiral restoration.Comment: LATTICE99 (finite temperature and density), 3 pages, 3 figure
Responses of hadrons to chemical potential at finite temperature
We present a framework to compute the responses of hadron masses to the
chemical potential in lattice QCD simulations. As a first trial, the screening
mass of the pseudoscalar meson and its first and second responses are
evaluated. We present results on a lattice with two
flavors of staggered quarks below and above . The responses to both the
isoscalar and isovector chemical potentials are obtained. They show different
behavior in the low and the high temperature phases, which may be explained as
a consequence of chiral symmetry breaking and restoration, respectively.Comment: 10 pages, 11 figure
Resonant enhanced diffusion in time dependent flow
Explicit examples of scalar enhanced diffusion due to resonances between
different transport mechanisms are presented. Their signature is provided by
the sharp and narrow peaks observed in the effective diffusivity coefficients
and, in the absence of molecular diffusion, by anomalous transport. For the
time-dependent flow considered here, resonances arise between their
oscillations in time and either molecular diffusion or a mean flow. The
effective diffusivities are calculated using multiscale techniques.Comment: 18 latex pages, 11 figure
Heavy Hadron Spectroscopy
I review recent theoretical advances in heavy hadron spectroscopy.Comment: Plenary talk at the XXXIII International Conference on High Energy
Physics (ICHEP 06), Moscow, Russia, July 26 - August 2, 2006; 11 page
Time-dependent quantum many-body theory of identical bosons in a double well: Early time ballistic interferences of fragmented and number entangled states
A time-dependent multiconfigurational self-consistent field theory is
presented to describe the many-body dynamics of a gas of identical bosonic
atoms confined to an external trapping potential at zero temperature from first
principles. A set of generalized evolution equations are developed, through the
time-dependent variational principle, which account for the complete and
self-consistent coupling between the expansion coefficients of each
configuration and the underlying one-body wave functions within a restricted
two state Fock space basis that includes the full effects of the condensate's
mean field as well as atomic correlation. The resulting dynamical equations are
a classical Hamiltonian system and, by construction, form a well-defined
initial value problem. They are implemented in an efficient numerical
algorithm. An example is presented, highlighting the generality of the theory,
in which the ballistic expansion of a fragmented condensate ground state is
compared to that of a macroscopic quantum superposition state, taken here to be
a highly entangled number state, upon releasing the external trapping
potential. Strikingly different many-body matter-wave dynamics emerge in each
case, accentuating the role of both atomic correlation and mean-field effects
in the two condensate states.Comment: 16 pages, 5 figure
Magnetic Fields in Quasar Cores II
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA)
telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of
1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g.,
3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the
jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be
produced by cluster or galactic-scale magnetic fields, but rather must be the
result of magnetic fields organized over the central 1-100 pc. The RMs of the
sources studied to date and the polarization properties of BL Lacs, quasars and
galaxies are shown to be consistent so far with the predictions of unified
schemes. The direct detection of high RMs in these quasar cores can explain the
low fractional core polarizations usually observed in quasars at centimeter
wavelengths as the result of irregularities in the Faraday screen on scales
smaller than the telescope beam. Variability in the RM of the core is reported
for 3C 279 between observations taken 1.5 years apart, indicating that the
Faraday screen changes on that timescale, or that the projected superluminal
motion of the inner jet components samples a new location in the screen with
time. Either way, these changes in the Faraday screen may explain the dramatic
variability in core polarization properties displayed by quasars.Comment: Accepted to the ApJ. 27 pages, 9 figures including figure 6 in colo
Pion Propagation near the QCD Chiral Phase Transition
We point out that, in analogy with spin waves in antiferromagnets, all
parameters describing the real-time propagation of soft pions at temperatures
below the QCD chiral phase transition can be expressed in terms of static
correlators. This allows, in principle, the determination of the soft pion
dispersion relation on the lattice. Using scaling and universality arguments,
we determine the critical behavior of the parameters of pion propagation. We
predict that when the critical temperature is approached from below, the pole
mass of the pion drops despite the growth of the pion screening mass. This fact
is attributed to the decrease of the pion velocity near the phase transition.Comment: 8 pages (single column), RevTeX; added references, version to be
published in PR
One-Flavour Hybrid Monte Carlo with Wilson Fermions
The Wilson fermion determinant can be written as product of the determinants
of two hermitian positive definite matrices. This formulation allows to
simulate non-degenerate quark flavors by means of the hybrid Monte Carlo
algorithm. A major numerical difficulty is the occurrence of nested inversions.
We construct a Uzawa iteration scheme which treats the nested system within one
iterative process.Comment: 11 pages, to appear in proceedings of the workshop "Numerical
Challenges in Lattice QCD", Springer Verla
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