5,549 research outputs found
ENCORE: An Extended Contractor Renormalization algorithm
Contractor renormalization (CORE) is a real-space renormalization-group
method to derive effective Hamiltionians for microscopic models. The original
CORE method is based on a real-space decomposition of the lattice into small
blocks and the effective degrees of freedom on the lattice are tensor products
of those on the small blocks. We present an extension of the CORE method that
overcomes this restriction. Our generalization allows the application of CORE
to derive arbitrary effective models whose Hilbert space is not just a tensor
product of local degrees of freedom. The method is especially well suited to
search for microscopic models to emulate low-energy exotic models and can guide
the design of quantum devices.Comment: 5 pages, 4 figure
Lifshitz-point critical behaviour to
We comment on a recent letter by L. C. de Albuquerque and M. M.
Leite (J. Phys. A: Math. Gen. 34 (2001) L327-L332), in which results to
second order in were presented for the critical
exponents , and
of d-dimensional systems at m-axial Lifshitz points.
We point out that their results are at variance with ours. The discrepancy is
due to their incorrect computation of momentum-space integrals. Their
speculation that the field-theoretic renormalization group approach, if
performed in position space, might give results different from when it is
performed in momentum space is refuted.Comment: Latex file, uses the included iop stylefiles; Uses the texdraw
package to generate included figure
Effective Spin Couplings in the Mott Insulator of the Honeycomb Lattice Hubbard Model
Motivated by the recent discovery of a spin liquid phase for the Hubbard
model on the honeycomb lattice at half-filling, we apply both perturbative and
non-perturbative techniques to derive effective spin Hamiltonians describing
the low-energy physics of the Mott-insulating phase of the system. Exact
diagonalizations of the so-derived models on small clusters are performed, in
order to assess the quality of the effective low-energy theory in the
spin-liquid regime. We show that six-spin interactions on the elementary loop
of the honeycomb lattice are the dominant sub-leading effective couplings. A
minimal spin model is shown to reproduce most of the energetic properties of
the Hubbard model on the honeycomb lattice in its spin-liquid phase.
Surprisingly, a more elaborate effective low-energy spin model obtained by a
systematic graph expansion rather disagrees beyond a certain point with the
numerical results for the Hubbard model at intermediate couplings.Comment: 20 pages, 10 figure
Detection of Exotic Massive Hadrons in Ultra High Energy Cosmic Ray Telescopes
We investigate the detection of exotic massive strongly interacting hadrons
(uhecrons) in ultra high energy cosmic ray telescopes. The conclusion is that
experiments such as the Pierre Auger Observatory have the potential to detect
these particles. It is shown that uhecron showers have clear distinctive
features when compared to proton and nuclear showers. The simulation of uhecron
air showers, and its detection and reconstruction by fluorescence telescopes is
described. We determine basic cuts in observables that will separate uhecrons
from the cosmic ray bulk, assuming this is composed by protons. If these are
composed by heavier nucleus the separation will be much improved. We also
discuss photon induced showers. The complementarity between uhecron detection
in accelerator experiments is discussed.Comment: 9 page 9 figure
Neutrino Telescopes as a Direct Probe of Supersymmetry Breaking
We consider supersymmetric models where the scale of supersymmetry breaking
lies between 5 GeV and 5 GeV. In this class of
theories, which includes models of gauge mediated supersymmetry breaking, the
lightest supersymmetric particle is the gravitino. The next to lightest
supersymmetric particle is typically a long lived charged slepton with a
lifetime between a microsecond and a second, depending on its mass. Collisions
of high energy neutrinos with nucleons in the earth can result in the
production of a pair of these sleptons. Their very high boost means they
typically decay outside the earth. We investigate the production of these
particles by the diffuse flux of high energy neutrinos, and the potential for
their observation in large ice or water Cerenkov detectors. The relatively
small cross-section for the production of supersymmetric particles is partially
compensated for by the very long range of heavy particles. The signal in the
detector consists of two parallel charged tracks emerging from the earth about
100 meters apart, with very little background. A detailed calculation using the
Waxman-Bahcall limit on the neutrino flux and realistic spectra shows that
km experiments could see as many as 4 events a year. We conclude that
neutrino telescopes will complement collider searches in the determination of
the supersymmetry breaking scale, and may even give the first evidence for
supersymmetry at the weak scale.Comment: 4 pages, 3 figure
Mechanisms for Spin-Supersolidity in S=1/2 Spin-Dimer Antiferromagnets
Using perturbative expansions and the contractor renormalization (CORE)
algorithm, we obtain effective hard-core bosonic Hamiltonians describing the
low-energy physics of spin-dimer antiferromagnets known to display
supersolid phases under an applied magnetic field. The resulting effective
models are investigated by means of mean-field analysis and quantum Monte Carlo
simulations. A "leapfrog mechanism", through means of which extra singlets
delocalize in a checkerboard-solid environment via correlated hoppings, is
unveiled that accounts for the supersolid behavior.Comment: 12 pages, 10 figure
X, Y and Z States
Many new states in the charmonium mass region were recently discovered by
BaBar, Belle, CLEO-c, CDF, D0, BESIII, LHCb and CMS Collaborations. We use the
QCD Sum Rule approach to study the possible structure of some of these states.Comment: Contribution for the proceedings of the "XII Quark Confinement and
the Hadron Spectrum - CONF12" conferenc
Astrophysical Neutrino Event Rates and Sensitivity for Neutrino Telescopes
Spectacular processes in astrophysical sites produce high-energy cosmic rays
which are further accelerated by Fermi-shocks into a power-law spectrum. These,
in passing through radiation fields and matter, produce neutrinos. Neutrino
telescopes are designed with large detection volumes to observe such
astrophysical sources. A large volume is necessary because the fluxes and
cross-sections are small. We estimate various telescopes' sensitivities and
expected event rates from astrophysical sources of high-energy neutrinos. We
find that an ideal detector of km^2 incident area can be sensitive to a flux of
neutrinos integrated over energy from 10^5 and 10^{7} GeV as low as 1.3 *
10^(-8) * E^(-2) (GeV/cm^2 s sr) which is three times smaller than the
Waxman-Bachall conservative upper limit on potential neutrino flux. A real
detector will have degraded performance. Detection from known point sources is
possible but unlikely unless there is prior knowledge of the source location
and neutrino arrival time.Comment: Section added +modification
EXPERIMENTAL ANALYSIS OF THE TRANSIENT BEHAVIOR OF AN ENGINE COOLING RADIATOR
The aim of this work is to study the behavior of an engine cooling radiator in a transient state, with circulation of heated water. An experimental apparatus was constructed with the radiator inside a wind tunnel. The water is pumped from a small water tank to a heater, passes through the heat exchanger, and returns to the tank. The tests were carried out with constant flowrates of water and air, and the heater was turned on and then off according to a pulse function. The temperatures of the radiator, the air and the water were measured on several points with thermocouples and a thermal camera. The flow of water and air were measured as well. A fast dynamics because of the radiator was observed, whereas a slow one was noticed due to water heating in the tank. The steady state was reached after 15 min. These results might be useful in the project of a vehicle cooling system under transient conditions
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